Epilepsy Surgery
Introduction
Epilepsy is one of the most common neurologic diseases in the world, and is present in up to 4% of the world's population. Treatment of epilepsy with medications is a major effort of the World Health Organization, as many patients with epilepsy never receive the treatment that would render them seizure free. Epilepsy surgery is even less likely to be considered in developed or developing countries because of a lack of resources or a lack of recognition by the physician that a treatable syndrome exists.
For excellent patient education resources, visit eMedicine's Brain and Nervous System Center. Also, see eMedicine's patient education article Epilepsy.
Photograph of a patient positioned for lumbar drain insertion prior to corpus callosotomy.
This article addresses some of the most common types of epilepsy surgeries performed and explains the syndromes for which they are most useful. The type of evaluation required to prepare a patient for epilepsy surgery is discussed. Common procedures and imaging modalities used to aid with the diagnosis and localization of epilepsy prior to epilepsy surgery are also discussed. Advanced diagnostic techniques, namely, intracranial electrode placement, is covered in some technical detail. The following 4 surgeries are presented as models on which a preliminary understanding of the various surgical strategies can be gained:
- Anteromedial temporal resection (AMTR): This is the most frequently practiced surgery for a common and well-described disorder, namely medial temporal lobe epilepsy. It serves as a model for other focal resections.
- Corpus callosotomy: This is the only applicable surgery for generalized epilepsy syndromes.
- Modified hemispherotomy
- Multiple subpial transection (MST): This is a newer type of epilepsy surgery, and has limited applications that are still being developed.
Reasons for considering surgical intervention
Although intracranial surgery involves inherent risks, these risks do not equal the risks of uncontrolled seizures. The morbidity and mortality of seizures include accidental injury; cognitive decline; sudden unexplained death in epilepsy (SUDEP); and psychological, social, and vocational impairment.
- Accidental injuries commonly include fractures, burns, dental injuries, lacerations, and head injuries.
- Mortality rates for patients with nonconvulsive and convulsive seizures far exceed those for age-matched controls. Among patients with poorly controlled epilepsy, SUDEP can reach a rate of 1 death per 500 patients per year. Risk factors are poorly controlled seizures and low serum antiepileptic drug (AED) levels.
- Cognitive decline and memory loss over time has been demonstrated to occur in patients with certain epilepsy syndromes who have recurrent convulsive seizures or episodes of status epilepticus.
- Both depression and anxiety are very common among patients with medically refractory epilepsy.
- Intractable epilepsy prevents driving, and reduces fertility and marriage rates.
- Vocational issues include inability to be employed or, if employed, underemployment.
Criteria for surgery
A candidate for epilepsy surgery must have not attained acceptable seizure control with sufficient trials of AEDs and have a reasonable chance of benefiting from surgery. Adequate AED trials must be considered within the context of the patient's circumstances and form of epilepsy. The precise numbers and type of AEDs to try before recommending surgery are unknown for the various epilepsy syndromes.
However, the American Academy of Neurology, the American Association of Neurological Surgeons, and the American Epilepsy Society has recommended the following practice parameters:
- Patients with disabling complex partial seizures, with or without secondarily generalized seizures, who have failed appropriate trials of first line antiepileptic drugs, should be considered for referral to an epilepsy surgery center, although criteria for failure of drug treatment have not been definitely established (level A rating)
- Patients referred to an epilepsy center for the reasons stated above who meet established criteria for an anteromedial temporal lobe resection and who will accept the risks and benefits of this procedure, as opposed to continuing pharmacotherapy, should be offered surgical treatment (level A rating).3
Strategy for a surgical workup
The presurgical evaluation for epilepsy has changed substantially in the past few decades, most notably since the advent of long-term video-EEG monitoring in the late 1970s, advanced neuroimaging, and subspecialty epilepsy centers. The presurgical evaluation requires input from many members of an integrated team, which includes neurologists, neurophysiologists, neuropsychologists, social workers, radiologists, nurses, and epilepsy neurosurgeons. Aspects of the presurgical evaluation include the patient's history and physical examination findings, social circumstances, seizure syndrome and severity, and diagnostic testing (see Diagnostic Phase).
A surgical plan is usually developed at a multidisciplinary team conference. This allows open discussion among multiple experts so that the surgical approach is unique and is tailored to the individual's personal needs and epilepsy syndrome. When all presurgical information points to a unifying location and theory regarding focal seizure onset (also referred to as concordant data), then the patient may proceed directly to resective surgery. When data are inadequate to define a resective strategy, then diagnostic intracranial electrodes may be considered to further define the syndrome or site of seizure onset prior to any resective surgery.
Diagnostic Phase
A presurgical diagnosis is made after a classification of the patient's seizure types and specific epilepsy syndrome. The International League Against Epilepsy (ILAE) recognizes approximately 10 types of recurrent seizures and approximately 40 forms of epilepsy syndromes. Both classification schemes reflect the fact that seizures and epilepsies naturally fall into 2 major groups, based on the site of seizure onset in the brain, either (1) partial-onset (focal, localization-related) or (2) generalized-onset.5,6
The signs and symptoms (semiology) experienced by the patient and the EEG pattern recorded at ictal onset determine a seizure diagnosis. This process begins by recording a careful history. For example, an event that is initiated with a blank stare and arrest of motion and then progresses to the development of automatisms (ie, automatic repetitive semipurposeful movements) is likely a complex partial seizure.
For initial AED therapy, a presumptive diagnosis based on history may suffice. However, even under the best circumstances, a diagnosis based solely on the history can be incorrect. Therefore, when proposing surgery for intractable epilepsy, the most accurate way to determine the epilepsy syndrome diagnosis and brain location of seizure origin is with long-term video-EEG monitoring (VEEG).
The following is a simplification of the international classification of epileptic seizures:
- Partial seizures (seizures that begin locally)
- Simple partial seizures (consciousness not impaired; SPS)
- Complex partial seizures (consciousness impaired; CPS)
- Partial seizures that secondarily progress to generalized tonic-clonic seizures (SGTCS)
- Generalized seizures (seizures that arise diffusely)
- Absence seizures
- Atypical absence seizures
- Clonic seizures
- Tonic seizures
- Tonic-clonic seizures
- Myoclonic seizures
- Atonic seizures
- Unclassified seizures
A simplification of the previous international classification of the epilepsy syndromes has been proposed and is presented as follows:
- Focal (partial, localization-related) syndromes
- Idiopathic (some are hereditary) - Benign childhood epilepsy with centrotemporal spikes, childhood epilepsy with occipital paroxysms, autosomal dominant nocturnal frontal lobe epilepsy, and familial temporal lobe epilepsies
- Symptomatic (to known cause or lesion) - Temporal lobe epilepsies (mesial, lateral), frontal lobe epilepsies (several locations), parietal lobe epilepsies, occipital lobe epilepsies, and Rasmussen's encephalitis
- Generalized syndromes
- Idiopathic (most are hereditary) - Benign neonatal familial convulsions, childhood absence epilepsy, juvenile absence epilepsy, juvenile myoclonic epilepsy, generalized epilepsy and febrile seizures plus, and various progressive myoclonic epilepsies
- Symptomatic (or probably symptomatic) - West syndrome (infantile spasms) and Lennox-Gastaut syndrome
- Mixed - Continuous spike waves in slow sleep and acquired epileptic aphasia (Landau-Kleffner syndrome [LKS])
- Special situations
Because seizures may result from cortical lesions or malformations, neuroimaging can often help identify and localize this damage and, therefore, the site of seizure onset.
- Skull radiography: Routine skull films are of little value.
- CT scanning: MRI has replaced routine CT scanning because of superior imaging. The one exception is that CT scanning demonstrates intraparenchymal calcium and acute bleeding better than MRI. This may be helpful in distinguishing certain types of tumors or CNS syndromes, such as tuberous sclerosis.
- MRI: Brain MRI unquestionably is the best structural imaging study. Every surgical evaluation should include a complete study with special thin-cut coronal magnified views perpendicular to the axis of the temporal horn. These views can demonstrate mesial temporal sclerosis.
- Positron emission tomography (PET): Unlike MRI or CT scanning, PET scanning demonstrates brain glucose metabolism rather than structure. The typical finding from an interictal scan is hypometabolism in the region of the epileptic focus and, if the scan is obtained during a seizure, the typical finding is hypermetabolism from the focus.
- Single-photon emission tomography (SPECT): SPECT scanning helps visualize blood flow through the brain and, therefore, has been evaluated as another method for localizing the epileptic focus.
- Interictal SPECT scans are less accurate than ictal scans. However, ictal scans are problematic because the tracer must be injected within the initial seconds of seizure onset. This requires that the radionucleotide be available on the monitoring ward 24 hours per day with personnel licensed (under state law) to administer intravenous injections.
- A newer methodology that has greater accuracy than either ictal or interictal SPECT scanning is subtraction ictal-interictal SPECT co-registered to MRI (SISCOM). This requires obtaining scans (separated by at least 48 h to accommodate radionucleotide washout) during an interictal period and within seconds of seizure onset. These scans are then subtracted from one another with the use of specialized computer software. This leaves a better indication of the cortical area of ictal onset. This subtracted scan can then be co-registered onto the patient's MRI to provide support for the location of the focus.
- Magnetoencephalography/magnetic source imaging (MEG/MSI): MEG is a noninvasive type of imaging based on the brain's ability to produce small magnetic dipoles with neuronal discharges. When large groups of neurons fire synchronously, as in an interictal epileptiform discharge, this creates a miniscule magnetic dipole that can be sensed with sophisticated imaging equipment and complicated computer analysis. This map of the epileptiform discharge can be useful for diagnostic purposes and for presurgical planning of intracranial electrode placement.
The most useful test in epilepsy diagnosis is the EEG. An essential mistake is to place too much value on an isolated individual interictal recording. Assuming that a normal finding from the interictal scalp electrode EEG precludes the diagnosis of epilepsy is erroneous. The presence or absence of epileptiform discharges is highly variable, has no relationship to seizure frequency (for most epilepsies), and may be affected by antiepileptic drugs (AEDs). Thus, EEGs may need to be repeated several times before epileptiform discharges are observed. By definition, "epileptiform discharges" are interictal patterns that include spikes, spike-and-slow-wave complexes, sharp waves, and sharp-and-slow-wave complexes. The reader should refer to the second international glossary of EEG terms for precise definitions.
When the waking scalp EEG fails to demonstrate evidence of epilepsy, but the diagnosis is still suspected, a sleep EEG is recommended. Epileptiform discharges commonly activate during non–rapid eye movement sleep in some epilepsies. On fewer occasions than in the past, sphenoidal and additional extracranial electrodes are used to help reveal epileptiform (interictal) and ictal discharges.
Although the standard scalp EEG is helpful in making a diagnosis of epilepsy, it is not usually used when the physician makes major surgical decisions. This is because the distribution of interictal EEG discharges may not correctly localize epileptic foci. This error occurs for several reasons. First, discharges can be multifocal, although one focus can be the origin of all seizures. Further, because the EEG consists of volume-conducted potentials that originate over a relatively large area of cortical gray matter, some discharges can shift apparent location within or between hemispheres, and others may appear widely or even diffusely over the scalp. In addition, to obtain the most accurate data possible, recording sufficient numbers of the patient's typical seizures is important.
Like interictal discharges, ictal discharges vary somewhat. Also, more than one seizure focus or psychogenic or physiologic nonepileptic seizure may be found when numerous episodes are recorded.The latter may greatly affect a decision to proceed with surgery. Therefore, all surgical candidates should undergo long-term video-EEG monitoring prior to surgery to record several typical seizures.
Features of the scalp EEG ictal discharge, other than just location, can be helpful in the presurgical evaluation. For example, the authors reported that the frequency of the initial ictal discharge in the scalp EEG correlates with the degree of hippocampal pathology in temporal lobe epilepsy.
Neuropsychological testing
Neuropsychological testing, also known as neurocognitive testing, is the process of empirically testing an individual's functioning across a variety of cognitive domains, including attention, concentration, language, visuospatial skills, verbal and visual memory, and executive abilities (ie, problem solving, organization, strategic planning), as well as personality and emotional functioning. Neuropsychologists use a variety of standardized tests to assess all areas of cognitive functioning. An individual's performance on these standardized tests is then analyzed using normative data, and their performance is then compared with their peers and with what is expected based on estimates of their premorbid level of cognitive functioning. Patterns of cognitive strengths and weaknesses then provide information about brain-related deficits in cognitive functioning.
Well established is the idea that performance on specific neuropsychological tests is subserved by various areas of the brain, thus providing lateralizing and localizing information about brain functioning. For example, a right-handed individual's performance on verbal memory tests (ie, story memory, verbal paired associates learning, and list-learning tasks) is largely subserved by left mesial temporal structures. Similarly, a right-handed individual's performance on visual memory tests (ie, figural memory and complex figure memory tasks) is largely subserved by right mesial temporal structures. Thus, an individuals cognitive strengths and weaknesses on neuropsychological testing can indicate areas of brain dysfunction, which provides additional information that can help localize an epileptogenic focus.
Additionally, an individual's cognitive strengths can provide information about possible cognitive risks involved in undergoing a neurosurgical resection to treat medically-intractable epilepsy because a risk of increased memory deficits after resection of mesial temporal structures exists.Routinely, all epilepsy surgical candidates undergo extensive neuropsychological testing. Neuropsychological testing is tailored to best assess the type of epilepsy with which each individual presents, often focusing heavily on cognitive functioning that has likely been most affected by a hypothesized epileptogenic focus. As a result, testing is often tailored to uniquely assess the cognitive strengths and weaknesses for each individual and can vary across individuals.
Neuropsychological testing can also be used for nonoperative or postoperative epilepsy patients to assess their level of cognitive functioning in order to assist with vocational and cognitive rehabilitation in the context of their neurological disorder.
Intracarotid amobarbital (Wada) test
The intracarotid amobarbital test was developed by Jun Wada to preoperatively determine which hemisphere contains language function. Although this continues to be a use of the Wada test, functional MRI for language now provides a noninvasive way to more accurately lateralize and localize language functioning. The primary use of the Wada test is to assess language lateralization and the ability of the contralateral mesial temporal structures to support memory postoperatively when AMTR is being considered to treat medically-intractable epilepsy.
The procedure is conducted by individually cannulating each internal carotid artery. After contrast arteriography verifies that blood flows to the corresponding hemisphere and not to the brainstem or contralateral side, a dose of sodium amobarbital (sufficient to impede hemispheric function) is injected. If the drug produces a contralateral hemiparesis, function of that hemisphere is assumed to be minimized.
Language lateralization is determined by conducting a comprehensive screen of various components of language, including expressive language, receptive language, naming, repetition, and complex syntactical comprehension. If an individual is able to engage in all aspects of language while they demonstrate hemiparesis, language function is assumed to not be represented within that hemisphere. Assessing a variety of aspects of language is important because some individuals possess a bilateral representation of language, such that some aspects of language are subserved by one hemisphere while other aspects of language are subserved by the contralateral hemisphere.
Memory functioning is assessed by presenting the patient with visual and auditory items (often using multiple sensory modalities to maximize the chance of encoding) while the hemisphere is anesthetized. Following recovery of hemispheric function, which is confirmed by return of gross and fine motor function, as well as language if possible, the patient is then tested for their free, cued, and recognition recall of the items with which they were presented during anesthetization. If the patient is able to accurately recall 75% of the items presented to them during anesthetization, that provides evidence that the contralateral hemisphere should be able to support memory after ATMR of the hemisphere that was anesthetized.
If the patient is unable to accurately recall a sufficient number of items, that information provides evidence that the patient may experience significant memory difficulties postoperatively. Importantly, patients should undergo neuropsychological testing prior to undergoing a Wada procedure, because that testing provides important information about their baseline level of memory functioning and more accurately informs the results of the Wada test.
The deficiencies of this evaluation for memory function directly relate to the multiple problems of targeting a drug effect to specific brain structures via cerebral blood flow. Injection of a drug into the internal carotid artery does not assure drug effect in the basal temporal area in general or the hippocampal region specifically (an area that subserve memory function). This is due to variations in the direct blood supply to the hippocampus and inequalities in delivery when the drug is injected into the blood stream.
Intracranial EEG recordings
Intracranial EEGs recordings, also known as chronic electrocorticography (ECoG) is an invasive procedure that is performed when noninvasive presurgical evaluation has not led to definitive localization of a seizure syndrome and surgical plan. The presurgical team, which includes a neurosurgeon, neurologist, neuropsychologist, social worker, and neuroradiologist, considers all previous accumulated data to determine an appropriate strategy for placement of intracranial electrodes. Any combination of intracranial strips, grids, and/or depth electrodes may be tailored to answer the specific questions posed by the case history and presentation specific to that particular patient, depending on the needs of the patient, experience of the monitoring team, and resources available for use.
The following are examples of instances that may require invasive intracranial monitoring:
- Seizures are lateralized but not localized (eg, a left-sided, widespread frontal-temporal onset).
- Seizures are localized but not lateralized (eg, ictal EEG patterns that appear maximally over both temporal lobes).
- Seizures are neither localized nor lateralized (eg, stereotyped complex partial seizures with diffuse ictal changes or initial changes obscured by artifact).
- Seizure localization is discordant with other data (eg, EEG ictal scalp data discordant with neuroimaging [MRI, PET, SPECT/SISCOM, MEG] or neuropsychological data).
- Relationship of seizure onset to functional tissue must be determined (eg, seizures with early involvement of language or motor function).
- Relationship of seizure onset to lesion must be determined (eg, dual pathology or multiple intracranial lesions).
- Seizures are clinically suspected, but video-EEG is inadequate for defining them (eg, simple partial seizures with no detectable scalp EEG ictal discharge or suspected epileptic seizures with unusual semiology that suggests psychogenic seizures [pseudo-pseudo seizures])
Definitions
Depth, strip, and grid electrodes are implantable intracranial devices used to record the electrocorticogram (ECoG) chronically, and to stimulate the cortex to determine function (see Image 1). Depth electrodes are multicontact, thin, tubular, rigid or semirigid electrodes that penetrate the brain substances for the purpose of recording from deep structures (see Image 2). Intracranial strip electrodes are a linear array of 2-16 disk electrodes embedded in a strip of silastic,or they can be tubular in structure, similar to depth electrodes.Grid electrodes are parallel rows of similar numbers of electrodes that can be configured in standard or custom designs according to the preferences of the surgeon and the abilities of the manufacturer. Grid and strip electrodes are designed to be in direct contact with brain neocortex.
Examples of various grid electrodes available for specific needs. These range in size and number of contacts
Examples of 3 depth electrodes with varying numbers of contacts. Note: The stylus is in place and is removed once the electrode has been inserted.
In most cases, electrodes are placed in the subdural space, although they may occasionally be used in the epidural space. All of these electrode types are constructed from biologically inert materials (ie, silastic, stainless steel, platinum). Platinum electrodes are more easily seen on fluoroscopic images than are stainless steel electrodes and are compatible with MRI so that postoperative diagnostic and localizing neuroimaging studies can be obtained.
Diagnostic Phase - Intracranial Recording
Strip Electrodes
Strip electrodes are used most often to lateralize the side of seizure onset in frontal and temporal lobe epilepsy, but they may also be used to obtain survey studies over all cortical surfaces of the brain. They are usually implanted while the patient is under general anesthesia, according to the preoperative plan created by the epilepsy monitoring team. Electrodes can be directed safely over long distances within the calvaria by surfing electrodes over the brain with a gentle fluid pulse. Fluoroscopy is used to confirm placement prior to closure of the wound.Electrode wires are tunneled under the skin with a 13-guage passing needle designed for that purpose (Ad-Tech Medical; Racine, Wis), to exit the skin several inches away from the burr-hole incision to decrease the risk of infection. Cerebrospinal fluid (CSF) may leak from around the electrode wires during the first 3 days after implantation. This can be minimized if the scalp exit sites for the electrode tails are directed superiorly toward the vertex of the skull. The suggested risk of infection from a CSF leak differs from center to center; however, completely sealing the skin with a foreign body in place is difficult and CSF leaks are not uncommon. Because of this, the dressing is changed as often as needed.
Some authors suggest placing a cable-retaining suture in the scalp, both to attempt to secure the electrode and to decrease CSF leak. The lead author has not found it helpful in decreasing CSF leak, and all electrode companies manufacture quick-release connectors designed to break apart easily if tugged.
Patients are monitored with electrodes in place in the monitoring unit for 2-7 days to record their typical seizures. Subdural strip electrodes are removed through the skin without an open surgical procedure by gentle traction on the electrodes, under conscious sedation or general anesthesia.
Although strip electrodes can be inserted epidurally,this practice is not advisable for routine cases because the exposure is limited to the lateral convexities of the brain. The epidural space in the temporal fossa does not allow the electrode to be advanced medially enough to record from the parahippocampal gyrus, and electrodes cannot be placed over mesial frontal lobe cortex. In most exploratory investigations, these locations should be sampled. However, epidural placement may be the most reasonable option when recording from a patient with a prior craniotomy because scarring may obliterate the subdural space.
Subdural Grid Electrodes
Arrays of electrodes more than one column wide are considered intracranial grids (see Image 1). Practically speaking, electrode arrays that are 2-3 contacts wide cannot be easily passed for any substantial distance through a burr hole and require a craniotomy for placement. Once the decision to proceed with a craniotomy is made, grid arrays of 5-8 rows (20-64 contacts) are usually used to maximize coverage over the craniotomy site. The craniotomy site is determined based on data gathered during the presurgical evaluation; usually, a large craniotomy is performed to accommodate up to an 8 X 8-cm grid. Prophylactic antibiotics and dexamethasone are routinely administered. Mannitol is not used unless necessary because the putative space created by a fluid shift could adversely contribute to hematoma formation after closure.Once placed, the grid is sutured to the dura to prevent motion. Often, one or more strip electrodes are added to sample adjacent areas or lobes, such as the interhemispheric fissure or basal temporal lobe. The electrode tails of grids are similarly tunneled to strip electrodes, toward the vertex, to avoid CSF leaks. At some centers, the bone flap is frozen under sterile conditions until the patient returns to the operating room (OR) for grid removal. However, the author prefers to leave the bone flap in place to decrease the risk of hematoma formation at the craniotomy site.
After recovery in the postanesthesia care unit, the patient is transferred to the video-EEG monitoring suite, where the patient is hooked up on the day of surgery and a formal head dressing is placed. Acute nursing care is provided in the monitoring suite for the first 24 hours after craniotomy, similar to the level practiced in a neurological step-down unit.
The grid is removed when sufficient data have been obtained to determine the site of ictal onset or, alternatively, to determine that no more recording is likely to lead to satisfactory localization. If resective surgery is planned, then the relationship of the grid to the underlying cortex must stay unchanged while the craniotomy is reopened. The dura is opened, leaving the grid-stabilizing sutures intact and keeping all relationships between electrode contacts and unique underlying cortical topography (eg, blood vessels) undisturbed. Once these relationships have been documented and the surgeon has extrapolated the mapped data to the underlying cortex, the grid is removed and discarded.
Resective surgery is performed, with the neurophysiologist or pathologist present in the OR as necessary. If resection is not performed at the time of grid removal (eg, because of hemorrhage, edema, patient preference, or insufficient data), then pertinent landmarks may be documented with digital photography or frameless stereotaxy for reoperation at a later date.
Higher complication rates for intracranial grid electrode placement have been associated with an increased number of electrode contacts, increased length of the monitoring period, placement of burr holes in addition to the craniotomy, and multiple cable exit sites.At the authors' institution, antiepileptic drugs (AEDs) are stopped on the morning of surgery (except for benzodiazepines and barbiturates, which are given in reduced doses). Video-EEG monitoring continues until the epilepsy team believes adequate ictal data have been obtained, usually in 2-8 days.
Depth Electrodes
Depth electrodes are used most commonly for recording from the hippocampus and amygdala. The approach preferred by the authors is to place electrodes via the occipital, parasagittal route.This trajectory allows for simultaneous implantation into the amygdala and anterior and posterior hippocampus using a single multicontact electrode. Placement is performed with either a frameless system, discussed below, or a stereotactic frame with adequate clearance at the back of the head. MRI is used with both frame-based and frameless stereotactic placement to allow direct visualization of the target and trajectory.Indications for depth electrode placement are expanding as neuroimaging becomes more sophisticated and more complex epilepsy syndromes are identified. This is particularly true among the malformations of cortical development, particularly when the dysplastic lesion is subcortical. Depth electrode recordings into hypothalamic hamartomas and periventricular nodular heterotopia have shown ictal onsets beginning within the lesions, which subsequently spread to produce clinical seizures.
Depth electrodes are most often used in conjunction with other subdural strip or grid electrodes so that multiple brain areas are sampled simultaneously to avoid false localization based on insufficient data collection. Specific intracranial EEG ictal discharge frequencies, locations, and patterns can suggest, preoperatively, certain types of histopathological findings
Intracranial Electrode Recordings and Removal
At most epilepsy centers, rooms with hard-wired EEG and video telemetry instruments are only used for long-term monitoring and not for general medical or surgical patients. Patients may be taken directly from the recovery room to one of these monitoring rooms, or may stay in the ICU for the first night of monitoring. Antiepileptic drugs are tapered or withdrawn and video-EEG recordings are begun the day of electrode implantation.The relative accuracy of ictal versus interictal electrographic activity remains somewhat controversial. In general, interictal spikes are usually more diffuse than the ictal onset zone, and bilateral interictal spikes can occur in patients who ultimately do well after a unilateral resection, and therefore, do not preclude a good surgical outcome. Obtaining ictal recordings is usually preferable in order to confirm the significance of interictal abnormalities, as ictal recordings are considered more accurate that interictal data. One possible exception to this general guideline might be when recording from a focal cortical dysplasia; distinctive interictal epileptiform patterns have been identified that may provide enough data to guide a resection based solely on interictal data.
When monitoring a lesion with intracranial electrodes, seizure outcome is best when both the lesion and the ictal onset zone are completely resected; outcome is compromised when either the lesion or the ictal onset zone is incompletely resected. In the case of nonlesional epilepsy, seizure freedom is more difficult to achieve, even in cases in which the ictal onset zone has been well studied with intracranial electrodes.
The number of seizures required to consider an intracranial study complete depends on the specific issues involved with treating a particular patient. In general, an arbitrary number of 3 typical clinical seizures has been considered the minimum number to be captured; however, exceptions to this rule abound For example, a patient with a posterior temporal or parietal lesion and scalp EEG localization to the anterior temporal lobe that is delayed compared with clinical onset might be considered for an intracranial study to confirm the clinical suspicion that seizures are falsely localized to the anterior temporal lobe. In such a case, 1-2 seizure onsets with intracranial EEG ictal onset directly over the lesion might provide sufficient data to proceed to surgical resection.
On the other hand, a patient with nonlesional epilepsy and bilateral ictal onset over both temporal areas on scalp EEG monitoring would require many more than 3 seizures to be recorded to exclude bilateral temporal onset and/or establish a predominant side of onset.
The type of ictal onset recorded may influence the number of seizures required. Fewer seizures need to be recorded in patients who have identical ictal onset patterns over the exact same electrode contacts in every seizure. More seizures need to be recorded in patients with multifocal ictal onsets in different electrodes from seizure to seizure. Initial ictal changes at the beginning of a seizure are more important than late changes and the propagation patterns of the seizure. Seizures that occur seconds to minutes after a previous seizure may be disregarded as being potentially misleading.
When intracranial monitoring is complete, most subdural strip and depth electrodes can be removed percutaneously in the OR after administrating a conscious sedation protocol or general anesthesia. Because of concern over contracting virally transmitted disease (eg, AIDS, Creutzfeldt-Jakob disease, hepatitis), recording electrodes are commonly discarded after a single use.
New computer-assisted prediction paradigms are being created to analyze ictal onset and changes in the background electrical state, interictal spike frequency, confluence analysis, and chaos theory in order to predict seizure occurrence minutes to hours before ictal onset. Intracranial monitoring will likely need to adapt to accommodate these new technologies in the near future.
Staged procedures
The following are reasons a second intracranial study may be considered:
- Strip electrode survey study for lateralization and localization to a lobe, with a planned return at a later date for definition of the ictal onset zone and cortical mapping as necessary
- Reimplantation of a second grid because of failed localization secondary to sampling error (Seizures may occur on the margin of the grid, be diffuse, or show variable propagation that makes seizure localization uncertain.)
- Recurrent seizures after a previous intracranial study and resection
Therefore, when planning a second intracranial study, the surgeon should anticipate a difficult entry and should use many of the strategies used for reoperation in other craniotomies, including enlarging the bone flap until pristine dura is encountered and opening the dura away from the previous operative site and away from functional cortex. If adhesions are encountered, an operating microscope should be reserved and used early in the dissection.
The authors of this article routinely plan to spend 1-3 hours under the operating microscope when scheduling a reoperation for epilepsy. Even with tedious dissection, adhesions can limit the distal passage of electrodes and limit the effectiveness of a repeat intracranial study.
Consecutive grid placement
Several authors have suggested an alternative to delayed reoperation. These authors advocate immediate reimplantation during the same hospitalization if the findings from the first grid were not diagnostic. Doyle refers to this as a 3-stage procedure, and Lee describes the same technique as a double grid. This technique has certain advantages over delayed return for implantation of a second grid. Firstly, adhesions do not obscure the subdural space, which can limit grid reimplantation. Secondly, cortical injury can be avoided because adhesions do not need to be dissected from functional tissue. Thirdly, intracranial EEG changes can be compared with the previous study while the subtleties of the previous intracranial electrocorticogram (ECoG) are still fresh in the minds of the evaluating team.
However, others feel that placing a grid again immediately after a first operation is likely to alter the network of epileptogenesis and that resultant seizures with second grid implantation might not best represent the patient's typical seizures.
Doyle advocates performing a limited resection of the ictal onset zone seen with the first grid because a partial resection of the epileptogenic region may help identify which additional areas are still contributing to seizure onset once the major site of ictal origin has been removed. Favorable seizure control was achieved with no apparent increase in surgical morbidity in 42 three-stage procedures when compared with 369 traditional grid protocols and in 18 double procedures compared with 165 routine intracranial procedures.
Documenting the Intracranial Study
Each intracranial study is unique to the patient for whom it is designed. Even routine intracranial cases have subtle variations in electrode placement based on the patient's anatomy. Some sort of documentation of the intracranial study is advisable for a number of reasons, including confirmation of the accuracy of placement, communication with the neurophysiology team, and correlation with gyral anatomy or intracranial lesions. The possible options available for documentation, beginning with the simplest and proceeding to the more complicated, are discussed in this section.When placing intracranial strip electrodes, the most vital piece of documentation regarding the study begins in the OR. As each electrode is inserted, the operating surgeon describes the identifying characteristics of that electrode (eg, length, color coding, scalp exit site) and its intracranial position to an assistant, usually an OR nurse, who documents this information directly in the operative record or chart notes so that no confusion is encountered when the electrodes are eventually connected. Most EEG technologists appreciate a line diagram handwritten by the surgeon in the chart. This simple step can eliminate many potential sources of human error, particularly with extensive intracranial surveys, and facilitates communication between all members of the team.
Another simple way to document the operative technique is for the monitoring team to use an anatomical brain diagram and transparencies of the grid montage to create a mock-up of the surgery. The image created is compared with fluoroscopic images taken at surgery so that a relatively accurate rendition of the electrode placement is available within minutes on the day of the surgery. These images can be quite helpful in interpreting seizure onset and propagation during EEG monitoring.
In more complicated cases, a member of the monitoring team is present in the OR to take digital photographs of the exposed cortex before and after grid placement. Digital photography helps identify the relationship of a grid to the sylvian fissure and is one of the best methods to document the fine anatomy of the brain, including sulcal and arteriolar anatomy that cannot be seen with advanced imaging techniques.
More sophisticated imaging techniques have been developed and are being used with increasing frequency as advanced imaging software becomes more available. Cranial MRI with a head coil can be obtained safely in patients with intracranial grids and strip electrodes in place if a few safety considerations are kept in mind. Most electrode manufacturers endorse platinum electrodes as MRI compatible, although stainless steel electrodes have also been used without apparent patient injury. Because each epilepsy center has different procedures, checking the recommendations of the particular electrode manufacturer prior to obtaining an MRI is advisable. In all cases, current loops can theoretically be created within a magnetic field if the electrode tails are allowed to contact one another. Therefore, all electrode tails should be isolated before obtaining MRI.
Most MRI workstations or software packages allow for 3-dimensional reconstruction of images, which are often more useful than traditional MRIs or CT scans. In addition, most frameless stereotaxic navigation systems allow for image reconstructions of an MRI that can be merged with preoperative anatomic, functional, or angiographic imaging to create an accurate rendition of the grid in relation to relevant operative anatomy.
Cortical Mapping
Often, in addition to defining the location of the epileptogenic cortex, the surgeon must determine its relationship to functional cortex. This requires mapping the cortex underlying an implanted grid electrode. The technique is similar to that performed acutely in the OR and requires a testing protocol appropriate to the cortical region being investigated. Cortical stimulation is performed using commercially available constant-current generators. Cortical mapping is performed by selecting 2 adjacent electrodes (1-cm intervals) because bipolar stimulation provides more precise control of current flow. Bipolar pulses at 50 Hz are used for language, motor, and sensory mapping.Extraoperative cortical mapping has several advantages over acute intraoperative mapping. Functional mapping may be performed in multiple sessions if necessary. For example, if a seizure that impairs function is generated during mapping, the patient may be allowed to recover for several hours (or days) until proceeding with further mapping. Advanced paradigms may be performed over hours or days that would not be possible in the acute intraoperative setting. Once mapping is completed, the patient, family, and surgeon have time to discuss the potential risks and benefits of surgery and to make decisions to accept or reject a functional loss that might be associated with surgery.
During brain stimulation, brain mapping is performed by a neuropsychologist or physician, who may test language or motor function. A clinical neurophysiologist reviews the ECoG during stimulation to ensure that any disruption of neurological function is due to the stimulation and not an afterdischarge. Afterdischarge potentials are repetitive spike discharges or electrical seizures directly provoked by electrical stimulation and may limit the ability to map the brain or may lead to a seizure. The amount of current needed to produce an effect varies from patient to patient and between cortical regions. Enough current should be used to produce a reliable effect without causing afterdischarge. Occasionally, pain can result from current spread to the dura or a nearby cortical vessel. In such cases, a particular contact may not be suitable for mapping.
Surgeons are often encouraged to be present for intracranial, extraoperative mapping, particularly early in their careers. Some of the slight variations in mapping technique and interpretation have subtle ramifications for the surgeon and are different from those that a neurologist or neuropsychologist might appreciate. On some occasions, mapped cortical regions vary from what one would expect from classic anatomic studies, particularly in areas of cortical malformations.
On other occasions, mapping different pairs of electrodes in a specific region (eg, motor or language areas) might allow the surgeon to appreciate the orientation of a crucial region relative to the orientation of the grid or to adjacent contacts. Finally, subtle errors in naming or language may be present extraoperatively, when the patient is off AEDs, and these errors may change when the patient is reloaded with AEDs Such subtleties of extraoperative mapping can be useful to the surgeon if observed personally prior to a resection.
Implanted grid arrays are excellent tools for identification of the position of sensorimotor cortex through somatosensory evoked potentials (SSEPs). Allison and colleagues have reviewed the rationale and technique,and others have discussed the clinical experience with subdural grids for this purpose.SSEPs can be used during acute recording in the OR, using subdural strip electrodes to identify primary motor cortex. The strip electrode must be positioned to traverse motor and sensory cortex, and it may need to be repositioned several times during intraoperative recording to optimize the signal. Therefore, if SSEPs are planned during extraoperative monitoring, using a subdural grid to increase the surface coverage by the electrode array is advantageous in order to optimize the location of motor cortex (see Images 8-9).
Craniotomy that shows an intracranial grid in place. The grid lies over left primary motor and sensory cortex, near the vertex of the skull. The patient is positioned laterally, and the face is at the right. The intracranial study was performed to document seizure onsets in a patient who had a previous oligodendroglioma removed. Seizures began just posterior to leg motor cortex, in primary sensory cortex.
The primary motor cortex is located with the use of extraoperative somatosensory evoked potentials and intraoperative cortical stimulation, as well as the grid described in Image 8. The Penfield instrument in the field is positioned over primary motor cortex.
Sometimes, extraoperative mapping indicates that the ictal onset zone is close to or overlies critical motor or speech areas. On such occasions, using the advantages of awake operative language mapping may be helpful at the time of grid removal and resection of the epileptic focus. Although extraoperative mapping has many advantages, the accuracy of its spatial resolution is limited to 1 cm, namely, the distance between 2 electrode pairs. Sometimes, awake intraoperative mapping helps confirm which electrode is the contact that directly overlies cortical function. This can be particularly important when the 2 electrodes span a sulcus; in such cases, awake mapping may allow the surgeon to determine which gyrus is involved in function and which gyrus is not.In such cases, the epileptic zone may be resected up to the pial margin without disturbing function, as long as the vascular structures within the pia are preserved.
Complications
Published series of infection rates from all types of intracranial electrodes range from 0-12%. The morbidity of surgery depends on the type of electrode implantation; intracranial strip electrodes have the lowest morbidity, and intracranial grid placement has the highest morbidity.The most common cause of morbidity in subdural strip placement is infection. One randomized study found a 0.85% rate of infection between groups treated with antibiotics prior to surgery, compared with a 3% infection rate when no antibiotics were given. No difference in infection rates was noted between patients who received antibiotics for the duration of strip electrode implantation versus those who received only a single preoperative dose; therefore, at the authors' institution, a single dose of antibiotics is given immediately before strip or grid implantation.
Other complications of intracranial strip placement include cortical contusion, cerebral edema, brain abscess, subdural empyema and subdural hemorrhage, placement of electrodes into the brain parenchyma, accidental extraction of electrodes, and superficial wound infection. Many of these complications are minor and cause no long-term problems; permanent neurological deficit is seen in less than 1% of patients who undergo intracranial strip electrode implantation.
Complications of grid implantation include infection, transient neurological deficit, hematoma, cerebral edema with increased intracranial pressure, and infarction. Transient neurological deficits can occur secondary to edema or hematoma associated with the grid. In most cases, if neurologic compromise is evident, the grid should be removed immediately. Cerebral edema is more likely to occur with an increase in the duration of the monitoring session or with a greater number of intracranial electrodes Some authors have been concerned that pediatric patients are more likely to develop increased intracranial pressure because, theoretically, less space is available to accommodate the mass of an intracranial grid however, others have not found this to be a concern.
Complications from depth electrodes include intraparenchymal hemorrhage, subarachnoid hemorrhage, arterial spasm, and misplacement of the electrode. The rate of permanent neurological deficit from occipital depth electrode placement has been reported at less than 1%.The risk of hitting the brainstem or posterior cerebral artery with occipitally inserted depth electrodes may be decreased by (1) targeting tip placement in the lateral amygdala and lateral hippocampus, (2) making sure the occipital burr hole is not too medial, and (3) confirming the trajectory with an image guidance system prior to electrode placement.
Alternatively, one can attempt to place the depth electrode into the lateral ventricle rather than into the hippocampus. To do so, a rigid guide cannula is placed into the ventricle (verified by identifying CSF flow from the cannula), and then the depth electrode is placed into the ventricle. The electrode then lies in the temporal horn adjacent to the hippocampus, with its tip entering the targeted amygdala.
Ictal recording from the ventricle usually conducts signal well, with only occasional failure The signal obtained amplifies (1) both hippocampal and parahippocampal ictal onsets and (2) intraparenchymal amygdala onset. The risk of subarachnoid hemorrhage may be minimized if depth electrodes are placed under direct visualization through a burr hole rather than using a closed twist-drill technique, which helps avoid draining veins during placement, and by using image-guided stereotaxy and contrast-enhanced MRI to avoid surface veins at the entry point.
Some surgeons prefer orthogonal placement of depth electrodes, which has been facilitated by frameless stereotactic techniques. Several systems allow placement of orthogonal depth electrodes via twist drill or burr holes in the temporal fossa. Stereotaxy is required to avoid vascular injury to vessels of the middle cerebral artery and to gain accurate placement into the amygdala and hippocampus. Electrodes must be secured at the skin to ensure migration of the electrodes does not occur as the medial trajectory of these electrodes is towards the brainstem.
Appropriate Surgical Strategy
Surgery may be considered as either definitive and palliative. Definitive surgery carries a significant chance of producing complete, or at least 70-90%, improvement in seizures. The goal of palliative procedures is to decrease seizure frequency, but rarely results in seizure freedom.
In general, definitive surgeries physically remove seizure-producing cortex from the brain. Examples are resections of small seizure-producing tumors, vascular abnormalities, cortical malformations, or lesions such as mesial temporal sclerosis.
Palliative surgeries usually disrupt pathways involved in seizure production and propagation or attempt to disrupt seizures with the use of electrical stimulation; thus, the potential for continued seizures always remains.
A general approach to epileptogenic lesions
Tumors that cause epilepsy are frequently low-grade astrocytomas, oligodendrogliomas, or gangliogliomas. Commonly, they are well circumscribed, and excellent outcomes can be achieved with a lesionectomy that includes the immediate surrounding abnormal cortex.
However, there is a temptation to rush to surgery, making the assumption that the tumor is causing the seizures. The more cautious epilepsy surgeon may consider the possibility of alternative diagnoses, such as dual pathology, (the presence of hippocampal atrophy and tumor in the same patient), or look for evidence for concomitant cortical dysplasia, depending upon the suspected tumor pathology.
The presurgical evaluation in the setting of tumor pathology is to determine that the EEG findings and the tumor are not discordant, which can occur approximately 10-15% of the time. In such cases, the lesion may not be solely responsible for the epilepsy, and a more extensive resection may be required to control the epilepsy. When the tumor and the lesion are concordant, the most common mistake when treating these tumors is to remove only the gross tumor and not the immediate surrounding tissue, thus leaving epileptogenic tissue in place and resulting in clinical seizure resumption. If seizures continue after tumor removal, an assumption is that the tumor was not completely removed, regardless of MRI evidence of a clean resection.
Small vascular abnormalities, such as cavernous hemangiomas surrounded by hemosiderin, can be extremely epileptogenic. Removal of the vascular abnormality and surrounding hemosiderin-stained cortex may be all that is necessary for an excellent seizure outcome in approximately 80% of patients, provided the presurgical evaluation is concordant.
Large vascular abnormalities (eg, high-flow arteriovenous malformations) are commonly associated with seizures. Unlike the smaller vascular lesions, the relationship between the structural lesion and the epileptogenic cortex is not always clear, and simple lesionectomy often fails to stop the seizures. Because of the technical difficulties involved, implanting a large grid electrode over these lesions for the purpose of mapping is usually not feasible. Therefore, treatment of such lesions should seldom be approached purely as an epilepsy surgery. One successful approach is to stage surgeries; the vascular abnormality is removed with the intent of further investigation and possible surgery if seizures remain.
Epileptogenic congenital malformations of cortical development, such as cortical dysplasias, heterotopia, schizencephalic clefts, and the various forms of phakomatoses, are very difficult to treat surgically and usually require more extensive evaluations and tailored resections based on implanted grid electrodes.Seizure outcome after resection of such malformations is variable and directly relates to the complexity of the lesion.
Traumatic encephalomalacia is treatable surgically, with varied results. The difficulty with these lesions is that cortical damage often extends much further than the area of obvious anatomical damage, or multiple regions of the brain are affected simultaneously.
Controversial issues in definitive surgery
With regard to performing an acute intraoperative electrocorticogram (ECoG), some surgeons believe that epileptiform discharges recorded acutely during surgery indicate which part of the brain is epileptogenic; therefore, these surgeons use the ECoG to define boundaries for cortical resections. This approach is referred to as a tailored resection in that no 2 operations are identical. Because anesthetic agents affect the reliability of acute ECoG recordings and prevent the surgeon from mapping cortical function (eg, language), tailored resections are performed best with the patient under local anesthesia.
Other surgeons believe that an acute, interictal, ECoG is not reliable enough to determine surgical boundaries and can be inaccurate; these surgeons proceed with a standardized surgery, wherein tissue important for generating seizures is removed without considering the ECoG for some surgeries. If a question is to be answered with ECoG, then the patient receives an intracranial study, and long-term monitoring with intracranial electrodes is performed to capture ictal ECoG, and a "tailored resection" is based on intracranial ictal recording rather than short-term, less accurate, interictal data.
For anteromedial temporal resection (AMTR), no scientific data support the belief that ECoG-guided resections produce results superior to standard surgeries. A more rational approach is that the surgeon should recognize the specific syndromes for which a standardized operation is very effective. The best example is complex partial seizures of mesial temporal lobe epilepsy with hippocampal sclerosis. Such patients have excellent seizure and language outcomes after standard surgeries while under general anesthesia. Likewise, excellent results are expected from the removal of a well-circumscribed capillary hemangioma from a patient with complex partial seizures.
In contrast, patients with epilepsy secondary to other etiologies in other brain regions may need extensive cortical mapping and an ictal ECoG. In such cases, implantation of a large subdural grid electrode for long-term monitoring (with an ECoG) and mapping is preferred because it yields results superior to performing an acute interictal ECoG.
Operative Technique - Anteromedial Temporal Resection
Anteromedial Temporal Resection for Hippocampal Sclerosis
Anteromedial temporal resection (AMTR) is the most commonly performed surgery for epilepsy. It has the clearest indications and best results. Thus, AMTR is used as a criterion standard against which other procedures can be judged and strategies extrapolated.Indications
Candidates for AMTR have the following:
- Complex partial seizures with semiology typical of mesial temporal lobe epilepsy
- MRI evidence of unilateral hippocampal atrophy and increased T2-weighted signal (At some centers, volumetric measurements of the hippocampus are routinely obtained.)
- Unilateral temporal lobe hypometabolism on PET scans if MRI findings are nonlesional in nature
- EEG confirmation that seizures begin over the temporal area ipsilateral to the hippocampal atrophy or PET scan evidence of hypometabolism
An issue that remains unclear is how much time must elapse, or how many AEDs must be found to be inadequate, before surgery is recommended. For this reason, the US National Institutes of Health sponsored a large prospective study entitled Early Randomized Surgery for Epilepsy Trial (ERSET). ERSET randomly compared AMTR against 2 additional years of aggressive AED management. The results of ERSET are expected to be available in 2009.
Preoperative considerations
Because valproic acid can be associated with bleeding disorders, the authors choose to discontinue this medication at least 3 weeks prior to surgery and replace it with another medication, if possible. On the morning of surgery, the patient receives the usual medication dosage with a few sips of water. 1 g of cephalosporin can be administered intravenously 1 hour before the incision is made. No further antibiotics are administered. Adults are administered 10 mg dexamethasone intravenously 1 hour before surgery (pediatric dose is adjusted appropriately). Intravenous steroids are continued postoperatively at 4 mg every 6 hours for the first 24 hours, then discontinued).
The anesthetic technique for a routine AMTR is general anesthesia. Anesthesia is maintained with consideration to the need for an electrocorticogram (ECoG), depending on the particular details of the case.
For operative positioning, the patient is placed supine on an alternating air mattress with the head held in the Mayfield 3-pin holder. The sagittal midline and axial axis of the head should angle 20° and 30°, respectively, to horizontal. Positioned properly, the zygoma is the highest point of the head (see Image 3).
Method for positioning the patient for temporal lobectomy. The head is angled so the malar prominence is the highest portion of the patient's head.
For the opening, minimal hair is shaved, only enough to make a standard question-mark curvilinear frontal temporal incision (see Image 4). The temporalis muscle is incised directly beneath the skin and reflected with the scalp. The bone flap should be based low in the middle fossa, extending just above the sphenoid wing but within the confines of the temporalis muscle fan. A rongeur or high-speed drill is used to enlarge the bone opening towards the middle fossa floor and anteriorly into the sphenoid wing for several centimeters. The dura is opened by a cruciate incision to optimize temporal tip exposure and should extend approximately 1 cm above the Sylvian fissure (see Image 5). The remainder of the operation should proceed using the operating microscope.
A pterional incision is made, exposing the temporalis muscle and skull.
After retracting the muscle and performing a small craniotomy, the dura is opened to expose the temporal tip and a small portion of the suprasylvian cortex.
A distance of 3.5-4 cm from the temporal tip is measured along the middle temporal gyrus posteriorly from the sphenoid wing. This marks the posterior limit of the proposed lobectomy, regardless of speech laterality or side of operation. An incision is made in the sulcus between the superior and middle temporal gyri and is carried mesially until the temporal ventricular horn is entered. A small cotton pledget is placed into the ventricle to maintain orientation. Using an irrigating bipolar coagulator and suction device, the middle and inferior temporal gyri are removed as a single surgical specimen. At some centers, the superior temporal gyrus is removed; at others, it is not. If removed, then one must be careful to maintain an intact pia along the sylvian fissure and basal temporal regions. The superior temporal gyrus is removed to the level of the ventricle, and then the amygdala is partially removed.
The remainder of the inferior temporal structures are removed piecemeal mesially until the parahippocampal gyrus is encountered. At this point, only the mesial-temporal structures remain, and the ependymal surface of the hippocampus should be easily identified (see Image 6).
The lateral cortex is resected, leaving the hippocampus (H). The tentorium (T) and choroid plexus (CH) should be identified.
The hippocampus and uncus are removed en bloc for pathologic examination. An incision into the parahippocampal gyrus is made through the choroidal fissure from the level of the posterior boundary of the cerebral peduncle anteriorly until the tip is reached (see Image 7). This serves as the superior-mesial margin of the resection. The posterior incision is made at the level of the posterior margin of the cerebral peduncle laterally to the lateral hippocampal margin and then brought anteriorly and laterally until it meets the lateral resection margin. The hippocampus is separated carefully from the intact pial surface.
The hippocampus is removed as a separate en bloc specimen for pathologic evaluation. Resection is carried back to the level of the collicular plate. The peduncle and third nerve should be visible through intact pia.
As the hippocampus is rolled posteriorly, care is taken to coagulate and cut the numerous small vessels that arise from the posterior communicating and cerebral arteries without damaging the vessels that supply the peduncle and thalamus, causing traction hemiplegia and hemianopsias. With the hippocampus removed, the pial bank overlying the tentorial incisura and cerebral peduncle should be intact.
Below this pial barrier should be the cerebral peduncle, posterior cerebral artery, posterior communicating artery, and third nerve. Pial bleeding is controlled carefully by surgical and gel foam at a low setting. Best results are obtained with removal of the hippocampus to the level of the superior colliculus. The resection cavity is irrigated completely free of blood, and the dura is closed in a watertight manner. No drain is needed, and blood loss should not exceed 300 mL.
Postoperative management
The patient is moved to the ICU for overnight observation unless the patient can be returned to a seizure monitoring room in the epilepsy unit for careful monitoring. The night of surgery, the patient sits at the bedside and performs deep breathing for pulmonary toilet. Should a fever occur, an incentive spirometer is used. The Foley catheter and arterial line are removed as soon as possible, usually before leaving the recovery room. If kept overnight in the ICU, the patient is moved to a surgical floor the next morning. The intravenous drip is converted to a saline lock as soon as the patient takes oral fluids. Ambulation is encouraged, as is sitting in a chair. Unless a problem arises, the patient is discharged on the third postoperative day.
If the patient is taking multiple AEDs, an attempt is made to reduce the patient to non-toxic doses of effective medication or diminish the most poorly-tolerated medication some time after surgery. If the patient is seizure free after about 2 years, the provider may discuss with the patient the pros and cons of discontinuing multiple AEDs. Patients are strongly urged to continue medications for many years postoperatively as there is a l in 3 chance of having a seizure in the 5 years after withdrawing medications after epilepsy surgery.
Complications
In a large surgical series compiled prospectively by Kraemer, serious complications occurred in less than 4% of patients. Two cases of hemiparesis occurred in 160 temporal lobectomies (1.25%), due to damage of the perforators to the anterior choroidal artery that supplies the internal capsule. Paralysis was present immediately and resolved completely in one patient, but it prevented another patient from returning to work. Use of the operating microscope, careful attention to en-passant blood vessels around the brainstem and thalamus, minimal retraction of the mesial structures and restraint of the bipolar cautery on the choroid plexus can minimize the risk of this complication occurring.
The anticipated visual-field deficit from AMTR is a contralateral superior quadrantanopsia from damage to the Meyer loop as it courses anteriorly around the temporal horn of the lateral ventricle. When present, this rarely causes significant disability and is often unnoticed by the patient unless a formal visual-field evaluation is performed. Even if present, a superior quadrantanopia is not a barrier to driving or work.
In the series by Kraemer noted above, 3 unanticipated visual-field deficits occurred; one patient had an infarct to the optic radiation, and 2 others developed homonomous hemianopsia when the temporal lobe resection extended beyond 7 cm from the temporal pole (1.8%). Homonomous hemianopsia can occur from vascular damage to the geniculate body or optic tract by a mechanism similar to what causes hemiparesis. Again, careful microsurgical technique can minimize this risk. Minimal lateral cortical and maximal medial resection can minimize the risk of injury to the Myers loop.
Infection occurred in the above series in 2% of patients. All infections occurred locally within the suture line and required removal of the bone flap and prolonged intravenous antibiotics. No infection resulted in a permanent neurological injury, and bacterial meningitis was not encountered.
Occurrences of cranial nerve III or IV palsies are less common if surgeons use the operating microscope. The rate of ocular motor paresis following AMTR has decreased markedly; however, it may occur in up to 20% of cases. Because cranial nerve III lies directly beneath the pial surface of the uncus, it can develop temporary dysfunction from mild manipulation during dissection. Cranial nerve IV can be damaged if the current of the bipolar cautery is set too high when coagulating near the edge of the tentorium, from traction on the tentorium during the surgery, or possibly even by drainage of CSF during the procedure.
Temperature may be elevated for the first few days after most craniotomies. However, if fever continues longer than approximately 72 hours in the setting of good pulmonary toilet, aseptic or bacterial meningitis should be suspected. A sudden headache associated with a sharp rise in temperature can, in some cases of aseptic meningitis, mar an uneventful convalescence. Diagnosis is made if findings from lumbar puncture demonstrate sterile xanthochromic cerebrospinal fluid (CSF) under pressure (with several hundred to several thousand leukocytes per cubic mm) and CSF cultures are negative. Treatment includes an antipyretic and dexamethasone.
Verbal memory problems have been noted, particularly in patients with speech-dominant temporal lobe resection. This is more severe than deficits in visual-spatial memory that are thought to occur after nondominant AMTR. Patients without hippocampal sclerosis are at greatest risk for this complication. The most consistent and reliable clinical indicator for or against this complication is age at first seizure; if the patient's first seizure (including a febrile seizure) occurred before age 6 years, the risk of increased memory problems postoperatively is slight. A history of severe alcohol abuse also places a person at higher risk for global memory problems after AMTR.
Operative Technique - Corpus Callosotomy
Corpus callosotomy for epilepsy was first reported in 1940 when Van Wagenen and Herren described 10 patients who underwent the operation between February and May and whose cases were followed until July 1939. The rationale was based solely on Van Wagenen's observation that, as a tumor that involved the corpus callosum grew, the patient's generalized seizures became less common and less severe, with increased preservation of consciousness. Results in this first group of patients were encouraging enough to warrant further evaluation of the surgery. Van Wagenen subsequently reported 14 additional patients in whom surgery occasionally included sectioning the anterior or fornix commissure.
Interest in callosotomy remained dormant until the 1960s, when Bogen and colleagues published a series of articles on the clinical and neuropsychological outcome of the surgery. In 1970, Luessenhop et al described a group of patients treated with corpus callosotomy as an alternative to hemispherectomy. In the mid 1970s, Wilson et al reported the Dartmouth series of callosotomies. Since then, a renewed interest in epilepsy surgery has prompted a reevaluation of the clinical usefulness of callosotomy.
Rationale
According to published data, the aim of this operation is to disrupt one or more major CNS pathways used in seizure generalization. The rationale assumes disruption of this pathway decreases the frequency and severity of either primary or secondary generalized seizures. As a result, callosal sectioning has been applied to almost all seizure disorders.
Approaches
Despite renewed interest in epilepsy surgery in general and corpus callosotomy in particular, reported seizure outcomes from this operation vary greatly. This is due to a lack of standardization of the anatomical structures disconnected, the forms of epilepsy treated, patient selection criteria, and the preoperative evaluations. Also, over the years, diagnostic equipment and surgical techniques have evolved and improved.
In his first group of operations, Van Wagenen approached the corpus callosum through a large right frontoparietal craniotomy. The structures cut varied among patients and included part or all of the corpus callosum with and without unilateral division of the fornix. In contrast to Van Wagenen (who cut the entire callosum through one opening), Bogen and Vogel used 2 separate craniotomies to cut the anterior or posterior portions of the callosum and often included the anterior commissure. In a later series of patients, Bogen completely sectioned the corpus callosum, the anterior commissure, the hippocampal commissure, and the massa intermedia. Since 1962, numerous other published series have described sectioning of various combinations of corpus callosum, massa intermedia, anterior and hippocampal commissures, and unilateral fornix. The wide variability of structures sectioned within individual surgical series makes comparison of outcomes and complications problematic.
In recent years, only the corpus callosum has been sectioned. The extent of sectioning necessary for maximum seizure control with minimum risk of disconnection syndrome (ie, mutism, left-sided apraxia that resembles hemiparesis, bilateral frontal lobe reflexes) is not known. Clearly, the best seizure results are achieved with a complete callosal section. However, the risk of disconnection is greatest with a complete section. Thus, an 80-90% section that spares the splenium seems optimal.
Indications
Indications for corpus callosotomy have not been clearly defined, other than the patient must experience medically refractory seizures. Moreover, unlike anteromedial temporal resection (AMTR) for complex partial seizures, no clear and consistent indicators help to identify patients likely to benefit from surgery. Overall, callosotomy seems to lessen the frequency of primarily and secondarily generalized seizures, ie, tonic, clonic, tonic-clonic, and atonic. Callosotomy significantly improves atonic seizures, but having atonic seizures does not guarantee benefit from surgery. Complex partial seizures can be ameliorated somewhat, but the results are far more capricious.
Some epileptologists believe that people with epilepsy who have mental handicaps should not be considered for callosotomy because it seldom renders patients seizure free and these patients may benefit less than patients of normal intelligence. The presence or absence of mental handicaps is not a reliable predictive factor for outcome. This author has personally observed such patients have gratifying results.
This surgery is not performed with the same goals as resective surgery, in which a seizure-free outcome is more likely and expectations are higher. The usual aim of callosotomy is to reduce seizure frequency and associated morbidity. The additional goals of social or vocational rehabilitation, applicable to resective surgery, are often not realistic expectations after callosotomy.
Preoperative considerations
All patients should have an MRI performed to uncover any structural lesion that could help identify an etiology for epilepsy. In addition, a good MRI allows the surgeon to visualize preoperatively the anatomic relationships to the corpus callosum. For example, coronal images can alert the surgeon to a singular or so-called simian pericallosal artery, which is a worthwhile point to know before dissection. Long-term video-EEG monitoring is essential for an epilepsy diagnosis. If, after a well-conceived workup, the patient has tonic, clonic, tonic-clonic, or atonic seizures and is not a candidate for a focal resection of a clearly identifiable epileptogenic region, then corpus callosotomy should be considered.
The usefulness of neuropsychological testing on every patient considered for surgery is debatable. If such testing is needed for long-term postoperative planning or if specific issues need to be addressed (eg, memory integrity), then specific testing should be performed. Routine extensive neuropsychological testing is not required to determine a patient's candidacy for surgery.
The usefulness of routine Wada testing is questionable. If a special situation requires speech lateralization, then the test should be performed (eg, if specifically concerned about mixed cerebral dominance). In 1990, Sass et al reported that patients with mixed cerebral dominance for handedness and language (ie, a right-handed person with right-hemisphere language dominance) are at risk for postcallosotomy language impairments. This complication appears to involve primarily speech and writing and spares verbal comprehension. However, more data are needed for a full understanding of postoperative language deficits.
Surgical procedure
The procedure is performed from the vertex of the head, with the patient lying in the right lateral decubitus position. The head is positioned so that the left hemisphere is superior and the right hemisphere is inferior. This allows the left hemisphere to be supported out of the field by the falx cerebri, and the right hemisphere to be allowed to sag away from the midline by gravity, as CSF is removed. The bone is removed in the midline over the sagittal sinus.
The most dangerous complication of this operation is air embolism, which is most likely to occur from a tear in the superior sagittal sinus during the initial stage of craniotomy. In addition, bleeding from the sagittal sinus can be extensive, with significant blood volume loss accumulating rapidly. This is especially critical in children. Therefore, one policy for pediatric cases is to not begin surgery without transfusable blood available in the OR.
In the authors' experience, patients undergoing callosotomy with narcotic anesthesia are often slow to arouse immediately after surgery and are therefore difficult to evaluate neurologically. Thus, the authors have abandoned narcotic anesthesia in favor of inhalation agents. The authors induce anesthesia with an appropriate amount of propofol followed by an inhalation agent of choice. A lumbar drain can be implanted to allow CSF drainage for improved exposure until the callosum is sectioned.
The supine position favored by most surgeons necessitates frontal lobe retraction. The authors place the patient in the lateral decubitus position to allow gravity to pull the dependent hemisphere gently away from the falx. In this position, with CSF drainage and a PCO2 of 30 mm Hg, brain retraction should be minimal. The Mayfield head holder is applied, and the neck is secured in a neutral position. The operating table is tilted at a head-up incline of approximately 15° (see Images 10-11). A rectilinear, U-shaped. vertex scalp incision is centered over the junction of the coronal and sagittal sutures. A 4-hole bone flap which straddles the sagittal and coronal sutures is elevated. From this point, an operating microscope is used. To provide superior stability, the authors prefer to operate while seated, using a sterile, draped Mayo
stand for elbow support.
Illustration of lateral position for insertion of a lumbar drain.
Photograph of a patient positioned for lumbar drain insertion prior to corpus callosotomy.
The dura over the dependent hemisphere is opened to the edge of the sagittal sinus (see Image 12). The dural flap is pulled tight with retention sutures to provide maximum exposure of the interhemispheric fissure. Lysis of midline adhesions between the arachnoid and dura is performed using bipolar cautery. Attempts are made to preserve bridging veins, but 1 or 2 veins (anterior to the coronal suture) can be sacrificed, if necessary. Moist cottonoid strips are placed over the medial frontal cortex of the dependent frontal lobe, and any additional adhesions between the cortex and falx are cut with bipolar cautery. In this manner, dissection is carried down to the corpus callosum, which is identified only after clear visualization of both pericallosal arteries. Without this verification, an inexperienced surgeon may mistake the cingulate gyrus for the callosum.
Surgical site preparation for corpus callosotomy. The right hemisphere is inferior, and the left hemisphere is superior. In this position, the left hemisphere will be held out of the field by the falx cerebri, and the right hemisphere will fall away from the field, minimizing the need for traction.
After both pericallosal arteries are separated and protected and the callosum has been exposed, the callosum is opened along the midline of the body. This incision is carried deep until the cavum septum pellucidum is entered, leaving the ventricular ependyma intact. In rare instances, one major pericallosal artery supplies both hemispheres and makes the dissection more difficult because the artery must be manipulated from side to side without damaging branches to either hemisphere. Bipolar cautery is used to cut the callosum. Care is taken to stay within the cavum septum pellucidum. The entire rostrum, genu, and body are divided, and dissection is carried posteriorly until only the splenium remains intact.
The lumbar drain is removed, and the PCO2 is allowed to rise to 40 mm Hg. Nitrous oxide anesthesia should be discontinued at this point. The wound is irrigated generously to replace most of the drained CSF. If mannitol was used earlier, intravenous fluid should replace the volume loss from diuresis. The dura is closed, dural tack-up sutures are secured, and the craniotomy is closed in layers. Blood loss should not exceed 150 mL. Experience has suggested that patients fare better postoperatively if the net fluid balance for the surgery is positive at the end of the case.
Postoperative care
The patient is observed in an ICU for the first evening following surgery. During this time, neurologic parameters may fluctuate and be complicated by the disconnection syndrome. The patient may not verbalize readily or respond quickly and may have unexplained pupillary inequality. These findings may prompt CT scanning to be performed, which can help rule out a clot or tension pneumocephalus. By the second postoperative day, the patient's normal baseline neurological status should begin to return. MRI in the midsagittal plane is an excellent method for evaluating the extent of sectioning.
Seizure outcome
The patient is maintained on the same anticonvulsant regimen as before surgery. Seizures may increase transiently during the postoperative first week. Although seizure outcome varies from patient to patient, the authors' results seem typical of other published reports.
In general, this operation should decrease but not stop seizures for most patients. A 60-70% decrease can be expected for more than 80% of patients. Approximately 10-15% of patients receive no worthwhile benefit.
The major problem with callosotomy is its unpredictable and/or incomplete seizure control. Nevertheless, for patients with frequent seizures (especially frequent status epilepticus) or seizure-related injuries, this may provide significant benefit. Although never objectively documented, these patients are often observed to continue to improve in seizure control over the years after surgery. Whether this is a long-term effect of surgery or the natural history of epilepsy is not clear.
Operative complications
Early series reported a 50% rate of hydrocephalus and aseptic meningitis, with several deaths. This may be due partly to opening ependyma into the ventricles and can be minimized by entering only the cavum septum pellucidum. Other potentially serious operative complications include excessive bleeding from the superior sagittal sinus, frontal lobe cerebral edema, and venous infarction from sacrificing major bridging veins.
Neurologic outcome
Neurologic sequelae are few. Transient left-sided hemiparesis has been reported. Lateral decubitus positioning may eliminate much of this problem. Temporary bladder incontinence has been associated with damage to the cingulate gyrus.
Numerous psychological studies have been performed following various degrees of midline commissurotomy. Although results can demonstrate problems in transfer of interhemispheric information, the changes do not seem to hinder patients in activities of daily living. Many of the earlier studies involved patients with more extensive commissurotomies, rather than partial or complete callosotomy. Memory difficulties have been reported after complete callosotomy.
A deterioration of impulse control and an increase in aggressive outbursts has been observed following callosotomy in some children with mental handicaps. Although no formal measurements of this behavior have been reported, the authors found that some patients are more alert and more aware of their surroundings after surgery and, hence, may be frustrated more easily.
Callosotomy disrupts EEG bilateral synchrony but does not eliminate epileptiform discharges. Corpus callosotomy may be used diagnostically to help define the side of seizure onset in some patients with frontal lobe epilepsy and secondary bilateral synchrony. However, the authors had little success with this approach when tried on 6 patients.
In 1993, Spencer et al reported more intense focal seizures or emergence of focal seizures after corpus callosotomy, especially in patients with asymmetric bilateral epileptogenic foci. The authors have observed one patient in whom this occurred, but seizures were reduced with alterations in medications so that the overall benefits of surgery were favorable.
Operative Technique - Multiple Subpial Transections
Multiple subpial transection (MST) is a nonresective surgical technique for abolishing epileptiform discharges and correlative seizures from epileptogenic cortex.
Rationale
All partial epileptic seizures originate from a restricted region of cortex considered (for lack of a better term) the focus. The exact mechanisms responsible for generation of focal seizures are not completely understood but are believed to be due to abnormally discharging neurons and abnormal interneuronal synchrony.
Whether epileptogenic neurons fire in action potential bursts solely because of abnormal interneuronal synchrony or because of intrinsic abnormalities (or a combination of both) is unknown. However, recordings from chronic animal models and human cortex have demonstrated that seizures are associated with periods of increased interneuronal synchrony within cortex as well as between cortex and subcortical nuclei. Theoretically, if this intracortical synchronization can be disrupted, the focus' epileptogenic potential can be reduced or eliminated. The rationale for placing parallel slices through cortex is to permanently disrupt side-to-side intracortical synchronizing neural networks.
Because the neocortex is organized in functional columnar units, right-angle cuts to the pial surface should not disrupt cortex-subcortical input-output interactions. Therefore, the MST technique theoretically is ideal for treating epileptogenesis while preserving intrinsic cortical function.
Indications
The most effective surgical treatment of partial (focal) seizures has been removal of the seizure-producing cortex from the brain. However, this cannot be performed if the cortex also serves an indispensable function such as speech. Consequently, MST is the only acceptable surgical treatment of a focus within such cortex.
In 1989, Morrell et al reported on the use of MST to treat seizures from chronic (Rasmussen) encephalitis that affected the speech-dominant hemisphere. They applied MST to speech and motor cortex and resected or disconnected the remainder of the hemisphere. The authors' results in these cases were not good, and the authors have abandoned this indication.
In 1997, Patil et al reported on the application of MST to bilateral seizure foci in very difficult cases otherwise not considered for surgical intervention.
MST has been used as surgical treatment of Landau-Kleffner syndrome (LKS), a form of acquired aphasia associated with epileptiform spiking (but not necessarily seizures) in the central neocortex.
Experimental studies
In 1995, Sugiyama et al reported a study on MST-induced histological changes and its effects on interneuronal cortical spread in acutely kindled rats. They showed the transection severed horizontal fibers but left vertical fibers and neuronal cell bodies preserved. Cortical hyperactivity across the transacted zone was reduced, and afterdischarge propagation was also inhibited. These results suggest that MST interrupts not only neuronal synchronization but also excitatory interneuronal conduction.
In 1994, Tanaka et al reported MST effects on propagation in a cat kainic acid model. They showed that MST suppressed epileptic activity of the cortical surface. However, residual hypermetabolic areas (defined by carbon 14–labeled deoxyglucose autoradiography) were observed in the deep layer of the cortex and caudate nucleus and in the contralateral sensorimotor cortex. These results suggest that seizure propagation uses side-to-side connections more than cortical subcortical connections in neocortical epilepsy.
Clinical studies
In 1989, Morrell et al reported on MST in 32 patients but were able to report 5-year seizure outcome in only 20 patients.Complete seizure control was obtained in 11 (55%) patients. Taken in the context of extratemporal epilepsy, these results are comparable with cortical resection. Of 16 patients, Morrell et al reported no major complications that involved the precentral gyrus, 6 that involved postcentral cortex, 5 that involved the Broca area, and 5 that involved the Wernicke area.
Evaluation of the antiseizure efficacy of MST was confounded by the inclusion of patients with progressive neurological diseases and structural lesions, such as tumors. For example, whether continued seizures in a patient with chronic encephalitis represented failure of MST or progression of the disease is unknown. Whether seizure control after MST and lesionectomy was attributable to lesionectomy or MST is unknown. On the other hand, this pioneering report documented the minimal cognitive or behavioral sequelae of the procedure.
Subsequent reports support the findings of Morrell et al. In 1991, Shimizu et al reported 12 cases of MST with fair results.In 1995, Sawhney et al reviewed 21 cases of MST, with 18 patients having epilepsy and 3 having LKS. Of the 18 patients with epilepsy, 8 demonstrated focal abnormalities on MRI, including 6 with chronic encephalitis and 1 with a tumor. Cortical resection plus MST was performed in 12 patients, with 11 showing worthwhile seizure decreases. None of the 21 patients developed chronic neurological deficits. In 1997, Devinsky et al reported on MST in the language cortex of 11 patients. In 1996, Pacia et al reported a series of 21 patients. Only 3 of these underwent MST alone; the other 18 had additional cortical resections.
In 1997, Hufnagel et al reported less satisfactory results in a series of 22 patients with a mixture of resections and MST. Devinsky et al reported 13 patients with MST in language cortex whose cases were followed for longer than 1 year (11 patients had additional resective surgery). Although 11 of 13 patients had greater than 90% seizure reduction, MST patients had significantly poorer postoperative naming, verbal fluency, and oral reading than patients with dominant temporal lobe resections that spared language function.
These additional reports make assessing the true antiseizure effect of MST alone difficult because of the inclusion of patients with progressive neurodegenerative diseases or a combination of lesionectomy and cortical resection plus MST.
In 1995, Wyler et al reported on 6 patients who underwent only precentral and/or postcentral MST without resection.In a longer follow-up of that series, 2 patients (30%) were seizure free, and 3 patients (50%) showed greater than 90% seizure reduction. One patient did not benefit significantly. Four patients had MST of primary language cortex. Patients with MST of precentral language areas showed subjective improved fluency, and those in postcentral speech areas showed no deficits. Wyler changed the technique from Morrell's original description by pointing the instrument tip downward (away from the pia) rather than toward the pia. He believes this is technically easier and results in less subpial hemorrhage. A special knife has been developed (Ad-Tech Medical, Racine, Wis) for this purpose.
In 1997, Patil et al reported on patients undergoing MST only from large multilobar areas or bilateral areas of cortex with good results. They also reported a series of patients with bilateral foci treated with MST plus other minimally resective procedures.
MST applied to LKS
Perhaps the most sensitive validation of the antiepileptic effects of MST with preservation of underlying cortical function is its application to patients with LKS. LKS, also known as acquired epileptiform aphasia, is characterized by acquired verbal auditory agnosia in childhood with epileptiform discharges in central regions in most cases and superior temporal regions in some. Children often become mute and unresponsive to verbal commands after initially acquiring rudimentary language. Some children show no overt seizures in spite of obvious epileptiform discharges that become more plentiful during sleep; at times these discharges are so diffuse and continuous that they resemble the epilepsy syndrome of continuous spike waves in slow-wave sleep. Antiepileptic drug (AED) therapy, for the purpose of regaining speech, is usually ineffective. Steroid therapy may provide temporary improvement but seldom a long-lasting benefit.
In 1995, Sawhney et al reported on 3 preoperatively mute patients with LKS who showed "substantial recovery of speech" months after MST. In 1995, Morrell et al reported on 14 children with LKS who were treated with MST.Seven recovered age-appropriate speech and were taking regular classes in school. Four have shown marked improvement. Thus, 11 of 14, none of whom possessed communicative language for at least 2 years preoperatively, are now speaking.
These results have several important implications. First, they add additional evidence that this novel technique is effective in eliminating epileptiform electrical activity from cortex while preserving function. Second, they support the concept of early surgical intervention in well-defined epileptic childhood syndromes for fear that delays result in irreversible cognitive and/or behavioral sequelae.
The authors have applied MST to several children with LKS with mixed benefit. Interestingly, in most cases, the disruptive behavior displayed by these children improves after surgery.
Technique
MST is still a relatively new surgical technique. As such, many questions remain unanswered. The following description is admittedly highly biased by the author's personal experience, but it has proven effective in the author's experience. Over time, the following recommendations will be improved on as many of the questions are answered.
As with all surgery for partial epilepsy, the margins of the epileptic focus must be clearly defined. Although interictal recordings define the foci of LKS cases (because most children with LKS have no seizures), all other cases should have long-term ictal recordings using a subdural grid electrode. Because most patients considered for MST have primary motor cortex involvement, the focus is near the central sulcus or, more commonly, the junction of the central sulcus with the sylvian fissure. Thus, a craniotomy that provides adequate exposure of this area is required whenever possible. A 64-electrode contact grid, centered over the suspected region of ictal onset, is recommended.
Within the first few hours of monitoring, somatosensory evoked potentials (SSEPs) are performed to orient the grid sensory-motor cortex with respect to the grid. Direct cortical mapping by electrical stimulation is performed as needed for each individual. Once sufficient ictal recordings identify the focus, the patient is returned to surgery for grid removal and MST.
Often, the ictal region is no more than a few square centimeters and is usually bordered by sulci (ie, contained within 1-2 gyri). Seizure propagation is commonly observed along the long axis of the involved gyri. The entire region of ictal onset should undergo MST, as well as the 1-2 cm bordering the ictal zone. If a discrete ictal zone is not evident from long-term recordings, MST results are not as good.
The authors' preference is to use a specially designed MST knife (see Image 13; Ad-Tech Medical, Racine, Wis) with the point angled downwards, rather than upwards as originally described. In addition, the cutting portion of this knife is sharpened to a blade to minimize the excessive damage from using blunt instruments such as a right-angled dissector. The actual cuts should be performed under direct vision through the operating microscope.
Incision of the dura mater, adjacent to the sagittal sinus.
After protecting the surrounding cortex with cotton patties, the insertion point can be either at the side or at the crest of the gyrus. After a small pial spot is cauterized, the knife blade is inserted and pushed subpially towards the gyrus edge, making a right-angled cut to the long-axis of the gyrus. The horizontal arm to the blade should be barely visible through the pia at all times. If the insertion point is centered in the gyrus, then, after the first half-cut, the instrument is removed and replaced and the remainder of the slice is completed. Parallel cuts then are made 5 mm apart until the entire proposed ictal zone and surrounding area have been sliced.
Take care when encountering a gyrus that curves. This is because the outer length of the curve is much longer than the inner length. When this is encountered, the authors suggest staggering the cut lengths so that slices converging at the center of the curve do not all join at a common point or come so close together as to severely damage cortex. Pial bleeding at the blade insertion point is usually controlled with bipolar cautery or a small square of thrombin-soaked Gelfoam. Significant subpial hemorrhage should not occur.
Functional Hemispherectomy
Functional hemispherectomy is a highly effective procedure for treating epilepsy in well-selected candidates who suffer from catastrophic epilepsy. Patients who are candidates for functional hemispherectomy are those persons who have injury and seizures limited to one hemisphere of the brain. Seizures occur at such a high frequency to interfere with cognition and impair quality of life. The goal of surgery is to isolate the affected brain from the healthier hemisphere to allow it to function without the burden of seizures or interictal discharges to impair brain function.
Most patients upon whom hemispherotomy has been performed have been children, although some previous reports include adults. Etiologies of the catastrophic epilepsies leading to hemispherotomy include Rasmussen’s encephalitis, other encephalitides, prenatal ischemia (often with porencephaly), Sturge-Weber syndrome, cortical dysplasia, HHE, hemimegalencephaly, malignancy, or Tuberous Sclerosis. Preferentially, the pathology is limited to a single hemisphere of the brain, as recovery after surgery depends upon the remaining hemisphere’s ability to take over cognitive and language functions that may have previously been within the domain of the injured hemisphere.
Timing of surgery is dependent on the syndrome causing the epilepsy, severity of the epilepsy and the age of the patient. If seizures begin early in infancy, then hemispherectomy before the second or third year of life offers the best chance of transfer of neurologic function to the opposite hemisphere. When the onset of seizures begins somewhat later in life, timing of surgery can be more controversial, as the transfer of language function may be less complete in older children.
However, delay of surgery can lead to decline in cognitive ability, as accumulating evidence suggests that seizures themselves may delay cognitive development.The timing of surgery depends upon the severity of the seizures, the natural history of the illness affecting the patient, and the response to antiepileptic drug treatments. When the syndrome presents early in life, many centers recommend proceeding with surgery rapidly to avoid loss of cognitive abilities due to an epileptic encephalopathy, which may impair learning and cognition. However, patients with late onset syndromes can respond to surgery with improvements in neurologic function as well, suggesting that plasticity after hemispherectomy is not limited to early childhood.
In 1950, Krynauw first described an operation for children with infantile hemiplegia in which the entire hemisphere (excluding basal ganglia) was anatomically removed from the cranium (anatomical hemispherectomy). In 1969, Falconer and Wilson reported on long-term follow-up of early cases and disclosed symptomatic hydrocephalus and progressive hemosiderosis as 2 potentially lethal complications. As these became known, enthusiasm for hemispherectomy declined and remained low until 1983, when Rasmussen reported that preserving disconnected frontal or occipital lobe(s) lessened the incidence of these complications. From this observation, functional hemispherectomy was developed.
In a functional hemispherectomy, cortex is disconnected from all subcortical structures and the interhemispheric commissures are divided, but the brain remains in place. Excellent postoperative seizure control and good quality of life can be achieved among children treated with hemispherectomy.
This procedure still involved considerable time in the operating room and often required blood transfusions, leading to further refinement of the functional hemispherectomy, frequently referred to as the hemispherotomy, by a number of authorsThese variations all include the following in their techniques; disconnection of the corona radiata and the internal capsule, resection or disconnection of the mesial temporal lobe structures, intraventricular callosotomy and deafferentation of the medial-basal occipital and frontal lobes. These refinements in functional hemispherectomy have decreased the time needed for the operation and the blood loss associated with the operation, making the operation less morbid and easier to perform.
Still, complications from functional hemispherotomy can be formidable, with patients who have had the procedure developing hemogenic meningitis, ventriculitis, CSF leak, hydrocephalus or less commonly, stroke, infection, coma or postoperative hemorrhage.The procedure must be performed in centers with experienced surgeons with advanced equipment available for intraoperative and postoperative support of the patient and complications if they should arise.
Functional hemispherectomy has a seizure free outcome of between 54-90%, depending on the series reported and type of syndrome for which the procedure is performed. Some patients have minor seizures after surgery that are not considered disabling. When overall improvement includes nondisabling seizures, the percentage of patients who are significantly improved by functional hemispherectomy rises to 80-90% in most series.
Some authors have found the underlying pathologic substrate to have a bearing on postsurgical outcome, with patients with migrational disorders and hemimegalencephaly doing less well than other disease processes. In addition, higher presurgical cognitive ability, shorter duration of seizures and better postoperative seizure control are more likely to correlate with higher postoperative developmental gains.Seizure freedom is important in improving postoperative cognition, but quality of life scores have improved even when some minor seizures persist.
Physicians and parents are often concerned about the motor consequences of functional hemispherectomy. Usually, the decision to proceed to hemispherectomy is taken after the patient has a complete hemiparesis. However, in the case of a progressive disease process, such as Rasmussen’s encephalitis, some centers advocate early surgery, prior to the development of complete hemiparesis, to avoid developmental delay that might be caused by epileptic encephalopathy.
Several series have reported on motor outcomes after functional hemispherectomy. Most children remain unchanged by surgery, with a return to baseline motor function in the contralateral leg within one year of surgery. The exception to this generalization is that increased tone in the contralateral hand compromises function of that hand and does not return to baseline. A small percentage of children will have a worsening of hemiparesis, and a few will have an improvement in motor function. Most patients (89%) are ambulatory after surgery: those patients who walked prior to surgery are likely to be ambulatory as well after surgery.
Overall, gains in activities of daily living, quality of life scale, and social interactions outweigh the increased motor deficit in the hand which occurs after functional hemispherectomy.
Multimedia
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 | Media file 2: Examples of 3 depth electrodes with varying numbers of contacts. Note: The stylus is in place and is removed once the electrode has been inserted. |
 | Media file 3: Method for positioning the patient for temporal lobectomy. The head is angled so the malar prominence is the highest portion of the patient's head. |
 | Media file 4: A pterional incision is made, exposing the temporalis muscle and skull |
 | Media file 5: After retracting the muscle and performing a small craniotomy, the dura is opened to expose the temporal tip and a small portion of the suprasylvian cortex. |
 | Media file 6: The lateral cortex is resected, leaving the hippocampus (H). The tentorium (T) and choroid plexus (CH) should be identified. |
 | Media file 7: The hippocampus is removed as a separate en bloc specimen for pathologic evaluation. Resection is carried back to the level of the collicular plate. The peduncle and third nerve should be visible through intact pia. |
 | Media file 8: Craniotomy that shows an intracranial grid in place. The grid lies over left primary motor and sensory cortex, near the vertex of the skull. The patient is positioned laterally, and the face is at the right. The intracranial study was performed to document seizure onsets in a patient who had a previous oligodendroglioma removed. Seizures began just posterior to leg motor cortex, in primary sensory cortex. |
 | Media file 9: The primary motor cortex is located with the use of extraoperative somatosensory evoked potentials and intraoperative cortical stimulation, as well as the grid described in Image 8. The Penfield instrument in the field is positioned over primary motor cortex. |
 | Media file 10: Illustration of lateral position for insertion of a lumbar drain. |
 | Media file 11: Photograph of a patient positioned for lumbar drain insertion prior to corpus callosotomy. |
 | Media file 12: Surgical site preparation for corpus callosotomy. The right hemisphere is inferior, and the left hemisphere is superior. In this position, the left hemisphere will be held out of the field by the falx cerebri, and the right hemisphere will fall away from the field, minimizing the need for traction. |
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