You are in: eMedicine Specialties > Neurosurgery > MEDICAL TOPICS Epilepsy SurgeryArticle Last Updated: Sep 13, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 10
  Author: David Vossler, MD, Director of Epilepsy Center and Clinical Neurophysiology Laboratories, Seattle Neuroscience Institute at Swedish Medical Center, Clinical Associate Professor, Department of Neurology, University of Washington School of Medicine David Vossler is a member of the following medical societies: American Academy of Neurology, American Clinical Neurophysiology Society, and American Epilepsy Society Coauthor(s): Diana L Kraemer, MD, Consulting Staff, Epilepsy Center, Seattle Neuroscience Institute at Swedish Medical Center; Allen R Wyler, MD, Medical Director, Northstar Neuroscience, Inc Editors: Scott C Dulebohn, MD, Assistant Professor, Department of Surgery, Division of Neurosurgery, University of Minnesota College of Medicine; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Ryszard M Pluta, MD, PhD, Associate Professor, Neurosurgical Department Medical Research Center, Polish Academy of Sciences at Warsaw, Poland; Senior Researcher, Surgical Neurology Branch, National Institute of Neurological Disorders and Stroke, NIH; Herbert H Engelhard III, MD, PhD, Director, UIC Neuro-Oncology Program, Chief, Division of Neuro-Oncology, Associate Professor, Department of Neurosurgery, University of Illinois at Chicago; Allen R Wyler, MD, Medical Director, Northstar Neuroscience, Inc Author and Editor Disclosure Synonyms and related keywords: epilepsy surgery, anteromedial temporal resection, AMTR, corpus callosotomy, multiple subpial transections, MST, intracranial surgery, long-term EEG-video monitoring, long-term electroencephalogram–video monitoring, neuroimaging, epileptogenic focus, strip electrode, grid electrode, depth electrode INTRODUCTIONSection 2 of 10
  A single review article devoted to the subject of surgery for the treatment of epilepsy can only serve as a brief introduction. The field of epilepsy surgery is too large a topic to be adequately covered in this article. This assertion is evidenced by the facts that postresidency fellowships are available for neurosurgeons that wish to subspecialize in epilepsy surgery and that many neurosurgeons perform epilepsy surgery almost exclusively. More comprehensive information is available in several encyclopedic textbooks published on this subject. For excellent patient education resources, visit eMedicine's Brain and Nervous System Center. Also, see eMedicine's patient education article Epilepsy. This article is divided into several sections. Firstly, the seizure types, epilepsy syndromes, and indications for surgery are discussed. Secondly, intracranial diagnostic techniques commonly used in epilepsy surgery are covered. Finally, 3 surgeries are presented as models on which a preliminary understanding of the various surgical strategies can be gained.
First presented in this article are definitions of basic concepts; then, the general workup of patients with routine diagnostic tests is discussed. Specific surgical techniques follow. 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 death; and psychological, social, and vocational impairment.
The above factors clearly suggest that continued medical therapy after failure to control seizures with aggressive trials of antiepileptic drugs (AEDs) is not optimal treatment of certain forms of epilepsy. In several retrospective trials and one prospective, randomized, controlled trial for a well-defined syndrome with a known favorable surgical outcome, mesial temporal lobe epilepsy, the morbidity and mortality associated with surgery has been demonstrated to be less than that associated with the disorder.1, 2, 3 In addition, surgery yields a better quality of life and reduced depression and anxiety as soon as 3 months after AMTR, compared with continued medical therapy.2 This improved quality of life is specifically related to the occurrence of complete seizure freedom in both the medical and surgical study groups. Criteria for surgery A candidate for epilepsy surgery must (1) have not attained adequate seizure control with sufficient trials of AEDs and (2) 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. In 1996, Engel emphasized that the strategy of trying all combinations of drugs is not an acceptable approach to patients with syndromes known to have excellent chances of benefiting from surgery. As one example, with the advent of 8 completely new AEDs since 1993, an estimated 300 years would be required to try all medications in all combinations. Far more importantly, recent studies of patients with new-onset seizures have shown that only 64% have seizure freedom by the time they try their third AED.4 Thus, after 3 different AEDs have failed to control seizures, more than 35% of patients continue to have seizures. Therefore, the decision to proceed with surgery must take into consideration both the chance of seizure freedom with additional AED trials and the adverse long-term effects of uncontrolled seizures. 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 PHASESection 3 of 10
A presurgical diagnosis is made after a classification of the seizure types and specific epilepsy syndrome that affect the patient. 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) focal (partial, localization-related) or (2) generalized.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. However, even under the best circumstances, a diagnosis based solely on the history can be incorrect. The most accurate method of seizure diagnosis is with long-term video-EEG monitoring. Below is a simplification of the international classification of epileptic seizures.
An epilepsy syndrome diagnosis combines the seizure type with its associated MRI, physical examination, genetic, and other features. For example, if the seizure described above (1) has correlative EEG epileptiform patterns (interictal spikes or sharp waves) and ictal discharges over the right temporal lobe, (2) occurs in a patient who had a febrile seizure as a child but no family history of epilepsy, and (3) is associated with ipsilateral atrophy and increased signal of the hippocampus on an MRI, it is likely a complex partial seizure of right mesial temporal lobe epilepsy. The greatest value of a syndrome diagnosis is to provide a prognosis. In the above example, if the patient is right-handed with normal intelligence, he or she has excellent odds of becoming seizure free after right-sided anteromedial temporal resection (AMTR). A simplification of the previous international classification of the epilepsy syndromes has been proposed and is presented as follows:7, 6
Structural and metabolic brain imaging Because seizures may result from cortical lesions or malformations, neuroimaging can often help identify and localize this damage and, therefore, the focus.
EEG evaluation 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. 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.8 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.9, 10 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.11 Neuropsychological testing Routinely, all surgical candidates undergo extensive neuropsychological testing. Neuropsychologists lack consensus as to what constitutes an adequate preoperative test battery; consequently, testing is not well standardized among centers. A test battery usually contains a personality inventory (eg, Minnesota Multiphasic Personality Inventory), intelligence quotient tests, memory and language function tests, and other tests, depending on the interests of the neuropsychologist. The rationale for such testing is to help localize an epileptogenic focus on the basis that subtle deficits in cognitive functioning might provide additional localization that the neurologic examination misses. Certain tests and abnormal findings have value in demonstrating lateralization of dysfunction to one hemisphere and, occasionally, to one lobe of the brain. Perhaps the best example is the testing of material-specific memory and abilities among patients with suspected temporal lobe epilepsy. In such cases, deficits in verbal memory and other verbal abilities (eg, object naming, word list generation) are common when the seizure focus lies in the left temporal lobe in a right-handed patient. Similarly, deficits in nonverbal memory and abilities suggest right temporal lobe epilepsy in a right-handed person. These tests can also be used for other purposes, such as formulating postoperative vocational goals. Intracarotid amobarbital (Wada) test The intracarotid amobarbital test was developed by Jun Wada to preoperatively determine which hemisphere contains language function. This remains its primary use. It has also been used to test memory function within each hemisphere when considering AMTR. The test is accomplished 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. If speech persists in the face of this hemiparesis, language function is assumed to not be represented within that hemisphere. 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 (both are areas thought to be involved in memory retrieval). 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 The following are examples of instances that may require invasive intracranial monitoring:
The general understanding of epilepsy and its treatment techniques are limited, at best. Invasive intracranial monitoring and surgery do not result in seizure freedom in every patient. The onus is on the surgeon to question where electrodes should be placed based on the presurgical information available and to consider what other alternative diagnosis should be included or excluded to obtain the best possible outcome. The concept that epilepsy consists of a focus that can be removed has evolved into a more unified theory in which the neural network, environment, genetic predisposition, and epileptogenic substrate all must be considered during the evaluation of the patient with epilepsy, if surgery is to be effective.13, 14 Definitions Depth, strip, and grid electrodes are implantable intracranial devices used to record the electrocorticogram (ECoG) 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,15 or they can be tubular in structure, similar to depth electrodes.16 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. 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 RECORDINGSection 4 of 10
Strip ElectrodesStrip 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. The electrode wires are tunneled with a 13-gauge passing needle designed for that purpose (Ad-Tech Medical; Racine, Wis), to exit the skin several cm from the burr-hole incision. At the authors' center, a cable-retaining suture is not traditionally placed in the scalp, as has been suggested by others17, 18, because this increases the risk of breaking the electrode cable if it is pulled violently during a seizure. Instead, Ad-Tech Medical quick-release connectors are used because these break apart easily if tugged. 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. Subdural strip electrodes may be removed through the skin without an open surgical procedure. Although strip electrodes can be inserted epidurally,19, 20 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 ElectrodesArrays 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. Electrode tails 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 authors prefer 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.21, 22 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 ElectrodesDepth 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.23, 24 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.16, 25, 26, 27, 28 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 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.29 Intracranial Electrode Recordings and RemovalAt 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 are taken directly from the recovery room to one of these monitoring rooms, and recordings are begun the day of electrode implantation. The relative accuracy of ictal versus interictal electrographic activity remains somewhat controversial. 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. However, interictal spikes are usually more diffuse than the ictal onset zone, and bilateral interictal spikes do not preclude a good surgical outcome. Obtaining ictal recordings is usually preferable in order to confirm the significance of interictal abnormalities. 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.32 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 might 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.32, 33 When intracranial monitoring is complete, most subdural strip and depth electrodes can be removed percutaneously in the OR after administrating a conscious sedation protocol. 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.34, 35, 36, 37, 38, 39, 40 Staged procedures The following are reasons a second intracranial study may be considered:41, 42, 32, 43, 44, 45
These procedures are typically performed months to years after the first intracranial study, either to give the wound time to heal or because seizures recur at some variable time after resective surgery. Often, an interim presurgical evaluation is performed, which includes video-EEG monitoring, ictal SPECT scanning, MRI, and/or magnetoencephalography, to reexplore the suspected cause of a person's epilepsy and prepare a more effective intracranial study. Secondary grid implantations can be quite troublesome because dural adhesions are the rule rather than the exception. 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.46, 47, 48 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. 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 protocols48 and in 18 double procedures compared with 165 routine intracranial procedures.47 Documenting the Intracranial StudyEach 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.49 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. MRI 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.50, 51, 52 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.18, 52 Cortical MappingOften, 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.53, 54 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. Stimulation occurs from testing performed by a neuropsychologist or physician, and 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.55 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.56 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,56, 57, 58, 59 and others have discussed the clinical experience with subdural grids for this purpose.53, 54, 60, 61, 62 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). 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.60 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. ComplicationsPublished 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.15, 16, 17, 21, 22, 63, 53, 23, 26, 64, 65, 66, 67, 68, 69, 70 The most common cause of morbidity in subdural strip placement is infection. One randomized study by Wyler and coworkers 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.68, 70 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. 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.21, 22 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;71 however, others have not found this to be a concern.21, 72, 31, 73, 74 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%.75, 76 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.29 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. APPROPRIATE SURGICAL STRATEGYSection 5 of 10
The 2 broad types of surgery are definitive and palliative. Definitive surgery carries a significant chance of producing complete, or at least 90%, improvement in seizures. The goal of palliative procedures is to decrease seizure frequency. 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. 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 ECoG is not an accurate indicator for defining surgical boundaries; these surgeons proceed with a standardized surgery, wherein tissue important for generating seizures is removed without considering the ECoG. 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. 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. The most common mistake when treating these as simple brain 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 is all that is necessary for an excellent seizure outcome in approximately 80% of patients.77 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 not always 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.55 Seizure outcome after resection of such malformations is variable and directly relates to the focal extent 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. Sometimes, an entire hemisphere is epileptogenic. Commonly, this is due to infantile hemiplegia syndrome. Typically, these patients show a large cavitating infarction involving all or several branches of the middle cerebral artery. Far less common are patients with hemimegalencephaly, a dysplastic hemisphere, or Rasmussen encephalitis. For these patients, removal of the hemisphere is beneficial for controlling seizures. 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.78 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.79 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. Several techniques have been reported; however, the details of these are beyond the scope of this article. OPERATIVE TECHNIQUE - ANTEROMEDIAL TEMPORAL RESECTIONSection 6 of 10
Anteromedial Temporal Resection for Hippocampal SclerosisThe greatest emphasis is on anteromedial temporal resection (AMTR) because this is the most commonly performed surgery. It also 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:
Although AMTR for mesial temporal lobe epilepsy has now been shown to be more effective than continued medication, studies to date have included patients who have had epilepsy that has been refractory to medical treatment for a prolonged period. Most patients who are finally referred for AMTR have had epilepsy for approximately 20 years. Because continued complex partial and generalized tonic-clonic seizures have such negative effects on vocational, educational, social, cognitive, and psychological areas and because these seizures can cause injury and even sudden death, most epileptologists believe that AMTR should be offered to good surgical candidates earlier. An issue that remains unclear is how much time must elapse, or how many medication failures must occur, before surgery is recommended. For this reason, the US National Institutes of Health is sponsoring a large prospective study entitled Early Randomized Surgery for Epilepsy Trial (ERSET). ERSET directly compares early AMTR against 2 more years of prospective aggressive antiepileptic drug (AED) management. To enroll, patients must not have achieved seizure control with a minimum of 2 major medications and must have had seizures with outwardly observable signs at a rate of 6 or more per year for less than 2 years. See the ERSET Web site for additional details and a list of US study sites to which patients should be referred. 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. Although the use of prophylactic antibiotics is debated, 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 48-72 hours; then, convert to a methylprednisolone dosing regimen (eg, Medrol Dosepak). 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.80 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). For the opening, minimal hair is shaved, only enough to make a standard curvilinear frontal temporal incision similar to the pterional approach to the circle of Willis (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 easily identified sylvian fissure (see Image 5). The remainder of the operation should proceed using the operating microscope. A distance of 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. 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 is 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 hippocampus and uncus are removed en bloc for pathologic examination. An incision is made along 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. 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 barrier should be the cerebral peduncle, posterior cerebral artery, posterior communicating artery, and third nerve. Pial bleeding is controlled carefully by bipolar cautery 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 heparin 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 2 AEDs, the least effective drug is tapered after 1 year. If the patient is seizure free at 2 years, the remaining drug can be tapered. One seizure during or after withdrawal results in resumption of single-medication therapy. 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. Using the operating microscope, with careful attention to all en-passant blood vessels, and a minimal amount of retraction on medial structures 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 still occurs 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. 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 are negative after culture. 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. Surprisingly, the extent of hippocampal resection is not related to poor memory outcome. OPERATIVE TECHNIQUE - CORPUS CALLOSOTOMYSection 7 of 10
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. However, on average, patients who are mentally handicapped fare less favorably. 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 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 sodium pentothal followed by an inhalation agent of choice, oxygen, and nitrous oxide. 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 25 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 vertex scalp incision is centered over the junction of the coronal and sagittal sutures. A 4-hole bone flap 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. 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. 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. Postoperative care Whether the patient is observed in an ICU or returned to a high-level neurological nursing floor depends on the particular hospital. In any case, the patient needs to be watched particularly closely for the first 24 hours. 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 hem |