Mapping, Disconnection, and Resective Surgery in Pediatric Epilepsy

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Chapter 57 Mapping, Disconnection, and Resective Surgery in Pediatric Epilepsy

Brent O’Neill, Jeffrey G. Ojemann, Matthew Smyth, Johannes Schramm

Children present many unique challenges to the epilepsy surgeon necessitating the use of special techniques and equipment. A large portion of adult epilepsy surgery treats mesial temporal sclerosis, while in children, extratemporal epilepsy is more common and developmental lesions are frequently encountered. The scope of surgical treatment of pediatric epilepsy may involve mapping the site of seizure onset and surrounding essential brain functions, resection of a seizure focus, or disconnection of the majority or even the entirety of a hemisphere.

The incidence of epilepsy is higher in children than adults with about 5% of children experiencing a seizure before the age of 20.^1,² The majority of these (80%) will never have another seizure, and therefore not meet the diagnosis of epilepsy.² Among children with epilepsy, 20% will be refractory to medical therapy even with the numerous new medications available.³ Epilepsy surgery provides a powerful treatment modality for the subgroup of children with refractory epilepsy who are candidates. Determining appropriate surgical candidates requires a team specialists and a number of diagnostic modalities.

Another important distinction from adult epilepsy is the dynamic, developing nervous system of children. Repetitive seizures and anticonvulsive medications present noxious stimuli that may inhibit brain development.⁴ Seizures may additionally hamper socialization and school integration, causing deleterious impacts beyond the physiologic.^5,⁶ However, brain plasticity could also benefit the child in recovering from resective or disconnective surgery. While these factors weigh significantly in the decision to pursue surgical treatment and the timing of such treatment, each child and family must assess their particular situation with the advice of the epilepsy team to help them weigh the risks of ongoing epilepsy, the risks of surgery, and the likelihood of seizure control with surgery.

Mapping

All evaluations for resective epilepsy surgery focus on identifying a localized source of seizure onset. Noninvasive tools used to accomplish such localization include seizure semiology, neurologic exam (including neuropsychology), scalp electroencephalogram (EEG), and imaging. In the most straightforward of cases, all modalities of localization identify a single, safely-resectable source of seizure onset. In such cases, invasive mapping is unnecessary. In some cases, many of the nonoperative localization modalities give equivocal results or discordant localization. In these cases, operative mapping can reveal an otherwise obscure epileptic focus.

In addition to identifying a seizure focus, mapping can define the extent of a subtle or diffuse epileptic focus such as cortical dysplasia. Such mapping guides the extent of surgical resection.

Finally, mapping can localize neurologic function, defining the relationship between functional brain tissue and an epileptic focus. Such information predicts what if any neurologic deficit will be induced by resection, vital information for a family and care team in deciding whether to proceed with resection. Advances in functional imaging are increasingly able to localize neurologic function, often supplanting or supplementing invasive mapping.

Nonoperative Localization

Attempts at nonoperative localization of an epilepsy focus include an array of specialized testing and a team of trained personnel to administer and interpret them. History (particularly of seizure semiology), neurologic exam (including neuropsychology to elucidate subtle cognitive deficits), prolonged video electroencephalography, and advanced imaging are all vital elements of the epilepsy surgery workup.

Semiology

The clinical semiology of a seizure provides the first clue to localizing its onset. Penfield described an assortment of mental, sensory, and physical aspects of seizures specific to various brain regions.⁷ These range from mental phenomena such as déjà vu and ill-defined epigastric rising to olfactory and gustatory auras to motor convulsions. The first manifestation of a given seizure most accurately reflects the area of onset, whereas seizure spread can lead to later involvement of other areas outside to true site of seizure onset.

Seizures of temporal lobe origin are the most likely to have auras. Those originating in the mesial temporal structures classically begin with an ill-defined epigastric sensation accompanied with panic or autonomic disturbance. Neocortical temporal seizures commonly have auditory, visual, or perceptual hallucinations. Head turning, posturing, automatisms, and behavioral arrest are common accompaniments of temporal seizures.

Frontal lobe seizures are often brief, occur in clusters, and may include complex motor posturing, versive eye movements, or vocalization. They can rapidly spread yielding early generalization, drop attacks, or early signs of temporal lobe activation.

Parietal lobe seizures often present with somatosensory symptoms but may have abdominal symptoms such as nausea or a gustatory sensation. Occipital seizures may be heralded by visual phenomena including scotoma.⁸

Neuropsychology

The classic neuropsychology workup of an epilepsy patient consists of a variety of standardized tests and questionnaires that establish a profile of the patient’s cognitive, emotional, and behavioral abilities. This testing quantifies the patient’s abilities, deficits, and coping strategies. Such information can localize areas of dysfunctional cortex, which may be the site of seizure onset. Neuropsychology also predicts the deficits likely to be incurred by resection of a given focus and the impact that such deficits will have on the individual’s life.⁹

In many emerging technologies such as functional magnetic resonance imaging (fMRI), the neuropsychologist plays a vital role in administering verbal and functional tasks and assessing the patient’s cooperation with such tests. As these tests are very dependent on cooperation from the child while in the MRI machine, the neuropsychologist plays a vital role in extracting and interpreting information from an otherwise uncooperative patient.^9,¹⁰

Electroencephalography

Electrophysiologic data are acquired by placing electrodes on the scalp and recording electrical differences between them. Interictal EEG can reveal epileptiform discharges such as spike and sharp waves, providing clues to the lateralization and localization of seizure onset, but prolonged video EEG provides an electrophysiologic picture of the seizures themselves. Video EEG data can confirm that spells are seizures (as opposed to breath-holding or other nonepileptic spells) and in many instances begin to localize the onset of the seizure. Most centers want to record several typical seizures and will continue monitoring until this is accomplished.¹¹

Ictal video EEG will precisely localize the seizure focus in about a third of patients with temporal lobe epilepsy.¹² Precise localization is even less common in those with extratemporal epilepsy where seizures tend to spread rapidly. Rapid spread from the orbitofrontal or posterior parietal cortex can falsely localize to the temporal lobe.¹³

Dense-array EEG is a newer technology that holds promise for more precise seizure localization. The dense array consists of 256 electrodes held to the head by an elastic mesh, giving substantially more information and spatial resolution than the standard 32-electrode array.¹⁴

Imaging

Surgical epilepsy cases are sometimes divided into lesional and nonlesional. Classically lesional cases from well-circumscribed pathologies such as cavernoma, dysembryoplastic neuroectodermal tumors (DNETs), and ganglioglioma have a significantly better prognosis than nonlesional cases. The identification of a structural abnormality on MRI is a strong predictor of localization.¹⁵ When the scalp EEG confirms seizure onset from the lesion, localization is strongly suggested, and resection often proceeds without invasive mapping.

The distinction between lesional and nonlesional epilepsies has blurred a bit as improved imaging has revealed cortical dysplasias, focal sclerosis, or other pathologies previously noted only by histology. Higher-field MRI, diffusion tensor imaging, and other MRI methods (e.g., MR spectroscopy and fMRI) may also show abnormalities. Voxel-based MRI postprocessing has been recently shown to help visualize blurred gray–white matter junctions or abnormal extension of cortical bands otherwise not recognizable on MRI.¹⁶ A negative MRI may still allow for further surgical planning. In a recent series of pediatric temporal lobectomies, half of those eventually shown to have histologically confirmed mesial temporal sclerosis had normal hippocampus on MRI.¹⁷ In such cases, other imaging such as positron emission tomography (PET), single-photon emission computed tomography (SPECT), and magnetoencephalography (MEG) can help to further define the extent of the epileptic-onset zone and the functionality within and surrounding the malformation.

PET provides a tomographic image of brain glucose utilization, while SPECT images blood flow. Both can reveal areas of metabolic derangement (hypometabolism interictally and hypermetabolism during a seizure) that can represent a seizure focus.^18,¹⁹ MEG collects minute magnetic potentials generated by bands of synchronized neural currents, a technique that can reveal epileptic foci as well as functional circuits (e.g., motor units).²⁰ The specific role for these various modalities is not determined and considerable variability exists across programs.²¹

Invasive Monitoring

Invasive mapping can identify a focus of seizure onset, define the extent of an epileptic zone, or localize neurologic functions. The intraoperative mapping needed is determined by the preoperative workup and must be determined on an individualized basis.

In some patients, no definitive focus can be identified on preoperative studies, and invasive monitoring is needed to lateralize. This may occur in a child with classic temporal lobe epilepsy but evidence for bilateral onset on scalp EEG recordings. In this setting, bilateral strip or depth electrodes may reveal lateralized onset with rapid spread and allow the child to be a candidate for temporal lobectomy.

In the setting of infiltrative low-grade tumors or focal cortical dysplasia, invasive EEG recording may be used to define the relationship of epilepsy onset to the lesion. This may guide the surgeon to resect epileptogenic tissue beyond the bounds of the imaging abnormality. Such infiltrative tumors and cortical dysplasias can have functional neurologic tissue within them. If these lesions are anatomically near motor or speech cortex, mapping of these functions can further define this relationship. It may be found that a lesion has displaced the motor fibers in such a way that resection can be accomplished while preserving function or an aberrant localization of speech function may be present precluding violation of a typically ineloquent area of cortex.

Direct cortical recording and stimulation mapping can be accomplished via intraoperative electrocorticography (ECoG) or via implantation of grid, strip, and/or depth electrodes for extraoperative study. The choice of technique depends on the information needed and the familiarity of the epilepsy team with the various techniques.

Some centers approach poorly defined lesions and occult lesions on MRI with MEG, PET, and ECoG alone, avoiding grid placement entirely.²² While good results from this approach²² have been published, it is difficult to directly compare it to the liberal use of prolonged grid monitoring. At our center, both ECoG and extraoperative grid monitoring are a part of the armamentarium. The approach used is determined on a case-by-case basis depending on the information from the preoperative workup and what vital questions remain.

Electrocorticography

A significant amount of information about the epileptogenicity and function of an area of tissue can be obtained from intraoperative electrical monitoring. ECoG data are obtained by placing electrodes on the exposed brain and observing the EEG record for epileptiform discharges (spike and sharp waves).^23,²⁴ Anesthetic considerations are important to avoid suppressing or altering the EEG record. Benzodiazepenes should be avoided except at induction or after ECoG. Inhalational agents above 0.5 MAC will dampen the record beyond interpretation. A combination of sevoflourane and dexmetomidate is preferred at our (BO and JGO) institution, but a total intravenous anesthetic is also possible.²⁵

Some functional information can be obtained in the anesthetized patient. A distinct pattern of cortical activity (phase reversal) of evoked-somatosensory responses can localize the central sulcus. Direct stimulation of motor cortex can elicit motor responses that are visualized or recorded on EMG. Awake craniotomy in cooperative older children (sometimes as young as 10) can be used to map language cortex.²⁵ Current is passed through a hand-held bipolar stimulator (Ojemann stimulator) to inhibit an area of cortex while the patient is performing speech tasks. Speech arrest induced during this procedure reflects an area of vital speech function.²⁴

ECoG guidance adds little risk to an epilepsy resection, and can avoid the second operation and likely some morbidity of implanted electrode monitoring. The information gained, however, is limited to the anesthetized, interictal state. Time constraints also limit electrophysiologic data.

Implanted Electrodes (Grids, Strips, Depths)

Implanted electrodes allow for EEG monitoring and stimulation mapping outside of the operating room. Recordings include the awake, drowsy, and sleep states and ideally will include several of the patient’s seizures. Monitoring typically continues for 5 to 7 days but can be extended if additional information is needed. Very brief intracranial monitoring (24 or 48 hours) can be a useful technique in toddlers who would not tolerate prolonged implantation.

Intracranial monitoring electrodes are typically placed in the subdural space but epidural placement is employed at times, and yields comparable information.^26,²⁷ Placement below the skull dramatically increases the amplitude of cortical electrical signals collected and diminishes muscle artifact, the main source of noise. Implanted electrodes also greatly increase the spatial resolution of the EEG. A typical grid consists of 64 electrodes on an 8×8 cm array, whereas scalp EEG typically employs 32 electrodes to cover the entire head.

Effective definition of an epileptic focus relies on placement of electrodes as close to the region of interest as possible while electrodes covering distant sites, including the opposite hemisphere can exclude multifocal onset of seizures.

Implanting electrodes additionally allows for prolonged functional mapping outside the operating room by stimulating through the same electrodes.² By applying current across two neighboring electrodes, the intervening area of cortex (about a centimeter of tissue) is inhibited. Deficits observed can identify the function of the cortex and the deficits likely to be incurred by resection. Lower stimulation of motor cortex may produce muscle contraction rather than inhibition. Many children who cannot tolerate with awake intraoperative mapping will cooperate with extraoperative mapping.³⁰ Additionally, the mapping can take place during complex activities such as drawing, writing, or playing a musical instrument potentially identifying important integrative functions.

Cortical stimulation for mapping can induce seizure activity, causing inhibition beyond the intended stimulation both temporally and spatially. The EEG must be monitored for after-discharges reflective of stimulation induced seizure.

Grid electrodes consist of a broad array of contacts that can cover a large area of cortex. They can be particularly helpful in defining an epileptic zone within a lobe or defining the relationship of a lesion to seizure onset. Convexity sites are particularly amenable to grid placement.³¹ Extraoperative language and motor mapping are typically performed through grids as they give ample coverage of the hand and face motor area as well as the typical language sites.

Strip electrodes consist of a single or double row of electrodes spaced along a flexible strip that can be safely passed along the subdural space. Coverage of the interhemispheric cortex, orbitofrontal cortex, and mesial temporal lobe can be achieved by passing strip electrodes, all locations poorly monitored by scalp recordings.^28,³² A strip passed along the undersurface of the temporal lobe will typically place the distal electrode just at the parahippocampal gyrus, providing good monitoring of the mesial temporal structures.³³ Placement of strip electrodes requires only a burr hole, allowing for limited monitoring of distant sites, including the contralateral hemisphere without requiring a craniotomy.

Distant and mesial temporal recordings can also be obtained with depth electrodes. These electrodes line a narrow probe that passes directly into the region of interest. Some centers also place depth electrodes directly within or along the deep margins of a lesion infiltrating the white matter (e.g., cortical dysplasia) to evaluate the depth of resection necessary.^28,³⁴ These various electrodes are often used in combination to monitor all areas of interest.

Complications

Electrode implantation procedures require two separate operations (occasionally more) with a prolonged hospitalization between. The most common complications are cerebrospinal fluid (CSF) leakage, fever, and infection. Minor CSF leak is common while electrodes are in place, but rarely problematic. While low-grade fevers are common and wound infection much less so, suspicion should remain high in order to recognize and treat infections promptly.³⁵

Cerebral edema combined with the mass effect of the implants can cause elevated intracranial pressure. This occurs more commonly with more electrodes implanted. Mannitol and steroids typically will control intracranial pressure, but on rare occasions, grids may need to be removed.

Strip electrodes have fewer complications than grids, 1% overall compared to 3% to 4%. Strips also appear to be safer than depths. Isolated cases of permanent neurologic deficit and death have been reported.³⁵^–³⁹ The risk of complications, as well as the additional hospitalization time, need to be balanced against the information gained by invasive monitoring.

Disconnection

Disconnective operations include hemispherotomy, the modern iteration of hemispherectomy, to isolate a diffusely pathologic, epileptogenic hemisphere and lobar or multilobar disconnection, for pathology that affects multiple lobes (Fig. 57-1).

FIGURE 57-1 Hemispherectomy. Axial (A), coronal (B), and sagittal (C) images of a 13-year-old patient 1 day after peri-insular hemisphereotomy. The thick white arrows show the area of central resection, while the smaller arrows demonstrate the disconnecting cuts.

Trans-Sylvian Hemispheric Disconnection or Functional Hemispherectomy

Hemispheric deafferentation, hemispheric disconnection, hemispherotomy, or functional hemispherectomy are synonyms for surgical procedures that aim to disconnect all cortical structures of one hemisphere from the deeper lying structures of the brain, i.e., the basal ganglia by combination of disconnective steps and additional more or less extensive resective steps. The various terms mirror the development in the last 20 years away from classic anatomic hemispherectomy to procedures that combine more and more disconnective steps with less and less resective steps. In the last 15 years, several procedures have been described ⁴⁰^–⁴⁴ that step by step have replaced anatomic hemispherectomy and Rasmussen’s functional hemispherectomy technique.

Indication

These procedures are primarily indicated in patients with pre-existing unihemispherical damage and typical neurologic deficits such as hemianopia and hemiparesis combined with drug resistant epilepsy. If these cases occur in early infancy, are combined with holohemispheric dysplasias or extensive damage and severe drug resistant seizures they are frequently called “catastrophic epilepsy.” Other typical diagnoses seen in these patients include hemimegalencephaly, multilobar cortical dysplasia, and various disorders of gyration such as polymicrogyria or lisencephaly. Hemimegalencephaly (HME) is a quite rare malformation of the cortical development arising from an abnormal proliferation of anomalous neuronal and glial cells that leads to the hypertrophy of the whole affected cerebral hemisphere. Epilepsy typically presents in infancy with severe, drug-resistant seizures. Many infants with HME undergo hemispherotomy before 2 years of age. HME patients have more operative complications and worse seizure outcomes than other hemispherotomy patients.^45,⁴⁶

Frequently hemispheric damage is due to perinatally acquired brain defects or intrauterine hemorrhagic damage. Perinatal stroke is in many ways an ideal pathology for hemispherotomy as patients typically have little function stored in the affected hemisphere and rarely incur new deficits from the operation. Development and cognition often improve when seizures are controlled and epilepsy medicines are decreased or discontinued. Many of these children have cystic encephalomalacia in continuity with the ventricular system or separated by a thin membrane. This expanded space gives additional surgical access and easier visualization of the deep anatomy that needs to be disconnected, but this anatomy can be distorted. Care must be taken to maintain orientation.

Hemispherotomy may also be considered in diseases such as Sturge-Weber syndrome, a sporadically occurring phakomatosis consisting of unilateral facial port-wine stain in the V1 distribution, glaucoma, and unilateral leptomeningeal angioma. The intracranial angioma causes progressive cortical atrophy and calcification. Epilepsy occurs in 75% to 90% of Sturge-Weber patients, usually presenting in infancy. In some patients, the angioma is focal enough to be resected, but most are candidates for hemispherotomy.^47,⁴⁸

Other pathologies potentially amenable to hemispherotomy include Rasmussen’s encephalitis, and postencephalitic and post-traumatic hemispheric damage. Rasmussen’s encephalitis or chronic focal encephalitis is a chronic T-lymphocyte inflammation that remains localized to one hemisphere. Presentation is usually between 5 and 10 years of age with acute, drastic onset of focal seizures and progressive loss of hemispheric function. Anticonvulsants typically have little success in controlling seizures. Medical treatments to control inflammation, including steroids, intravenous immunoglobulin (IVIG), tacrolimus, and plasmapheresis are the first line of therapy, although their efficacy is limited.^49,⁵⁰

Timing of hemispherotomy has been debated in Rasmussen’s patients particularly. Permanent loss of hemispheric function eventually occurs in a large majority from the disease, prompting recent recommendations for early hemispherotomy. Some evidence supports improved cognitive outcomes with early surgery,^46,⁵¹ but this must be balanced against the acute loss of hemispheric function from surgery performed before hemispheric dysfunction is fully actualized.

As hemispherotomy disconnects the motor and visual cortices, hemiparesis and visual field defects are inevitable consequences of the operation. The procedure is more appealing if these deficits are already present or if they are an inevitable outcome of the underlying pathology. Postoperative hemiparesis affects the upper extremity more than the lower. Ability to walk is almost always maintained.^52,⁵³ Improved development and cognition commonly follow hemispherotomy in cases where seizures stop and anticonvulsants can be weaned.^52,⁵⁴^–⁵⁶

Diagnostics

The most important presurgical diagnostics include MRI and EEG recording of ictal events. MRI may show larger defects, frequently in the distribution of the middle cerebral artery, and malformations of cortical development. MRI findings include gross malformations of the hemisphere, ectopic gray matter, disorders of gyration, extensive postencephalitic damage, or holohemispheric atrophy. In the ideal case all ictal activity can be localized to the affected hemisphere. However, hemispherectomy may be indicated even in the setting of contralateral EEG or even MRI abnormalities. MRI abnormalities on the opposite hemisphere are particularly concerning for predicting poor success.^57,⁵⁸

If the disease has started after the fourth year it is important to demonstrate transfer of language to the other hemisphere for which the intracarotid amobarbital test, or possibly functional MRI,¹⁰ is useful. However, the association between speech outcome and hemispherectomy is multifactoral with congenital etiologies having a more favorable outcome, even with age considered.⁵⁹

Choice of Approach

It has become clear that the disconnective procedures carry the same success rate, may be even higher success rates as those procedures that involve larger resective steps.^60,⁶¹ In the 10 to 15 years of available follow-up for these disconnective procedures the complications known from anatomic hemispherectomy have not been reported thus far, and in particular no cases of cerebral hemosiderosis, which was reported for earlier procedures. The availability of CT and MRI-scanning has also minimized the potential problem of shunt malfunctions. However, postoperative hydrocephalus is also much rarer in disconnective procedures such as trans-sylvian keyhole deafferentation or trans-sylvian keyhole disconnection, or the alternatives described by others.^40,^43,⁴⁴ Other advantages of the trans-sylvian keyhole disconnection procedure favored by us have been confirmed by other series.^43,^60,⁶² The keyhole procedures are less well suited for cases with extensive hemispheric malformation such as hemimegalencephaly. In these cases we combine the trans-sylvian approach with a perisylvian window technique such as has been described by Villemure and Mascott.⁴⁴

Surgical Technique

Preoperatively, anticonvulsive medication is not discontinued. Dexamethasone is given in cases with normal brain volume or hemimegalencephaly, not in cases with large central cysts and huge ventricles. Testing of clotting mechanisms and blood typing should be done and before skin incision a prophylactic antibiotic is given. The placement of an arterial line is important as well as a central venous line in small babies. As always in pediatric neurosurgery, keeping the patient warm during surgery is important.

The patient’s head is brought into a horizontal and slightly downward pointed position. The skin incision starts right before the tragus and can be linear or slightly curvilinear to allow for a craniotomy flap of 4×4 or 5×6 cm, depending on the size of the cranium. This small craniotomy size is possible because one just has to reach in front and behind the corpus callosum, which in children is no longer than 7 cm. Neuronavigation may be helpful to place the craniotomy in an ideal position, that is, the upper border at the level of the corpus callosum and the lower about 1 cm below the level of the M1. The sylvian fissure is opened and the temporal and frontal opercula are dissected away from the insular cortex (if those opercula still exists) and thus the insular cortex is exposed. Due to atrophic processes or postinfarction cyst built-up orientation may be more difficult. The tree of the M2–M3 branches is a good guide, although one should keep in mind that in cases with perinatal infarction, the M2 and M3 branches are smaller in diameter. In some cases the block of basal ganglia, thalamus, and insular cortex is considerably smaller than in healthy brains. If multiple cysts are present, these vascular structures inside the ventricle the choroid plexus are helpful as guides to orientate the surgeon.

Once the insular cortex has been exposed, the temporal horn is opened from its anterior tip to the trigone, approaching it through the inferior circular sulcus of the insular cistern. As the next step, the cella media and the frontal horn are also exposed by a transcortical incision along the circular sulcus following the outline of the circular sulcus to its superior part underneath the frontal operculum. In that way finally the whole ventricular system is exposed through a U-shaped incision along the circular sulcus from the temporal horn tip through the trigone and forward again to the tip of the frontal horn.

At this stage the basal ganglia bloc is disconnected from its cortical input but the four lobes are still connected through their mesial and basal structures to the deeper lying brain structures.

The mesial disconnection begins with removal of the uncus, transecting mesially through the amygdala to the choroidal fissure in the temporal horn and then taking the hippocampus/parahippocampus en bloc for accurate histology. Using the tentorial rim as a guideline the mesial disconnection is then carried backward around the trigone through the mesial brain structures. Always leaving the mesial arachnoid intact one exposes step by step the falcotentorial edge, works upward through the trigone to the inferior rim of the falx, to the cella media. Now working very close to the midline one in fact does a paramesial callosotomy all the way anteriorly to the frontal horn. The frontal and parietal opercula are reflected with self retraining retractors, looking through the exposed ventricle one can easily disconnect the callosal fibers close to the mesial ventricular roof or wall.

The most difficult step considering orientation is the fronto-basal disconnection. This is facilitated by creating a disconnection line through the basal part of the frontal lobe. A dissecting plane through the fronto-basal cortex and white matter along the M1 branch is created starting on the surface of the brain always aiming for the major M1 branch and following it down toward the midline. After a short distance the carotid bifurcation is reached and exactly in the same direction as the outward bound M1, one then follows the inward bound A1 to the interhemispheric fissure. Leaving the basal and mesial arachnoid intact one disconnects the fronto-basal brain tissue. After reaching the midline, the A1 turns into the A2 and now one is following the ascending pericallosal artery around the anterior bend of the corpus callosum. In this way one will finally reach the posterior disconnection line which originally went from the trigone to the anterior aspect of the frontal horn.

Important guide structures for the temporo-mesial, occipital, and parietal disconnection are the rim of the tentorium, and later the falco-tentorial margin and finally the inferior rim of the falx. In the frontal part of the brain the guide structure for the surgeon are the M1, the A1, and then the pericallosal artery. The two transection lines will meet at the roof of the anterior horn where the surgeon, following the pericallosal artery from the fronto-basal area will meet the transection line that was created when following the inferior rim of the falx from the trigonal area.

In the event of preserved insular cortex, we prefer its routine removal by CUSA, since postoperatively there will be no discussion about the importance of persisting insular cortex if case persisting seizures are observed.

Postoperative Care

Patients spend one or two nights on intensive care, particularly very small babies, and patients with hemimegalencephaly stay one night more. Wakefulness, verbal response, and motor response are classically monitored, as are pulse frequency, blood pressure, and temperature. In cases with more than expected blood loss, especially in hemimegalencephaly patients with larger blood loss, replacement of erythrocytes and coagulation factors will have been done during surgery and should be completed on intensive care. We have seen the occasional case with electrolyte disorder, so these also need to be followed. Postoperative anticonvulsive medication remains the same as preoperative. In case postoperative seizures are observed they need to be recorded and described carefully. In some patients deterioration of motor function of the leg makes postoperative treatment in a rehabilitation unit mandatory. An early postoperative MRI nicely demonstrates completeness of transection because fresh blood acts like a contrast medium.

Complications may be similar to all craniotomies, that is, subdural hemorrhage, epidural hemorrhage, or infection of ventricular space and bone flap. Dreaded complications would occur if the midline is transgressed, particularly in the area of the septum pellucidum where the contralateral fornix is close. Patients frequently develop pyrexia, which may last for a few days or for more than a week and they are usually noninfectious.⁶³ An elevated cell count may also be caused by contamination of CSF with blood.

As with all intraventricular surgeries a certain rate of patients needing a shunt or developing intraventricular cysts or adhesions appears to be unavoidable. Large series of hemispherotomy (20 to 83 patients) report rates of hydrocephalus ranging from 2% to 16%, commonly presenting months after the operation. Hemimegalencephaly and widespread cortical dysplasia patients are more likely to develop hydrocephalus than others. Infections and hemorrhages have also been reported, at times as often as 5% for each.^42,^53,^64,⁶⁵

In the Bonn groups pediatric series, one of us (JS) has reviewed 93 children with a minimum follow-up of 1 year (mean 100 months, range 12–265 months) with five shunts required (5.4%) and a few cases with hygromas and meningitis. Severe blood loss, not infrequently seen in anatomic hemispherectomy or with Rasmussen’s functional hemispherectomy technique was never seen with the trans-sylvian technique. There was one death in the total series, a hemimegalencephalic 5-year-old boy was found dead in his bed on day 6 without recognizable intracranial complications.

A certain degree of deterioration in motor function, especially of the affected hand pincer movement and sometimes in movement of the leg, is unavoidable, whereby ability to walk is typically regained after rehabilitation in those few cases where significantly deterioration in walking ability occurred.^52,⁵³

Outcome

Cognitive and developmental outcomes have garnered significant attention recently with quite positive results. An increasing number of studies show improved postoperative development following hemispherotomy as fewer seizures and anticonvulsant medications improve cognitive abilities and social integration.^52,⁵⁴^–⁵⁶ Early surgery has been increasingly recommended to avoid seizures during the critical periods of brain and social development. These points must be balanced against the increased anesthetic and blood loss risks of young children⁶⁶ and the potential for ongoing development or radiologic revelation of a pathology (as with cortical dysplasia or Sturge-Weber syndrome) that would have argued against hemispherectomy.

Good seizure outcomes have been demonstrated from all forms of hemispheric surgery, with more than two thirds of patients seizure-free. Rasmussen’s encephalitis, Sturge-Weber, and perinatal stroke patients consistently have better seizure outcomes, with reported rates ranging from 73% to 93% seizure-free. Case series of cortical dysplasia and hemimegencephaly are seizure-free between 63% and 80% of the time.^53,^64,⁶⁵

In Schramm’s series (mentioned above) of 93 pediatric and juvenile patients with a minimum follow-up of 1 year, Engel class 1 outcome was 88%. Other all-pediatric series have reported similar results.⁶⁷ Thus, hemispherotomies continue to be a successful type of epilepsy surgery.

Palliative Disconnection—Callosotomy

For children with generalized seizures, and drop attacks in particular, corpus callosotomy is a palliative disconnection consideration. The disconnection will, in theory, prevent the spread and generalization of a seizure. Callostomy has been considered for drop attacks, primary and secondary generalized seizures, and medically refractory mixed seizure types like Lennox-Gastaut.⁶⁸^–⁷² Traditionally, anterior callosotomy has been the preferred option to avoid disconnection syndromes, but recent reports in the literature suggest that a one-stage complete callosotomy may be a better choice for initial surgical treatment in some patients.⁷³^–⁷⁵

The surgical approach for both anterior two thirds or complete callosotomy can be done through the same incision placed over the coronal suture with the patient in the supine position in a head holder. Brain relaxation may be used to minimize the degree of frontal retraction. Frameless stereotactic navigation can optimize flap location, trajectory, and avoidance of large midline venous complexes. A midline craniotomy, biased to the right, is performed. The dura is opened based medially until the interhemispheric fissure is visualized. Cortical veins are respected and the frontal lobe retracted until pericollosal vessels found. Azygous vessels must be considered as the dissection proceeds to find the avascular midline. The pearly white callsoum is then exposed and divided. In an anterior two-thirds procedure, the anterior genu is disconnected to just before the splenium. The callosum is sectioned staying within the leaves of the septum and ideally preserving the ependymal lining of the ventricles. In complete callosotomies, the resection is taken more posteriorly and the internal cerebral vein and vein of Galen that lie just anterior to the splenial reflection of the callosum are preserved by remaining inside the adjacent pial membrane.

When analyzed by seizure type, atonic spells, myoclonic seizures, and absence seizures appear to be the seizure types most affected by a corpus callosotomy.⁷⁵^–⁷⁸ Cognitive and psychosocial outcomes have been demonstrated by family surveys and other assessments administered following surgery.^79,⁸⁰ Operative complications of callosotomy can include hydrocephalus, aseptic meningitis, and cerebral edema.⁷⁵

Resections

Resective operations for pediatric epilepsy include a diverse array of underlying pathologies and diverse prognoses. Resection of circumscribed lesions in ineloquent cortex can have low risk, good prognosis, and straightforward decision making, while other cases may require an involved search for the seizure focus and difficult decision making about whether to proceed with surgery.

Temporal Lobe Resections

The results of pediatric temporal lobe resections are less consistent than adult series and the pathologies more varied. Structural lesions are present in as many as half of pediatric temporal lobectomy candidates.^15,⁸¹ Hippocampal sclerosis does occur in pediatric patients, but often in association with other neocortical pathology.⁸² The presence of such dual pathology alters the surgical approach, and the suspicion of such will at times mandate invasive monitoring.

Common temporal lobe pathologies in children include tumors, vascular lesions, and dysplasias. In the subset of patients with a well-circumscribed, neocortical lesion such as DNET, ganglioglioma, or cavernous malformation, the decision must be made whether to resect mesial temporal structures. Lesionectomy alone can yield seizure freedom, but often mesial structures are also involved and may need to be included ⁸³^–⁸⁵ typically this is a consideration in the case of a prolonged seizure history. Dysplasia in the anterior temporal lobe can often be seen in the setting of hippocampal changes in the young patient (Fig. 57-2).

FIGURE 57-2 Temporal lobe pathology with hippocampal involvement. Preoperative (A to C) and postoperative (D and E) T2 images of 9-year-old boy with intractable epilepsy due to temporal lobe cortical dysplasia. Electrocorticography revealed epileptiform spikes extending to much of the posterior hippocampus. An extensive temporal lobe resection was performed.

Technique

Many different terms are applied to temporal lobectomies. An “aggressive” temporal lobectomy would typically be employed only when extensive dual pathology is present. In what historically might be considered a standard anterior temporal lobectomy, the lateral cortex is resected back 4 cm on the dominant side and 6 cm on the nondominant side.⁸⁶ Compared to selective procedures, this “standard” temporal lobectomy would be less desirable for cases of isolated medial temporal pathology.⁸⁷

So-called “selective” approaches cover a variety of strategies. When the lateral temporal lobe is suspected to be pathologic, the resection can be tailored to address the specific imaging or electrophysiologic abnormalities.^88,⁸⁹ Such a “tailored-selective” approach may use ECoG or prolonged monitoring through grids and strips to identify the extent of the epileptic zone when a diffuse lesion is present. In children old enough for consideration of language preservation, functional MRI, invasive mapping, or in some adolescents, awake language mapping may be necessary to identify and preserve speech cortex⁹⁰ and such considerations may also modify the surgical approach.

In any access to mesial structures, the temporal horn must be accessed, this may be through middle temporal,⁹¹ inferior temporal, or basal temporal^92,⁹³ in the various “anatomic-selective” approaches that address isolated mesial temporal pathology. Approaches through sylvian fissure access to the hippocampus are also described primarily in adult populations.^94,⁹⁵ The amygdalohippocampectomy can then be performed. While these approaches are often appropriate in adult patients where hippocampal sclerosis is usually isolated and evident on imaging, they are applicable to only the pediatric case that displays these clear preoperative criteria for isolated mesial temporal sclerosis.¹⁷ More tailored resections may be indicated, varying the amount of lateral resection, depending on preoperative imaging and other evaluations.

The hippocampal resection is carried back beyond the choroidal point between 1.5 to 3 cm, or until the lateral brain stem is visualized beyond the arachnoid. Direct hippocampal recording has been described to further tailor the operation.⁹⁶ Care must be taken to maintain a subpial resection as the oculomotor nerve, carotid artery, optic nerves, and brain stem lie medial to the hippocampus.

Complications

Some degree of superior quadrant visual field defect is expected from interruption of Meyer’s loop pathways running through the temporal lobe, even for many selective approaches,^93,^97,⁹⁸ but small deficits typically goes unnoticed by the patient and on bedside exam. Dominant temporal lobectomies in rare instances cause severe anomia and more commonly cause a variable decline in verbal ability on neuropsychology testing and difficulty with naming specific proper nouns.⁹⁹ Good verbal memory preoperatively and a normal MRI are risk factors for decline in language function in adults,¹⁰⁰ but the effects in children are less studied. “Selective” procedures appear to have better cognitive outcomes than “standard” temporal lobectomy, but an anatomic-selective versus a tailored-selective approach have not been directly compared. Damage to medial structures can occur, including the brain stem, cranial nerves, or major vessels. Respect for the pial boundary between the temporal lobe and these structures helps to prevent such complications. CSF leak, infection, and hemorrhage complicate temporal lobectomy at similar rates as other craniotomy procedures.¹⁰¹

Outcomes

Outcomes in pediatric patients undergoing temporal lobe surgery for intractable epilepsy are difficult to compare for several reasons. Temporal lobe pathology is less common in pediatric epilepsy series than adult, and the pathologies vary between different series, with some institutions reserving surgery for lesional resections while others take a more aggressive approach. The outcome measures also vary among the reported series. The inclusion of adolescents, who more commonly have mesial temporal sclerosis, can also affect the outcome of a series.

In three recent series of pediatric temporal lobe operations for epilepsy, the rate of Engel class 1 or 2 outcome has varied from 63% to 88.5%.^17,^102,¹⁰³ Cortical tumors have the best seizure outcome in most series.¹⁰³

Mesial temporal sclerosis (MTS) confirmed on pathology also bore a favorable prognosis in a recent multi-institution study.¹⁷ MTS was identified histologically in 53% of their series of children younger than 14 years with nontumor temporal lobe pathology. Children with MTS had Engle class 1 or 2 outcome 77% of the time as compared to 57% in those without MTS, a group consisting mostly of cortical dysplasia and gliosis.

Despite the added complexity of dual pathology cases, some series report similar outcomes in cases with preoperatively recognized dual pathology to other children undergoing temporal lobe epilepsy surgery.¹⁰³ Negative predictors of seizure outcome include developmental delay, multifocal EEG, and multiple seizure types. All of these factors point to diffuse seizure onset.^17,¹⁰³

Extratemporal Resections

Extratemporal epilepsy foci are far more common in children than in adults where mesial temporal sclerosis dominates. Many authors group lateral temporal sources of seizure with extratemporal sources under the category of neocortical epilepsy as distinct form MTS. This taxonomy better reflects the underlying pathology and more starkly divides pediatric from adult epilepsy populations, but when planning a surgical approach we find the geographical categorization of temporal versus extratemporal epilepsy more useful (particularly with the possibility of dual pathology). Extratemporal epilepsy foci occur most commonly in the frontal lobe followed by the parieto-occipital regions.¹⁰⁴

Search for a lesion with brain imaging is a key component to any epilepsy evaluation. MRI is the mainstay of this evaluation. Particularly with higher resolution MRIs, subtle areas of cortical dysplasia or migrational disorders can be identified to focus the further evaluation. Common lesions seen in surgical pediatric epilepsy include low-grade tumors, malformations of cortical development, and less commonly, vascular lesions and acquired gliosis (from trauma, infection, or other cause).

Developmental tumors such as gangioglioma, ganglocytoma, and dysembryoplastic neuroectodermal tumor (DNET) most commonly present with seizure. Epilepsy from such lesions has a very high likelihood of intractability, reaching 90% by 10 years after diagnosis.¹⁰⁵ Resection of a low-grade tumor provides very good seizure control, with Engle class 1 or 2 in as many as 95% in some series.¹⁰³ Often after scalp EEG confirmation of the involvement of the lesion in seizures, lesionectomy is undertaken with or without electrocorticography guidance.^106,¹⁰⁷ Complete resection of the lesion is an independent predictor of good outcome.¹⁰⁸

Malformations of cortical development appear on imaging as variations in the depth of sulci, distribution of sulci, thickness of the cortex, or presence of gray matter in the depths of white matter. Focal cortical dysplasia is the most common of these amenable to epilepsy surgery.¹⁰⁹ It histologically consists of disorganization of lamellar structure, large neurons, neuronal heterotopias extending into the deep white matter, usually tailing toward the lateral ventricle, and balloon cells with focal gliosis. Cortical dysplasias are most commonly located at extratemporal sites.¹¹⁰ High-resolution MRI or PET may reveal lesions that are otherwise occult.

Vascular malformations including cavernous malformation and arteriovenous malformation (AVM) are a less common cause of pediatric epilepsy. Cavernous malformations are well circumscribed lesions surrounded by an area of hemosiderin-stained brain. For either, lesionectomy outcome resembles that of low-grade tumor resection.¹¹¹

Technique

When a diffuse lesion such as FCD is identified, the extent of the epileptogenic zone often requires better characterization. Type 2 dysplasias are typically visible on MRI, but PET or MEG may show areas of abnormal metabolism that are more extensive than the visualized malformation. Functional imaging may also reveal type 1 cortical dysplasia which is occult on MRI.¹¹² Several approaches have been taken for cortical dysplasia. Tuberous sclerosis in particular is managed differently across institutions.

Invasive seizure monitoring and cortical mapping are often employed to tailor the resection of diffuse lesions to include the full seizure focus and avoid important functional tissue (Fig. 57-3). In a two stage approach to mapping such a lesion, a grid is typically placed over the lesion to precisely localize seizure onset. Strip electrodes are useful in monitoring otherwise hard to sample areas such as the interhemispheric fissure, the orbitofrontal cortex, or at times, the contralateral hemisphere. Depth electrodes can define the epileptogenicity of a tail of gray matter extending toward the ventricle from a cortical dysplasia. In some cases, a replacement of the subdural electrodes immediately after an initial resection.