Unique Challenges in Extratemporal Lobe Epilepsy

ETLE refers to epilepsy arising outside the temporal lobe and may result from a variety of causes including neoplasms, vascular malformations, malformations of cortical development, and encephalomalacias related to prior ischemic or traumatic events (Fig. 52.1). In addition to a variety of causes, the epilepsy may arise in the frontal, parietal, or occipital lobes and rapidly spread to involve large regions of cortex. These unique factors result in a broad spectrum of imaging findings, complex seizure semiology, and potentially diffuse electroencephalography (EEG) data.

FIGURE 52.1 A, T2-weighted magnetic resonance imaging (MRI) scan demonstrating right frontal cavernous angioma. B, Preoperative MRI and intraoperative photo of left perirolandic oligodendroglioma. C, T2-weighted MRI of left hemimegalencephaly.

Seizure semiology in ETLE may be complex depending on the site of seizure origin. The nature of ETLE lends itself to areas of ictal onset that may not necessarily result in clinical manifestations until the symptomatogenic zone becomes activated. With extratemporal areas of ictal onset, the location that is suggested by semiology has a greater propensity to be a result of secondary spread, resulting in potential diagnostic confusion. Similarly, EEG data may be particularly difficult to interpret because of rapid spread and artifact from muscle movement during the seizure. Deep cortical areas such as the interhemispheric and insular regions are particularly difficult to record epileptic activity from on-scalp EEG. Patients whose seizures start in these areas can present a particular challenge, as electrical seizure activity may be absent on standard EEG arrays, or it may falsely localize as originating in another area entirely.

Magnetic resonance imaging (MRI) findings in ETLE are generally broken down into nonlesional and lesional categories. The goal of MRI evaluation is to identify and characterize any potentially epileptogenic lesion(s). For those patients without MRI lesions, defining a potential area for surgical resection is inherently more challenging, often necessitating the use of adjuvant investigative techniques. Although the radiographic presence of a lesion may prognosticate a better outcome after surgery, more information is usually necessary prior to surgical removal. The lesion’s location and its relationship to functional cortex must be defined in a hypothesis that agrees with the semiology and EEG data.

Surgical Indications

Intractable disabling epilepsy, a localizable epileptogenic zone (defined as the area of tissue that must be removed to prevent further seizures), and a low risk of postoperative deficits are considered the three basic tenets that must be met before a patient can be considered a candidate for epilepsy surgery.² It is important to recognize that medical intractability makes a patient a candidate for further investigation regarding the appropriateness to undergo surgery for epilepsy, but it does not guarantee that surgical management should ultimately be offered. Although no clear definition of medical intractability exists, recent reports in the literature suggest that successful treatment with anticonvulsant therapy is unlikely once two different medicines fail to control seizures.³ The optimal surgical patient will be highly motivated with a social support system in place, and additional coexisting psychiatric illness will be well controlled. Neurocognitive risks must be assessed and discussed with each patient, and risks should be deemed acceptable from both the perspective of the patient and the treatment team. A high probability of improving a patient’s overall quality of life must be confirmed before undertaking the procedure.⁴

Preoperative Evaluation

The starting point for any patient evaluation should include a detailed history and physical examination, with special focus on seizure semiology. Lateralizing and localizing signs, such as ictal eye movements, motor or sensory features, presence or absence of ictal speech, and the presence or absence of aura, should be noted. Video-electroencephalography (VEEG) data allow correlation of a patient’s behavior during a seizure with the changes recorded on scalp EEG. It allows for confirmation of the epilepsy and may provide helpful lateralization/localization to aid in the surgical hypothesis.

MRI studies for patients with suspected extratemporal lesions should include axial T2 sequences, axial and coronal fluid-attenuated inversion recovery (FLAIR) sequences, a sagittal T1 sequence, and a high-resolution (thin-cut) coronal magnetization prepared rapid gradient echo (MPRAGE) sequence.⁵ Although 1.5-tesla magnet strength is considered acceptable, 3-tesla or higher machines and the use of surface coils over the region of interest may add to the diagnostic yield. Additional studies such as subtraction single-photon emission computed tomography (SPECT), [¹⁸F]-fluorodeoxyglucose (FDG), positron emission tomography (PET), or magnetoencephalography (MEG) provide complementary information and may help to formulate the surgical hypothesis (Fig. 52.2). These studies may also be used as an adjunct in patients with multilobar lesions or discordant data.⁶ Functional MRI (fMRI), MEG, or intracarotid sodium amytal testing may be performed for localization of language and sensorimotor cortex (Fig. 52.3). After review of all available data, a likely cause should be determined for the patient’s seizures; an underlying cause has been correlated with the probability of postoperative seizure freedom across a number of surgical case series.^7,8

FIGURE 52.2 Example of left temporoparietal and bitemporal hypometabolism seen on [¹⁸F]-fluorodeoxyglucose (FDG) positron emission tomography (PET) scanning.

FIGURE 52.3 Left hemispheric language dominance documented by word generation, listening, and rhyming tasks during functional magnetic resonance imaging (fMRI).

The goal of the presurgical evaluation is the creation of an individualized surgical hypothesis. This is best done by a multidisciplinary team skilled in the treatment of epilepsy. All the data gathered should be presented to the epileptologist, neurosurgeon, neuropsychologist, psychiatrist, social worker, and neuroradiologist on the team. Initially, each available diagnostic study is analyzed independently of the other investigative modalities. No single study can measure the epileptogenic zone and so all data must be considered together with the video-EEG data, seizure semiology, and MRI, which is perhaps the most important. Adjuvant testing such as PET, SPECT, MEG, neuropsychological testing, and fMRI help to round out the surgical strategy.

The strongest hypothesis is formed when the localization predicted by individual studies is concordant. Any discrepancies must be evaluated and resolved in relation to surgical planning. Discrepancies between semiology, EEG, or MRI are considered the most troubling. If localization differs based on these studies, or if the MRI is nonlesional, then potentially clarifying information should be sought with either FDG-PET or ictal SPECT. When discrepancies cannot be resolved, additional methods of strengthening the working surgical hypothesis should be considered. Chronic extraoperative intracranial recordings may provide critical information in such patients (Fig. 52.4). These may also be used when noninvasive evaluation leads to a hypothesis involving a large cortical area and further delineation of the epileptogenic zone is desired. A working hypothesis of the ictal onset zone based on noninvasive data helps guide the placement of the intracranial electrodes in an effort to confirm it prior to surgical resection.

FIGURE 52.4 A, Example of subdural grid electrode insertion over left hemisphere. B, Sagittal magnetic resonance imaging (MRI) reconstruction with electrode colocalization. C, Anteroposterior (AP) and lateral skull film localization of subdural electrodes.

Invasive intracranial electrodes also have a distinct advantage in mapping functional cortex. Consequently, their placement should be considered when epilepsy is thought to arise from within or near eloquent cortex. They are especially useful for localizing functional cortex in those patients unable to tolerate awake craniotomy (Fig. 52.5).

FIGURE 52.5 Example of cortical stimulation map displaying stimulation identified language and face motor sites.

It is also not uncommon that after a full noninvasive presurgical evaluation, the concordance of data results in a hypothesis that would preclude the patient from undergoing surgical treatment. Even though the patient may still have intractable disabling epilepsy, there is no longer a localizable epileptogenic zone, or the localized epileptogenic zone may confer a high risk of unacceptable postoperative deficit. This possibility should be introduced and discussed with the patient at the onset, not the conclusion, of the surgical evaluation.

Surgical Procedures

There is a broad range of surgical techniques for the treatment of extratemporal epilepsy. These procedures can be loosely divided into resective techniques and palliative techniques.⁹