Recognition of Potential Surgical Candidates and Video-Electroencephalographic Evaluation

Gregory D. Cascino

In the United States, 1% to 4% of the population has epilepsy—i.e., two or more unprovoked seizures, and approximately 180 000 patients are newly diagnosed each year (1,2). Worldwide, epilepsy affects an estimated 50 million people (1,2). Rates of unprovoked seizure activity are highest in early childhood and increase again with age (1,2). Partial or localization-related epilepsy characterized by focal seizure activity is most common in adults with recurrent, unprovoked seizures (1, 2, 3), with more than 90% of the incident cases of epilepsy in adults involving focal seizures (1,2). The most frequently occurring seizure type is a complex partial seizure of medial temporal lobe origin (1, 2, 3). Approximately 45% of patients with partial epilepsy have medically refractory disorders (3). A minority who fail to respond to first-line therapy will be rendered seizure free with antiepileptic medications approved since 1993 (4, 5, 6). In one population-based study, approximately 10% of 470 newly diagnosed patients whose initial antiepileptic drug was unsuccessful achieved control with subsequent medical therapies, but nearly one third of those who were followed up for 5 years developed medically refractory seizures (6). The early response to antiepileptic drug therapy is highly predictive of long-term medical outcome, and the most “effective” drug for partial epilepsy is often the first medication used (6). A seizure disorder is likely to be considered “refractory” after the first two drug regimens (6).

INTRACTABLE EPILEPSY

Approximately 20% to 30% of all patients with epilepsy have seizures that are physically and socially disabling (1, 2, 3, 4, 5, 6, 7, 8, 9, 10), and these individuals are at significantly greater risk for long-term morbidity and mortality than a control population (11). Intractable epilepsy may be associated with psychosocial deprivation, inability to operate a motor vehicle, unemployment or under-employment, limited educational opportunities, and need for a caregiver. Often, it may be a “catastrophic” disorder accompanied by progressive neurocognitive impairment and behavioral alterations (3, 4, 5, 6). Unfavorable seizure control significantly increases the cost of epilepsy (12,13). Each year, prevalent cases of epilepsy cost the United States approximately $12 billion, and almost 90% of these mostly indirect expenses are related to the care of patients with recurrent seizures and intractable epilepsy (13). Treatment of intractable partial epilepsy includes antiepileptic drugs, vagus nerve stimulation (VNS), and epilepsy surgery (6,14,15). The rate of response, i.e., at least a 50% reduction in seizures, with medical therapy and VNS is approximately 30% to 40% (14); complete control occurs in fewer than 10% of patients (6,14). Surgical ablation of the epileptic brain tissue in patients with a localization-related epilepsy is the most effective
way to significantly reduce seizure activity and improve quality of life (7, 8, 9, 10,15, 16, 17). Epilepsy surgery may be appropriate for patients who have failed to respond to “two well-tolerated treatment regimens” (6). This chapter emphasizes the importance of preoperative electrophysiologic findings in the evaluation of potential surgical candidates.

SURGICAL TREATMENT OF EPILEPSY

Identification of Patients

Epilepsy surgery is a highly effective and safe alternative for selected patients with intractable partial epilepsy (7, 8, 9, 10, 15, 16, 17) that aims to reduce seizure tendency and allow the individual to become a participating and productive member of society (3,7). Other goals include avoidance of neurologic morbidity and reduced exposure to toxic reactions of antiepileptic drugs. Potential candidates have physically and medically disabling seizures that significantly impair quality of life, and these patients should be identified early in medical treatment and selected for diagnostic evaluation (7, 8, 9, 10). Focal corticectomy, the most common operative procedure in the adult patient, involves resection of the epileptic brain tissue in the anterior temporal lobe (17). In patients with intractable partial epilepsy, a focal cortical resection includes the site of seizure onset and initial seizure propagation, with excision of the pathologic lesion (15, 16, 17). Patients with medial temporal lobe epilepsy and substrate-directed or lesional epilepsy may achieve a significant reduction in seizures (10,17) but are less likely to respond to antiepileptic drugs and may have a pathologic substrate underlying the epileptogenic zone. Magnetic resonance imaging (MRI) may demonstrate structural abnormalities and plays a pivotal role in the selection and evaluation of patients for alternative forms of therapy (7,18, 19, 20, 21). Its sensitivity and specificity in identifying posttraumatic alterations, vascular anomalies, tumors, malformation of cortical development (MCD), and mesial temporal sclerosis (MTS) have been confirmed (18, 19, 20, 21). The preoperative evaluation is designed to identify the site of seizure onset and to localize functional cerebral cortex by means of scalp-recorded ictal EEG monitoring and neuropsychological studies; visual perimetry and cerebral arteriography (Wada test) are performed as needed.

Medial Temporal Lobe Epilepsy

Epilepsy surgery has been compared favorably to antiepileptic drug therapy as regards seizure outcome and quality of life in patients with intractable temporal lobe epilepsy (17). The most common localization of the epileptogenic zone in adults is the mesial temporal lobe. Approximately 70% of patients who undergo a temporal lobe resection become seizure free during long-term follow-up (8,17). More than 90% of patients with concordant unilateral temporal lobe epileptiform discharges and a neuroimaging alteration experience an excellent outcome after anterior temporal lobectomy (8). Partial seizures involving the “detonator structures” of the temporal lobe, i.e., amygdalohippocampal formation, may be associated with a characteristic aura and ictal behavior (3,15, 16, 17) and involve neurocognitive impairment (3,8). The hallmark lesion of medial temporal lobe epilepsy is MTS (18, 19, 20, 21), and the surgically excised hippocampus almost invariably shows focal cell loss and gliosis (7,8,18, 19, 20, 21). Optimally, MRI in adults with partial epilepsy should include temporal lobe coronal or oblique coronal images using T1-weighted and T2-weighted sequences (18, 19, 20, 21). MRI usually reveals atrophy of the hippocampal formation, reflecting neuronal loss, with a signal intensity change in patients with MTS. Fluid-attenuated inversion recovery sequences increase the sensitivity of MRI to detect a signal change (20).

Substrate-Directed or Lesional Epilepsy

Patients with lesional epilepsy may have a primary brain tumor, vascular anomaly, MCD, or a structural intra-axial abnormality, e.g., posttraumatic encephalomalacia, as the etiology of the seizure disorder (7,9,10,16). A low-grade glial neoplasm, cavernous hemangioma, and focal cortical dysplasia are commonly encountered (7,9,10,16). The MRI almost invariably shows an abnormal structure, and the seizure types are classified as substrate-directed partial epilepsy. The diagnostic yield of MRI in substrate-directed epilepsy depends on the underlying pathologic findings (7,9,10,16,18, 19, 20, 21); however, the test’s high sensitivity and specificity for identifying tumors and vascular malformations have been demonstrated (7,9,10,16). In patients with MCD, the MRI may detect an intra-axial structural abnormality suggesting the likely site of seizure onset and the pathologic lesion (7,9,10). In patients with an MRI-identified foreign-tissue lesion, the electroclinical correlation essentially confirms the epileptogenicity of the structural abnormality (9,10). Concordance between the pathologic substrate and the ictal-onset zone indicates a highly favorable operative outcome in selected individuals, with more than 80% of patients with a low-grade glial neoplasm or a cavernous hemangioma becoming seizure free after surgery (9,10). In contrast, fewer than 40% of patients with focal cortical dysplasia have an excellent operative outcome (22).

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