Chapter 33: Interpreting Grid and Strip Electrode EEG

Introduction

Long-term video-electroencephalographic (EEG) monitoring may be used to confirm the diagnosis of a seizure disorder, classify seizure-type(s), assess response to therapy, and evaluate patients for surgical treatment of epilepsy.^1–7 In adult patients, video-EEG monitoring (VEM) is mainly used for diagnostic classification (i.e., epilepsy vs nonepileptic spells) and to evaluate surgical candidacy in patients with medically refractory partial seizure disorders. The latter indication is usually restricted to patients with intractable partial epilepsy being considered for a focal cortical resection.¹ VEM is essential in determining the localization of the epileptogenic zone (i.e., the site of seizure onset and initial seizure propagation) in adult patients being considered for surgical ablative procedures.⁶ The high diagnostic yield of VEM in adult patients with recurrent and unprovoked spells has been confirmed.³ Recognition of the ictal EEG pattern may be pivotal in making the diagnosis of epilepsy in such patients. The scalp-recorded interictal EEG study, neurologic history and examination, and current neuroimaging procedures may not always permit appropriate spell classification. The potential disadvantages of video-EEG recordings include the inherent cost of the electrophysiological study and hospitalization and the need for special resources and personnel.³ The ictal EEG patterns may also be difficult to interpret because of myogenic and movement artifacts (e.g., eye blinking). The presence of a subtle epileptiform discharge may be difficult to distinguish from the background. In certain seizure types, for example, auras and simple partial seizures, there may not be a definite scalp-recorded EEG alteration.^1–6,8–11 Finally, patients may not have a typical clinical spell during VEM. The prolonged interictal EEG study may not prove to be a reliable indicator of spell classification.¹²

Surgical Treatment of Epilepsy

Partial or localization-related epilepsy is the most common seizure disorder.^1–3 The most frequently occurring seizure type in the adult patient is a complex partial seizure of mesial temporal lobe origin.^1–3 Approximately 45% of patients with partial epilepsy will experience medically refractory seizures that are physically and socially disabling.^12–14 A minority of patients who fail to respond to first-line antiepileptic drug (AED) therapy will be rendered seizure-free with newer medical treatments introduced in the past decade.^15–18 Epilepsy surgery is an effective alternative form of therapy for certain patients with intractable partial epilepsy.^{13,14,19–22} Patients with mesial temporal lobe epilepsy (MTLE) and lesional epilepsy may be favorable candidates for epilepsy surgery and have a surgically remediable epileptic syndrome.^23–26 The majority of these patients experience a significant reduction in seizure tendency following surgical ablation of the epileptic brain tissue.¹⁹ The hallmark pathology of MTLE is mesial temporal sclerosis.^14,26 The surgically excised hippocampus in these patients almost invariably shows focal cell loss and gliosis.^{13,14,16–18} Patients with lesional epilepsy ...