Review
The role of EEG in epilepsy: A critical review

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Abstract

Electroencephalography (EEG) is the most specific method to define epileptogenic cortex. Its sensitivity and specificity depend on several factors such as age and recording procedures, for example, sleep recordings and activation procedures (hyperventilation, photic stimulation). EEG reveals characteristic findings in several epilepsy syndromes. Rarely, epileptiform discharges are recorded in healthy, particularly young individuals. Ictal video/EEG recording is considered to be critical in localizing the epileptogenic zone. A careful analysis of the first clinical signs and symptoms of a seizure and of the evolution of the seizure symptomatology can provide important localizing clues. Although surface EEG recordings are less sensitive than invasive studies, they provide the best overview and, therefore, the most efficient way to define the approximate localization of the epileptogenic zone. Invasive recordings are used in patients in whom the epileptogenic zone either cannot be located with noninvasive diagnostic methods or is adjacent to eloquent cortex. The most commonly used invasive electrodes are stereotactically implanted depth electrodes and subdural strip or grid electrodes. Foramen ovale and epidural electrodes are of intermediate invasiveness, but less sensitive. Invasive electrodes are subject to sampling errors if misplaced and should be used only after exhaustive noninvasive evaluations have (1) failed to localize the epileptogenic zone and (2) led to a testable hypothesis regarding this localization. Invasive EEG studies are associated with additional risks that are justifiable only if there is a good chance of obtaining essential localizing information and on a potentially resectable area.

Introduction

Electroencephalography (EEG) is the most useful diagnostic procedure for epilepsy, and with the advent of more sophisticated methods of imaging structural damage, epilepsy is now actually one of the few common clinical problems routinely demanding EEG evaluation [1]. EEG can answer three main questions in the diagnostic workup of patients suspected of having epilepsy [1]:

  • Does the patient have epilepsy?

  • Where is the epileptogenic zone?

  • How good is therapy?

Section snippets

Does the patient have epilepsy?

Interictal epileptiform discharges (IEDs) in EEG help to differentiate between epileptic and other nonepileptic paroxysmal attacks. The following EEG patterns are considered epileptiform [2], [3]:

  • Spikes

  • Sharp waves

  • Benign epileptiform discharges of childhood

  • Spike–wave complexes

  • Slow spike–wave complexes

  • 3-Hz spike–wave complexes

  • Polyspikes

  • Hypsarrhythmia

  • Seizure pattern

  • Status pattern

There is no difference in the diagnostic information between sharp waves and spikes [4] (Fig. 1). Rarely, the

Where is the epileptogenic zone?

Several characteristic EEG patterns are associated with well-defined epilepsy syndromes (Table 2). EEG, thus, helps to define certain epilepsy syndromes, which is important for selection of therapy and assessment of prognosis of the epilepsy. We are used to differentiating between ictal and interictal EEG findings. However, although this distinction is clinically important, it is very arbitrary. It is easy if, for instance, a clonic seizure is observed during an EEG discharge, but it may be

How good is therapy?

Where the usefulness of EEG as an aid to diagnosis of seizure disorders is established, its role as a guide to monitoring treatment is much less certain [69]. The EEG may reflect unspecific central nervous side effects of the standard AEDs such as carbamazepine, valproate, phenytoin, benzodiazepines, and phenobarbital [70]. All standard AEDs may result in slowing of the dominant rhythm and increased slow activity. Carbamazepine, in particular, is often associated with apparent deterioration of

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