The role of EEG in epilepsy: a critical review.

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.