Electroencephalography and video-electroencephalography

Electroencephalography (EEG) is a specific investigation to support the diagnosis of epilepsy, demonstrating interictal epileptiform activity in the majority of individuals with epilepsy. The EEG can also help classify the epilepsy as focal or generalized, and can suggest certain epileptic syndromes. However, epileptiform activity is absent in the first EEG in approximately half of affected individuals and the interictal EEG remains normal in about 10%. Prolonged EEG recordings increase the odds of finding epileptiform activity and also provide the opportunity to capture ictal discharges for more definitive diagnosis and classification. Video-EEG (V-EEG) additionally allows correlation of clinical and EEG abnormalities and analysis of seizure semiology. V-EEG has become an essential element of the presurgical evaluation for localization of the epileptogenic zone, but confidence in the EEG data requires congruence with other presurgical tests, particularly magnetic resonance imaging. Interictal slow activity defines a functional deficit zone, interictal epileptiform discharges define the irritative zone, ictal EEG onset defines the ictal onset zone, and the analysis of seizure semiology on video helps define the symptomatogenic zone. The EEG pattern and frequency at ictal onset can improve localization and may help predict seizure outcome. The EEG is also crucial in the diagnosis of nonconvulsive status epilepticus. The EEG has pitfalls and limitations, and has to be used in conjunction with clinical and imaging data to avoid misdiagnosis of epilepsy.