Epileptic seizures may begin hours in advance of clinical onset: A report of five patients

Mechanisms underlying seizure generation are traditionally thought to act o ver seconds to minutes before clinical seizure onset. We analyzed continuou s 3- to 14-day intracranial EEG recordings from five patients with mesial t emporal lobe epilepsy obtained during evaluation for epilepsy surgery. We f ound localized quantitative EEG changes identifying prolonged bursts of com plex epileptiform discharges that became more prevalent 7 hr before seizure s and highly localized subclinical seizure-like activity that became more f requent 2 hr prior to seizure onset. Accumulated energy increased in the 50 min before seizure onset, compared to baseline. These observations, from a small number of patients, suggest that epileptic seizures may begin as a c ascade of electrophysiological events that evolve over hours and that quant itative measures of preseizure electrical activity could possibly be used t o predict seizures far in advance of clinical onset.