Asymmetry of the background EEG develops, with loss of normal physiological rhythms and progressive slow activity appearing over the affected hemisphere. The EEG may be normal at presentation in a minority of patients.
Epileptiform activity is frequent over the affected hemisphere in the interictal EEG. There may be focal epileptiform activity that becomes widespread. The appearance of contralateral epileptiform activity may correlate with emerging cognitive decline.
EEG abnormality is enhanced by sleep deprivation and in sleep.
Focal ictal EEG discharges may be seen with focal seizures. There may be multifocal seizures. Epilepsia partialis continua may not be accompanied by surface EEG change.