Background

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.

Interictal

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.

Activation

EEG abnormality is enhanced by sleep deprivation, in drowsiness and in sleep.

Ictal

Focal ictal EEG discharges may be seen with focal seizures, these are often frequent and captured on EEG. Seizures may arise from several foci within the affected hemisphere. Epilepsia partialis continua may not be accompanied by surface EEG change. Due to ipsilateral atrophy, seizures recorded on EEG may appear to have higher amplitude on the contralateral side.