False lateralization by surface EEG of seizure onset in patients with temporal lobe epilepsy and gross focal cerebral lesions.

Medically intractable temporal lobe seizures developed in 3 patients with radiological and clinical evidence of a gross focal cerebral lesion acquired early in life. All had bilateral independent epileptogenic discharges from the lateral and inferomesial regions of both hemispheres. Scalp and sphenoidal electroencephalographic (EEG) recordings suggested that the seizures originated from the side contralateral to the known cerebral lesion. Because the lateralizing evidence presented by the ictal EEG conflicted with the clinical data, depth electrodes were implanted stereotaxically to determine the side of onset of the seizures. These studies showed that the seizures originated from the limbic structures of the damaged hemisphere in all 3 patients. At operation the mesial temporal lobe structures showed gliotic changes in all. The patients have remained seizure-free for 3 to 13 years postoperatively. These findings suggest that depth electrode recordings may be required to clarify the lateralization of seizure onset in such cases. Extracranial EEG findings must be interpreted with caution in epileptic patients who have gross focal lesions.