The significance of bilateral EEG abnormalities before and after hemispherectomy in children with unilateral major hemisphere lesions.

The rate of seizure relief following hemispherectomy varies between 50 and more than 80%. There has been particular debate concerning the significance of bilateral electroencephalography (EEG) abnormalities in influencing prognosis. This study was set out to determine the frequency of bilateral EEG abnormalities and their relationship to underlying pathology and outcome. We investigated 28 children with unilateral hemisphere lesions, who underwent hemispherectomy. Interictal and ictal EEGs before and after hemispherectomy were reviewed. Post-operative outcome with respect to seizures was noted. Bilateral EEG abnormalities were seen in 75%, but were more common in children with malformations of cortical development than in patients with acquired cerebral lesions, and were found more often in interictal than in ictal records. Post-operative EEG abnormalities were variable and did not consistently predict outcome. Short-term outcome was similar, irrespective of aetiology. With longer term follow-up, only 47% of children with developmental abnormalities were still seizure-free in contrast to 77% of children with acquired abnormalities. Although the incidence of bilateral EEG abnormalities in patients with major unilateral hemisphere lesions is high, these findings alone should not preclude further consideration for hemispherectomy. Our findings emphasise that the aetiology of the lesion plays a major role in determining outcome.