BILATERAL HIPPOCAMPAL ATROPHY IN MEDIAL TEMPORAL-LOBE EPILEPSY

Quantitative evidence of hippocampal atrophy has been correlated with site of seizure onset, hippocampal neuronal loss, and seizure relief a fter resection. Most studies have quantified hippocampal atrophy using ratios or differences between right and left hippocampal values. Howe ver, bilateral hippocampal atrophy may remain undetected by these tech niques. To assess the frequency and implications of bilateral hippocam pal atrophy, we studied absolute hippocampal volumes in 53 temporal lo bectomy patients who had undergone intracranial electroencephalogram r ecordings preoperatively. Coronal images were constructed perpendicula r to the longitudinal axis of the hippocampus. Atrophy was defined as >2 SD below control values in the volume of the posterior 1.5 cm of th e hippocampus, Five of 53 patients (9%) had bilateral hippocampal atro phy; four of these cases were undetected by ratios. Surgery was perfor med on the side of ictal onset in all five patients; four have been se izure-free for >2 years. These results suggest that (a) mesial tempora l sclerosis can be present bilaterally and may go undetected by hippoc ampal ratio or difference measures; (b) absolute hippocampal volume va lues as well as ratios are needed to detect all patients with bilatera l hippocampal atrophy; and (c) temporal lobectomy is not contraindicat ed in patients with bilateral hippocampal atrophy, but success depends on electroencephalographic documentation of the side of predominant i ctal onset.