Neuropathological findings following temporal lobectomy related to surface and deep EEG patterns

Interictal and ictal characteristics of preoperative EEG recordings, derived from limited surface montages, and medial temporal lobe sites, were compared with the results of pathological studies done on resected lobes obtained from 44 patients with complex partial seizures. Pathological material was divided into four groups: (a) sclerosis (mesial temporal or restricted to pes hippocampi); (b) neoplasia (mainly hamartomas); (c) miscellaneous lesions; and (d) no significant lesions. Interictal EEG correlates of no pathology included bilaterally synchronous surface spikes (with or without simultaneous deep spikes) and independent surface spikes (with or without simultaneous deep spikes) on the sides of lobectomy. Ictal EEG correlates of no pathology included unilateral surface or surface/deep onsets, bilaterally synchronous surface onsets, more than one onset location, and suppression at onset. Focal onsets correlated with sclerosis. Frequent interictal spike activity in the nonlobectomized lobe and fast buildup at onset of ictus suggested neoplasia. Many of the EEG correlates of no pathology are known to correlate with poor postsurgical seizure relief, due probably in part to the fact that absence of pathology in the resected specimen is a negative prognostic sign. Patients with sclerosis could be distinguished from patients with no demonstrable pathology with 81% cross-validation classification accuracy using a distribution-independent, nonlinear classifier. Both interictal and ictal EEG measures were used by the classifier, and one may conclude that ictal and interictal EEG recordings contain nonredundant information for predicting the presence and type of underlying pathology.