Abstracts

RATIONALE: To describe EEG patterns in secondarily generalized seizures (SGS) and determine whether any cortical areas are preferentially involved.
METHODS: Electroclinical correlation was accomplished in 21 SGS in 13 patients with mesial temporal origin who had comparable bilateral intracranial electrode placements. Placements included bilateral frontal and lateral temporal subdural strips and bilateral occipito-mesial temporal depth electrodes. EEG data was separated into 10 categories and correlated with clinical events. Continuous spiking, spike wave-activity and low voltage fast activity were considered major ictal activity (MIA). Disorganized ictal slowing below 12 Hz (DIS) was not considered ictal activity per se, but rather remote ictal effects. Concordance between bilateral corresponding electrodes was examined.
RESULTS: During bilateral asymmetric tonic posturing with arm extension contralateral to seizure origin and arm flexion ipsilateral (n=5) MIA was observed in the mesial temporal region ipsilateral to seizure origin with variable involvement of the lateral temporal (n=3) and frontal regions (n=4). During bilateral symmetric tonic arm extension (n=11) MIA was predominantly ipsilateral (lateral temporal n=9, frontal n=5, mesial temporal n=5) with contralateral DIS in 9 patients. Two patients with large hemispheric lesions had predominantly contralateral frontal, lateral temporal and occipital MIA. During symmetric clonic activity (n=13) bilateral frontal MIA was frequent (n=11; 85%). Three patients (23%) had bilateral DIS and no MIA, 5 patients (38%) had unilateral MIA and the remaining patients had bilateral MIA in the mesial temporal structures during symmetric clonic activity.During symmetrical tonic or clonic activity no patient had symmetric EEG activity ([gt]50% symmetric EEG patterns). Concordance rate was higher during symmetric clonic (22.4% symmetric)than tonic activity (12.0% symmetric, p=0.07). EEG patterns were consistent with minor variations in patients (n=7) with more than one SGS, if clinical signs were identical.
CONCLUSIONS: 1. During SGS,the cerebral cortex is not uniformly or extensively involved with electrical seizure activity during the process of generalization or during sustained generalization.
2. During sustained bilateral tonic motor activity, seizure activity predominantly involves cortex ipsilateral to the side of seizure origin.
3. Bilateral frontal lobe involvement is late, occurring predominantly in the clonic phase.
4. The lack of consistency and limited extent of cortical involvement is consistent with the hypothesis that SGS involve predominantly subcortical structures that are not sampled during invasive EEG recording.
Support: The research was sponsored by the Tiffany-Blake Fellowship of the Hitchcock Foundation.