Abstracts

Patients with normal MRI (Non-lesional) and medically intractable extratemporal epilepsy make up a disproportionate number of non-excellent outcomes from epilepsy surgery. We investigate the usefulness of intracranial EEG (iEEG) in the identification of surgical candidates. Twenty-seven consecutive patients (1990-2001) with normal MRI, extratemporal epilepsy who had iEEG and subsequent surgery were identified. The implantation of intracranial electrodes was determined by seizure semiology, interictal and ictal scalp EEG, and in some SPECT and PET studies. The anatomic location of the seizure onset zone, as determined by iEEG, and subsequent surgical resection was divided into frontal, parietal, and occipital. The seizure onset iEEG pattern was characterized by the waveform morphology, frequency of discharge and spatial extent (focal, regional, lobar, multilobar). The Engel classification system was used to identify excellent (Class I, and IIA) and non-excellent (Class IIB, IIC, IID, III and IV) outcome from surgery. The presumed seizure onset zone as determined by iEEG was frontal lobe in 25 (92%) patients and parietal lobe in 2 (7%) patients. The iEEG seizure onset frequency was a beta or gamma frequency discharge in 11(40%) of patients, and in 7(63%) of these patients the ictal discharge was focal ([lt] 5 electrodes). In 16(60%) the seizure onset frequency was less than beta frequency ([lt]12 Hz), and of these patients only 3(18%) had a focal onset. A total of 11 (41%) patients had excellent outcome, but none had been able to discontinue medications at follow-up (average follow-up 36 months). Of the patients who achieved excellent surgical outcome, 8(72%) had focal beta or gamma frequency seizure onset patterns compared to 2(13%) of the non-excellent outcome patients [p[lt]0.005]. Focal high frequency (beta or gamma frequency) seizure onset discharge on intracranial EEG seizure may identify patients likely to have an excellent outcome from surgery for non-lesional extratemporal epilepsy. (Supported by the National Institutes of Health (K23 NS047495-01))