Scalp and intracranial EEG in medically intractable extratemporal epilepsy with normal MRI

Abstract

Purpose. To investigate EEG and SPECT in the surgical outcome of patients with normal MRI (nonlesional) and extratemporal lobe epilepsy. Methods. We retrospectively identified 41 consecutive patients with nonlesional extratemporal epilepsy who underwent epilepsy surgery between 1997 and 2007. The history, noninvasive diagnostic studies (scalp EEG, MRI, and SPECT) and intracranial EEG (iEEG) monitoring was reviewed. Scalp and iEEG ictal onset patterns were defined. The association of preoperative studies and postoperative seizure freedom was analyzed using Kaplan-Meier analysis, log-rank test, and Cox proportional hazard. Results. Thirty-six of 41 patients had adequate information with a minimum of 1-year followup. Favorable surgical outcome was identified in 49% of patients at 1 year, and 35% at 4-year. On scalp EEG, an ictal onset pattern consisting of focal beta-frequency discharge (>13-125 Hz) was associated with favorable surgical outcome (P = 0.02). Similarly, a focal fast-frequency oscillation (>13-125 Hz) on iEEG at ictal onset was associated with favorable outcome (P = 0.03). Discussion. A focal fast-frequency discharge at ictal onset identifies nonlesional MRI, extratemporal epilepsy patients likely to have a favorable outcome after resective epilepsy surgery.

Figure 1

Kaplan-Meier survival curve. A favorable surgical outcome was reported by 49% of patients after 1 year and by 35% after 4 years of followup.

Figure 2

Scalp EEG-recorded extratemporal onset seizure. The bipolar montage-recorded seizure onset (red horizontal line) shows a focal ictal beta (FIB) frequency discharge from the left frontocentral (Fz, F3, Cz) head region.

Figure 3

Scalp EEG-recorded extratemporal onset seizure. The Laplacian montage-recorded seizure onset (red horizontal line) shows a theta frequency rhythmic discharge from the right frontotemporal (F4, F8, T8) head region.

Figure 4

Kaplan-Meier survival curves comparing patients with scalp-recorded focal ictal beta frequency onset (>13 Hz) (red line) and low frequency (≤13 Hz) (blue line).

Figure 5

Subdural intracranial EEG-(iEEG) recorded frontal lobe onset seizure. Referential montage iEEG from 6 × 6 (36-contact) subdural grid (contacts 35 and 36 removed because of not recording) and a 1 × 8 (8 contact) subdural strip. Gamma frequency oscillation at seizure onset (red box) primarily involving contacts 21, 22, and 23 with early spread to contacts 12, 13, 28, 29, and 30.

Figure 6

Kaplan-Meier survival curves comparing patients with intracranial EEG recorded fast frequency oscillation at seizure onset (>14 Hz, red line) and low frequency onset (<14 Hz, blue line).