Abstracts

Rationale: In patients with refractory nonlesional frontal lobe epilepsy, identification of the epileptogenic zone (EZ) is frequently arduous. Intracranial EEG (icEEG) recordings are frequently required to better delineate the EZ but the presence of an extensive network of connections allowing rapid ictal spread may result in bilateral homologous regional (or extremely diffuse) electrical ictal patterns. Fast continuous spread of ictal activity is associated with significantly poorer surgical outcome. We report a case where callosotomy performed after a first non-lateralizing icEEG study allowed adequate identification of the EZ. Methods: An 18-year old left-handed patient with no seizure risk factors and normal development, had onset of seizures at age 12 years. Seizures were characterized by lower extremity hypertonia, fall to the ground, and inability to stand up for the following 15 seconds, recurring up to 100/day. After failing 8 anticonvulsants, a presurgical evaluation was performed. Video-EEG disclosed central spikes. EEG-fMRI showed bold activations over the right frontal medial and cingulate gyri. MEG revealed medial and lateral sources in front of the central sulcus bilaterally. PET disclosed mild hypometabolism of the left frontomedial area and ictal SPECT revealed multiple activation sites. Neuropsychology testing showed no significant deficits. An intracranial study was performed, sampling the cingulate, medial frontal, orbitofrontal, middle and superior frontal gyri bilaterally using subdural strip electrodes. Results: Numerous seizures were recorded characterized by low voltage fast activity over both supplementary motor areas (SMA). Interictal and ictal activity from the right SMA mirrored that of the left SMA. Careful analysis of ictal manifestations or spreading epileptic activity was not helpful in lateralizing the EZ. A decision was made to perform an anterior partial callosotomy and continue with further icEEG recordings. Sectioning of anterior transcallosal fibers allowed to identify the EZ to the right SMA. Resection of the right SMA led to seizure-freedom.Conclusions: When icEEG recordings reveal bilateral synchronous focal or regional ictal activities, a partial anterior callosotomy followed by further icEEG recordings may be helpful in the adequate localization of the EZ. The strategy described above (icEEG ? anterior callosotomy ? icEEG ? resective surgery) is not unlike the strategy adopted by rare paediatric epilepsy groups that perform callosotomy in multistage epilepsy surgery [callosotomy (without prior icEEG) ? icEEG ? additional surgery)] for patients with mostly cryptogenic or symptomatic refractory epilepsy. The absence of a mirror focus after callosotomy despite high seizure frequency and long duration of epilepsy is of note.