Abstracts

RATIONALE: Brain mapping via subdural grid stimulation may identify eloquent cortex during pre-surgical evaluations. Seizures may be triggered, but the importance of these seizures in pre-surgical planning has not been described. METHODS: Review of clinical, EEG, surgical and outcome characteristics of three consecutive cases in which brain mapping (pulse duration 0.5ms, rate 50Hz, peak-to-peak output 3-10 mA) reproducibly triggered seizure symptoms. RESULTS: Three patients (2/3 male, age range 33-37 years) with medically refractory neocortical epilepsy (duration 15-24 years; seizure frequency >10/day to 4/month) were evaluated. Each had a single stereotypical partial seizure pattern. The non-invasive evaluations including standard EEG, MRI brain (non-lesional 2/3 patients) and scalp-recorded video-EEG failed to precisely localize seizure onset. Interictal subdural spike foci involved 2 (1 patient) or 3 (2/3 patients) left hemisphere lobes. Spontaneously generated seizures had diffuse or multifocal initial ictal activity involving 3 lobes (2/3 patients). No spontaneous seizures were recorded with the subdural grid in one patient. Brain mapping within a focal cortical zone reproducibly triggered seizures (range 2-5 seizures, median 3) with similar EEG and clinical features to the spontaneous seizures. The trigger zone included only a small portion of the cortex with initial ictal activity. Due to the high risk of injury to eloquent cortex a resection of the initial ictal regions was not advised. A localized cortical resection defined by the trigger zone was performed. Size of cortical resection varied from 2cm x 2cm to 3cm x 5cm (2 posterior temporal, 1 posterior frontal). There was no post-operative neurological morbidity. Three of 3 (100%) were seizure free at follow-up (range 6-36 months, median 24 months). CONCLUSIONS: Triggered seizures during brain mapping can provide valuable localizing information. The cortical trigger zone is not always similar to the region of first ictal activity. When a trigger zone is present in our patients it has been utilized in pre-surgical planning to define the cortical resection and to minimize the risk of neurological morbidity.