Abstracts

Rationale: Extra- operative intracranial stimulation has been used for identifying and localizing the epileptogenic foci in patients with medical refractory epilepsy. In our current study we present our findings regarding the observed motor, sensory, emotional, and visceral responses from a series of patients with medically intractable epilepsy undergoing invasive preoperative evaluation. Methods: Fifty patients (28 males and 22 females) underwent electrical stimulation to rule out the existence of eloquent cortex in the vicinity of the ictal focus and also to validate focus localization by reproducing clinical components of their habitual seizures. The insertion techniques of depth and subdural electrodes have been previously described. Stimulation studies occurred after sufficient seizures were recorded and seizure medication had been restarted. A Grass 88 or Grass 12 stimulator was used. Balanced biphasic square waves of 1.0-12.0 milliAmps were utilized. Stimulus trains were 3 seconds. Pulse width was usually 0.5 milliseconds. A Tailairach/Tournoux grid was superimposed on postimplant skull x-rays to aid in electrode contact localization. Responses obtained from primary motor or sensory areas were not included in the study. Responses produced by after discharges were not analyzed. Results: A total of 235 responses were obtained,however only 95 were analyzable. Of 51 mesial frontal responses, 27 (53%) were sensori-motor and 24 (47%) were psycho-affective or viscero-autonomic. Of 22 insulo-opercular responses, 7 (32%) were sensori-motor, and 15 (68%) were psycho-affective or viscero-autonomic. Of 22 mesio-basal temporal responses, 3 (13.6 %) were sensori-motor and 19 (86.4 %) were psycho-affective or viscero-autonomic. Of 20 sensori-motor responses, 18 (90%) were of mesial frontal origin. Of 28 dysphoric affective responses, 26 (93%) were of right-sided origin. All 3 euphoric responses were left-sided in origin. Of 14 visceral responses, 9 (64%) were epigastric and were of mesial frontal (4), insulo-opercular (3) and baso-mesial temporal (2) origin. Of 5 autonomic responses, 4 were of mesial frontal and 1 of mesio-basal temporal origin. Reports of a “spaced out” feeling occurred only with frontal stimulation (6 cases). Conclusions: Our study suggests that stimulation responses may be of localizing or lateralizing value in epilepsy surgery candidates without evidence of a lesion on MRI and nonlocalizing scalp EEG.