Medically intractable, localization-related epilepsy with normal MRI: Presurgical evaluation and surgical outcome in 43 patients

Purpose: High-resolution magnetic resonance imaging (MRI) plays a crucial r ole in the presurgical evaluation of patients with medically refractory par tial epilepsy. Although MRI detects a morphologic abnormality as the cause of the epilepsy in the majority of patients, some patients have a normal MR I. This study was undertaken to explore the hypothesis that in patients wit h normal MRI, invasive monitoring can lead to localization of the seizure-o nset zone and successful epilepsy surgery. Methods: A series of 115 patients with partial epilepsy who had undergone i ntracranial electrode evaluation (subdural strip subdural grid, and/or dept h electrodes) between February 1992 and February 1999 was analyzed retrospe ctively. Of these, 43 patients (37%) had a normal MRI. Results: Invasive monitoring detected a focal seizure onset in 25 (58%) pat ients, multifocal seizure origin in 12 (28%) patients, and in six patients, no focal seizure origin was found. Of the 25 patients with a focal seizure origin, cortical resection was performed in 24, of whom 20 (83%) had a goo d surgical outcome with respect to seizure control. Six of the 12 patients with multifocal seizure origin underwent other forms of epilepsy surgery (p alliative cortical resection in two, anterior callosotomy in two, and vagal nerve stimulator placement in two). Conclusions: Successful epilepsy surgery is possible in patients with norma l MRIs, but appropriate presurgical evaluations are necessary. In patients with evidence of multifocal seizure origin during noninvasive evaluation, i nvasive monitoring should generally be avoided.