XCISION of the epileptogenic zone, that is, the site of seizure onset and initial seizure propagation, has been shown to be an effective alternative treatment in persons with intractable epilepsy. 19 The most common surgical strategy for partial or localization-related epilepsy is focal resection of the anterior temporal lobe and mesial temporal structures. In a previous evidence-based study, au- thors revealed that approximately 70% of patients undergo- ing epilepsy surgery become seizure free after temporal lo- bectomy. Patients with extratemporal seizures may be less favorable operative candidates. 8 Factors predictive of a less satisfactory surgical outcome include a nondiagnostic pre- operative MR imaging study and the absence of a specific histopathological entity in the excised cortex. 8,13 Approximately 20% of persons who undergo surgical treatment for a partial seizure disorder do not experience a significant reduction in seizure tendency, and thus the sur- gery is considered a failure. 13 Potential reasons for an unfa- vorable surgical outcome include subtotal resection of the epileptogenic zone or incomplete excision of the patholog- ical lesion. 8,13,19 Treatment options in these patients include AED therapy, vagus nerve stimulation, and enrollment in an investigational study for epilepsy. 3 Less than 10% of patients with intractable epilepsy will be seizure free with AED therapy or vagus nerve stimulation. 3,7 Persons with J. Neurosurg. / Volume 105 / July, 2006 J Neurosurg 105:71–76, 2006 Subtraction ictal single-photon emission computed tomography coregistered to magnetic resonance imaging in evaluating the need for repeated epilepsy surgery NICHOLAS M. WETJEN, M.D., GREGORY D. CASCINO, M.D., A. JAMES FESSLER, M.D., ELSON L. SO, M.D., JEFFREY R. BUCHHALTER, M.D., PH.D., BRIAN P. MULLAN, M.D., TERENCE J. O’BRIEN, M.D., FREDRIC B. MEYER, M.D., AND W. RICHARD MARSH, M.D. Departments of Neurosurgery and Neurology; and Division of Nuclear Medicine, Department of Diagnostic Radiology, Mayo Clinic, Rochester, Minnesota Object. The aim of this study was to determine whether ictal single-photon emission computed tomography (SPECT) is useful in localizing the site of seizure onset in patients in whom surgery for intractable epilepsy failed and who are being considered for repeated surgery. Methods. Subtraction ictal SPECT coregistered to magnetic resonance imaging (SISCOM) studies were retrospec- tively analyzed in 58 patients who were being evaluated for possible repeated resection for intractable partial epilepsy between January 1, 1996, and October 31, 1999. All patients had persistent seizures subsequent to an initial resection and underwent another excision. The SISCOM-demonstrated abnormalities were classified as concordant, discordant, or indeterminate, compared with the localization of the epileptogenic zone revealed on video electroencephalography monitoring. The ability of SISCOM to predict operative outcome was also determined in patients who had undergone repeated surgical procedures. The SISCOM studies revealed a localized hyperperfused alteration in 46 (79%) of 58 patients. Forty-one (89%) of these 46 patients had a SISCOM-demonstrated alteration in the hemisphere of the previous epilepsy surgery. Imaging changes in 33 (72%) of the 46 patients were at the site of the previous focal cortical resection. Eight (17%) of the 46 had SISCOM-demonstrated abnormalities remote from the lobe in which surgery had been performed but in the ipsilat- eral hemisphere. The hyperperfusion focus was in the contralateral hemisphere in the remaining five patients (11%). The site of the epileptogenic zone was concordant with the SISCOM focus in 32 (70%) of 46 patients. Twenty-six pa- tients underwent repeated resection and were followed up for a mean of 44 months thereafter; 11 of these patients (42%) had a significant reduction in seizure tendency. Only five patients (19%) were seizure free. Ten (50%) of 20 pa- tients with a concordant SISCOM focus compared with none (0%) of three patients with a discordant focus had a favor- able surgical outcome (p = 0.23). Conclusions. The SISCOM method might be useful in the evaluation of, and the surgical planning for, patients with intractable partial epilepsy in whom previous resective treatment has failed and who are being considered for reoper- ation. KEY WORDS • partial epilepsy • surgical treatment • repeated surgery E 71 Abbreviations used in this paper: AED = antiepilepsy drug; ATL = anterior temporal lobectomy; EEG = electroencephalogra- phy; MR = magnetic resonance; SISCOM = subtraction ictal single- photon emission computed tomography coregistered to MR imaging; SPECT = single-photon emission computed tomography.