Abstracts

To identify patient categories in those not proceeding to resective surgery after subdural grid evaluation (SDE), and to correlate them with the results of noninvasive investigation. Retrospective review of medical records of all patients (n = 140, aged 15 years and over) undergoing SDE over an 8-year period at the Cleveland Clinic Foundation, when a single surgeon (WB) performed all grid placements. Patient clinical details, and results of scalp interictal and ictal EEG, cranial MRI, ictal SPECT, interictal PET, and interictal and ictal subdural grid recordings, were obtained. 25/140 patients (=17.9%) did not proceed to subsequent resective surgery; 22 were selected for further analysis. Reasons for patients not proceeding to resective surgery were (i) ictal onset confirmed over eloquent cortex (8/22 = 36.4%; Category I ), (ii) no seizures despite prolonged monitoring (2/22 = 9%; Category II), (iii) complications related to grid placement (2/22 = 9%; Category III), and (iv) nonlocalizing (diffuse or multifocal) ictal onsets (10/22 = 45.5%; Category IV).
The majority of Category IV (8/10 patients) had nonlesional MRIs. A subgroup analysis compared these patients with those nonlesional MRI patients who underwent resective surgery after SDE (22 patients). A simple scoring system to quantify the degree of concordance between the remaining noninvasive investigations (ictal EEG, interictal epileptiform EEG discharges, interictal PET and ictal SPECT) was devised. The range of scores was 0 (no concordance; generalized ictal and interictal EEG) to 3 (perfect concordance; focal ictal and interictal EEG). A statistically significant difference (p [lt] 0.001; 2-sample [italic]t[/italic] test) was found between the mean scores of the two subgroups. 55% of patients not proceeding to resection after SDE had causes that could not have reasonably been anticipated in advance (Categories I-III). Resective surgery on the remainder was precluded by diffuse, or multifocal, ictal onsets on SDE. The majority (80%) of these patients had nonlesional MRIs. A simple scoring system to quantify the localizing value and mutual concordance of four noninvasive investigational parameters (interictal and ictal EEG, PET and SPECT) delineated, on average, these patients from those nonlesional MRI patients whose SDEs subsequently allowed for surgical resection. Segregation of SDE patients into subgroups based on the noninvasive data may allow for prognostication of invasive evaluation.