Abstracts

Rationale: Extra-operative electrical cortical stimulation mapping (ESM) is frequently used to identify language, motor, and sensory cortex prior to surgical resection, with the goal of preserving function postoperatively. Although attempts are made to avoid stimulation-evoked afterdischarges (ADs) and seizures, the frequency and clinical impact of these events is unclear. By qualifying and quantifying the frequency of ADs and seizures associated with ESM, we sought to determine the extent to which these events 1) interfere with mapping, and 2) simulate the semiology and localization of patients' typical seizures. Methods: A retrospective review of clinical documentation was conducted for 66 patients with refractory focal epilepsy (mean age 30.6 years ± 12.1) who underwent subdural electrode implantation and ESM at Columbia University Comprehensive Epilepsy Center between 1997 and 2012. Regions implanted included temporal (60.6%), frontal (31.8%), parietal (10.6%), and occipital cortex (10.6%). Fifty implants (75.8%) involved the dominant hemisphere. For each patient, we determined 1) rate and type of stimulation-evoked epileptiform activity, 2) rate of evoked events that interfered with mapping, and 3) rate of evoked events that were similar to patients' typical seizures. Interference was defined as cessation of the mapping session. Results: Of 1713 electrode pairs stimulated, afterdischarges were provoked at 695 pairs (40.6%) in 54 patients (81.8%). The mean number of affected pairs per patient was 11.6 ± 9.4, or a mean 40.4% of pairs tested. 30.6% of ADs in 21 patients (31.8%) spread to neighboring electrodes without producing clinical seizures. These ADs interfered with ESM in only 2 patients (3%). ADs evolved into clinical seizures at 57 pairs (3.3%) in 23 patients (34.8%). Seizures were classified as simple partial in 8 patients (12.1%), complex partial in 8 patients (12.1%), secondarily generalized in 6 patients (9.1%), and were unspecified in 3 patients (4.5%). Simple partial seizures did not interfere with ESM in any patients. Complex partial seizures and unspecified seizures each interfered in 3 patients (4.5%) and generalized seizures interfered in all 6 patients (9.1%). Of the 23 patients with stimulation-evoked seizures, 15 (65%) had seizures that were clinically similar to their baseline seizures. Five patients had seizures originating only within their typical seizure onset zones, 4 patients had seizures originating only outside the seizure onset zone, and 14 patients had seizures originating both within and outside the seizure onset zone. Conclusions: Stimulation-evoked afterdischarges and partial seizures occur frequently during ESM. While they may have an impact on the quality of functional localization, ADs and partial seizures were rarely noted to disrupt the mapping procedure. Secondarily generalized seizures, however, are more disruptive. Although stimulation-evoked seizures are undesirable, they may provide valuable localizing information as they often replicate patients' habitual events.