The overall complication rate of 6% is comparable with other IHE series (1). There was, however, a slight difference regarding complication type. There was no infection in our series or the series by Abuelem et al. (1), while Bekelis et al. (5) described a 4% infection rate in their study. On the other hand, we had to interrupt the MG-262 procedure in 2 patients because of bleeding difficulties, and in 2 additional cases contusions were noted on the postoperative computed tomography scan. Thus the overall hemorrhage complication rate was 4% (n = 4). Such complications were
not reported in the other 2 IHE studies 1 and 5. A slightly lower bleeding complication rate was reported after implantation of depth electrodes (2.5%) (13). However, the 2 hemorrhages in the current analysis occurred intraoperatively, were superficial in nature, and could be managed without long-term sequelae, whereas bleeding complications after depth electrodes are mostly intraparenchymal hemorrhages with a reported rate of permanent deficits of 1% to 2.6% 8 and 13.The rate of transient deficits was 2%, which was the same as the one reported by Bekelis et al. (5) and lower than the 4% and 11.2% transient deficits described by Arya et al. (3) and Hamer et al. (15), respectively. Behrens et al. (4) described a transient deficit rate of 0.7%. However, in their study there was also a 0.7% permanent neurologic deficit rate. This was not seen in this cohort.
Different potential risk factors have been reported to increase the complication rate of subdural electrode insertion, including number of electrodes 3, 15, 16 and 36, number of burr holes and contacts 15 and 36, duration of invasive EEG monitoring 3 and 15, and patient age (15). Interestingly, none of these factors was significantly associated with increased complication rate in mutation patient cohort. This phenomenon could possibly be explained by a relatively low complication rate in a specific subgroup of presurgical invasive diagnostics as presented here. Nevertheless, this is an important observation because an increased number of IHEs (P = 0.005) and IHE contacts (P = 0.03) increased the rate of focus detection, while not significantly changing the complication rate (P = 0.26).
The overall postoperative seizure-free rate was 53% (n = 46). Prognostic factors associated with worse seizure outcome were negative MRI scans (P < 0.01), use of MST (P < 0.01), and nonspecific histology (P < 0.001). Similar results have previously been reported concerning the prognostic value of these features by other groups as well 6, 9 and 31.
An essential part of the results reported in this study represent specific diagnostic benefits of IHE when used in selected patients. By analyzing this information together with seizure outcome data, we were able to identify where IHEs were particularly useful as part of the invasive preoperative diagnostics and how this influenced the final seizure outcome.