Abstracts

Rationale: At the Yale Comprehensive Epilepsy Program (CEP), CIEEG has evolved over 35 years, combining neocortical surface coverage with depth electrodes. Data supports the existence of epileptogenic networks that may extend beyond the ictal onset zone. The proof for epileptogenic networks requires a broader coverage of the brain and therefore an expanded array of intracranial electrodes. With an increased number of electrodes, patient safety must be maintained. This study reviews the evolving technique for placement of subdural grids, strips and depth electrodes and addresses seizure localization, control and associated morbidities. Methods: We reviewed the records of 419 consecutive patients undergoing evaluation at the Yale CEP by two surgeons with CIEEG from 1990 to 2016, concentrating on fronto-temporal relationships. Demographic information and information about the pre-surgical work up and surgical methods for electrode placement were reviewed. The success of the intracranial EEG in the localization of the seizures, surgical resection or neuromodulation treatment, complications and seizure outcomes were noted.  Results: 419 patients underwent CIEEG with implantation of subdural grids, strips and depths.  Electrodes chosen for evaluation changed from primarily strip and depth electrodes in the early 1990’s to combined studies of grids, strips and depth electrodes with increasing contact numbers with time. Recent availability of robotic depth electrodes have provided for pure depth electrode studies in some patients in the past three years. Seizures were localized in the majority of patients (71%) who went on to have resection or neuromodulation. In addition to localizing seizure onset, the combination of surface and depth electrodes provide increasing evidence for epileptogenic networks, with this study emphasizing the fronto-temporal network. Despite the increasing numbers of electrodes placed over the years, averaging roughly 100 contacts per patient in the 1990’s to over 200 contacts per patient in the 2000’s, the morbidity from the procedure has not increased. Bone flap infection has accounted for 40% of the morbidity. In lieu of replacing the bone flap during electrode implantation, titanium mesh prevented elevated intracranial pressure pressures as well as the consequences of leaving the bone flap off completely. Also, delayed bone flap replacement for four to six weeks after electrode explantation has greatly decreased the most common morbidity.  Conclusions: CIEEG has evolved over the past 35 years at the Yale CEP. The types of electrodes and technical placement have changed over time. The number of contacts per patient has increased as data has emerged, potentially identifying epileptogenic networks and has increased our understanding of epilepsy. The data offer insight on how best to decide when surface and depth recordings must be studied simultaneously. The study also demonstrates that technical modifications have led to a decrease in the morbidity and mortality associated with intracranial studies.   Funding: None