Defining the epileptogenic zone using subdural or depth electrode recordings in a series of 443 consecutive patients undergoing intracranial presurgical evaluation

Intruduction & Objectives

Intracranial recordings are used to define the epileptogenic region in complex cases with insufficient or inconsistent localizing information from non-invasive studies, and particularly in non-lesional cases to prove uniregional seizure origin and to plan surgical interventions. The type of intracranial investigation differs between centers without universal agreement as to the optimal diagnostic procedure. We here retrospectively analyzed outcomes of subdural and depth electrode recordings at the Freiburg epilepsy center to compare their diagnostic yield.

Methods

443 consecutive patients (225 female; 227 male; mean age at implantation: 28.6 y (range2-67 y)) undergoing intracranial presurgical evaluation were analyzed using electronic charts and information from MR imaging and intracranial electrophysiology.

347 patients were lesional on MRI, 96 non-lesional. 147 patients underwent purely subdural evaluation, 160 patients depth electrode recordings, and 136 a combined approach. Patients were analyzed as to whether (1) the seizure onset region could be identified with certainty or approximately, (2) surgery was recommended, and (3) how the brain region of implantation influenced these results.

Results

(1)In lesional epilepsy, 63% of patients undergoing subdural recordings, 69% of patients with depth recordings, and 61% ofpatients with combined recordings were considered to have a clear definition of the seizure onset zone. Rates were lower in non lesional epilepsy, with a clear seizure onset zone in 54% of patients undergoing subdural recordings, 55% of patients with depth recordings, and 55% of patients with combined recordings were considered to have a clear definition of the seizure onset zone.

(2) In lesional epilepsy, 86% of patients undergoing subdural recordings, 64% of patients with depth recordings, and 83% of patients with combined recordings proceeded to surgery with a mean Engel I outcome of 60.8%. In non-lesional epilepsy, 63% of patients undergoing subdural recordings, 57% of patients with depth recordings, and 55% of patients with combined recordings proceeded to surgery with a mean Engel I outcome of 54.2%.

(3)In both temporal and frontal implantations ofl esional patients, the rate of patients proceeding to surgery was higher following subdural recordings than in those undergoing depth recordings only, although the proportion of patients with clearly identified channels with seizure onset was at least as high with depth electrode recordings.

Conclusions

In both, non-lesional and lesional patients and in different rain regions, subdural and depth electrode recordings showed high rates of successful identification of the seizure onset zone.

In subdural recordings, patients in whom seizure onset was defined only approximately proceeded to surgery more frequently; resulting proportions of operated patients obtaining complete seizure control were similar with either diagnostic approach and almost as high in non-lesional cases as in patients with imaging-defined lesions.

Either implantation strategy thus proved valid and rewarding in properly selected patients, with significantly higher surgical rates when subdural recordings were used.