Effect of epilepsy magnetic source imaging on intracranial electrode placement

Abstract

Objective: Intracranial electroencephalography (ICEEG) with chronically implanted electrodes is a costly invasive diagnostic procedure that remains necessary for a large proportion of patients who undergo evaluation for epilepsy surgery. This study was designed to evaluate whether magnetic source imaging (MSI), a noninvasive test based on magnetoencephalography source localization, can supplement ICEEG by affecting electrode placement to improve sampling of the seizure onset zone(s).

Methods: Of 298 consecutive epilepsy surgery candidates (between 2001 and 2006), 160 patients were prospectively enrolled by insufficient localization from seizure monitoring and magnetic resonance imaging results. Before presenting MSI results, decisions were made whether to proceed with ICEEG, and if so, where to place electrodes such that the hypothetical seizure-onset zone would be sampled. MSI results were then provided with allowance of changes to the original plan.

Results: MSI indicated additional electrode coverage in 18 of 77 (23%) ICEEG cases. In 39% (95% confidence interval, 16.4-61.4), seizure-onset ICEEG patterns involved the additional electrodes indicated by MSI. Sixty-two patients underwent surgical resection based on ICEEG recording of seizures. Highly localized MSI was significantly associated with seizure-free outcome (mean, 3.4 years; minimum, >1 year) for the entire surgical population (n = 62).

Interpretation: MSI spike localization increases the chance that the seizure-onset zone is sampled when patients undergo ICEEG for presurgical epilepsy evaluations. The clinical impact of this effect, improving diagnostic yield of ICEEG, should be considered in surgery candidates who do not have satisfactory indication of epilepsy localization from seizure semiology, electroencephalogram, and magnetic resonance imaging.

Figure

Diagram for evaluation and treatment pathways of all patients identified as epilepsy surgery candidates after inpatient seizure monitoring with video and scalp electroencephalography (VEEG) and brain magnetic resonance imaging (MRI) for five years (2001-2006) at the University of Alabama at Birmingham (UAB). The cohort of cases enrolled for observational study (N=160) was considered to have localization-related epilepsy that was potentially amenable to unifocal resection. Specifically excluded were patients (N=95) with clinically defined mesial temporal lobe epilepsy (MTLE) supported by localized scalp VEEG (including supportive semiology) and concordant evidence of unilateral hippocampal sclerosis (HS) on MRI and patients considered to require simple lesionectomy–both reflecting groups of patients that are widely treated without additional localization tests including seizure recordings with chronic implanted intracranial electrodes (ICEEG). ^† Three patients did not complete the study: one patient died as a result of myocardial infarction unrelated to a seizure. Two patients on further evaluation did not meet eligibility requirements. ^‡ Three cases had multiple subpial transection procedures and one had a vagal nerve stimulator implantation without further evaluation. * Prior to presenting magnetic source imaging (MSI) results at “Conference 1”, decisions were made as to whether to proceed with ICEEG, and if so, where to place electrodes such that the hypothetical seizure onset zone would be sampled. MSI results were then provided with allowance of changes to the original plan.