Extraoperative Subdural Grid Electrode Stimulation for Primary Motor Cortex Localization in Medically Intractable Epilepsy.

Abstract

Background and purpose: Extraoperative subdural grid electrode (SDE) implantation with cortical stimulation mapping (CSM) is a cornerstone of presurgical evaluation in medically intractable epilepsy, enabling localization of epileptogenic and eloquent cortex. Precise identification of the primary motor cortex (PMC) is essential to minimize postoperative motor deficits, particularly in pediatric patients. This study evaluated the clinical utility of extraoperative CSM for PMC localization and examined factors associated with stimulation amplitude thresholds in a large cohort.

Methods: We retrospectively reviewed 297 patients with medically refractory epilepsy who underwent SDE implantation with extraoperative CSM. Motor mapping was performed using standardized stimulation parameters. Postoperative motor outcomes were assessed and correlated with preservation of mapped PMC. Associations between stimulation amplitude thresholds and demographic, clinical, radiological, pathological, and antiseizure medication (ASM)-related factors were analyzed.

Results: The PMC was successfully identified in all patients. Although motor responses were most commonly localized to the precentral gyrus, atypical PMC localization was observed in 11.1% of patients. Preservation of mapped PMC was associated with a significantly lower rate of persistent postoperative motor deficits. Younger age demonstrated a significant inverse correlation with stimulation amplitude threshold (p=0.001), with children younger than 7 years requiring higher stimulation amplitudes. This association remained significant after controlling for ASM burden.

Conclusions: Extraoperative CSM using SDEs is a reliable and clinically impactful method for PMC localization in medically intractable epilepsy. Functional motor areas may deviate from classical anatomical landmarks, underscoring the necessity of direct functional mapping.