Subdural to Epidural Direct Wave Monitoring Provides Insight into Limitations of Epidural Monitoring in Two Patients Undergoing Spine Tumor Surgery

Introduction: Subdural spinal electrode placement for D-wave recordings surpasses epidural placement in impedance, waveform clarity, and acquisition rates. We hypothesize that instances of successful subdural but not epidural D-wave recordings may be due to desynchronized neuronal firing, spinal cord edema, or distorted corticospinal tracts, compounded by technical limitations inherent to epidural electrode recordings. We share our experience using subdural D-wave recordings during lesionectomy, complimented by epidural recordings, to demonstrate CST continuity post-dural closure and to show post-resection attainability when initial epidural baselines were unavailable.

Methods: For intradural lesions, a spinal electrode is placed epidurally, distal to the lesion after laminectomy for baseline establishment. After durotomy, the same electrode is repositioned subdurally. The electrode remains subdural throughout lesionectomy, removed just before dural closure, and then reinstated epidurally to evaluate CST integrity during closure or to attempt epidural recordings if baselines were unestablished.

Results: We present two-cases as proof of concept. Case 1, a 46-year-old female with a ventral C2 meningioma, showed stronger baseline subdural D-wave recordings compared to epidural. The patient experienced limb MEP loss during resection with a 35% attenuation of the subdural D-wave. At dural closure, epidural placement showed D-wave recordings within established baseline epidural/subdural limits. Transient post-operative weakness resolved by day 2. Case 2, an 80-year-old female with a C5-C6 intramedullary cavernous malformation, saw robust subdural, but no baseline epidural D-wave recordings. The surgery was uneventful with no neurological complications. Epidural placement during closure successfully acquired a D-wave recording.

Conclusion : Subdural D-wave recordings exhibit superior reliability to epidural counterparts. Placing the electrode epidurally during closure provides insights into CST continuity post-lesionectomy and highlights the limitations to acquiring epidural baselines versus subdural electrode placement, with epidural recordings becoming feasible after the spinal cord is decompressed. Larger cohort studies are needed for robust validation.