Optimization of direct cortical stimulation using tibial versus median nerve sensory mapping during midline brain tumor resection: illustrative case

Abstract

Background: During brain tumor resection, neurophysiological mapping and monitoring help surgeons locate, characterize, and functionally assess eloquent brain areas in real time. The selection of mapping and monitoring targets has implications for successful surgery. Here, the authors compare direct cortical stimulation (DCS) as suggested by median nerve (MN) with posterior tibial nerve (PTN) cortical sensory mapping (SM) during mesial lesion resection.

Observations: Recordings from a 6-contact cortical strip served to generate an MN and a PTN sensory map, which indicated the strip was anterior to the central sulcus. Responses exhibited an amplitude gradient with no phase reversal (PR). DCS, elicited through a stimulus probe or contact(s) of the strip, yielded larger responses from the corresponding sensory mapped limb; that is, PTN SM resulted in larger lower limb muscle responses than those suggested by MN SM.

Lessons: SM of the MN and PTN is effective for localizing eloquent cortical areas wherein the PTN is favored in surgery for mesial cortical tumors. The recorded amplitude of the cortical somatosensory evoked potential is a valuable criterion for defining the optimal location for DCS, despite an absent PR. The pathway at risk dictates the specifics of SM, which subsequently defines the optimal location for DCS.

Keywords: case report; direct cortical stimulation; maximum amplitude criteria; midline tumor; motor mapping; phase reversal; sensory mapping.

FIG. 1

MRI showing abnormality located in the right mesial superior frontal convolution: axial T2-weighted (A), axial FLAIR (B), coronal FLAIR (C), and sagittal FLAIR (D) sequences.

FIG. 2

Sensory mapping of the cortical representation of the left MN and PTN. Cortical potentials after left MN stimulation obtained from a strip electrode placed directly on the surface of the brain (A). Although no PR was observed, the response amplitude varied along the strip, increasing from contact A1 to A2 and decreasing systematically from A2 to A6, suggesting contact 2 as optimal for DCS. Cortical potentials after left PTN stimulation obtained from the strip at the same location as in panel A (B). Once again, no PR was observed. The response amplitude increases from contact A1 to A3, decreasing from A3 to A6, suggesting contact 3 as optimal for DCS. In both panels, traces A1 through A6–FPz are those obtained from the strip, and the two upper traces are scalp recordings that served as reference. The red trace is the baseline. Scale is 10 msec/dv and 8 μV.

FIG. 3

Motor responses elicited by DCS through the strip at 11.4 mA. MEPs obtained from stimulation through contact 2, resulting in robust responses from upper extremity muscles (left [L] adductor policis brevis and brachioradialis muscles, [A]), and from stimulation through contact 3, resulting in robust responses from lower extremity muscles (L gastrocnemius, tibialis anterior, and adductor hallucis muscles, [B]). The pattern of maximally activated muscles is consistent with the optimal location for DCS suggested by the sensory map for MN and PTN, respectively. Scale is 10 msec/dv and 200 μV. Stimulation was monopolar with the current return placed in the scalp at FPz.