F111. Isolating C8 nerve root technique with focal digital stimulation

Abstract

The American Clinical Neurophysiology Society guidelines recommend eliciting upper extremity somatosensory evoked potentials (SSEP) via median or ulnar nerve stimulation at the wrist. However, when monitoring cervical spine surgeries, it may be more appropriate to use ulnar nerve stimulation as the root contribution to the cortical SSEP originates at levels below C7, while median nerve SSEPs have contributions possibly as high as C5. We postulate that wrist stimulation reflects a combination of activated axons from both the median and ulnar nerves secondary to supramaximal stimulation typically used intraoperatively. It’s therefore difficult to precisely determine the level of SSEP nerve root entry. Isolated ulnar stimulation targeting only C8-T1 roots could remedy this issue. The authors tested the feasibility and reliability of pure ulnar C8-T1 activation through 5th digit stimulation. This prospective study consented and enrolled eight patients without known evidence of peripheral neuropathy or cervical radiculopathy undergoing routine intraoperative neurophysiologic monitoring (IONM) including SSEP. Standard adhesive gel electrodes were used for wrist stimulation over the median and ulnar nerves, as well as the 5th digit. Ulnar nerve stimulating electrodes were placed at the wrist and 5th digit on the contralateral side of arterial line placement. Cortical SSEPs (montage of CPc-Fz and CPc-CPi) were analyzed for amplitude, latency and morphology. The intensity, pulse width, and repetition rate of stimulation to elicit the SSEP were also analyzed. SSEPs were obtained throughout surgery to ensure reproducibility. Stimulation of the median nerve at the wrist provided the highest amplitude cortical SSEP. Signals were consistently present in the ulnar nerve at wrist and 5th digit for each patient. The mean ulnar nerve amplitudes at the wrist and 5th digit were 42% and 78% smaller compared to the median amplitude, respectively. The average percent standard deviation of amplitudes for median nerve SSEP was 13.42% compared to 17.87% and 17.35% from ulnar nerve at the wrist and 5th digit, respectively. These data confirm 5th digit stimulation may be a reproducible method of isolating C8-T1 root activation. Despite higher amplitude responses from median wrist stimulation, 5th digit ulnar stimulation remained consistent during surgical procedures and could be used to detect conduction changes. This technique avoids coactivation of the median nerve SSEP which can occur when stimulating for an ulnar nerve SSEP at the wrist, thereby preventing false negative responses. This was further confirmed during this study by the identification of a case, which was subsequently excluded from analysis, of a patient determined to have a preexisting ulnar neuropathy, diagnosed by outpatient nerve conduction studies. Intraoperative cortical SSEP was correctly absent from 5th digit stimulation and incorrectly present from wrist stimulation of the ulnar nerve in this patient.