Effect of Tourniquet-Related Nerve Ischemia on Response to Handheld Nerve Stimulation in Ulnar Nerve Transposition.

Abstract

Tourniquet-related nerve ischemia has been well studied in several reconstructive procedures, but the time-course of impaired response to intraoperative stimulation is unclear. The present study evaluated ischemic effects on conduction during ulnar nerve transposition and examined the relationship between intra- and pre- operative diagnostics. We hypothesized that intraoperative ischemia would have minimal impact on conduction. Thirty patients scheduled for anterior transposition were enrolled after preoperative examination, electrodiagnostic testing, and ultrasound. Demographic and symptom severity data were recorded. A handheld biphasic nerve stimulator was used intraoperatively to assess minimum amplitude and pulse duration needed for muscle response. Measurements were taken at 15-minute intervals after placement. Changes in threshold amplitude and pulse duration were calculated between each 15-minute interval; no significant difference was found in the change of either value (p = 0.70 and 0.178). A weak negative correlation existed between preoperative CMAP amplitudes and average intraoperative pulse duration, which increased to a moderate correlation when compared to 45-minute pulse duration (r=-0.62, p<0.01). Preoperative ulnar nerve cross-sectional area (CSA) demonstrated no significant correlation with average pulse duration but a moderate correlation with pulse duration at 45 minutes (r=0.63, p=0.01). Tourniquet use did not prevent effective intraoperative stimulation of the ulnar nerve for at least 45 minutes. The window for meaningful stimulation with tourniquet usage appears to be greater than previously thought. Preoperative nerve CMAP amplitude and CSA does appear to influence pulse duration required after 45 minutes of ischemia, suggesting that injured nerves are more susceptible to ischemia. 1.