Poster 38: A novel recording technique of the serratus anterior compound muscle action potential and establishment of normal values

Abstract

Objective: To establish an optimal technique for electrodiagnostic recording of the serratus anterior compound motor action potential (CMAP) while stimulating the long thoracic nerve in the axilla. Design: Observational study. Setting: Academic electromyography laboratory. Participants: 15 healthy volunteers (mean age, 28.3y). Interventions: The investigators stimulated the long thoracic nerve in the axilla just anterior to the mid axillary line. The serratus anterior CMAP was recorded using a self-adhesive, 8×0.5cm, “ring” electrode (Oxford) as E1, which was positioned with the proximal end at the nipple level (5cm distal to the bipolar stimulator) in line with the bipolar stimulator and the anterior superior iliac spine. A self-adhesive, motor (1cm) electrode was used as E2 and placed over the seventh rib distal to the pectoralis muscle. Main Outcome Measures: The onset latencies and amplitudes were recorded for both sides. Results: For the right long thoracic nerve distal motor latency, the mean ± SD was 2.3±0.5ms (range, 1.7–3.5ms). The left long thoracic nerve distal motor latency was 2.3±0.4ms (range, 1.5–3.2ms). The side-to-side mean latency difference was 0.3±0.2ms. The right serratus anterior CMAP amplitude mean ± SD was 3.8±1.9mV (range, 1.6–9mV). The left serratus anterior CMAP amplitude mean was 3.9±1.9mV (range, 1.2–3.6mV). The side-to-side mean amplitude difference was 0.6±0.6mV. Conclusions: The latencies, side-to-side comparisons, and amplitude data are consistent with other motor nerve conduction findings in the literature. Stimulating the long thoracic nerve with a standard bipolar stimulator in the axilla will provide a reliable technique by which to measure the long thoracic distal motor latency and serratus anterior CMAP amplitude.