Gender, BMI and side-to-side differences in spinal accessory nerve conduction from the upper and middle components of the trapezius muscle

Abstract

Introduction: There is a variety of testing methods described in the literature for the spinal accessory nerve (SAN). This study aims to evaluate side-to-side, gender, and BMI differences with surface recording from the upper and middle trapezius using a standard distance to the upper trapezius.

Methods: Subjects underwent bilateral SAN conduction testing with the active recording electrodes over the superior border of the upper trapezius, midway between the acromion and the C7 spinous process, and over the middle trapezius 3 cm medial to the vertebral border of the scapula.

Results: Mean latency values were 2.17 ± 0.22 msec and 3.14 ± 0.40 msec for the upper and middle trapezius, respectively. Mean amplitude values were 8.02 ± 2.2 mV for the upper trapezius and 3.96 ± 1.77 mV for the middle trapezius. The mean side-to-side latency difference was 7.8% for the upper and 9.5% for the middle trapezius, while the mean side-to-side amplitude difference was 18.2% for the upper and 37.6% for the middle trapezius. BMI had a significant inverse effect on upper and middle trapezius amplitudes such that both males and females with lower BMI had larger amplitudes. There was a significant gender difference for upper and middle trapezius latency with faster latency values observed in females.

Conclusions: SAN conduction with surface recording from the upper and middle trapezius is well tolerated. Side-to-side differences may be the best way to evaluate both amplitude and latency, so bilateral testing is essential in light of anatomical variation and BMI effects on amplitude.