Gender, side to side and BMI differences in long thoracic nerve conduction velocity: A novel technique.

Objectives: The purpose of this study was to describe a new technique to record long thoracic nerve conduction velocity through the axilla as well as to assist in establishing normative values for latency and amplitude of the long thoracic nerve and to evaluate side to side, gender and BMI differences.

Methods: 26 healthy subjects (12 males/14 females) participated in the study with data collected bilaterally resulting in 52 limbs studied. Surface recording was over the serratus anterior muscle with the recording electrode located on the rib closest to a distance within a standardized range of 22-24 cm distal to the acromion. Stimulation was delivered at the mid axillary line, then again in the supraclavicular region. A caliper was used to measure the distance between the two stimulation sites in order to calculate the nerve conduction velocity.

Results: The normal value (mean + 2 SD) for distal latency is <2.7 msec, while the normal value for velocity (mean - 2 SD) is >61.0 m/s. Absolute amplitude values were not calculated. Side to side difference normal values for distal latency, amplitude and velocity are 0.7 msec, 70.3% and 8.5% respectively. A two way analysis of variance (ANOVA) revealed a significant gender and BMI difference in both distal (0.02) and proximal amplitude (0.05) means. There was no significant interaction between gender and BMI for latency or velocity values for either stimulation site.

Conclusions: The distal latency values are not significantly different from those reported previously, however long thoracic nerve conduction velocity has not been described before and would be an appropriate way to monitor velocity through the proximal portions of the brachial plexus. Men who qualified as overweight with a BMI greater than 25 demonstrated a larger amplitude when compared with average weight men and women. In contrast women with BMI greater than 25 demonstrated a much smaller amplitude when compared with overweight men and average weight men and women.

Significance: Absolute amplitude normal values are not reported as BMI may impact the ability to record an accurate amplitude for both men and women. Long thoracic nerve conduction velocity and latency appear to be more reliable measures.