Axillary motor nerve conduction study: Description of technique and provision of normative data

Abstract

Background Axillary nerve lesions can commonly occur secondary to trauma or brachial plexopathy. Our aim was to describe our technique of axillary nerve motor conduction studies and provide the respective normal values. Methods Active electrode was positioned over the most prominent portion of the middle deltoid, approximately 5–7 cm distal to the acromion. Reference electrode was positioned over the acromion. Ground electrode was placed between the active and the reference electrodes. Supramaximal stimulation was at the Erb’s point. Results A total of 154 participants (61% male, age range 18–84) were included. There was a significant positive correlation between the subjects’ age and the onset latency (Spearman’s rho 0.312, p < 0.001) and a significant negative correlation between the participants’ age and the CMAP (Spearman’s rho −0.481, p < 0.001). For the total male population the lower normal value for the CMAP was 7.6 mV and the higher normal value for the onset latency was 5.0 ms. For the total female population the respective normal values were 6.5 mV and 3.5 ms. In order to detect an axillary nerve lesion, asymmetry of >40% in the CMAPS between the symptomatic and the asymptomatic side show a sensitivity of 95.2% and a specificity of 96.6%. Conclusion We described our technique of axillary nerve motor conduction studies and provided the respective normal values stratified for age and gender. When suspecting an axillary nerve lesion it is always worth performing axillary motor NCS bilaterally and compare the CMAPs.