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Abstract
Background and aims The aim of this study was to ascertain the relationship between median nerve cross-sectional area (CSA) and depth from skin with height, weight, BMI and mid-arm circumference in healthy volunteers.
Methods After formal assessment from the Health Research Authority, UK, data was collected from 21 healthy volunteers (7 males, 14 females), median age 39 years (range 22–63). A SonoSite S-Nerve™ ultrasound machine was used to take an image of the median nerve at the mid-forearm using a 5–13MHz linear array probe. The inbuilt callipers were used to measure the nerve’s CSA and depth. Height, weight, and mid-arm circumference was also recorded. The relationship between variables was evaluated using Spearman’s rank correlation coefficient with STATA™ software.
b) Annotated image: MN=median nerve, FDS=flexor digitorum superficialis, FCR=flexor carpi radialis, FDP= flexor digitorum profundus, FPL=flexor pollicis longus.
Results The mean median nerve CSA was 0.09 cm² (range 0.06–0.17). The correlation between nerve CSA and weight was positive and highly significant (rho=0.4132, p=0.0065). There was no significant correlation between CSA and BMI (rho=01524, p=0.3352) or mid-arm circumference (rho=0.232, p=0.1391). Median nerve depth showed statistically significant positive correlations with weight (rho=0.5814, p=0.0001), BMI (rho=0.7343, p=0.0000), and mid-arm circumference (rho=0.6855, p=0.0000). The correlation with height was not significant (rho=-0.0536, p=0.7362).
Conclusions This study suggests that median nerve CSA, which determines the dose of local anaesthetics [1], correlates strongly with weight. The depth of the nerve from the skin correlated most directly to the subject’s BMI and mid-arm circumference.
a) Ultrasound image of the median nerve with surrounding structures at the mid-forearm.