Ultrasonographic Reference Values for Assessing the Normal Median Nerve in Adults

Several studies have evaluated the cross-sectional area of the median nerve at the wrist, but none have examined other sites along the median nerve. Nerve enlargement has been demonstrated in entrapment, hereditary and acquired neuropathies, as well as with intraneural masses, and cross-sectional area reference values at sites along the nerve will help in the evaluation of these conditions. In addition, muscle intrusion into the carpal tunnel has been implicated in carpal tunnel syndrome, but the normal amount of muscle intrusion has not been quantified. Fifty asymptomatic volunteers (100 arms) were evaluated to determine the mean cross-sectional area of the median nerve at 6 sites and the mean amount of muscle intruding into the carpal tunnel. The cross-sectional area of the nerve was consistent along its course (7.5 to 9.8 mm(2)). The amount of muscle within the carpal tunnel varied greatly, with the mean area of flexor digitorum being 15.5 mm(2) and lumbricals 13.5 mm(2). These reference values are necessary for advancing the field of neuromuscular ultrasound, because they facilitate studies of the median nerve in conditions such as entrapment, hereditary neuropathy, acquired neuropathy, and intraneural masses. J Neuroimaging 2009;19:47-51.