The median palmar cutaneous nerve in normal subjects and CTS

Objective: The neurophysiological confirmation of carpal tunnel syndrome (CTS) relies on detecting abnormal median nerve transcarpal conduction in the presence of unaffected comparator nerves. We compare the palmar cutaneous median branch (PCBm) with the ulnar sensory nerve conduction to digit 5 (US5) as comparator nerves for diagnosing CTS. Methods: In a prospective case control study of patients with clinically defined carpal tunnel syndrome and normal subjects, we determined and compared the PCBm and US5 conduction velocity. Results: We examined 57 hands with clinically defined CTS and 59 control hands. Comparison showed highly significantly slowed PCBm conduction (p< 0.0001) but not for US5 conduction (p = 0.488). Using a 3 percentile cut-off for abnormality derived from controls, PCBm conduction velocity was abnormal in 46% of CTS hands. Conclusions: The high frequency of PCBm nerve conduction abnormality in CTS suggests that this nerve should not be used as a comparator nerve for the neurophysiological diagnosis of CTS. This finding may help explain some of the extension of sensory symptoms outside the median nerve distribution in CTS. Significance: In CTS frequent abnormality of PCBm conduction makes this a poor comparator nerve and may explain extension of sensory symptoms beyond the median nerve.