Purpose: Diabetic patients without symptoms of carpal tunnel syndrome (CTS) may frequently show a prolongation of their median distal latency. Previous authors have reported that diabetic polyneuropathy (DPN) and CTS were differentiated by evaluating the sensory conduction velocity (SCV) distal to the palm. This study aimed to clarify this issue.
Methods: The subjects consisted of 30 DPN patients without clinical CTS and 50 CTS patients without diabetes mellitus. An antidromic sensory conduction study stimulating the median nerve at the wrist and palm was performed. The SCV distal to the palm (SCVpf) and across the carpal tunnel (SCVwp) and the sensory nerve action potential amplitude following palmar stimulation (AMP-p) were evaluated.
Results: The SCVwp was significantly lower than the SCVpf in DPN patients, implying the frequent presence of subclinical lesion at the carpal tunnel. The SCVpf was similar for both groups. The AMP-p was disproportionately low in relation to SCVwp in DPN patients, as compared with CTS patients.
Conclusions: The polyneuropathic feature of DPN is primarily expressed as axonal loss. The diabetic axonal loss index has been introduced, which enabled to determine the co-presence, isolated presence, or absence of DPN/CTS in the electrophysiological sense.