Usefulness of distal motor latency measurement after palmar stimulation in advanced carpal tunnel syndrome.

The measurement of distal motor latency (DML) after palmar stimulation distal to the carpal tunnel (Palmar-DML) was investigated before carpal tunnel release (CTR) in 48 advanced carpal tunnel syndrome (CTS) with thenar atrophy and absence of motor and sensory responses after wrist stimulation. This allowed measurement of the direct effects of stimulation, in assessing the pathologic condition of the median nerve. Palmar-DML was measurable in 47 of 48 hands preoperatively, which responded to CTR with improvement of thenar atrophy at 1 year postoperatively. Also, electrophysiologic recovery presenting reappearance of DML after wrist stimulation was shown in 43 hands of those 47 hands. Furthermore, sensory nerve conduction velocity was measurable in 24 hands. In contrast, in one hand with the absence of Palmar-DML, CTR neither relieved thenar atrophy nor induced measurable Palmar-DML. This study demonstrated that Palmar-DML was measurable in most advanced CTS, which suggested the major abnormality was focal demyelination. This was confirmed by electrophysiologic and clinical improvement obtained after CTR. Palmar-DML measurement was useful to elucidate the median nerve condition in advanced CTS.