Relation between distal motor latency delay and sensory action potential absence of the median nerve in carpal tunnel syndrome

• DML of APB correlated SCV in the index finger in the patients with CTS.
• SNAPs were absent when DML was longer than 7.7 ms in the most of patients.
• Knowing the cutoff value can inform clinician about other disorders.

ObjectiveTo elucidate the cutoff value for distal motor latency (DML) at which sensory nerve action potentials (SNAPs) are absent in carpal tunnel syndrome (CTS) patients.MethodWe examined 157 hands in 129 patients with CTS retrospectively. We classified the patients according to whether SNAPs were successfully obtained. Group A consisted of hands with SNAPs, while Group B consisted of hands without SNAPs. The cutoff value for DML was determined by receiver-operating characteristic curve analysis. We enrolled 130 hands with CTS for the analysis, because measurements were successful in 130 hands for compound muscle action potentials and in 82 hands for SNAPs from a total of 157 hands investigated.ResultsA significant correlation was observed between DML and SCV (P<0.0001, R*2 = 0.40). The most discriminative cutoff value for DML was 7.7 ms, resulting in a sensitivity of 79.6% and specificity of 79.3%.ConclusionCases in which SNAPs are evoked despite a DML longer than 7.7 ms should raise suspicion.