Determination of the median nerve residual latency values in the diagnosis of carpal tunnel syndrome in comparison with other electrodiagnostic parameters

BACKGROUND: Along with conventional electrodiagnostic studies, several other indexes including residual latency (RL) were introduced in patients with different types of peripheral neuropathies. RL is the time difference between measured and predicted distal conduction times. This study was performed to determine the values of the median nerve RL and to investigate its sensitivity and specificity in the diagnosis of carpal tunnel syndrome (CTS). MATERIALS AND METHODS: The study was carried out among 100 hands of 75 healthy volunteers and 64 patients who had a positive history of pain or paresthesia in upper extremities and 2 of 3 signs suggesting CTS. Information including age, gender and results of sensory and motor nerve conduction velocity, compound motor action potential of proximal and distal stimulation and RL were recorded for analysis. RESULTS: Normal range of the median nerve RL was found to be 1.03-2.65 (mean = 1.84 ± 0.41). The cut-off point of median RL was 2.37 for CTS diagnosis with sensitivity of 85.9% (95% of confidence interval [CI]: 84.4-87.5%) and specificity of 91.1% (95% CI: 87.8-92.2%). CONCLUSION: In mild cases of CTS, which conventional nerve conduction studies (NCSs) shows abnormalities only in sensory studies, RL may better demonstrate the effect on the median nerve motor fibers. We conclude that RL measurement of the median nerve may raise the sensitivity of NCSs for the diagnosis of CTS.