Reference values for nerve function assessments among a study population in northern India - III: Sensory and motor nerve conduction.

Objective: To identify reference values for normal sensory and motor nerve conduction in upper and lower limb peripheral nerves in a study population in India. The work was carried out in advance of the INFIR Cohort Study, a prospective study of individuals with newly diagnosed multibacillary MB leprosy, the objective being to identify early changes in nerve function predictive of new onset impairment and reactions. Methods: We assessed sensory nerve conduction in bilateral ulnar, median, radial cutaneous and sural nerves and motor nerve conduction in distal and proximal sites in bilateral ulnar, median and peroneal nerves among 315 healthy subjects. After adjustment for skin temperature and removal of outliers reference values were computed using regression analysis of log-transformed data. The analysis and resulting reference values were stratified by age and sex and based on the appropriate 5th or 95th percentiles. Results: Presented here are reference values for sensory nerve conduction velocity (SNCV), sensory nerve action potential (SNAP) amplitude and latency. Also for motor nerve conduction velocity (MNCV) and compound motor action potential (CMAP) amplitude at proximal sites and for amplitude and latency at distal sites. In each case percentiles are given by sex within four 10 year age bands. For males aged 55 years old, the reference value for ulnar SNCV was 43.6 m/sec and SNAP amplitude was 7.43 μV. Ulnar MNCV at the proximal site in the elbow was 50.8 m/sec and CMAP amplitude 7.25 mV and at distal sites in the wrist the amplitude was 7.14 mV and latency 3.1 msec. In the leprosy-affected cohort, the most common and therefore potentially the earliest impairment, is found in sensory nerve conduction amplitude of the sural nerve.