Median to radial latency difference test in mild carpal tunnel syndrome.

Electrodiagnostic assessment of carpal tunnel syndrome (CTS) is hampered by the normal variability of conduction velocity, amplitude and duration of evoked responses, and hand temperature. Comparative testing of other nerves in the same hand may control for these variables and improve sensitivity for detection of mild focal entrapment. In consecutive cases referred for evaluation, antidromic sensory latencies were recorded from the thumb over 10 cm segments of the median and radial nerves, and the difference between these latencies was calculated. Median sensory antidromic conduction to digit III, ulnar sensory conduction, and median motor latency measurements were also made. Three hundred and thirty-three upper limbs were studied in 262 patients. All tests were normal in 73 arms. Mild conduction slowing was identified in 86 median nerves. The median-radial latency difference was increased in 87% of these mild cases. The median-ulnar latency difference was diagnostic in 88%. Either median-radial or median-ulnar latency differences were abnormal in all confirmed cases. The routine median motor and sensory distal latency tests had sensitivities of 29% and 52%, respectively. Radial responses were elicited in 99% of the hands studied. The median-radial latency difference in digit I is a sensitive indicator of mild CTS that can be measured quickly with a minimum of discomfort.