Comparison of median and ulnar sensory nerve action potentials in the diagnosis of the carpal tunnel syndrome.

Recording of median and ulnar digital sensory nerve action potentials in normal subjects showed that the ratio of the median (index finger) to ulnar (little finger) potential amplitude was consistently greater than one. In 15 patients with the carpal tunnel syndrome (seven bilateral) this ratio was found to be less than one for all but two of the 22 clinically affected hands, including three of the four hands with a normal motor latency to threshold stimulation and four of the five hands with a normal sensory conduction. It is concluded that the estimation of the ratio of the median to ulnar sensory potential amplitude is a sensitive test in the diagnosis of the carpal tunnel syndrome and is particularly useful in those patients who show a normal motor latency and sensory conduction.