Median and Radial Sensory Latencies to Digit I as Compared with Other Screening Tests in Carpal Tunnel Syndrome

Fifty two patients with suspected carpal tunnel syndrome were evaluated electrodiagnostically by using a variety of screening tests. The criteria of normalcy were as follows: distal median motor latency lower than 4.2 ms, distal median sensory latency wrist-to-third finger at 14 cm higher than 3.2 ms for the onset and than 3.8 ms to the peak, median sensory conduction velocity wrist-to-palm at 7 cm higher than 40 m/s peak-to-peak and difference between median and radial sensory latencies wrist-to-first digit at 11 cm using an intermediate position as site of stimulation and measured peak-to-peak ("bactrian sign") higher than 0.4 ms. Other tests such as median-ulnar comparison using finger 4 were considered less reliable based primarily on the frequent association of carpal tunnel syndrome and Guyon canal entrapment. Carpal tunnel syndrome was confirmed in 49 of 52 suspected cases (94.2%) by at least one abnormal screening test. Bilateral involvement was noted in 65.3% of the cases (32 patients). The "bactrian" (two-hump camels) sign was the single most sensitive test used, being positive in 83.7% of cases. The second most sensitive test was median SNAP (wrist-to-finger) latency and the third, sensory conduction velocity across the carpal tunnel.