Sensitivity was 50 μV/division, low-frequency filter was 20 Hz, high-frequency filter was 2 kHz, sweep speed was 1 ms/division, and the machine used was a specially designed amplifier with a short blocking time (1 ms) and low noise (0.5-μV RMS at bandwidth of 2 kHz). Duration of pulse was not specified.

This study was performed in the supine position.

Following the antidromic method [1], signals were recorded from digit II (index finger) using ring electrodes (Fig. 1). The active electrode (A) was placed around the proximal interphalangeal joint of digit II; the reference (R) was placed on the distal interphalangeal joint of the same finger. Ground (G) electrode was placed around the forearm (author used a Velcro strap).

The median nerve was stimulated percutaneously at twelve (12) points between the midpalm and the distal forearm in 1 cm increments. The zero level represented the proximal edge of the transverse carpal ligament and was at the distal crease of the wrist. Each stimulus point was indicated by the distance in centimeters from the zero level assigning a “−”sign distally. The distal edge of the transverse carpal ligament then laid at or close to the “−3” level. At each stimulus site, the anode was 2 cm proximal to the cathode. In studying the mixed nerve, the antidromically recorded sensory potential may be obscured by superposition of the action potentials from distal muscles; sensory fibers were selectively activated with palmar stimulation distal to the origin of the recurrent motor fibers. Moving more proximally, possible superposition of the muscle action potential was easily recognized by a change in waveform of the elicited response. Moreover, stimulation further distally either failed to produce a twitch or caused adduction of digit I (thumb) with activation of a deep branch of the ulnar nerve.

Onset latency (ms) was measured in steps of 0.1 ms from the stimulus artifact to the onset of the initial negative phase (upward deflection) of the sensory nerve action potential (SNAP). Negative peak amplitude (μV) was determined from the baseline to the negative peak of the SNAP. All studies were done in a warm room with the temperature maintained at 26–28 °C. Skin temperature was checked over the forearm, and if necessary, the limbs were warmed with an infrared heat lamp to maintain the temperature at 34 °C or greater. Normal values were established from 122 hands of 61 patients (Table 1) with complaints unrelated to the carpal tunnel syndrome (CTS), 26 men and 35 women (age range 15–50 years, average age 43 years). Pathological values were obtained from 137 symptomatic hands of 105 patients (Table 2) with clinical signs of CTS (32 men and 73 women, age range 20–78 years, average age 48 years).