Sensitivity was 10 μV/division, sweep speed was 1 ms/division, duration of pulse was 100–200 μs, and the machine used was a one-channel electromyograph DISA 14A30. Low-frequency filter, high-frequency filter, and rate of pulse were not specified.

This study was performed in the supine position.

Following the orthodromic method [1], signals were recorded at two different points (Figs. 1 and 2): on the palm (R1) and at the wrist (R2). On the palm (R1), the subdermal electrodes (both active and reference electrodes) were inserted well outside the zone of compression in the carpal tunnel, 1.5–2 cm distally to the distal edge of the flexor retinaculum. To record from the palmar nerve of digit I (thumb), the electrode was placed 6 cm from the stimulating cathode (−) at the medial border of the thenar eminence. To record from digit III (middle finger), the electrode was placed 9 cm from the stimulating cathode (−) along a line pointing to the midline of digit III. The remote electrode was placed just below the skin either 2–3 cm proximally or transversely to the near-nerve electrode. At the wrist (R2), the electrode was placed near the nerve, proximally to the distal wrist crease. The remote electrode was placed at a transverse distance of 4–5 cm at the level of the near-nerve electrode. Ground (G) electrode was placed around the proximal phalanx when recording in the palm and at the wrist when recording proximal to the flexor retinaculum (the figure shows the ground electrode placed on the palm).

Stimulation was applied separately to two fingers: digit I and digit III. The stimulating electrodes (rings or uninsulated platinum needles) were placed medially and laterally at the proximal end of the distal phalanx of digit I or at the middle phalanx of digit III. The reference ring electrode was placed 2 cm distally from the active electrode. The stimulus was gradually increased until the response was a maximum using 0.1 or 0.2 ms of duration of the current pulse.

The short distances over which velocities were calculated required that they be measured as accurately as possible. Measurements of the conduction distance were all made with the hand stretched and placed in a standard position with digit I (thumb) abducted to 60°. The shortest latency was determined from the stimulus onset to the initial positive peak and the longest latency from the stimulus onset to the last separate component of the averaged sensory nerve action potential (SNAP). The sensory nerve conduction velocity (SNCV) over the digit–palm segment was calculated by the conventional method and measured in meter per second (m/s). In calculating SNCV over the palm–wrist segment, the distance was divided by the latency difference between the wrist and the palm. The amplitude of the sensory responses was measured peak to peak (peak to peak amplitude). Skin temperature was controlled (surface temperature of 37–38 °C). Normal values (Table 1) were obtained from 10 control subjects (5 women and 5 men, age range 32–57 years). The authors also studied 22 patients (Table 2) all hospitalized in neurological or neurosurgical wards (16 women and 6 men, age range 32–57 years): 13 patients had clinical signs and symptoms of carpal tunnel syndrome (CTS), 5 of which had both hands affected. In 9 patients, signs and symptoms had lasted for less than half a year, in 3 patients 6 months to 1 and a half years, and in 10 patients from 2 to 30 years.