Duration of pulse was 0.3 ms and the machine used was a Viking Myograph type IV (Nicolet Biomedical) electromyograph. Sensitivity, low-frequency filter, high-frequency filter, sweep speed, and rate of pulse were not specified.

This study was performed in the supine position, with the hand manually fixed by the examiner to reduce movements and artifacts.

Following the antidromic method [1], the sensory nerve action potentials (SNAPs) were recorded using ring electrodes placed distally to digit II (index finger) and to digit III (middle finger). The active recording electrode was placed around the first interphalangeal joint for both the digit II and digit III recordings. The reference (R) electrode was placed distally at least 3 cm from the active recording electrode. Digit II and digit III recordings were performed separately (Figs. 1 and 2). The ground (G) electrode was placed between the stimulation and recording sites (the figure shows ground electrode placed on the palm, at the base of digit II and digit III).

Following the antidromic method, stimulus was supramaximal on the palm (S1), at the wrist (S2), and at the elbow (S3). The stimulation on the palm (S1) was performed halfway the distance from the interphalangeal joint to the wrist, for both digit II and digit III. The stimulation at the wrist (S2) was delivered just medial to the flexor carpi radialis (FCR) tendon, and the stimulation at the elbow (S3) was performed at the cubital fossa. For all stimulations, the authors used duration of 0.3 ms, and they carefully determined the optimal stimulation site. In case of disturbing stimulus artifacts, the authors turned the stimulator (anodal rotation) or adjusted the stimulus duration of current to reduce the artifacts.

Onset latency (ms) was measured from the stimulus onset to the onset of the negative peak of the sensory nerve action potentials (SNAPs). Sensory nerve conduction velocity (SNCV) was calculated following conventional method: from the elbow to the wrist (forearm segment), from the wrist to the palm (wrist segment), from the wrist to the base of digit II and digit III (palm segment), and from the wrist to the base of digit II and digit III (hand segment). It was measured conventionally in meter per second (m/s). The authors calculated the differences between the SNCV among the palm–wrist, the forearm–wrist, and the forearm–hand segments; according to Padua et al. [2] the distoproximal ratio (wrist–palm segment) was calculated comparing antidromic palm–digit III (Palm) and wrist–palm (Wrist) SNCV, as follows:

The authors used Padua’s formula, but they preferred the sensory antidromic method, and they also extended the nerve conduction study to digit II (Padua et al. in 1996 performed sensory orthodromic method stimulating distally digit III and recording signals proximally on the palm and at the wrist). Kasius et al. [1] calculated the ratio between NCVs among all the tested segments as follows:

For the digit II recordings, the lower limits of normal for NCVs were ≥45.3 m/s for the wrist segment and for the palm segment, ≥47.8 m/s for the hand segment, and ≥51.2 m/s for the forearm segment. For the digit III recordings, the lower limits of normal for NCVs were ≥46.4 m/s for the palm segment, ≥44.8 m/s for the wrist segment, ≥48.2 m/s for the hand segment, and ≥51.2 m/s for the forearm segment. Distances between the recording electrode (placed around the interphalangeal joint) and the stimulation site (on the palm, at the wrist, and at the elbow) were measured in a straight line using a measuring tape. Surface temperature of the hand, recorded at the site of the recording electrode, was measured and maintained at or above 31 °C. Normal values (Table 1) were determined in 47 control subjects (30 women, 63.8 %; 24 left hands, 51.1 %; and 23 right hands, 48.9 %; mean age 41.04 ± 12.2 years). Pathological values (Table 2) were obtained from 157 consecutive patients (122 women, 77.7 %; 71 left hands, 45.2 %; and 86 right hands, 54.8 %; mean age 48.87 ± 13.7 years) with clinically defined carpal tunnel syndrome (CTS).