and Mario Di Napoli1
(1)
Neurological Service, S. Camillo de’ Lellis General Hospital, Rieti, Italy
Original Settings
Sensitivity was 20 μV/division, low-frequency filter was 10 Hz, high-frequency filter was 5 kHz, sweep speed was 2 ms/division, duration of pulse was 0.1 ms (0.05 for digit III to reduce stimulus artifact), and the machine used was a Medelec Sapphire 4ME. Rate of pulse was not specified.
Position
This study was performed in the supine position, with the hands and forearms placed on an arm support with the wrist in a neutral position and the digits relaxed.
Recording
Following the orthodromic method [1], the sensory nerve action potentials (SNAPs) were recorded using recording electrodes mounted on a plastic bipolar bar, placed on the palm and wrist (Fig. 1). The interelectrode distance was 10 mm and the active electrode (A) was in the distal position; the reference (R) was placed proximally. Ground (G) electrode was attached to the distal region of the wrist (on the distal wrist crease).
Fig. 1
Orthodromic sensory nerve action potentials (SNAPs) recorded at the palm (upper trace) and at the wrist (lower trace), stimulation of digit III
Stimulation
Stimulation was applied to the digit III (S), using stimulating ring electrodes, the cathode (−) was placed over the proximal phalanx and the anode (+) over the middle phalanx of the same digit. Supramaximal stimuli of 0.1- or 0.05-ms duration were used to reduce stimulus artifact.
Measurements
Onset latency (ms) was measured from the stimulus onset to the onset of the negative peak of the SNAP. The SNAPs were averaged at least 16 times. Distances were measured in relaxed hands (test position) with a flexible tape measure. All studies based on measurement of the sensory nerve conduction velocity (SNCV) over relatively short distances (i.e., digit to palm, palm to wrist) are more subject to measurement error than those that evaluate conduction over longer tracts of the median nerve, therefore all measurements used to calculate the distoproximal ratios must be made with extreme care and under standardized conditions (i.e. hand position, electrodes position, hand temperature). All values were rounded to the nearest millimeter. Skin temperatures were maintained at > 31 °C with an infrared lamp. For the segmental study of the orthodromic SNCV in the median nerve with calculation of the distoproximal ratio, the SNCV (m/s) from the palm to wrist (P–W) segment is calculated comparing the palm–wrist distance (mm) with digit III–wrist and digit III–palm latencies (ms) as follows:
The calculation of the distoproximal ratio allows comparison of the SNCV in the distal median nerve segment (SNCV digit III–wrist) with that of the proximal segment (SNCV palm–wrist) of the same nerve. For the authors, in normal hands, the SNCV in the proximal segment (palm–wrist) was higher than the SNCV of the distal segment (digit III–palm) so that the distoproximal ratio was always < 1.0 (upper normal limit: 0.98). This ratio is reversed (>1) in a patient with CTS. There was no correlation between patient age and the distoproximal ratio, digit III–palm, or palm–wrist. The distoproximal ratio is calculated comparing digit III–palm and palm–wrist SNCV as follows:
Normal values (Table 1) were obtained from 40 control hands of 36 asymptomatic subjects (25 women and 11 men, age range 19–79 years, mean age 43.7 years), and pathological values (Table 2) were recorded in 50 symptomatic hands from 43 patients (31 women and 12 men, age range 23–80 years, mean age 45.2 years) with signs and symptoms of carpal tunnel syndrome (CTS).
Table 1
Normal values [1]
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