and Mario Di Napoli1
(1)
Neurological Service, S. Camillo de’ Lellis General Hospital, Rieti, Italy
Original Settings
Sensitivity was 50–100 μV/division, sweep speed was 1 ms/division, duration of pulse was 0.1 ms, rate of pulse was 2–5/s, and the machine used was a Medelec MS 6. Low-frequency filter and high-frequency filter were not specified.
Position
This study was performed in the supine position.
Recording
Following the orthodromic method [1], signals were recorded using surface electrodes at the wrist, proximally to the distal wrist crease (Fig. 1). The active electrode (A) was placed proximal to the crease; the reference (R) was placed proximally. The author used conventional surface electrodes of felt (8-mm “cushions” 25 mm apart) soaked in 0.9 % saline secured by band; using bar electrodes, the distance between active and reference electrodes was fixed. Ground (G) was placed on the palm of the hand, proximal to the recording electrodes at the distal wrist crease. Each recording consisted of 5–8 superimposed traces.
Fig. 1
Orthodromic sensory nerve action potential (SNAP) recorded at the wrist, stimulation on the palm
Stimulation
The author used conventional surface electrodes of felt (8-mm “cushions” 25 mm apart) soaked in 0.9 % saline to stimulate the median nerve on the palm (S). The stimulating electrode was handheld. Conventional stimulators can be used, careful checking for polarity of the stimulus to avoid an additional delay (0.35 ms estimated by the author). If the stimulating electrode is held too close to the thenar eminence, a motor response appeared starting after the sensory wave or during its falling phase. Better results can be obtained using duration of 0.1 ms and low intensity of stimulation (10–20 mA).
Measurements
Latency (ms) was measured from the stimulus onset to the onset of the negative peak of the response (onset latency) and from the stimulus onset to the negative peak of the response (peak latency). Amplitudes of sensory responses and distance were not measured; skin temperature was not controlled. Eklund studied 17 hands (Table 1) from 11 healthy subjects (age range 25–43 years).
Normal values | Mean ± SD | Range | Limit of normal |
|---|---|---|---|
Palm–wrist, onset latency (ms) | 1.20 ± 0.14 | 0.95–1.55 | |
Palm–wrist, peak latency (ms) | 1.61 ± 0.14 | 1.45–1.85 | 2.0 |
Comment
Eklund [1] performed the same procedure in a small number of patients with signs of carpal tunnel syndrome (CTS). In two of three hands with pronounced CTS (with a distal motor conduction of about 7 ms), it was possible to record a 10–15-μV sensory wave with the peak at 4.0–4.5 ms. In cases of slight or moderate symptoms (eight hands), the amplitude was reduced and the peak latency increased. Thus, it ranged from 2.0 to 3.3 ms with an average of 2.5 ± 0.46. The author found abnormal findings not only in CTS but also in polyneuropathy.
Daube [2] used this technique in 20 normal subjects (Table 2); the findings were compared with two groups of patients (Table 3): Group 1, 21 patients with clinical findings of CTS, and Group 2, 29 patients with a variety of clinical diagnoses. 7 of 25 (28 %) extremities with CTS had standard median distal sensory latencies of less than 3.4 ms and normal motor latencies. Only 2 of 25 extremities (8 %) had palmar latencies of less than 2.0 ms. All extremities with median latencies over 3.4 ms had palmar latencies of less than 2.0 ms. The author suggested the use of palmar latencies to improve the diagnostic accuracy of nerve conduction studies in CTS.
Table 2
Reference values [2]
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