Digit I – Wrist | Neupsy Key

and Mario Di Napoli¹

(1)

Neurological Service, S. Camillo de’ Lellis General Hospital, Rieti, Italy

Original Settings

Sensitivity was 5 μV/division and sweep speed was 1 ms/division. Low-frequency filter, high-frequency filter, duration of pulse, rate of pulse, and the machine used were not specified.

Position

This study was performed in the supine position.

Recording

Following the orthodromic method [1], sensory nerve action potentials (SNAPs) were recorded at the wrist (Fig. 1). For the median nerve (R1), the active recording electrode was placed proximally to the distal wrist crease; for the radial nerve (R2) the active recording electrode was placed over the superficial radial nerve at the proximal border of the “anatomical snuff box.” The reference electrodes were placed proximally. The ground (G) electrode position was not specified in the report; the figure shows the ground electrode placed on the palm.

Fig. 1

Orthodromic sensory nerve action potentials (SNAPs) recorded at the wrist median nerve (upper trace) and radial nerve (lower trace), stimulation of digit I

Stimulation

Following the orthodromic method, the stimulation was applied for both the median and radial nerves to digit I. For stimulation to digit I (S), following the method by Gilliatt and Sears [2], the author used silver strips (2–4 mm wide) covered by lint moistened in saline and firmly wrapped around the finger for stimulating the sensory fibers of the median nerve. The active stimulating electrode (cathode, −) was placed near the metacarpophalangeal joint (at the base of the digit – proximal to the recording site); the anode (+) was positioned distally in the region of the terminal interphalangeal joint (distal to the recording site). Supramaximal stimulation was used. No fixed distance was used between the stimulating and recording electrodes.

Measurement

Onset latency (ms) was measured from the stimulus artifact to the onset of the negative deflection of the evoked sensory nerve action potential (SNAP). Peak to peak amplitude (μV) was measured from the negative to the positive peak of the SNAP. Sensory nerve conduction velocity (SNCV) was calculated in the conventional method. The nerve conduction studies were done to establish the diagnosis of carpal tunnel syndrome (CTS), on the basis of abnormalities in any of the following: distal motor latency (>5 ms), amplitude of the median nerve sensory action potential <8 μV, and sensory nerve conduction velocity (calculated over the distance between the stimulating and recording cathodes) ≤33 m/s. Skin and room temperatures were not given.

Smith [1] studied the pathological two-hump waveform obtained in CTS patients from the simultaneous stimulation or recording of both the median and radial nerves. He performed the orthodromic sensory conduction study of the median and radial nerves in 41 CTS patients (9 bilateral cases, 24 cases occurred in patients aged 50 years or younger, 26 cases were in patients older than 50 years, age range 24–79 years, mean age 52 years). Normal and pathological data for median and radial nerve latencies, amplitudes and conduction velocities were not reported. For the author, with the recording electrodes placed on the “anatomical snuff box,” it was impossible to record a radial SNAP following the stimulation of digit III (middle finger). In some patients, he reported that it was necessary to distinguish between a true radial SNAP and the spread of potential from the median nerve. The distinction was made by comparing potentials recorded over the two nerves with SNAPs recorded at a site midway between the nerves (Fig. 2), and in some cases further confirmation was obtained by stimulating digit III and recording over the radial and median nerves. In 5 of the 41 CTS patients, the delayed median SNAPs were recorded over the radial nerve because of the spread of the median SNAP to the recording site over the radial nerve. In 7 of the 41 CTS patients, the stimulation of the digital nerves of digit I evoked a double potential from the recording site over the median nerve at the wrist. The earlier component had the same latency as the radial nerve potential, and recordings from a site between the nerves showed that there was the spread of potential from the radial nerve. In all these patients, no median SNAPs were recorded at the wrist after stimulation to digit II and to digit III.

Fig. 2

Median and radial nerve sensory orthodromic conduction study, recording at the wrist – at a site midway between the median and radial nerves

For the author, in a patient with clinical symptoms of carpal tunnel syndrome, it is important to confirm that the sensory conduction is normal in at least one nerve other than the median, in order to exclude the presence of a peripheral neuropathy. Smith preferred to study the distal segment of the radial nerve than the ulnar nerve because of the prevalence of a subclinical ulnar neuropathy. Furthermore, the median and radial nerves may be stimulated by electrodes in a common position (around digit I) reducing the duration of the examination. In conclusion, in patients with CTS, the spread of potential from the median nerve does not contribute to the amplitude of the SNAP recorded over a normal radial nerve. However, the radial nerve potential may be recorded by electrodes over the median nerve [3]. This may give a false negative of normal conduction in the median nerve, if only digit I is stimulated. Comparison of the radial and median SNAPs in affected and unaffected limbs shows that the spread of potential is a variable phenomenon, which is prominent in some patients and not detectable in others.

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