The effect of stimulus intensity on motor latency in the carpal tunnel syndrome

Abstract

In the measurement of motor latency it is customary to stimulate the nerve trunk with a shock that is strong enough to produce a maximal response. Thus the latency, measured to the first deflection of the muscle action potential, depends on the fastest conducting fibres in the nerve trunk. In a partial nerve lesion this means that a few unaffected fibres will be sufficient to give a normal latency despite the presence of other damaged fibres with reduced conduction velocities. position changed when necessary to give the best stimulation of the motor fibres as judged by the least stimulating voltage required to produce a threshold response. The anode was an electrode of similar size placed over the course of the median nerve 5 cm. above the distal wrist crease. The conduction distance was measured on the skin from the centre of the stimulating cathode to the point of entry of the recording needle. This varied in different subjects between 5 and 6 5 cm.The stimulus was a brief condenser discharge (time constant 100 psec.); it was delivered once per second through a 1:1 isolating transformer (output impedance 1,000 ohms). The voltage was continuously variable up to 375 volts. The muscle action potentials were amplified by a R-C coupled amplifier and displayed on a double-beam cathode ray oscilloscope, the second beam of which provided a time scale with 1 and 5 msec. intervals. Muscle action potentials were always examined using high amplification to ensure that small early deflections were not overlooked. The stimulus was triggered at a fixed point on the time base of the oscilloscope and the whole display was photographed on 70 mm. photographic paper. The motor latency was then measured from the beginning of the stimulus artefact to the first deflection of the muscle action potential.In a few patients, sensory nerve conduction was also examined. The stimulus was applied to the index finger, and action potentials were recorded from the median nerve at the wrist, using a technique similar to that described by Gilliatt and Sears (1958).In order to ensure that differences in nerve threshold and motor latency between control subjects and carpal tunnel patients were not due to differences in limb temperature, skin temperature over the thenar eminence was recorded by a surface thermistor in all the control subjects and in eight of the patients. In another 13 patients muscle temperature was measured by a needle thermistor buried in the hypothenar eminence. The mean skin temperature record in the control group was 31 1°C. (range 26 7°C. to 34 6°C.) and in the patients it was 30-9°C. (range 26-4°C. to 34 6°C.). The mean muscle temperature in the patients was 32 8°C. (range 27 2'C. to 35 4°C.) which may be compared with muscle temperatures ranging from 34 0°C. to 36 5°C., recorded from the proximal third of the forearm by Henriksen (1956).

Clinical and Electrophysiological Studies of Peripheral Nerve Function in Patients with Chronic Liver Disease

“…Both ulnar and median nerve action potentials were recorded in 15 patients, but only two patients were found to have abnormal results for both nerves. Approximately one-third of the values obtained for each sensory or mixed motor and sensory action potential proved to be abnormal, yet the same patients were not always Horizontal lines indicate the lower limits of the normal range for the distal motor latency, and 2 SD below the mean of the normal range for the motor nerve-conduction velocities (Gilliatt & Thomas, 1960;Preswick, 1963;Yap & Hirota, 1967). In this Figure represented in each of the abnormal groups.…”

“…Even in the presence of normal conduction velocity, increased distal motor latencies can reflect disturbance of function in the distal segment of the nerve, but these were normal for the ulnar and lateral popliteal nerves. In only one patient was the distal motor latency for the median nerve greater than 4.7 ms (Gdliatt & Thomas, 1960; Preswick, 1963;Yap & Hirota, 1967).…”

Comparison of median and ulnar sensory nerve action potentials in the diagnosis of the carpal tunnel syndrome

“…With supramaximal stimulation, the range was 3-1 to 4-6 msec with a mean of 3-67 msec (SD ± 038). As these values were obtained with surface electrodes no strict comparison could Preswick (1963). be made with those by PATIENTS WITH CARPAL TUNNEL SYNDROME The results of the patient group are given in Table 1.…”

“…These currently include the measurement of distal motor latency (Simpson, 1956;Preswick, 1963) and the estimation of digital sensory conduction and amplitude (Gilliatt and Sears, 1958). While the diagnostic value of these methods is considerably enhanced by their combined application there still remains a group of patients, particularly those with minor, intermittent symptoms, who escape confirmation (Preswick, 1963;Fullerton and Gilliatt, 1965).…”

Stimulus intensity and site of excitation in human median nerve sensory fibres

“…Dawson (1956), Preswick (1963), Pinelli (1964), and Gilliatt, Melville, Velate, and Willison (1965), and Buchthal and Rosenfalck (1966) observed shortening of latency with increasing stimulus intensity. These investigators interpreted their findings at least partly as displacement of the point of stimulation from the cathode towards the recording electrode.…”