Sensory Nerve Action Potentials in Patients With Peripheral Nerve Lesions

Abstract

In 1949 Dawson and Scott described a method of examining sensory impulses in human peripheral nerves. By their technique, the median or ulnar nerve at the level of the wrist was stimulated electrically using single shocks, and the action potential of the afferent volley recorded through surface electrodes placed over the nerve trunk at a higher level in the arm. The afferent volley contained both sensory impulses and antidromic impulses in motor fibres and gave rise to a small diphasic or triphasic potential with an amplitude of less than 50 microvolts and a duration of 2 to 3 milliseconds. To eliminate the contribution of motor fibres, Dawson (1956) modified his original technique and stimulated the digital nerves through small ring electrodes wrapped round the fingers, the afferent volley being recorded as before through surface electrodes placed over the median or ulnar nerve trunk in the arm. The sensory potentials recorded were similar in form to the mixed nerve action potentials obtained previously, but, being derived from digital nerves, they were considerably smaller in amplitude.In view of Dawson's findings it seemed worthwhile to investigate the behaviour of sensory action potentials in patients with peripheral nerve lesions, and the present paper is based on some 50 cases examined in this way. In general the technique has proved to be of considerable value in the clinical assessment of patients, and in certain cases it has provided more information about sensory nerve function than could be obtained by formal sensory testing.Methods These differed only in detail from those described previously by Dawson and Scott (1949) and by Dawson (1956). For stimulating the digital nerves, silver strips 2 to 4 mm. wide were used, covered by lint moistened in saline and firmly wrapped round the finger, with the cathode placed near the metacarpo-phalangeal joint and the anode in the region of the terminal interphalangeal joint. The stimulus itself was a brief condenser discharge (time constant approximately 70 microseconds) delivered through an isolating transformer. Surface recording electrodes were of the "saddle" pattern described by Dawson and Scott (see their Fig. 1) with an inter-electrode distance of 3 cm. Before applying the recording electrodes, the course of the median or ulnar nerve in the arm was usually mapped out by stimulation and observation of the motor response. In the few patients without any motor response to nerve stimulation the course of the nerve could not be determined in this way; in such a case recording electrodes were placed over the expected position of the nerve trunk and adjusted to give the largest response to sensory stimulation. The design of the recording electrodes was such that small lateral movements had little or no effect on the size of the recorded potential; they were thus particularly suitable for rapid tests on patients.Nerve action potentials were amplified in the conventional way and displayed on a cathode ray tube. For photography, the method of superimposition used b...

Mechanically and electrically evoked somatosensory potentials in normal humans

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Pressure palsy of the ulnar nerve with prolonged conduction block

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