Nociceptive muscle reflex responses in human forearms

Abstract

In healthy volunteers, 5-min noxious stimulation with rectangular electrical pulses applied transeutaneously to the phalanges enhanced tonic activity in the palmar and finger flexor muscles resulting in a specific eleetromyographie pattern consisting of two successive bursts of activity which appeared after a period of inhibition. In patients with chronic pain in the arm, signitieantly lower thresholds for the first and the second waves of reflex activity have bee~ found. This electromyographie pattern of the forearm muscle reflex responses is supposed to be similar to the nociceptive flexor reflex in the leg. It can be useful for objective assessment of the effectiveness of analgesia and pain syndrome therapy in patients with cervical spinal cord injury. Key words: electromyogram; nociceptive flexor reflex; pain; inhibition; excitabilityMethods of the reflex excitability estimation in the different parts of the neuromotor apparatus are now widely used for the pain syndrome diagnosis, choosing of appropriate analgesia treatment, and control of its effectiveness. The nocieeptive flexor reflex (RIII reflex) recordings are most frequently used in clinical studies [8] aimed at evaluating hyperalgesia which results from tissue damage [4][5][6] and to determine an analgetie effectiveness [3]. However, this method allows one only to explore the excitability of nociceptive neurons from the lumbosacral segments of the spinal cord [9,10].We attempted to find the nociceptive reflex responses in the forearm muscles induced by noxious transcutaneous electrical stimulation similar to RIII-reflexes in the shank and tibial muscles. 58 years) with pain irradiating from the shoulder blade to the arm and to the forearm, sometimes accompanied by finger paresthesia were included in the study. Pain originated from the compression lesions in cervical segments of the spinal cord in men and after mastectomy in women and casted from 3 months to 2 years. Electromyograms (EMGs) were recorded via bipolar surface electrodes from the following muscles: m. thenar, m. extensor pollicis brevis, m. extensor carpi radialis, m. extensor digiti communis, anterior head of m. deltoideus, and m. extensor carpi ulnaris. Circular electrodes (5 mm in diameter) were fixed to a muscle belly a 2 cm distance between them. The EMG was amplified in a TIESY electrophysiological system in the frequency band of 20-2000 Hz and recorded in the digital form with a sampling rate of 5 kHz. Electri, cal stimuli were applied to the I, II, IV, or V finger via surface electrodes attached to the 1st and the 2nd phalanges. The stimulus was a train of constant current rectangular pulses (0.2 msec) delivered at the rate of 0.2-1 Hz. A threshold current (5-10 mA) was determined individually by subjectively reported tactile sensation (electrical shock), which was not painful or unpleasant. At suprathreshold intensities (30-40 mA), the stimulus produced moderate bear-0007-4888/98/0009-0889520.00 9 MATERIALS AND METHODSKluwer Academic/Plenum Publishers