Validity of electrical stimulus magnitude matching in chronic pain.

OBJECTIVE To examine the validity of the PainMatcher in chronic pain. DESIGN Comparison of parallel pain estimates from visual analogue scales with electrical stimulus magnitude matching. PATIENTS Thirty-one patients with chronic musculoskeletal pain. METHODS Twice a day ongoing pain was rated on a standard 100-mm visual analogue scale, and thereafter magnitude matching was performed using a PainMatcher. The sensory threshold to electrical stimulation was tested twice on separate occasions. RESULTS In 438 observations visual analogue scale ranged from 3 to 95 (median 41) mm, and PainMatcher magnitudes from 2.67 to 27.67 (median 6.67; mean 7.78) steps. There was little correlation between visual analogue scale and magnitude data (r = 0.29; p < 0.0001). The mean sensory threshold was 3.67 steps, indicating that the PainMatcher, on average, stimulated at 2.1 times the perception threshold at matching point. CONCLUSION Electrical magnitude matching of chronic pain intensity elicited limited activation of nerve fibres at 2.0-2.2 times sensory threshold, indicating that the induced pain was evoked by coarse nociceptive Adelta fibres. While the visual analogue scale estimates covered the whole range of the instrument, the PainMatcher readings utilized only a small part of the instrument range and, importantly, had little or no relation to the visual analogue scale estimates. The validity of the PainMatcher procedure is doubtful.