The differential effect of methadone dose and of chronic pain on pain perception of former heroin addicts receiving methadone maintenance treatment.
The reports on pain perception among former heroin addicts receiving methadone maintenance treatment (MMT) vary with regard to pain and intolerance threshold, and perception of suprathreshold stimuli has not been previously evaluated. Our aim was to systematically assess perception of threshold and suprathreshold noxious and innocuous stimuli with special attention to the effect of MMT dose and the presence of chronic pain. Noxious and innocuous, thermal and mechanical thresholds and ratings of suprathreshold heat-pain stimuli were measured among 31 MMT subjects receiving high and low MMT dose, with and without chronic pain, and in 17 healthy controls. The characteristics of chronic pain were also evaluated. MMT dose and chronic pain differentially affected pain perception. Whereas MMT dose did not affect thresholds, chronic pain MMT subjects exhibited increased pain threshold and pain-free MMT subjects exhibited decreased pain threshold compared with controls. MMT in general was associated with decreased perception of suprathreshold pain; however, MMT subjects with chronic pain exhibited increased suprathreshold pain ratings. It appears that subjects receiving MMT are hyperalgesic but that chronic pain in these subjects interferes with threshold measurements, inducing an apparent hypoalgesia. On the other hand, chronic pain reduces the analgesic effect of methadone seen in pain-free MMT subjects, amplifying suprathreshold pain perception. Factors such as chronic pain and MMT dose should be taken into account in future studies on pain perception in this population. We show that the presence of chronic pain and methadone dose significantly affects perception of pain in former heroin addicts receiving MMT. Studying the alteration in pain perception in these subjects may contribute to understanding the high rates of chronic pain among them and may promote better treatment.