[Morphine-induced hyperalgesia, allodynia and myoclonus--new side-effects of morphine?]

During the last ten years hyperalgesia (H), allodynia (A) and myoclonia (M) has been reported at an increased frequency in human beings treated with morphine. The side effects are most common in cancer patients treated with high dose morphine, and has been reported for all routes of administration. The mechanisms are unknown, but human cases and experimental works have resulted in the following theories: 1) Morphine and morphine metabolites change the postsynaptic pain-transmission in dorsal horn neurones via non opioid-receptors (glycine and/or N-methyl-D-aspartate). 2) Morphine and morphine metabolites activate other opioid receptor populations. 3) Supplemental drugs in cancer management. 4) An abnormal metabolism of morphine or morphine metabolites. 5) A combination of one or more of the above-mentioned theories. The first mentioned theory is the most likely. The treatment of morphine induced H, A, and M seems to be to discontinue morphine administration and to initiate therapy with other opioids (fentanyl, sufentanyl, methadone or ketobemidone).