Pathophysiology and treatment of opioid-related myoclonus in cancer patients.

Myoclonus occasionally occurs in the perioperative setting and in patients on chronic opioid therapy. It appears to be dose-related in a unpredictable manner. Different mechanisms have been proposed to explain the occurrence of a series of neuromuscular disturbances probably sharing final common pathways. A neuroexcitatory opioid metabolite accumulation has been proposed to have a relevant role in determining myoclonus in patients treated with chronic opioid therapy for cancer pain, especially in the presence of renal impairment. The neurological status, previous oncologic treatment and concomitant therapy with neuroleptic drugs, the metabolic and hydration status should also have been considered. Adjuvant drugs, such as benzodiazepines or dantrolene may avoid the reduction of the opioid dose while maintaining an acceptable analgesia. Current practice suggests a change in opioid when pain control is not obtained at opioid doses resulting in unacceptable adverse effects, including myoclonus and hyperalgesia. A change in the type of opioid may be useful in patients who develop severe central adverse effects, even if these patients appear to have normal renal function or hydration status.