Opioid-induced muscle activity: implications for managing chronic pain

OBJECTIVE: To increase awareness of opioid-induced involuntary muscle hyperactivity and to present management options. CASE SUMMARY: A ventilator-dependent 71-year-old man presented with pain caused by metastatic lung cancer. Transdermal fentanyl therapy was titrated to 200 micrograms/h. Two days later a continuous morphine infusion was initiated because of frequent administration of oral morphine solution for breakthrough pain. The patient became progressively less responsive and began exhibiting involuntary muscle hyperactivity thought to represent breakthrough pain. Despite the inability to assess pain control effectively in this unresponsive patient, the morphine infusion rate was increased from 22 to 717 mg/h within 7 days. No change in muscle hyperactivity was observed. DISCUSSION: Over the last decade involuntary muscle hyperactivity has been documented as an adverse effect of chronic opioid therapy. The literature describing the incidence of this toxicity, possible risk factors for its development, and recommendations for its management are discussed. CONCLUSIONS: The occurrence of muscle hyperactivity in an unresponsive patient receiving chronic opioid therapy may represent opioid toxicity. Recommendations for managing opioid-induced muscle hyperactivity include reduction of the opioid dosage and/or administration of clonazepam therapy.