Myoclonus associated with treatment with high doses of morphine: the role of supplemental drugs.

OBJECTIVE--To estimate the prevalence of important side effects in patients with malignant disease who were receiving high doses of morphine as part of their palliative treatment. DESIGN--Data on patients were collected over 12 months. SETTING--Two palliative care units in Western Australia. PATIENTS--19 Patients with malignant disease who were receiving morphine either subcutaneously or orally as the main analgesic. 10 Patients receiving a total daily dose of morphine of at least 500 mg orally or 250 mg parenterally were enrolled in the study. The other 9 patients were enrolled after an important problem thought to be related to the morphine had been identified. All of the patients were taking drugs to supplement the treatment. INTERVENTIONS--The dose of morphine or route of administration, or both, was changed in three patients. MAIN OUTCOME MEASURE--Determination of the prevalence of side effects in the patients. Assessment of the relation of any side effects with the supplemental drugs taken by the patients. MAIN RESULTS--Plasma morphine and electrolyte concentrations were measured and a full history taken for each patient. Thirteen of the 19 patients had an important side effect; 12 of them had myoclonus and one had hyperalgesia of the skin. Plasma morphine concentrations were similar in patients with and without myoclonus, ranging from 158 to 3465 nmol/l and 39 to 2821 nmol/l respectively. Eight of the patients with side effects were taking an antipsychotic drug concurrently compared with none of those without side effects. A greater proportion of patients with side effects were taking the antinauseant drug thiethylperazine (6/13 v 2/6) and at least one non-steroidal anti-inflammatory drug (10/13 v 2/6), whereas a smaller proportion were taking a glucocorticosteroid (3/13 v 4/6). The estimated prevalence of important side effects in the total population of patients receiving palliative treatment in the two units was 2.7-3.6%. CONCLUSIONS--Myoclonus as a side effect of treatment with morphine is more likely to occur in patients taking antidepressant or antipsychotic drugs as antiemetics or as adjuvant agents or non-steroidal anti-inflammatory drugs for additional analgesia. If a patient develops myoclonus the best approach may be to change the supplemental treatment.