Opioid-related narcosis in a woman with myopathy receiving magnesium

An Asian multiparous woman weighing 47 kg, who suffered from a rare myopathy, congenital fibre type disproportion, was given morphine 10 mg intramuscularly for labour analgesia. After delivery, she had diastolic hypertension and proteinuria and was prescribed magnesium sulphate. Some hours later she became unresponsive with little respiratory effort. Blood gas analysis revealed a respiratory acidosis. Naloxone administration reversed the symptoms. Further doses were required as the respiratory depression recurred. Opioid-related narcosis is the most likely diagnosis in this case. Other possible differential diagnoses were magnesium overdose or a post-ictal state. The presence of a myopathy could render this patient susceptible to the respiratory effects of opioids. Other explanations for an exaggerated and delayed response to opioids include co-administration of other respiratory depressant drugs such as magnesium sulphate, co-morbidity such as renal impairment and genetic variability in the metabolism of morphine. Robust guidelines and highlighting patients with risk factors are required to prevent this complication from recurring.