Lesson of the week: Morphine induced allodynia in a child with brain tumour What is allodynia?

Morphine is the drug of choice for patients with severe acute or chronic pain, especially those with intractable pain associated with malignant diseases. The drug is metabolised by the liver into morphine-3-glucuronide and morphine-6-glucuronide. The glucuronides are mainly eliminated via bile and urine. 1 Morphine-6glucuronide binds to opiate receptors and can be detected in the cerebral fluid after systemic administration; its analgesic effect is 40 times as potent as that of morphine itself. Therefore, if patients complain of an insufficient analgesic response to standard morphine dosage, they should receive higher doses, which may exceed hundreds of milligrams of morphine hourly. In a few adults treated with higher dosages for prolonged periods of time, morphine induced hyperalgesia or allodynia develops. 5 6 Allodynia has been provoked in Sprague-Dawley rats. These animals are unable to metabolise morphine to morphine-6-glucuronide, and thus morphine-3glucuronide is the major morphine metabolite. A noticeable inverse relation was observed between the mean degree of analgesia and ratio of plasma morphine-3-glucuronide to morphine. Consequently, a raised morphine-3-glucuronide to morphine-6glucuronide ratio seems to be responsible for the phenomenon of hyperalgesia and allodynia.