Morphine induced allodynia in child with brain tumour

Editor—Heger et al remind readers that high doses of morphine may have paradoxical effects.¹ We are surprised, however, at the choice of patient they use to illustrate this lesson.

The diagnosis of pain in an infant depends solely on the observation of his or her behaviour.² It is particularly difficult to diagnose pain, let alone characterise it as allodynia, in a 9 month old infant with considerable neurological deficit and raised intracranial pressure. The authors attempt to justify the diagnosis of allodynia in just such a patient. Furthermore, high dose morphine is well reported as a cause of rigidity, catalepsy, akathisia, and myoclonus, which must add to the difficulty of interpreting pain on the basis of observation alone.³ Two inconsistencies in the case history undermine the speculative diagnosis.

Firstly, the signs of distress provoked by routine nursing that were interpreted as allodynia induced by morphine-3-glucuronide were also recorded before morphine was given. Secondly, when the morphine dose was reduced the patient received methotrimeprazine, dexamethasone, and dypirone, each of which could have eased the signs of distress. The patient's distress had resolved within a week with this new treatment regimen, yet the raised ratio of plasma morphine-3-glucuronide to morphine, which the authors interpret as a cause of her allodynia, remained high for at least 17 days.

We believe that to diagnose allodynia in this patient is to ignore the much greater likelihood that the signs were a consequence of her underlying medical condition. We therefore agree with the authors that “morphine induced hyperalgesia has not been reported in children so far.”