Undiagnosed intraoperative methaemoglobinaemia.

Methaemoglobinaemia is a rare but potentially dangerous haemoglobinopathy that is often underdiagnosed. It is one of the causes for unexplained cyanosis with dark-coloured blood, especially in the absence of cardiac or pulmonary pathology. Not uncommonly so, it is an incidental perioperative finding in cases of dark-coloured blood not improving with oxygen in apparently acyanotic patients. The present case report is of a child with deaf-mutism posted for cochlear implant surgery who presented with 'chocolate-coloured blood' in the surgical field, despite blood gas analysis showing a normal partial pressure of oxygen.