Acute Methaemoglobinaemia as an Overlooked Cause of Acute Dyspnoea in a Patient With Chronic Obstructive Pulmonary Disorder in the Emergency Department.

Methaemoglobinaemia is a condition characterised by excessive methaemoglobin (MetHb), which is an oxidised form of haemoglobin in the blood. MetHb is unable to bind oxygen, subsequently leading to hypoxia. Methaemoglobinaemia can either be congenital or acquired, often caused by exposure to oxidising agents like alkyl nitrates, commonly found in illicit drugs. This case report is of a 45-year-old man with chronic obstructive pulmonary disorder (COPD), who developed methaemoglobinaemia as a result of recreational use of alkyl nitrates. This patient presented with worsening dyspnoea and blood gases showing high MetHb, which was initially overlooked. His leukocyte and neutrophil levels were found to be elevated. The initial impression was an infective exacerbation of COPD (IECOPD). The suspicion of acute methaemoglobinaemia was made only once he disclosed his alkyl nitrate usage. He was then transferred to the high dependency unit and received hyperbaric oxygen and ascorbic acid, alongside treatment for IECOPD. Significant learning includes the importance of considering methaemoglobinaemia in dyspnoeic patients, consideration of methaemoglobinaemia as a differential diagnosis to IECOPD, and the importance of obtaining a thorough history of recreational substance usage in anyone with methaemoglobinaemia.