Methaemoglobinaemia due to alkyl nitrites in a patient with suspected traumatic injuries

A man in his 60s with no relevant previous medical history presented to an urban, major trauma centre by ambulance after being found with a head injury in a nightclub. The paramedics reported he was hypoxic, hypotensive and tachycardic with altered mental status. At the emergency department, he had oxygen saturations of 85% despite high-flow oxygen and was hypotensive at 88/43mmHg. We were concerned the patient was haemorrhaging given the lack of response to oxygen therapy and their hypotension. However, an arterial blood gas (ABG) established a diagnosis of methemoglobinaemia. Methylthioninium chloride was promptly administered, and the patient's condition improved. He later reported using recreational drugs, including alkyl nitrites ('poppers'). He was monitored until his fraction of methaemoglobin returned to normal baseline levels with serial ABGs. He was discharged 24 hours later. It was suspected that his use of alkyl nitrites was the most likely cause of methaemoglobinaemia.