Cyanotic and Unresponsive: A Diagnostic Challenge in Alkyl Nitrite-Induced Methaemoglobinaemia.

Methaemoglobinaemia is a potentially serious condition in which haemoglobin is oxidised to a form that cannot carry oxygen effectively. Recreational inhalation of alkyl nitrites ("poppers") may precipitate this condition and can be overlooked when the presentation is sudden and severe. We report a 53-year-old male of Chinese ethnicity, with multiple comorbidities, including learning disabilities, who collapsed in public and presented unresponsive, cyanotic, hypothermic, and profoundly hypoxic despite high‑flow oxygen. Arterial blood gas analysis revealed a high methaemoglobin level with a normal arterial oxygen tension, producing a marked saturation gap. A bottle of alkyl nitrites found in his possession supported a toxic cause. Intravenous methylene blue led to rapid clinical and biochemical improvement, and the patient made a full recovery. Safeguarding measures were implemented due to his cognitive vulnerability. This case highlights the diagnostic challenge of unexplained hypoxia, the essential role of co‑oximetry in confirming methaemoglobinaemia, considerations for enzyme deficiencies like G6PD, and the importance of harm-reduction strategies amid rising poppers use.