Methaemoglobinaemia: a rare cause of chest pain, hypoxia and cyanosis

A man in his 20s presented with severe left-sided chest pain, shortness of breath and acute confusion. Initial examination revealed central cyanosis, normal heart sounds, vesicular breath sounds in both lung fields and a bruised right calf. The patient reported that he had recently injured his right leg and that he had not taken medication prescribed for blood clots for several days. Peripheral oxygen saturations were 85% despite high flow oxygen via a non-rebreather mask. ECG revealed sinus tachycardia. Arterial blood gas sampling confirmed hypoxaemia. Given the history and the severe hypoxia, he was managed for pulmonary embolism initially. Closer inspection of his arterial blood results showed a methaemoglobin percentage of 20.4%. He was given intravenous methylene blue which resulted in resolution of his symptoms within 30 min. He subsequently confirmed that he was undergoing genetic testing for likely congenital methaemoglobinaemia.