Acquired methemoglobinemia after hydroxocobalamin administration in a patient with burns and inhalation injury.

CONTEXT: Hydroxocobalamin is an effective cyanide antidote. While erythema, hypertension, and chromaturia are recognized side effects, methemoglobinemia has not been reported. Methemoglobin levels are most accurately measured by co-oximetry. We describe an extensively burned patient who developed methemoglobinemia within an hour of hydroxocobalamin administration.

CASE DETAILS: A 47-year old man without genetic deficiencies or abnormal hemoglobin variants presented with 61% body surface area thermal burns and grade 1 inhalation injury sustained during a tugboat engine explosion. On admission, lactate was 9.24 mmol/L, methemoglobin 1%, and carboxyhemoglobin 0.2% by blood gas analysis with co-oximetry. Despite large-volume resuscitation, lactate remained elevated (7-8 mmol/L). Intravenous hydroxocobalamin (5 g) was administered at postburn hour 19 for possible cyanide toxicity. Immediately thereafter, he became hypertensive with reflex bradycardia. Lactate decreased to 5.51 mmol/L, methemoglobin rose to 4.10%, and oxygen saturation by pulse oximetry decreased to 74-80% (despite arterial oxygen saturation of 95% by cooximetry). Methemoglobin concentration peaked at 13.40% at postburn hour 33. Methylene blue was not administered.

CONCLUSIONS: Methemoglobinemia in our patient was temporally associated with hydroxocobalamin administration.