Methemoglobinemia as a cause of coma

A 50-year-old man presented to the emergency department in a coma with fixed and dilated pupils. Skin and mucous membranes were noted to be an ashen grey color. Initial vital signs were blood pressure of 104/70 mm Hg; pulse, 110; and respirations, 12. Initial arterial blood gases were pH of 7.25; PaCO2, 26.6 mm Hg; PaO2, 22.1 mm Hg; oxygen saturation, 15.2%; and methemoglobin level, 81.5%. Venous and arterial blood samples were chocolate brown, and it was noted that the color did not change when 100% oxygen was bubbled through the blood. Methylene blue 140 mg was given intravenously, and the patient gradually became more responsive. He was discharged three days later with no neurologic deficits. A comprehensive literature search revealed no reported cases of complete patient recovery with this high a methemoglobin level.