Coma, metabolic acidosis, and methemoglobinemia in a patient with acetaminophen toxicity

We present a case of early coma, metabolic acidosis and methemoglobinemia after substantial acetaminophen toxicity in the absence of hepatic failure. A 77-year-old female presented to the emergency department with a decreased level of consciousness. She was found unresponsive by a family member in her bed, and was reported to be acting normally when she was last seen eight hours earlier. Laboratory results on arrival were: pH 7.19, sodium 139 mmol/L, chloride 106 mmol/L, potassium 3.3 mmol/L, CO2 8 mmol/L, and an anion gap of 25. Both venous lactate (10.2 mmol/L) and methemoglobin (9.4 %) were elevated. The patient's acetaminophen concentration was markedly elevated at 7138 µmol/L (1078 µg/ml). Hepatic enzymes and coagulation tests were normal [alanine transaminase (ALT) 8 U/L, international normalized ratio (INR) 1.0]. Intravenous N-acetylcysteine (NAC) was initiated at a dose of 150 mg/kg over 15 minutes, followed by 50 mg/kg over the next four hours, followed by 100 mg/kg over the next 16 hours. Twenty-four hours after admission, the anion gap metabolic acidosis had resolved, and the methemoglobin was 2.1%. Aminotransferases peaked at 44 U/L and INR peaked at 1.9. A urine 5-oxoproline assay performed five days after admission was negative, suggesting no evidence of a 5-oxoprolinase deficiency. We describe the pathophysiology and discuss the literature on acetaminophen-induced coma and metabolic acidosis in the absence of hepatic injury; and propose mechanisms for associated methemoglobinemia.