A patient presents with headache, nausea, and dizziness after exposure to gas or smoke, with anion gap metabolic acidosis. What is the most likely diagnosis?

A patient presents with headache, nausea, and dizziness after exposure to gas or smoke, with anion gap metabolic acidosis. What is the most likely diagnosis?

• A. Cyanide poisoning
• B. Methemoglobinemia
• C. Carbon Monoxide poisoning
• D. Smoke inhalation injury

Answer: (C)

Exposure to smoke or gas with headaches, nausea, and dizziness is a classic presentation of carbon monoxide poisoning. Carbon monoxide binds strongly to hemoglobin, forming carboxyhemoglobin, which both reduces the blood’s capacity to carry oxygen and shifts the oxygen-hemoglobin dissociation curve to the left, making it harder for oxygen to be released to tissues. The result is tissue hypoxia even when the blood is oxygenated, so the body switches to anaerobic metabolism and produces lactic acid, leading to an anion gap metabolic acidosis. That lactic acidosis plus the neurologic symptoms from brain hypoxia fit CO poisoning particularly well after smoke exposure. Methemoglobinemia would present with cyanosis and a characteristic discrepancy between pulse oximetry and arterial oxygen, not primarily with this vignette. Cyanide poisoning can also cause lactic acidosis, but the most common and classic cause of these symptoms after smoke exposure is carbon monoxide poisoning. Smoke inhalation injury is broader but CO poisoning remains the leading specific diagnosis here, and management centers on high-flow oxygen now, with hyperbaric oxygen considered in severe cases.

Exposure to smoke or gas with headaches, nausea, and dizziness is a classic presentation of carbon monoxide poisoning. Carbon monoxide binds strongly to hemoglobin, forming carboxyhemoglobin, which both reduces the blood’s capacity to carry oxygen and shifts the oxygen-hemoglobin dissociation curve to the left, making it harder for oxygen to be released to tissues. The result is tissue hypoxia even when the blood is oxygenated, so the body switches to anaerobic metabolism and produces lactic acid, leading to an anion gap metabolic acidosis. That lactic acidosis plus the neurologic symptoms from brain hypoxia fit CO poisoning particularly well after smoke exposure.

Methemoglobinemia would present with cyanosis and a characteristic discrepancy between pulse oximetry and arterial oxygen, not primarily with this vignette. Cyanide poisoning can also cause lactic acidosis, but the most common and classic cause of these symptoms after smoke exposure is carbon monoxide poisoning. Smoke inhalation injury is broader but CO poisoning remains the leading specific diagnosis here, and management centers on high-flow oxygen now, with hyperbaric oxygen considered in severe cases.