Snapshot A 20-year-old woman with a history of celiac disease presents to the dermatologist with pruritic papules on her bilateral extensor elbows. She is started on dapsone for dermatitis herpetiformis, an itchy rash commonly seen in patients with celiac disease. A few weeks later, she reports feeling very tired, with frequent episodes of lightheadedness. At an urgent care clinic, she is noted to have pulse oximetry readings of 90% with no improvement on supplemental oxygen. A methemoglobin level is found to be > 3%. She is sent to the emergency room for treatment. Introduction Overview methemoglobinemia causes tissue hypoxia due to accumulation of methemoglobin in the blood methemoglobin login to view 1 more bullet Epidemiology demographics infants are at increased risk risk factors drugs login to view 5 more bullets infants are more susceptible to hemoglobin oxidation food well water Pathogenesis methemoglobin is the oxidized form of Hb, which has an affinity for cyanide oxygen does not bind as easily to the heme subunit with the ferric form of iron remaining heme sites has increased affinity for oxygen login to view 1 more bullet causes left-shift of the oxygen-hemoglobin dissociation curve login to view 2 more bullets Presentation Symptoms may be asymptomatic if levels are low headache lightheadedness fatigue shortness of breath seizures or coma at very high levels of methemoglobinemia Physical exam pulse oximetry shows decreased oxygen saturation level (< 90%) supplemental oxygen will not improve pulse oximetry reading tachycardia cyanosis altered mental status “chocolate brown” or blue blood Studies Arterial blood gas normal pO2 this indicates dissolved oxygen in the blood Pulse oximetry oxygen saturation 85-90% Methemoglobin level > 3% (normal values < 1%) measured with CO-oximetry device Differential Cyanide poisoning distinguishing factor arterial blood gas shows metabolic acidosis from the accumulation of lactic acid CO-oximetry is normal does not respond to methylene blue Treatment Medical 100% oxygen + methylene blue mechanism login to view 1 more bullet contraindications login to view 6 more bullets vitamin C indication login to view 2 more bullets Complications End-organ failure Acute respiratory distress syndrome