Case Report: Phenazopyridine-Induced Sulfhemoglobinemia in an 83-Year-Old Presenting with Dyspnea

Sulfhemoglobinemia is a rare dyshemoglobinemia that presents similarly to methemoglobinemia.

An 83-year-old woman with stage IV ovarian cancer presented to the Emergency Department after a near syncopal spell and was found to be cyanotic with a pulse oximetry reading of 71%. Pulse oximetry improved to only the mid-80s range with administration of high-flow oxygen. Her arterial blood gas on supplemental high-flow oxygen demonstrated a PaO₂ of 413 mm Hg and methemoglobin of 1.2%, but also noted the interference of the co-oximetry with sulfhemoglobinemia. Further history revealed that the patient had recently been started on phenazopyridine. The phenazopyridine was stopped, an exchange transfusion was offered but declined, and the patient was discharged to home hospice. WHY SHOULD AN EMERGENCY PHYSICIAN BE AWARE OF THIS?: The diagnosis of sulfhemoglobinemia can be challenging given that routine co-oximetry does not identify it. The clue to the diagnosis is that the cyanotic-appearing patient has a normal or elevated PaO₂ and seems to be less ill than expected, given the degree of cyanosis. Sulfhemoglobinemia does not reverse with the administration of methylene blue.