Unusual cause of persistent dyspnea in a patient with nephrotic syndrome: dapsone-induced methemoglobinemia

Acquired methemoglobinemia is a rare, but important cause of tissue hypoxemia, which often results from commonly prescribed medications. We report a case of an 83-year-old female with stage III chronic kidney disease, who was started on high-dose prednisone for nephrotic syndrome and dapsone for pneumocystis jirovecii pneumonia prophylaxis. A week after initiation of dapsone, this patient presented with shortness of breath, and an oxygen saturation gap while on supplemental O2 via nasal cannula, with no obvious cardiopulmonary cause. Methemoglobin level was elevated on arterial blood gas. Our patient was treated with IV methylene blue. Dapsone was determined to be the culprit and immediately discontinued. Patient responded with a rapid decrease in methemoglobin levels and resolution of symptoms. This case report highlights the importance of maintaining a high index of suspicion and careful review of medication history for prompt and successful management of methemoglobin poisoning, as not all patients present with classical signs of methemoglobinemia such as cyanosis and/ or dark red blood.