Case Presentation: Methemoglobinemia is a hematological condition that is most commonly acquired when substances cause oxidation of iron to the ferric state (Fe3+) from the ferrous state( Fe 2+). It is a clinical diagnosis based on effective history taking and signs of hypoxemia resistant to supplemental oxygen. Here we describe a patient who had been using 100 mg of dapsone daily for PJP prophylaxis for 3 weeks in the setting of chemotherapy treatment for Waldenstrom Macroglobulinemia.
Discussion: An 80-year-old woman presents to the emergency department from her oncologist’s office for 1 week of mild dyspnea on exertion, fatigue, palpitations, and lethargy. Patient recently started her chemotherapy regimen of Bendamustine and Rituximab in addition to dapsone for PJP prophylaxis three weeks prior to her hospital admission. On ambulation, the patient desaturated to 84% on room air and was then placed on 6 liters of nasal cannula with oxygen saturation going up to 85%. Patient was then switched to a BiPap machine and was continuing to saturate at 85%. Patient had refractory hypoxemia with an initial arterial blood gas showing a PO2 of 368 and a methemoglobin level of 23.7 indicating a PaO2-saturation gap. Routine lab work revealed that the patient was hyponatremic and had a chronic anemia but otherwise no concerning lab values were seen. Guidelines indicate a low dose of methylene blue can be given regardless of G6PD deficiency status. Patient was given a dose of methylene blue at a dose of 1mg/kg. Patient had a repeat ABG that showed a PO2 of 116 and a methemoglobin level of 5. Patient continued to improve clinically and had dapsone discontinued.
Conclusions: The diagnosis of methemoglobinemia should be suspected in anybody with cyanosis and hypoxia. Our patient was on a prophylactic dose of dapsone for a short period of time and presented with no cyanosis. Patients who undergo chemotherapy and are placed on a prophylactic dose of dapsone should undergo routine follow-up for evaluation of oxygen saturation. In a retrospective study of 138 cases of methemoglobinemia, 42% of cases were dapsone acquired with an average methemoglobin of 7.6%. Our patient had minimal symptoms and had a methemoglobin level of 23.7% The patient could have been started on Bactrim for PJP but had a sulfa allergy.