Dapsone-induced methemoglobinemia: “Saturation gap”—The key to diagnosis
A 36-year-old woman was diagnosed with immune thrombocytopenic purpura in July 2012 and started on oral prednisolone and dapsone 100 mg/day. About 3 months later, she was admitted to the hospital with fatigue, headache, arthralgia, and severe dyspnea. Physical examination was normal except for peripheral cyanosis. She had a platelet count of 1,000/cmm, hemoglobin of 7.9 gm%. SpO2 on room air was 84%, respiratory rate was 52/min. Owing to intolerance for non-invasive ventilation (NIV) and low saturation, the patient was intubated and ventilated.\nArterial blood gas (ABG) analysis revealed a pH of 7.43, PaO2-180 mmHg, PaCO2-32 mmHg, SaO2-99.1% on oxygen by facemask. In view of history of dapsone intake, cyanosis, low SpO2 with normal PaO2 and SaO2(on ABG) a diagnosis of methemoglobinemia was considered and dapsone was stopped, however, the facility for methemoglobin (methHb) estimation was not available. After 48 hrs of mechanical ventilation, she improved symptomatically and was extubated following a weaning trial. She was shifted to high dependency unit with oxygen 5l/min. The SpO2 continued to remain low ranging from 88-92% over the next 3 days though the SaO2 was normal. As the patient remained clinically stable, no further intervention was carried out and she made an uneventful recovery.