Abstract Introduction Methemoglobinemia is a rare hematologic disorder caused by oxidation of hemoglobin’s iron from the ferrous (Fe²+) to the ferric (Fe³+) state, impairing oxygen delivery and leading to tissue hypoxia. It can be congenital or acquired following exposure to oxidizing agents such as nitrates, local anesthetics, and certain medications. Phenazopyridine, a commonly used urinary tract analgesic, is an uncommon cause, with fewer than ten cases reported in the past 35 years and an estimated incidence of less than 1%. Although rare, methemoglobinemia can be life-threatening, emphasizing the importance of early recognition and prompt management. Case Presentation A 28-year-old female with a history of recurrent urinary tract infections and mood disorders presented with fatigue, dyspnea, and cyanosis of the lips and nails for one day. Home pulse oximetry revealed SpO2 of 86-88%. She had recently started antibiotics for a urinary tract infection and self-administered over-the-counter phenazopyridine (AZO) for symptom relief, ingesting 24 tablets (99.5 mg each) within a single day. In the emergency department, her vital signs were blood pressure 140/72 mmHg, pulse 95-97/min, respiratory rate 18/min, and SpO2 86% on room air. Despite supplemental oxygen, saturation remained at 86-88%. Imaging was negative for pulmonary embolism or other pulmonary pathology. Arterial blood gas on a non-rebreather mask showed a PaO2 of 250 mmHg, SpO2 86% and methemoglobin level of 12%. Given her symptomatic presentation, she was treated with 2 g of intravenous ascorbic acid. Methylene blue was deferred due to mild symptoms, unknown G6PD status, and potential interaction with buspirone and desvenlafaxine, which could precipitate serotonin syndrome. Six hours post-treatment, methemoglobin levels decreased to 4.2%, and oxygen saturation improved to 94-96% with ambulation. She was counseled to avoid further phenazopyridine use and discharged in stable condition. Discussion Methemoglobinemia should be suspected in patients with cyanosis and low oxygen saturation unresponsive to supplemental oxygen, particularly when a marked gap exists between SpO2 and PaO2. Phenazopyridine-induced cases often result from high-dose or prolonged use beyond two days. Clinical severity correlates with methemoglobin levels—cyanosis typically appears at 10%, neurologic and cardiovascular symptoms occur at 30%, and levels 50% may cause seizures, arrhythmias, or coma. Management includes discontinuation of the offending agent, oxygen therapy, and either ascorbic acid for mild or G6PD-deficient cases, or methylene blue for severe presentations. This abstract is funded by: none
Paul et al. (Fri,) studied this question.