Severe Prilocaine-induced Methemoglobinemia after Perianal Infiltration in an Undiagnosed Anemic Adult

Methemoglobinemia is an uncommon but potentially life-threatening cause of tissue hypoxia in the emergency Department (ED), often presenting with a characteristic mismatch between low pulse oximetry readings and normal arterial oxygen tension. Prilocaine is a well-recognized trigger due to its oxidizing metabolite o-toluidine, and risk is further amplified when high volumes are infiltrated into highly vascular mucosal sites.We describe a 30-year-old woman who presented to the ED one hour after external hemorrhoidectomy under local anesthesia with sudden-onset weakness, dizziness, and marked central and peripheral cyanosis. She had received 800 mg of prilocaine (≈11.4 mg/kg) and 15 mg of bupivacaine via perianal infiltration. On arrival, she was hemodynamically stable but had SpO₂ 75% on room air, unresponsive to oxygen. Arterial blood gas revealed pH 7.41, PaO₂ 110 mmHg, and lactate 3.8 mmol/L; co-oximetry showed MetHb 38%. Laboratory tests demonstrated microcytic iron-deficiency anemia (Hb 7.3 g/dL). Intravenous methylene blue 100 mg (≈1.4 mg/kg) was administered over 5 minutes with rapid clinical improvement; MetHb fell to 15% at 6 hours and 6.5% at 24 hours without evidence of hemolysis.This case highlights a high-risk triad of mucosal prilocaine use, weight-based overdose, and unrecognized anemia, underscoring the need for strict dosing vigilance, early consideration of dyshemoglobinemia, and timely antidotal therapy in the ED.