Incidental perioperative detection of congenital methemoglobinemia: A prospective case series from a South Indian referral centre.

Background: Congenital methemoglobinemia is a rare inherited disorder caused by impaired reduction of methemoglobin to functional hemoglobin, resulting in functional hypoxia despite normal arterial oxygen tension. Due to its often-mild clinical phenotype, diagnosis is frequently delayed or made incidentally. Methods: This prospective case series of surgical patients was conducted at a tertiary care center in South India over a one-year period. Patients incidentally diagnosed with methemoglobinemia during perioperative or acute medical evaluation were included. Diagnosis was established using arterial blood gas analysis with co-oximetry and confirmed biochemically by erythrocyte NADH-cytochrome b5 reductase activity. Results: Ten surgical patients were identified, all originating from a single geographic region. Methemoglobin levels ranged from 14.4% to 29.1%. Most patients were asymptomatic or mildly symptomatic, with diagnosis prompted by refractory hypoxemia and a characteristic saturation gap. Enzyme assays confirmed Type I congenital methemoglobinemia in all cases. Management was largely supportive, with methylene blue reserved for symptomatic individuals. Conclusions: This case series highlights incidental perioperative detection as a key diagnostic opportunity and suggests possible regional clustering of congenital methemoglobinemia. Increased clinical awareness and targeted screening may improve the diagnosis of this underrecognized condition.