Late-Onset Neonatal Cyanosis Secondary to Methemoglobinemia in a Preterm, Small-for-Gestational-Age Infant: A Case Report.

Methemoglobinemia is a rare cause of neonatal cyanosis stemming from hemoglobin iron oxidation, creating susceptibility due to immature enzymatic systems in newborns. This case study details a 32-week preterm infant presenting with unexplained desaturations (oxygen saturation (SpO2): 82-90%) on day of life 13, which were resolved with intravenous methylene blue and oral vitamin C after co-oximetry confirmed 18% methemoglobin. Prompt diagnosis and low-dose targeted therapy are essential for managing this condition in fragile, preterm neonates with persistent central cyanosis.