Beyond the Blue: A Case Report on Diagnostic and Therapeutic Challenges in Infantile Methemoglobinemia With Incidental Genetic Mutation and Thrombotic Complication.

Cyanosis in early infancy that is unresponsive to oxygen therapy is a life-threatening presentation requiring prompt diagnosis and intervention. While most cases are attributed to cardiopulmonary causes, hematologic disorders such as methemoglobinemia, although rare, remain important differentials. This report describes an eight-week-old male infant presenting with profound hypoxia, severe metabolic acidosis, and cyanosis. Methemoglobinemia was confirmed with markedly elevated levels. In the absence of oxidant exposure and a normal glucose-6-phosphate dehydrogenase (G6PD) screen, methylene blue was administered empirically with rapid clinical improvement. Rapid genomic sequencing revealed no pathogenic variants causative of methemoglobinemia. The case was further complicated by incidental venous sinus thrombosis and a transiently positive direct antiglobulin test. This case highlights the diagnostic and therapeutic challenges of methemoglobinemia in early infancy without a clear etiology, the cautious use of methylene blue pending enzymatic confirmation, and the interpretative limitations of rapid genetic testing in emergent settings.