Case report: neonatal cyanosis secondary to congenital methemoglobinemia, a cause to consider in newborn cyanosis.

Background: Congenital methemoglobinemia (metHb), a rare cause of neonatal cyanosis, results from the oxidation of ferrous to ferric iron in hemoglobin (Hb). The aim of this case report is to highlight the need to broaden the differential diagnosis of neonatal cyanosis and emphasizes the role of capillary blood gas analysis and genetic testing to exclude hereditary hematological causes. We report a previously unreported genetic mutation associated with congenital metHb. Case Description: We report the case of a term male neonate with cyanosis unresponsive to oxygen administration. Complementary studies were normal, including echocardiography, cerebral ultrasound, blood tests, blood culture, cytomegalovirus testing, glucose-6-phosphate dehydrogenase (G6PD) assessment, and Hb electrophoresis, with the exception of a mild left anterior pneumothorax. There was a discrepancy between arterial oxygen pressure (PaO Conclusions: MetHb is a rare cause of cyanosis in children. The acquired form is the most common, due to exposure to oxidizing substances that increase metHb production. Congenital forms, however, are more frequent in neonates, and their evolution depends on specific mutations. Genetic testing is crucial for family counseling. Clinicians should suspect metHb in cases of cyanosis and hypoxia without respiratory distress, especially when there is no improvement with oxygen therapy and normal PaO