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

Abstract

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 (PaO₂) and oxygen saturation by pulse oximetry. Ultimately, co-oximetry revealed a metHb level of 20%, confirming the diagnosis of metHb. Next generation sequencing (NGS) identified a compound heterozygous missense mutation in the cytochrome B5 reductase (CYB5R3) gene: c673C>T (p.Arg225Cys) and c977A>G (p.His326Arg), both considered pathogenic/probably pathogenic. While the first mutation is documented, the second is not previously reported as a cause of congenital metHb. This compound heterozygous genotype in the CYB5R3 gene may explain the phenotype observed in this patient with congenital metHb. Erythrocyte enzyme testing confirmed reduced CYB5R3 activity. Family genetic studies confirmed the patient's compound heterozygosity, with one mutation inherited from each parent and genetic counseling was offered. The patient has progressed favorably, achieving appropriate psychomotor development without neurological disorders. There has been a decrease in perioral cyanosis, with metHb levels dropping to 3%. Oxygen saturation reached normal levels (96%) by 2 years of age.

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 PaO₂, after excluding other more common causes such as respiratory, infectious and cardiological conditions.