Abstract Introduction Type 1 CYB5R3 deficiency is a rare cause of hereditary methemoglobinemia. Although it is typically diagnosed during childhood, it may also be identified in older adults. Case A 62-year-old male patient presenting with cyanosis and shortness of breath was diagnosed with Type 1 CYB5R3 deficiency. The diagnosis was supported by the chocolate-brown appearance of arterial blood, a methemoglobin (metHb) level of 17%, hypoxemia inconsistent with pulse oximetry findings, and the identification of a homozygous pathogenic mutation in CYB5R3 on sequence analysis. The patient is currently being followed with supplemental oxygen therapy and vitamin C treatment. Conclusion In patients presenting with cyanosis and hypoxemia, arterial blood gas analysis should be evaluated for methemoglobinemia, as pulse oximetry may provide misleadingly high oxygen saturation readings. Congenital causes must be considered even in elderly patients.
Afşin et al. (Fri,) studied this question.