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Introduction <p> 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.</p> Case <p> 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.</p> Conclusion <p> 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.</p>

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Hereditary methemoglobinemia diagnosed in adulthood: Type 1 cytochrome B5 reductase deficiency

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  • 发表日期：

    2026年6月19日

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  • Emine Afşin,
  • Muzaffer Şenol,
  • Ayşe Gürel,
  • Emine Durak

摘要

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.