A new mutation of congenital methemoglobinemia exacerbated after methylene blue treatment

Methylene blue is useful for the treatment of methemoglobinemia. However, even after the patient's methemoglobin (metHb) rate has improved, careful observation is important because they could have undiagnosed congenital methemoglobinemia. In this case, a 67‐year‐old man underwent gastrointestinal endoscopy with the use of lidocaine for local anesthesia. During the examination, he complained of dyspnea and had low SpO2 despite normal PaO2 and SaO2. He was transferred to our department as a suspected case of acquired methemoglobinemia