AB019. Hereditary methemoglobinemia case: estimation of tissue oxygenation

Background: This case report highlights the fact that clinical suspicion is the key prerequisite for methemoglobinemia diagnosis and should be high in every case of peripheral cyanosis with normal PaO₂.

Methods: We present the case of an adult patient presenting with clinical features suggestive of asthmatic bronchitis, due to respiratory tract infection, a low pulse oximetry reading and mild peripheral cyanosis of the fingernail beds. His physical examination and the concomitant clinical investigation (spirometry, cardiology evaluation, computed tomography angiography) did not lead to findings consistent with the low pulse oximetry reading (SpO₂). Subsequently, since the findings of the analysis of the arterial blood gas were normal, the patient was prescribed medication for his asthmatic bronchitis and was discharged. On his re-evaluation, despite clinical improvement, a low SpO₂ 86% was detected again, leading to a new ABGs analysis on a different blood gas analyzer. The results revealed a high methemoglobin (MethHb) level of 13%, suggestive of methemoglobinemia, along with normal SaO₂ and PaO₂. Consequently, the patient was referred to a haematology specialist for further evaluation.

Results: Hereditary methemoglobinemia diagnosis was reached on the basis of an elevated MethHb level in blood gas analysis on two different time points, with no prior oxidising agent exposure, normal physical and laboratory testing, and in view of the peripheral cyanosis preexistent for several years. Genetic confirmation of the diagnosis is not available in any laboratory in our country.

Conclusions: Presence of a wide discrepancy in haemoglobin oxygen saturation, as assessed by pulse oximetry (SpO₂) and arterial blood gas analysis (SaO₂), along with normal arterial blood partial pressure of oxygen (PaO₂) are findings highly suggestive of elevated methemoglobin concentration. Measurements of SpO₂ and SaO₂ are, due to technical reasons, unreliable in cases of methemoglobinemia, either hereditary or due to exposure to an oxidizing agent, and alternative methods for tissue oxygenation estimation have to be used.

Keywords: Respiratory infections; methemoglobinemia; peripheral cyanosis