Case conference

Methemoglobinemia secondary to aniline exposure

Acquired methemoglobinemia may cause cyanosis and tissue ischemia unresponsive to oxygen supplementation.

We performed a literature search to identify cases of acquired methemoglobinemia published between 1980 and 2020. Clinical, diagnostic, and treatment details were extracted from eligible cases.

A total of 76 reports involving 87 cases were analyzed. The median age at presentation was 32.5 with male to female ratio of 1.6. Cyanosis and SpO₂ <90 % were reported in 82 % and 60 % of cases, respectively. Dapsone or cocaine-based anesthetics were causative in 52 % of cases; most anesthetic-related cases occurred in the peri-procedural setting. Methylene blue (MB) and red cell transfusion were given in 71 % and 10 % of cases, respectively. Compared to MB untreated patients, MB treated patients were more likely to be cyanotic (91.9 % vs 54.2 %), had higher proportions (%) and levels (g/dL) of methemoglobin (MetHb) – 33.2 % vs 15.3 % and 3.1 g/dL vs 1.2 g/dL, respectively. We found that among cyanotic cases, the median MetHb level was 3.0 g/dL (0.4–12.3 g/dL) with 74 % of values $\ge$ 1.5 g/dL. An SaO2:SpO₂ ratio of >1 was not universally present, but always coincided with an [SaO₂-SpO₂] delta value greater than zero.

Cyanosis and hypoxemia were not universal findings of acquired methemoglobinemia in our series. In addition, not all patients had cyanosis at MetHb $\ge$ 1.5 g/dL or an SaO2:SpO₂ ratio of >1. All those with an SaO₂:SpO₂ >1 did, however, have a delta value greater than zero – a finding not previously reported which we feel holds diagnostic value.

Methemoglobinemia is an uncommon cause of cyanosis. A 28-year-old male presented to the emergency department cyanotic and short of breath after exposure to noxious automobile fumes. He did not improve with the administration of 100% oxygen therapy. The initial arterial blood gas with cooximetry was: pH of 7.38, Paco₂ of 43 mm Hg, Pao₂ of 118 mm Hg, measured oxygen saturation of 70%, and a methemoglobin level of 24.8%. Methylene blue was given (2 mg/kg intravenously) and the patient's symptoms resolved. On the following day he was discharged home without complication. A comprehensive review of the literature revealed no reported cases of methemoglobinemia secondary to accidental exposure to exhaust fumes.