Methemoglobinemia is a rare, potentially fatal, condition caused by elevated levels of methemoglobin in the blood. We report the case of a 27yr old woman with a history of vasculitis who developed methemoglobinemia at 34 weeks gestation.
Patient had been treated seven years prior to pregnancy for leukocytoclastic vasculitis with dapsone. Dapsone (4,4'-Diaminodiphenylsulphone) is an antibiotic and anti-inflammatory drug with known serious side effects. This patient developed acute dapsone hypersensitivity syndrome leading to neutropenic sepsis and intensive care admission. She was also treated for recurrent erythema multiforme treated with valaciclovir.
In this pregnancy, the patient booked with a history of a recent rash. At 31 weeks, she was noted to have low oxygen saturations after ingesting a throat lozenge containing benzocaine. Investigations revealed methemoglobinemia and she was treated with oxygen and vitamin C. Her rash worsened and she developed swollen painful joints consistent with a vasculitic flare and was commenced on prednisolone. Both the vasculitis and methemoglobinemia worsened and she became hypoxic with oxygen saturations of 91% on room air and a methemoglobin level of 12.1% (normal value: ~1%), requiring transfer to the High Dependency Unit. She was delivered by caesarean section at 34 weeks gestation. Her condition improved rapidly after delivery and she was discharged home on day 5 post LSCS with advice to avoid triggers of metHb.
Methemoglobin is an altered state of haemoglobin in which the ferrous (Fe++) irons of heme are oxidized to the ferric (Fe+++) state, which is unable to bind oxygen in a reversible fashion1. This leads to a left shift in the oxygen-haemoglobin dissociation curve. The remaining oxy-haemoglobin has increased oxygen affinity, while the circulating methemoglobin-containing haemoglobin molecules cannot release their oxygen – both of which result in tissue hypoxia.
Pre-existing conditions such as anaemia, heart disease, respiratory disorders, and coexistent glucose-6-phosphate dehydrogenase (G6PD) deficiency may exacerbate the toxicity of methemoglobinemia.