Symptomatic-low grade methemoglobinemia because of dapsone: a multiple hit hypothesis.

Methemoglobinemia occurs when hemoglobin is oxidized to form methemoglobin (MetHb) rendering it incapable of oxygen transport and leading to tissue hypoxia. Typically, protective mechanisms keep MetHb levels below 1%, but a variety of compounds are capable of inducing methemoglobinemia including dapsone. Normally, symptoms correlate with MetHb level and treatment with methylene blue is reserved for patients with significantly elevated MetHb levels. We present a case of symptomatic low-grade methemoglobinemia because of dapsone therapy. This case illustrates a multiple hit hypothesis where comorbid conditions prompt symptoms at a low MetHb level. Understanding the impact of comorbid conditions in exacerbating low-grade methemoglobinemia is important in diagnosing and managing this condition. Treatment with methylene blue should be guided by the whole clinical picture rather than by the MetHb level alone.