Under these conditions, oxygen binding by hemoglobin is severely impaired. The diagnosis of methemoglobinemia should be considered whenever cyanosis occurs in the absence of demonstrable cardiac or pulmonary disease. The disturbance in the normal balance between ferrous and ferric iron may be the effect of alterations of hemoglobin structure, abnormalities of red cell enzymes or exposure to oxidant drugs or chemicals. Inherited forms of methemoglobinemia may be characterized by chronic cyanosis. When acute methemoglobinemia results from an oxidant stress, oxygen delivery may be severely compromised and the patient becomes acutely ill.
Symptoms depend on the concentration of methemoglobin.
When methemoglobin comprises approximately 10 to 30 percent of the total hemoglobin, only cyanosis occurs. In cases where anemia is also present, oxygen delivery is further compromised and toxicity may be more severe at lower concentrations of methemoglobinemia. Accurate diagnosis and rapid therapy will prevent serious damage. Diagnosis of methemoglobinemia should be strongly suspected when oxygen administration fails to affect cyanosis. Treatment depends on clinical severity.