Methemoglobinemia

By Ilene B. Anderson, PharmD; and Susan Y. Kim, PharmD

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Clinical Presentation

The severity of signs and symptoms seen in Mhgb is directly proportional to the percentage of hemoglobin that has been oxidized to methemoglobin. The higher the methemoglobin formation, the more severe the findings. At concentrations < 10 to 15% methemoglobin, there may be no obvious abnormality other than cyanosis by physical examination. In Mhgb, the skin classically has a gray-blue discoloration. With higher methemoglobin concentrations, symptoms of anoxia become more pronounced. At Mhgb levels > 30 to 40%, the patient may complain of headache, dizziness, fatigue, tachycardia, exertional dyspnea, tachypnea, and blurred vision. At even higher levels, anaerobic conditions are associated with severe metabolic acidosis, weakness, dyspnea, and altered mental status. At methemoglobin levels > 60 to 70%, symptoms may include seizures, coma, cardiovascular collapse, and death.^11,12

The methemoglobin concentrations and corresponding symptoms described above apply to normal, healthy, nonanemic individuals. However, patients who are anemic, are immunocompromised, or have underlying cardiac, pulmonary, or hematologic conditions are at risk for more severe symptoms at lower methemoglobin concentrations. First and foremost, the anemic host has an even smaller amount of available hemoglobin, further compromising tissue oxygenation. For any given percentage of methemoglobin concentration, the concomitant presence of anemia dictates that, in absolute terms, a smaller amount of hemoglobin is available to oxygenate tissues. Thus, the methemoglobin concentration reported as a percentage out of context does not reflect the potential severity of the physiologic compromise that is present.

 

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