The history is important for distinguishing methemoglobinemia between cyanosis that is due to cardiopulmonary abnormalities and that from other causes of discoloration of the skin and mucous membranes. Acute methemoglobinemia can be life threatening and usually is due to toxic exposure or drugs. Therefore, obtaining a history of exposure to substances that can induce methemoglobinemia is important. In contrast, patients with hereditary methemoglobinemia are often asymptomatic despite the presence of cyanosis. The failure of 100% oxygen to correct cyanosis is suggestive of methemoglobinemia.
Symptoms are proportional to the level of methemoglobin.
- Less than 10% methemoglobin – No symptoms
- 10-20% methemoglobin – Skin discoloration only (most notably on mucus membranes)
- 20-30% methemoglobin – Anxiety, headache, dyspnea on exertion
- 30-50% methemoglobin – Fatigue, confusion, dizziness, tachypnea, palpitations
- 50-70% methemoglobin – Coma, seizures, arrhythmias, acidosis
- Greater than 70% methemoglobin – Death