methemoglobinemiaa

Methemoglobinemia, occasionally spelled methemoglobinemiaa, is a blood disorder in which a higher than normal fraction of hemoglobin is in the ferric (Fe3+) state as methemoglobin, reducing its ability to carry and release oxygen to tissues.

Normally less than 1% of hemoglobin is methemoglobin. Methemoglobin is continually reduced back to functional hemoglobin by enzymes, especially cytochrome b5 reductase (NADH-dependent). Excess production or impaired reduction—due to genetic defects or exposure to oxidants—causes the condition.

Causes include congenital deficiencies of cytochrome b5 reductase or HbM variants, and acquired factors such as nitrates, nitrites, aniline dyes, benzocaine, prilocaine, dapsone, sulfonamides, and other oxidizing agents.

Clinical features range from cyanosis to tissue hypoxia: headache, dizziness, fatigue, shortness of breath, tachycardia, and confusion at higher levels. Blood often looks chocolate-brown. Diagnosis uses co-oximetry showing elevated methemoglobin; pulse oximetry may be unreliable.

Treatment involves removing the triggering agent and providing supportive care. Methylene blue (1–2 mg/kg IV) is first-line for many cases, but is ineffective or harmful in G6PD deficiency. Alternatives include high-dose ascorbic acid or red-cell exchange for severe or refractory cases; in some situations, additional supportive measures may be required.

Prognosis is generally favorable with prompt treatment. Prevention focuses on avoiding known oxidants in susceptible individuals, particularly in infants and patients with congenital enzyme deficiencies.