methemoglobinæmi

Methemoglobinemia is a condition characterized by elevated levels of methemoglobin, a form of hemoglobin in which the iron is ferric (Fe3+) and cannot effectively bind or release oxygen. This impairs oxygen transport and delivery to tissues. The remaining normal hemoglobin may hold onto oxygen more tightly, shifting the oxygen dissociation curve to the left and worsening tissue hypoxia even when arterial oxygen tension is normal.

Causes of methemoglobinemia are divided into congenital and acquired. Congenital forms result from defects in the enzyme cytochrome b5 reductase (methemoglobin reductase) or from hemoglobin variants known as hemoglobin M. Acquired methemoglobinemia occurs after exposure to oxidizing drugs and chemicals, including dapsone, benzocaine, prilocaine, nitrates and nitrites, aniline dyes, chlorates, and certain antibiotics. Neonates and young infants are particularly susceptible due to lower reductive capacity.

Clinical features depend on methemoglobin percentage. Mild cases may present with cyanosis and chocolate-brown colored blood without severe symptoms. Moderate levels can cause headache, dizziness, fatigue, and tachycardia. Higher levels (roughly 50–70%) may lead to confusion, dyspnea, and hypotension; very high levels can be life-threatening. Diagnosis is made by co-oximetry to quantify methemoglobin. Pulse oximetry may be unreliable and arterial blood gas often shows a normal or near-normal PaO2 with reduced functional oxygen saturation.

Treatment focuses on removing the offending agent and supporting oxygen delivery. Mild cases may be observed. For significant symptoms or high methemoglobin levels, antidotal therapy with methylene blue (1–2 mg/kg IV) is used, repeating once if necessary; methylene blue should be avoided in individuals with G6PD deficiency. In such cases or when methylene blue is ineffective, alternatives include high-dose vitamin C, exchange transfusion, or hyperbaric oxygen. Prognosis is generally good with prompt treatment; congenital forms require ongoing management and avoidance of triggers.