methemoglobinememi

Methemoglobinemia is a disorder characterized by elevated levels of methemoglobin in the blood. Methemoglobin is an oxidized form of hemoglobin in which iron is ferric (Fe3+), and thus cannot bind oxygen effectively. Normally, small amounts are continuously reduced back to functional hemoglobin by enzymes such as NADH-cytochrome b5 reductase. When production exceeds reduction, tissues may suffer from hypoxia despite adequate oxygen in the lungs.

Causes of methemoglobinemia are divided into congenital and acquired. Congenital forms arise from deficiencies of methemoglobin reductase or from hemoglobin variants (Hb M). Acquired methemoglobinemia results from exposure to oxidizing agents such as nitrates and nitrites, certain drugs (for example, benzocaine, lidocaine, dapsone, nitrates in well water, phenazopyridine), and some chemicals. Individuals with glucose-6-phosphate dehydrogenase deficiency may be more susceptible to hemolysis if treated with certain therapies.

Symptoms reflect reduced oxygen delivery and may include cyanosis, shortness of breath, headaches, fatigue, dizziness, and tachycardia. In severe cases, confusion, seizures, or coma can occur. Blood often appears chocolate-brown rather than bright red. Diagnosis is typically confirmed by co-oximetry or spectrophotometric analysis that directly measures methemoglobin levels, along with clinical assessment.

Treatment focuses on removing the source of oxidant exposure and correcting hypoxia. Oxygen therapy is supportive. Most cases respond to methylene blue given intravenously (usually 1–2 mg/kg); however, methylene blue is contraindicated in individuals with G6PD deficiency and can cause hemolysis. Alternatives include high-dose vitamin C and, in severe or refractory cases, exchange transfusion or hyperbaric oxygen.