The presence of methemoglobin in the blood, resulting in cyanosis. a small amount of methemoglobin is present in the blood normally, but injury or toxic agents convert a larger proportion of hemoglobin into methemoglobin, which does not function reversibly as an oxygen carrier. methemoglobinemia may be due to a defect in the enzyme nadh methemoglobin reductase (an autosomal recessive trait) or to an abnormality in hemoglobin m (an autosomal dominant trait). (dorland, 27th ed)

Methemoglobinemia, or methaemoglobinaemia, is a condition of elevated methemoglobin in the blood. Symptoms may include headache, dizziness, shortness of breath, nausea, poor muscle coordination, and blue-colored skin (cyanosis)...