The congenital form of methemoglobinemia has an autosomal recessive pattern of inheritance.

Due to a deficiency of the enzyme diaphorase I (NADH methemoglobin reductase), methemoglobin levels rise and the blood of met-Hb patients has reduced oxygen-carrying capacity. Instead of being red in color, the arterial blood of met-Hb patients is brown. This results in the skin of Caucasian patients gaining a bluish hue. Hereditary met-Hb is caused by a recessive gene. If only one parent has this gene, offspring will have normal-hued skin, but if both parents carry the gene there is a chance the offspring will have blue-hued skin.

Another cause of congenital methemoglobinemia is seen in patients with abnormal hemoglobin variants such as hemoglobin M (HbM), or hemoglobin H (HbH), which are not amenable to reduction despite intact enzyme systems.

Methemoglobinemia can also arise in patients with pyruvate kinase deficiency due to impaired production of NADH - the essential cofactor for diaphorase I. Similarly, patients with Glucose-6-phosphate dehydrogenase (G6PD) deficiency may have impaired production of another co-factor, NADPH.

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