Definition of Pernicious anemia:

Pernicious anemia is a decrease in red blood cells that occurs when the body cannot properly absorb vitamin B12 from the gastrointestinal tract. Vitamin B12 is necessary for the formation of red blood cells.

Pernicious anemia is a type of megaloblastic anemia.

See also: Anemia

Causes, incidence, and risk factors:

Pernicious anemia is caused by a lack of intrinsic factor. Intrinsic factor is a protein produced by the stomach that helps the body absorb vitamin B12. When stomach does not have enough intrinsic factor, it cannot properly absorb the vitamin. Nerve and blood cells need vitamin B12 to function properly.

Very rarely, infants and children are born without the ability to produce enough intrinsic factor. Congenital (born with) pernicious anemia is inherited as an autosomal recessive disorder (you need the defective gene from each parent to get it). Most often, however, pernicious anemia and other forms of megaloblastic anemia in children result from other causes of vitamin B12 deficiency or other vitamin deficiencies.

Other causes of low levels of intrinsic factor (and thus of pernicious anemia) include atrophic gastric mucosa, autoimmunity against gastric parietal cells, and autoimmunity against intrinsic factor.

The onset of the disease is slow and may take decades. Although the congenital form occurs in children, pernicious anemia usually does not appear before age 30. The average age at diagnosis is 60.

Risk factors include:

• Family history of the disease
• Scandinavian or Northern European descent
• History of autoimmune endocrine disorders, including type 1 diabetes, hypoparathyroidism, Addison's disease, hypopituitarism, testicular dysfunction, Graves disease, chronic thyroiditis, myasthenia gravis, secondary amenorrhea, and vitiligo

In addition to pernicious anemia, other causes of vitamin B12 deficiency include:

• Certain drugs, including colchicine, neomycin, and para amino salicylic acid used for tuberculosis treatment
• Gastrointestinal disease (stomach removal surgery, celiac disease, Crohn's disease)
• Infection (intestinal parasites, bacterial overgrowth)
• Metabolic disorders (methylmalonic aciduria, homocystinuria)
• Nutritional problems (strict vegetarians without B12 supplementation, poor diet in infant, or poor nutrition during pregnancy)