Subacute combined degeneration

Alternative names:

SCD; subacute combined degeneration of the spinal cord

Definition:

A disorder caused by vitamin B12 deficiency and characterized by weakness and abnormal sensations, mental abnormalities, and vision difficulties.

Causes, incidence, and risk factors:

Subacute combined degeneration of the spinal cord is a result of vitamin B12 deficiency. Lack of this vitamin may also cause pernicious anemia, a blood disorder. Although pernicious anemia is more common, subacute combined degeneration may occur in people without symptoms of anemia. These two disorders may be caused by a dietary lack of vitamin B12 (rare), but they more commonly result from inability to absorb the vitamin from the intestines.

Subacute combined degeneration primarily affects the spinal cord, but it also includes damage to the brain, the nerves of the eye, and the peripheral (body) nerves. There is diffuse damage to the nerves. Initially, there is damage to the myelin sheath (the covering of the nerve that speeds transmission of nerve impulses), followed by destruction of the axon portion of the cell and eventual destruction of the entire nerve cell. The mechanism by which lack of B12 damages nerves is not clear. It is thought that lack of this vitamin causes an abnormal formation of fatty acids, which are used by the body to build cell membranes and the nerve myelin sheath.

The disorder affects approximately 1 out of 10,000 people. Risks include lack of dietary B12 (rare), inherited or acquired lack of the factor needed to absorb B12 from the intestines, and disorders of the small intestine, such as Crohn's disease. Risks also include pernicious anemia and malabsorption (inadequate absorption of nutrients from the intestinal tract) syndrome, which can occur after gastrointestinal surgery.