Subacute combined degeneration is progressive degeneration of the spinal cord due to vitamin B₁₂ deficiency. Symptoms include muscle weakness, clumsiness, tingling, and other abnormal sensations.

This disorder affects about 1 of 10,000 people, usually those older than 40. It is due to a deficiency of vitamin B₁₂, which usually also causes pernicious anemia. Usually, the deficiency is not related to diet but to the body's inability to absorb vitamin B₁₂.

Vitamin B₁₂ is necessary for the formation and maintenance of a fatty sheath (myelin sheath) that surrounds some nerve cells and that speeds transmission of nerve signals. In subacute combined degeneration, the sheath is damaged, causing sensory and motor nerve fibers from the spinal cord to degenerate. The brain, nerves of the eyes, and peripheral nerves are sometimes also damaged.

Symptoms

The disorder begins with a general feeling of weakness. Tingling, a pins-and-needles sensation, and numbness are felt in both hands and feet. These sensations tend to be constant and to gradually worsen. People may not be able to feel vibrations and may lose the sense of where their limbs are (position sense). The limbs feel stiff, movements become clumsy, and walking may become difficult. Reflexes may be decreased, increased, or absent. Vision may be reduced.

People who have this disorder may become irritable, apathetic, drowsy, suspicious, and confused. Their emotions may change rapidly and unpredictably. Rarely, dementia develops.

Diagnosis and Treatment

Blood tests to measure levels of vitamin B₁₂ can confirm the deficiency.

Recovery is more likely if the disorder is treated early. When treated within a few weeks after symptoms appear, most people recover completely. If treatment is delayed, the progression of symptoms may be slowed or stopped, but full recovery of lost function is less likely.

Most people are immediately given injections of vitamin B₁₂, which are continued indefinitely to prevent symptoms from recurring. Large doses of vitamin B₁₂ taken by mouth can be used if the deficiency is mild and symptoms of nerve damage have not developed.

Last full review/revision August 2007 by Michael Rubin, MD