Subacute combined degeneration of the spinal cord is a disorder that involves weakness, abnormal sensations, mental problems, and vision difficulties.

Subacute combined degeneration of the spinal cord is caused by a vitamin B12 deficiency. (For specific information on vitamin B12 deficiency, see the article on pernicious anemia.)

Subacute combined degeneration primarily affects the spinal cord, but it can also damage the brain, the nerves of the eye, and the peripheral (body) nerves. At first, the disease damages the covering of the nerves (the myelin sheath), which speeds nerve signaling. It later affects the entire nerve cell.

How a lack of vitamin B12 damages nerves is unclear. However, experts believe the lack of this vitamin causes abnormal fatty acids to form around cells and nerves.

You have a higher risk for this condition if you cannot absorb vitamin B12 from the intestines or if you have:

• Pernicious anemia
• Disorders of the small intestine, including Crohn's disease
• Malabsorptive conditions, which can occur after gastrointestinal surgery

Symptoms include:

• Abnormal sensations (tingling and numbness)
• Weakness of the legs, arms, middle of the body, or other areas

These symptoms slowly get worse and are usually felt on both sides of the body.

Other symptoms include:

• Clumsiness, stiff, or awkward movements
• Unsteady gait
• Change in mental state such as irribility, apathy, confusion, ordementia
• Decreased vision
• Depression
• Sleepiness
• Speech impairment (possible)

The health care provider will perform a physical exam. An exam usually shows muscle weakness and sensation disturbances on both sides of the body. Reflexes are often abnormal. Muscles may develop spasticity. You may have a reduced sense of touch, pain, and temperature.

Mental changes range from mild forgetfulness to complete dementia or psychosis. Severe dementia is uncommon, but in some cases, it is the first symptom of the disorder.

An eye exam may show damage to the optic nerve, a condition called optic neuritis. Signs of nerve inflammation may be seen during a retinal exam. There may also be abnormal pupil responses, decreased visual acuity, and other changes.

Blood tests, including acomplete blood count (CBC), are used to diagnose anemia or a B12 deficiency.

Early treatment improves the chances of a good outcome, so the disease should be treated promptly.

Vitamin B12 is given, usually by injection into a muscle. Injections are often given one a day for a week, then weekly for about 1 month, and then monthly. Vitamin B12 injections must continue throughout life to prevent symptoms from returning.

How well a person does depends on how long they had symptoms before receiving treatment. If treatment was received within a few weeks, complete recovery usually occurs. If treatment was delayed for longer than 1 or 2 months, recovery isn't as complete.

If you had symptoms for many months without receiving medical care, treatment may slow or stop your symptoms, but it is unlikely that you will recover function that you have already lost.

Untreated, the disorder results in progressive and irreversible damage to the nervous system.

Complications can include permanent, progressive loss of nerve functions.

Call your health care provider if abnormal sensations, muscle weakness, or other symptoms of subacute combined degeneration develop, particularly if there is a personal or family history of pernicious anemia or other risk factors. Early diagnosis and treatment improve the chance of a good outcome.

There is no known prevention. However, treatment prevents symptoms from returning.