Subacute combined degeneration

Subacute combined degeneration is a disorder of the spine, brain, and nerves 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 its effects on the brain and the peripheral (body) nerves are the reason for the term “combined”. At first, the disease damages the covering of the nerves (the myelin sheath). 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, or other areas

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

Other symptoms include:

Exams and Tests

The health care provider will perform a physical exam. An exam usually shows muscle weakness and sensation disturbances on both sides of the body, especially in the legs. 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 severe 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 a complete blood count (CBC), vitamin B12 blood level, or methylmalonic acid blood level are used to diagnose anemia or vitamin 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 once a day for a week, then weekly for about 1 month, and then monthly. Vitamin B12 supplements either by injection or high dose pills must continue throughout life to prevent symptoms from returning.

Outlook (Prognosis)

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. Any symptom improvement occurs within a year of treatment.

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

Possible Complications

Complications can include permanent, progressive loss of nerve and mental functions.

When to Contact a Medical Professional

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.


Some strict vegetarian diets may be low on vitamin B12 and supplementation can prevent the disorder.

Alternative Names

Subacute combined degeneration of the spinal cord; SCD


So YT, Simon RP. Deficiency diseases of the nervous system. In: Bradley WG, Daroff RB, Fenichel GM, Jankovic J, eds. Bradley: Neurology in Clinical Practice. 5th ed. Philadelphia, Pa: Butterworth-Heinemann Elsevier; 2008:chap 61.

Update Date: 4/30/2012

Reviewed by: Kevin Sheth, MD, Department of Neurology, University of Maryland School of Medicine, Baltimore, MD. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine;David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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