Subacute combined degeneration is caused by vitamin B12 deficiency. It mainly 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 nerve covering (myelin sheath) is damaged. Later, the entire nerve cell is affected.
Doctors do not know exactly how a lack of vitamin B12 damages the nerves. It is possible that the lack of this vitamin causes abnormal fatty acids to form around cells and nerves.
People are at high risk for this condition if vitamin B12 cannot be absorbed from their intestine or if they have:
These symptoms slowly get worse and are usually felt on both sides of the body.
As the disease worsens, symptoms may include any of the following:
Exams and Tests
The health care provider will perform a physical exam. The exam usually shows muscle weakness and sensation problems on both sides of the body, especially in the legs. Knee jerk reflexes are often decreased. Muscles may develop spasticity. There may be reduced senses 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 that may be ordered include:
Early treatment improves the chance of a good outcome.
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.
How well a person does depends on how long they had symptoms before receiving treatment. If treatment is received within a few weeks, complete recovery may be expected. If treatment is delayed for longer than 1 or 2 months, full recovery may not be possible.
Untreated, the disorder results in continued and irreversible damage to the nervous system.
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. This is particularly important if you or a family member has had pernicious anemia or other risk factors.
Some vegetarian diets, especially vegan, may be low in vitamin B12. Taking a supplement can prevent the disorder.
Subacute combined degeneration of the spinal cord; SCD
So YT, Simon RP. Deficiency diseases of the nervous system. In: Daroff RB, Fenichel GM, Jankovic J, Mazziotta JC, eds. Bradley's Neurology in Clinical Practice. 6th ed. Philadelphia, PA: Elsevier Saunders; 2012:chap 57.
Update Date 6/1/2015
Updated by: Daniel Kantor, MD, Kantor Neurology, Coconut Creek, FL and Immediate Past President of the Florida Society of Neurology (FSN). Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.