Subacute combined degeneration

Alternative names
Subacute combined degeneration of the spinal cord; SCD

Subacute combined degeneration of the spinal cord is a disorder caused by vitamin B-12 deficiency and characterized by weakness and abnormal sensations, mental problems, and vision difficulties.

Causes, incidence, and risk factors

Subacute combined degeneration of the spinal cord is a result of vitamin B-12 deficiency. Lack of this vitamin is often caused by pernicious anemia, an autoimmune blood disorder that prevents absorption of vitamin B-12.

Most often, anemia symptoms are the most common symptoms of the disorder, but subacute combined degeneration can also occur without those symptoms.

While vitamin B-12 deficiency can rarely be caused by diet (vegans, who eat no dairy or meat products, are most susceptible), the most common cause is an inability to absorb the vitamin from the intestines.

Subacute combined degeneration primarily affects the spinal cord, but it can also include damage to the brain, the nerves of the eye, and the peripheral (body) nerves. The damage is throughout the body. 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 eventually, destruction of the entire nerve cell.

How lack of vitamin B-12 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.

Risks include lack of dietary vitamin B-12 (rare), inherited or acquired lack of the factor needed to absorb vitamin B-12 from the intestines, antibodies to this factor (pernicious anemia) which prevents its absorption, and disorders of the small intestine, such as Crohn’s Disease as well as other malabsorptive conditions which can occur after gastrointestinal surgery.


  • abnormal sensations of the legs, arms, trunk, or other area       o tingling, “pins and needles”, or “shock-like” sensations       o numbness       o these symptoms progressively worsen       o usually symmetrical (on both sides of the body)  
  • weakness of the legs, arms, trunk, or other area       o progressively worsens       o usually symmetrical  
  • unsteady gait  
  • clumsiness, stiff, or awkward movements  
  • change in mental state       o irritability       o apathy       o sleepiness       o suspiciousness, paranoia       o confusion, disorientation, delirium       o Depression       o Dementia, intellectual deterioration  
  • vision changes       o decreased vision

Speech impairment may also be associated with this disease.

Signs and tests

An examination usually shows symmetrical sensorimotor polyneuropathy, with muscle weakness and sensation disturbance. Reflexes are often abnormal. Muscles may develop spasticity. There is a loss of position sense (awareness of body position) and sense of touch, pain, and temperature may be diminished.

Mental changes are variable, from mild forgetfulness to complete Dementia or psychosis. Severe dementia is uncommon, but in some cases, it is the initial symptom of the disorder. An eye examination may indicate damage to the optic nerve (optic neuritis), with abnormal pupil responses, inflammation or atrophy of the optic nerve (visible on retinal exam), decreased visual acuity, and other changes.

  • CBC (complete blood count) may show anemia. If Folic acid has been used to treat anemia, neurologic symptoms may develop even if the anemia is no longer evident and this may make diagnosis difficult.  
  • Serum (blood) levels of vitamin B-12 are low.

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

Vitamin B- 12 is given, usually by intramuscular injection, in large doses. Injections are often given daily for about 1 week, then weekly for about 1 month, then monthly. Vitamin B-12 injections must continue throughout life to prevent recurrence.

Expectations (prognosis)

Response to treatment depends on how long symptoms have been present before treatment. Treatment within a few weeks usually results in complete recovery. If symptoms have been present for longer than 1 or 2 months, partial recovery can be expected.

If symptoms have been present for many months, progression of symptoms may be slowed or stopped, but recovery of lost function is unlikely. Vitamin B-12 injections must be continued for life or symptoms will recur.

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

Complications can include permanent, progressive loss of nerve functions.

Calling your health care provider
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.

None is known, but treatment of the disorder prevents recurrence.

Johns Hopkins patient information

Last revised: December 4, 2012
by Harutyun Medina, M.D.

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