Subacute combined degeneration (SCD) is a disorder of the spinal cord, brain, and nerves. It causes weakness, abnormal sensations, mental problems, and vision difficulties.

Subacute combined degeneration of the spinal cord; SCD

SCD 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.

Health care providers 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:

• Pernicious anemia, a condition in which the body does not have enough healthy red blood cells due to vitamin B12 deficiency
• Disorders of the small intestine, including Crohn disease
• Problems absorbing nutrients, which can occur after gastrointestinal surgery, including weight loss surgery
• Use of nitrous oxide (laughing gas), especially if used as a recreational drug

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.

As the disease worsens, symptoms may include any of the following:

• Clumsiness, stiff or awkward movements.
• Mental changes, which can range from mild memory problems, irritability, apathy, and confusion, to severe dementia and psychosis. Severe dementia is uncommon, but in some cases it's the first symptom of the disorder.
• Decreased vision.
• Depression.
• Sleepiness.
• Unsteady gait and loss of balance.
• Falls due to poor balance.

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.

Early treatment improves the chance of a good outcome.

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, SCD results in continued and permanent damage to the nervous system.

Contact your provider if abnormal sensations, muscle weakness, or other symptoms of SCD 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 help prevent SCD.

Published Date: April 16, 2025
Published By: Joseph V. Campellone, MD, Department of Neurology, Cooper Medical School at Rowan University, Camden, NJ. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.

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