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# Subacute combined degeneration (vitamin B12)

[Spinal cord](https://durgical.com/en/anatomy/spinal-cord/) damage caused by vitamin B12 deficiency, with loss of position sense, unsteady gait and weakness. Early treatment can reverse it.

Updated 2026-11-01

## Structures involved

Tap one to see what it is and open it in 3D.

- [Spinal cord](https://durgical.com/en/anatomy/spinal-cord/)
- [Dorsal root ganglia](https://durgical.com/en/anatomy/spinal-ganglion/)
- [Mid-back nerve roots](https://durgical.com/en/anatomy/anterior-root-of-spinal-nerve-spine-and-pelvis/)

## Why it happens

- [Vitamin deficiencies: what each one does to tissue](https://durgical.com/en/general-pathology/vitamin-deficiencies-what-each-one-does-to-tissue/)
- [Absorption of iron, calcium and vitamin B12](https://durgical.com/en/physiology/absorption-of-iron-calcium-and-vitamin-b12/)
- [Somatosensory pathways: dorsal column and anterolateral](https://durgical.com/en/physiology/somatosensory-pathways-dorsal-column-and-anterolateral/)
- [Cobalamin (vitamin B12)](https://durgical.com/en/biochemistry/topics/cobalamin/)
- [Methionine synthase](https://durgical.com/en/biochemistry/enzymes/methionine-synthase/)

## What it is

Degeneration of the dorsal columns and lateral corticospinal tracts of the spinal cord caused by vitamin B12 deficiency, often with peripheral neuropathy. "Combined" refers to both sensory and motor tract involvement.

## Who gets it

Mainly seen in older adults with pernicious anaemia or poor B12 absorption, and in strict vegetarians. Cases linked to recreational nitrous oxide use in young people are increasingly reported. Low B12 levels are common in older adults and in long-term metformin users in Latin America.

## Causes

- Pernicious anaemia (autoimmune loss of intrinsic factor)
- Gastrectomy, bariatric surgery or ileal resection; Crohn disease and other malabsorption
- Strict vegan or vegetarian diet without supplements
- Long-term metformin or acid-suppressing drugs
- Nitrous oxide exposure, which inactivates vitamin B12
- Fish tapeworm infection (rare)

## Risk factors

- Older age and atrophic gastritis
- Autoimmune diseases (thyroid disease, type 1 diabetes, vitiligo)
- Gastric or ileal surgery
- Heavy alcohol use
- Recreational nitrous oxide use

## Pathophysiology

Vitamin B12 is a cofactor for methionine synthase and methylmalonyl-CoA mutase. Deficiency impairs myelin synthesis and causes accumulation of methylmalonic acid and homocysteine, leading to vacuolar demyelination, first of the dorsal columns (position and vibration sense) and then of the lateral corticospinal tracts (weakness, spasticity). It usually starts in the cervical and upper thoracic cord.

## Symptoms and signs

- Symmetric tingling and numbness, first in the feet and hands
- Loss of vibration and position sense with a positive Romberg sign
- Unsteady (sensory ataxic) gait, worse in the dark
- Weakness and spasticity of the legs with extensor plantar responses (Babinski)
- Brisk knee reflexes with reduced or absent ankle reflexes when neuropathy coexists
- Cognitive changes, irritability or depression; signs of megaloblastic anaemia and glossitis may be absent

## Red flags

- Rapidly progressive gait loss or weakness
- Neurological symptoms after nitrous oxide use
- Visual loss (optic neuropathy)
- Confusion, psychosis or dementia-like decline
- Bladder or bowel problems

## Diagnosis

**Clinical**
Combination of posterior column signs (loss of vibration and position sense, Romberg) and upper motor neuron signs (spasticity, Babinski) in a person at risk of B12 deficiency.

**Tests**
Low serum vitamin B12; elevated methylmalonic acid and homocysteine confirm deficiency when B12 is borderline. Complete blood count may show macrocytic anaemia (normal in many cases). Anti-intrinsic factor and anti-parietal cell antibodies for pernicious anaemia; folate, copper and syphilis serology to exclude other causes.

**Imaging**
Spinal MRI may show T2 hyperintensity in the dorsal columns of the cervical and thoracic cord (inverted "V" sign on axial images); brain and spinal imaging also exclude compression.

## Differential diagnosis

- Cervical myelopathy (spondylotic compression)
- Copper deficiency myelopathy
- Multiple sclerosis
- Tabes dorsalis (neurosyphilis)
- HTLV-1 associated myelopathy (tropical spastic paraparesis)
- Folate deficiency and other causes of peripheral neuropathy

## Goal of treatment

Replace vitamin B12 quickly and correct its cause to halt progression and reverse as much damage as possible, since lasting symptoms become permanent.

## Treatment

**Medical**
Parenteral vitamin B12 (cyanocobalamin or hydroxocobalamin) with frequent initial doses and then maintenance, usually lifelong in pernicious anaemia; high-dose oral therapy is an option for some.

**Medical**
Treatment of the cause: stopping nitrous oxide, managing malabsorption, and dietary supplementation in vegetarians.

**Rehabilitation**
Physiotherapy and gait training for ataxia and weakness; fall prevention.

**Follow-up**
Follow-up of neurological signs, blood count and B12 or methylmalonic acid; watch for hypokalaemia when severe anaemia is corrected.

## Complications

- Permanent paraparesis and sensory ataxia
- Falls and fractures
- Neuropsychiatric problems and cognitive decline
- Optic atrophy
- Gastric cancer risk in long-standing pernicious anaemia

## Prognosis

Depends on how long symptoms have lasted before treatment: deficits present for weeks usually improve, while those present for many months may be permanent. Early diagnosis gives the best recovery.

## Prevention

- Supplementation in vegans and strict vegetarians
- Periodic B12 checks in long-term metformin users, older adults and after gastric or bariatric surgery
- Avoiding recreational nitrous oxide

## Test yourself

Think of the answer, then tap the question to see it.

## Which two spinal cord tracts are damaged in subacute combined degeneration?

The dorsal columns (vibration and position sense) and the lateral corticospinal tracts (motor).

## Which two metabolites are raised in vitamin B12 deficiency?

Methylmalonic acid and homocysteine (folate deficiency raises only homocysteine).

## Which recreational gas can trigger subacute combined degeneration?

Nitrous oxide, which inactivates vitamin B12.

## What is the most common cause of vitamin B12 deficiency with subacute combined degeneration in older adults?

Pernicious anaemia, an autoimmune loss of intrinsic factor.

## Can the neurological damage occur without anaemia?

Yes. Neurological signs can appear with a normal blood count and normal red cell size.

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## Related conditions

- [Pernicious anaemia](https://durgical.com/en/conditions/pernicious-anaemia/)
- [Megaloblastic anaemia (B12 or folate deficiency)](https://durgical.com/en/conditions/megaloblastic-anaemia/)
- [Peripheral neuropathy (diabetic and other)](https://durgical.com/en/conditions/peripheral-neuropathy/)
- [Syphilis](https://durgical.com/en/conditions/syphilis/)
- [Multiple sclerosis](https://durgical.com/en/conditions/multiple-sclerosis/)
- [Degenerative cervical myelopathy](https://durgical.com/en/conditions/degenerative-cervical-myelopathy/)
- [Alcohol use disorder](https://durgical.com/en/conditions/alcohol-use-disorder/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Subacute Combined Degeneration](https://www.msdmanuals.com/professional/neurologic-disorders/spinal-cord-disorders/subacute-combined-degeneration) (MSD Manual Professional Edition, 2026); [Vitamin B12 deficiency anemia: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000574.htm) (MedlinePlus (U.S. National Library of Medicine), 2026).

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Source: https://durgical.com/en/conditions/subacute-combined-degeneration/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/subacute-combined-degeneration/