# Subacute Combined Degeneration

> Subacute combined degeneration for NEET-PG Medicine, covering dorsal column and corticospinal damage from vitamin B12 deficiency and nitrous oxide exposure.

- Canonical URL: https://prepelephant.com/topics/neet-pg/medicine/subacute-combined-degeneration
- Exam / course: NEET-PG · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-14
- Last updated: 2026-10-14
- Reference basis: Adams and Victor's Principles of Neurology; Harrison's Principles of Internal Medicine
- How to cite: "Subacute Combined Degeneration", PrepElephant, https://prepelephant.com/topics/neet-pg/medicine/subacute-combined-degeneration

## Direct answer

Subacute combined degeneration is a myelopathy in which the dorsal columns and lateral corticospinal tracts degenerate, most often from vitamin B12 deficiency. It causes loss of vibration and position sense, sensory ataxia, spasticity and a positive Romberg sign, frequently with a peripheral neuropathy. Pernicious anaemia and nitrous oxide exposure are the classic causes, and treatment is intramuscular cobalamin.

## What you must remember

- **Pathology:** Vacuolation and demyelination of the dorsal columns and lateral corticospinal tracts, which are the most vulnerable white matter in B12 deficiency.
- **Earliest sign:** Loss of vibration and joint position sense in the feet, often before any gait change appears.
- **Romberg sign:** Positive, reflecting posterior column loss, so the patient sways when the eyes are closed.
- **Blood picture:** Macrocytic anaemia, hypersegmented neutrophils and a raised mean corpuscular volume, although neurology can appear before anaemia.
- **Confirmatory tests:** Low serum B12 with raised methylmalonic acid and homocysteine, where methylmalonic acid is the more specific marker.
- **Nitrous oxide:** Oxidises cobalt in cobalamin and inactivates methionine synthase, precipitating a myeloneuropathy in susceptible people.
- **Folate warning:** Giving folic acid alone may correct the anaemia but can worsen the neurological damage.

## A diagnostic and treatment pathway

Subacute combined degeneration is recognised from a spinal pattern in a patient with a B12 deficiency state. The steps below run from clinical suspicion to lifelong replacement.

1. **Suspect from the sensory pattern:** Symmetric numbness and tingling in the feet, with vibration loss and sensory ataxia, should prompt a B12 level.
2. **Check the blood count and smear:** Macrocytosis and hypersegmented neutrophils support the diagnosis, but a normal count does not exclude it.
3. **Measure cobalamin with metabolic markers:** A low serum B12 with raised methylmalonic acid and homocysteine confirms the deficiency.
4. **Find the cause:** Intrinsic factor antibodies test for pernicious anaemia, and the diet and nitrous oxide history are taken.
5. **Start intramuscular replacement:** Hydroxocobalamin is given in a loading course followed by lifelong maintenance, and should not wait for imaging when the picture is clear.
6. **Exclude mimics:** Copper deficiency, HIV myelopathy and compressive lesions are checked when the picture is atypical or the response is poor.

## Where the exam tries to catch you

Most errors come from treating the anaemia as the whole disease or from missing the cord.

- **Trap:** Giving oral folic acid to a patient with macrocytosis and neurological signs — **Correct:** folate can mask the anaemia while the cord degenerates, so cobalamin must be replaced first.
- **Trap:** Waiting for a macrocytic anaemia before suspecting the cord — **Correct:** neurological signs can appear before the blood count changes, so dorsal column loss needs a B12 check.
- **Trap:** Attributing a sensory gait to peripheral neuropathy alone — **Correct:** spasticity, an extensor plantar response and brisk reflexes point to the lateral corticospinal tracts and confirm the combined pattern.

## Frequently asked questions

### Why does vitamin B12 deficiency damage the spinal cord?

Vitamin B12 is a cofactor for methionine synthase and methylmalonyl CoA mutase, so deficiency disrupts myelin synthesis. The leading explanation is that abnormal fatty acids are built into myelin of the dorsal columns and lateral corticospinal tracts. The result is degeneration of those long tracts in the cord.

### Which investigation best confirms B12 deficiency in subacute combined degeneration?

Serum methylmalonic acid is the most specific test for cobalamin deficiency, because it rises in B12 deficiency but not in folate deficiency. Serum cobalamin is usually low, but a borderline level still needs methylmalonic acid checked. Raised homocysteine accompanies the deficiency but is less specific.

### Can nitrous oxide cause subacute combined degeneration?

Nitrous oxide can cause subacute combined degeneration in susceptible people, because it oxidises cobalt in cobalamin and inactivates methionine synthase. Heavy recreational use or repeated anaesthetic exposure can trigger a neuropathy and myelopathy even when serum B12 looks normal. Stopping exposure and replacing cobalamin are both needed.

### What is the usual treatment for subacute combined degeneration?

Subacute combined degeneration is treated with intramuscular hydroxocobalamin, given as a loading course followed by lifelong maintenance injections. Neurological recovery is often incomplete and slow, so early treatment matters. Folic acid should never be given alone in this condition.