Medicine – Subacute Combined Degeneration of the Spinal Cord

Subacute combined degeneration of the spinal cord is a neurological complication most commonly caused by vitamin B12 deficiency. It affects multiple neurological pathways at the same time, particularly the dorsal columns and corticospinal tracts, and is often accompanied by peripheral neuropathy.

The term combined degeneration refers to simultaneous involvement of more than one major spinal cord pathway.

1. Main Cause

The classic cause is:

Vitamin B12 deficiency.

Vitamin B12 is essential for normal myelin maintenance and nervous-system function.

Deficiency can lead to progressive demyelination and axonal injury involving the spinal cord and peripheral nerves.

2. Common Causes of Vitamin B12 Deficiency

Important causes include:

Pernicious anaemia.

Malabsorption, including terminal ileal disease.

Previous gastric or ileal surgery.

Strict vegan diet without adequate supplementation.

Certain medications, such as long-term metformin or acid-suppressing therapy in some patients.

Nitrous oxide exposure, which can functionally inactivate vitamin B12.

3. Pathways Affected

The major neurological structures affected are:

Dorsal columns.

Lateral corticospinal tracts.

Peripheral nerves.

This combination explains the apparently mixed neurological findings.

4. Dorsal Column Involvement

The dorsal columns carry:

Vibration sensation.

Joint-position sense.

Fine discriminative touch.

Damage therefore causes loss of proprioceptive input from the limbs.

The patient may become unsteady, particularly when visual compensation is removed.

5. Sensory Ataxia

Loss of proprioception from dorsal-column disease produces sensory ataxia.

Patients may describe:

Unsteady walking.

Difficulty walking in the dark.

A feeling that they do not know where their feet are.

Examination may show:

Loss of vibration sense.

Loss of joint-position sense.

Positive Romberg test.

6. Positive Romberg Test

A patient with sensory ataxia may maintain balance while the eyes are open because vision compensates for impaired proprioception.

When the eyes are closed, this visual compensation is removed and the patient becomes markedly more unstable.

Therefore:

Dorsal-column disease → sensory ataxia → positive Romberg sign.

7. Peripheral Neuropathy

Vitamin B12 deficiency can also damage peripheral nerves.

This often causes symmetrical distal sensory symptoms in a:

Glove-and-stocking distribution.

Patients may develop:

Numbness.

Tingling.

Burning or altered sensation.

Distal weakness in more advanced disease.

8. Glove-and-Stocking Sensory Loss

A glove-and-stocking pattern means that sensory loss begins distally in the:

Feet and lower legs.

and later, if more severe, the:

Hands.

This pattern reflects a length-dependent peripheral neuropathy rather than isolated spinal cord disease.

9. Corticospinal Tract Involvement

The lateral corticospinal tracts carry upper motor neurone motor fibres.

Damage can therefore produce:

Spasticity.

Increased muscle tone.

Weakness of the legs.

Extensor plantar responses.

The lower limbs are commonly affected more prominently than the upper limbs.

10. Spastic Paraparesis

Bilateral corticospinal tract involvement may cause spastic paraparesis.

This means:

Weakness of both legs + increased tone + pyramidal signs.

Patients may develop a stiff, difficult gait as the disease progresses.

11. Absent Reflexes

One of the characteristic features of subacute combined degeneration is that tendon reflexes may be reduced or absent, especially at the ankles.

This occurs because of the accompanying peripheral neuropathy.

Therefore, even though corticospinal tract disease usually increases reflexes, peripheral nerve damage can reduce the reflex arc.

12. Extensor Plantar Responses

The plantar responses may be extensor, or Babinski positive.

This reflects corticospinal tract involvement.

Thus a classic apparently paradoxical combination may occur:

Absent ankle reflexes + extensor plantar responses.

13. Why Absent Reflexes and Extensor Plantars Can Coexist

These findings arise from damage at different levels.

Peripheral neuropathy → reduced/absent tendon reflexes.

Corticospinal tract damage → extensor plantar responses.

Therefore, the combination does not contradict itself.

It is actually a useful clue to a disorder affecting both peripheral nerves and central motor pathways.

14. Other Neurological Features

Additional manifestations of vitamin B12 deficiency can include:

Weakness.

Paraesthesia.

Gait disturbance.

Cognitive changes.

Mood disturbance.

Optic neuropathy in some cases.

Severe untreated deficiency can lead to permanent neurological damage.

15. Haematological Features

Vitamin B12 deficiency may also cause:

Macrocytic anaemia.

Macro-ovalocytes.

Hypersegmented neutrophils.

However, important neurological disease can occur even when anaemia is mild or absent.

Therefore:

Normal haemoglobin does not exclude neurological vitamin B12 deficiency.

16. Investigations

Useful investigations include:

Serum vitamin B12.

Full blood count and MCV.

Blood film.

Methylmalonic acid, which often rises in B12 deficiency.

Homocysteine, which may also be elevated.

Further testing should investigate the underlying cause, such as pernicious anaemia or malabsorption.

17. Pernicious Anaemia

Pernicious anaemia is an autoimmune cause of vitamin B12 deficiency.

Autoimmune destruction of gastric parietal cells leads to reduced intrinsic factor, impairing absorption of vitamin B12 in the terminal ileum.

Testing may include:

Intrinsic-factor antibodies.

Other autoimmune gastric markers may also support the diagnosis.

18. Treatment

Treatment requires vitamin B12 replacement.

When neurological involvement is present, treatment should not be delayed unnecessarily because prolonged deficiency may cause irreversible deficits.

Replacement is often given parenterally initially, depending on the cause and severity of deficiency.

The underlying cause should also be identified and treated where possible.

19. Important Folate Warning

Folate can improve the anaemia caused by vitamin B12 deficiency without correcting the neurological injury.

Therefore, giving folate alone to someone with unrecognised B12 deficiency may allow neurological disease to continue.

For this reason:

Vitamin B12 deficiency should be excluded or treated when clinically suspected before relying on folate replacement alone.

20. Subacute Combined Degeneration – Note Form

Cause: vitamin B12 deficiency.

Main spinal pathways affected: dorsal columns + corticospinal tracts.

Additional involvement: peripheral nerves.

Dorsal-column damage: loss of vibration and joint-position sense.

Clinical result: sensory ataxia and positive Romberg test.

Peripheral neuropathy: glove-and-stocking sensory loss.

Corticospinal damage: spastic paraparesis and extensor plantar responses.

Reflexes: may be absent because peripheral neuropathy interrupts the reflex arc.

Characteristic mixed pattern: absent reflexes + extensor plantars.

21. Characteristic Examination Pattern

A typical neurological examination may show:

Loss of vibration and proprioception.

Sensory ataxia.

Positive Romberg sign.

Distal glove-and-stocking sensory loss.

Spastic weakness of both legs.

Reduced or absent ankle reflexes.

Extensor plantar responses.

This combination strongly suggests simultaneous involvement of dorsal columns, peripheral nerves and corticospinal tracts.

Key Clinical Pattern

Think of subacute combined degeneration as:

Vitamin B12 deficiency → DORSAL COLUMNS + CORTICOSPINAL TRACTS + PERIPHERAL NERVES.

Therefore:

Dorsal columns → sensory ataxia + loss of vibration/proprioception.

Peripheral nerves → glove-and-stocking neuropathy + absent reflexes.

Corticospinal tracts → spastic paraparesis + extensor plantars.

The high-yield combination is:

Sensory ataxia + peripheral neuropathy + spastic paraparesis + absent reflexes + extensor plantar responses.