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Subacute Combined Degeneration of the Spinal Cord

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Overview

Subacute combined degeneration of the spinal cord (SCDS) describes the degeneration of the dorsal columns, lateral corticospinal tracts, and spinocerebellar tracts due to vitamin B12 deficiency or an inability to metabolise B12.

B12 is a cofactor for enzymes involved in preserving myelin, therefore, its deficiency leads to a loss of myelin (demyelination) which tends to affect the dorsal and lateral spinal cord.

Causes

  • Causes of vitamin B12 deficiency:
    • Dietary insufficiency (e.g. strict vegan diets) – liver stores of B12 can last for up to 3 years
    • Malabsorption (e.g. coeliac disease or pernicious anaemia)
  • Other causes:
    • Nitrous oxide use – this impairs the ability to use B12
      • Patients may have normal B12 levels but features of SCDS
    • Replacing folate before B12 in B12 and folate deficiency:
      • B12 should be replaced first

Features

  • Features due to dorsal column involvement:
    • Paraesthesia (tingling, burning, and sensory loss) – usually seen first
    • Impaired fine touch, proprioception, and vibration sense
  • Features due to lateral corticospinal tract involvement – UMN features (as UMNs in the corticospinal tract terminate in the spinal cord):
    • Muscle weakness
    • Spasticity
    • Hyperreflexia, although ankle reflexes are absent
    • Positive Babinski sign
  • Features due to spinocerebellar tract involvement:
    • Sensory ataxia
      • Due to the spinocerebellar tracts conveying proprioception from the muscles to the cerebellum
    • Positive Romberg’s test

Investigations

  • Full blood count (FBC):
  • Serum B12 and folate:
    • Confirms B12 deficiency
    • May identify folate deficiency, which can mimic B12 deficiency
  • Methylmalonic acid (MMA) and homocysteine:
    • Done if B12 is normal
    • These are intermediates of B12 metabolism
    • MMA is only elevated in B12 deficiency
    • Homocysteine is elevated in both B12 and folate deficiency

Management

Management involves vitamin B12 therapy. This may lead to partial or full recovery depending on how long SCDS has been ongoing.

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