Neurosurgery notes/Other specialities/Neurology/Post column dysfunction/Subacute degeneration of the posterior column

Subacute degeneration of the posterior column

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Post column dysfunction

Function

  • Cofactor for methionine synthase (transfers CH₃ groups as methylcobalamin) and methylmalonyl-CoA mutase. Important for DNA synthesis.

Deficiency

  • Macrocytic, megaloblastic anemia; hypersegmented PMNs; paresthesias and subacute combined degeneration (degeneration of dorsal columns, lateral corticospinal tracts, and spinocerebellar tracts) due to abnormal myelin. Associated with ↑ serum homocysteine and methylmalonic acid levels, along with 2° folate deficiency. Prolonged deficiency → irreversible nerve damage.

Notes

  • Found in animal products.
  • Synthesized only by microorganisms. Very large reserve pool (years) stored in liver.
    Deficiency: malabsorption (sprue, enteritis, Diphyllobothrium latum, achlorhydria, bacterial overgrowth, alcohol excess), lack of intrinsic factor (pernicious anemia, gastric bypass surgery), absence of terminal ileum (surgical resection, Crohn disease), or insufficient intake (veganism).
  • Anti-intrinsic factor antibodies diagnostic for pernicious anemia.
  • Folate supplementation can mask hematologic symptoms of B12 deficiency, but not the neurologic symptoms.

Numbers

  • Most common in patients older than 40 years and especially older than 60 years

Pathophysiology

  • A sequela of any cause of vitamin B12 deficiency
    • Crohn disease and other causes of terminal ileitis
      • (Vitamin B12 is primarily absorbed at the terminal ileum)
    • Use of proton pump inhibitors
      • (Acid is required to release vitamin B12 from meat)
    • Gastrointestinal surgery
      • (Especially if affecting the terminal ileum)
    • Coeliac disease
    • Atrophic gastritis
    • Abuse of nitrous oxide (usually from recreational abuse)
    • Methotrexate
    • Etoh
    • Strict vegans
  • Build up of methylmalonic acid → damage myelin

Investigation

  • Blds
    • Vitamin B12 levels may be normal in up to 30% of patients, and thus looking at levels of other more sensitive and specific biomarkers may be utilised instead
      • Holotranscobalamin or active vitamin B12 (low)
      • Methylmalonic acid (high)
      • Homocysteine (high)
  • MRI
      • MRI spine may be normal in 60-85% of cases
      • Inverted "V" sign: When abnormal, most commonly there is symmetric bilateral high signal within the dorsal columns.
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