Vitamin B12 deficiency

Vitamin B₁₂ (cobalamin) plays an essential role in enzymatic reactions responsible for red blood cell (RBC) formation and proper myelination of the nervous system. Thus, a deficiency of vitamin B₁₂ may lead to megaloblastic anemia and a wide range of neurological disturbances. Deficiency may be caused by malabsorption, malnutrition, or increased demand. The most common underlying cause of vitamin B₁₂ deficiency is pernicious anemia, an autoimmune disorder characterized by the absence of intrinsic factor (IF). IF is a protein that is crucial for vitamin B₁₂ absorption. Patients present with signs of anemia (e.g., fatigue) and/or neurological manifestations such as paresthesia, spasticity, ataxia, and neuropsychiatric disorders. After detecting low serum vitamin B₁₂ levels, the diagnostic approach consists of identifying the underlying cause by measuring autoantibodies and possibly conducting a Schilling test. When testing patients with suspected vitamin B₁₂ deficiency, it is important to remember that folate deficiency also causes megaloblastic anemia. However, only vitamin B₁₂ deficiency can be accompanied by neuropathy and exhibits elevated levels of methylmalonic acid (MMA). Treatment consists of parenteral supplementation; depending on the underlying cause, long-term supplementation may be needed.

• Malabsorption
  • ↓ Intrinsic factor (IF)
    • Atrophic gastritis due to
      • Autoimmune atrophic gastritis: most common cause of vitamin B₁₂ deficiency
      • H. pylori infection
    • Gastrectomy ^[1]
  • Reduced uptake of IF-vitamin B₁₂ complex in terminal ileum due to:
    • Alcohol use disorder
    • Crohn disease, celiac disease
    • Pancreatic insufficiency
    • Surgical resection of the ileum
    • Diphyllobothrium latum (tapeworm) infection
    • Bacterial overgrowth
    • Enteritis
    • Achlorhydria
• Malnutrition
  • Anorexia nervosa
  • Strict vegan diets: occurs only after years of a strict diet that excludes all animal products (unlike folate deficiency, which occurs within a few months of insufficient intake)
• Increased demand: e.g., during pregnancy, breastfeeding, fish tapeworm (Diphyllobothrium latum) infection, and leukemia ^[1]
• Drugs: metformin, proton-pump inhibitors, colchicine

Dysfunctional biochemical reactions

• Physiological function: Vitamin B₁₂ is a water-soluble cofactor for enzymatic reactions of DNA synthesis (via methionine synthase) and odd-chain fatty acid metabolism (via methylmalonyl CoA mutase). A deficiency of vitamin B₁₂ leads to enzyme dysfunction.
• Dysfunctional methionine synthase (normally converts homocysteine to methionine, thereby demethylating N5-methyl-THF to THF)
  • ↓ Tetrahydrofolate (THF; cofactor in purine synthesis) → ↓ DNA synthesis → large, nucleated hematopoietic cells, including megaloblasts; → megaloblastic precursors undergo apoptosis or are phagocytosed by macrophages → pancytopenia (including megaloblastic anemia)
  • ↓ Methionine → neuropathy ^[2]
  • ↑ Homocysteine → endothelial damage → predisposes to cardiovascular disease
  • Can also cause secondary folate deficiency
• Dysfunctional methylmalonyl CoA mutase
  • Methylmalonyl CoA cannot be converted to succinyl CoA → accumulation of methylmalonyl CoA and its precursor propionyl CoA, as well as their associated odd-chain fatty acids, which cannot be completely metabolized
  • Propionyl CoA replaces acetyl CoA in neuronal membranes → demyelination → neurological manifestations

Folate deficiency also leads to low levels of THF, resulting in megaloblastic anemia.

Pernicious anemia

• A type of vitamin B₁₂ deficiency caused by autoantibodies against intrinsic factor and/or gastric parietal cells (type II hypersensitivity reaction)
  • Antiparietal cell antibodies: target gastric parietal cells
    • Causes ↓ acid production and atrophic gastritis
    • ↓ Intrinsic factor production → ↓ vitamin B₁₂ absorption in terminal ileum
  • Anti-IF antibodies: bind intrinsic factor and block the vitamin B₁₂ binding site
• Associated with other autoimmune diseases (e.g., hypothyroidism, vitiligo)
• Increases the risk of gastric cancer

• Signs of anemia (e.g., fatigue, pallor) ^[3]
• Mild scleral icterus and/or jaundice
• Neurological disturbances are generally symmetrical
  • Peripheral neuropathy: tingling, numbness, pins-and-needles sensation, coldness (especially in the lower extremities)
  • Subacute combined degeneration of spinal cord: symmetrical demyelination of the spinal cord tracts occurs in vitamin B₁₂ deficiency due to insufficient vitamin B₁₂-dependent fatty acid synthesis and production/maintenance of myelin
    • It manifests with the following symptoms:
      • Paresthesia, impaired proprioception, loss of vibratory sensation, tactile sensation, and position discrimination due to demyelination of the dorsal columns
      • Spastic paresis due to demyelination of the lateral corticospinal tracts (axons of upper motor neurons)
      • Gait abnormalities (spinal ataxia) resulting from the damage of spinocerebellar tracts and dorsal columns
    • Long-term deficiency can lead to irreversible neurological damage.
  • Neuropsychiatric disease; (e.g., reversible dementia, depression, paranoia) ^[3]
  • Worsening vision
  • Autonomic dysfunction: impotence and incontinence
• Glossitis

The Spinocerebellar tracts, lateral Corticospinal tracts, and Dorsal columns are affected in Subacute Combined Degeneration.

Always consider vitamin B₁₂ deficiency when evaluating patients with dementia.

Hematological findings

• Signs of megaloblastic anemia
  • ↓ Hb
  • ↑ MCV (macrocytic), ↑ MCH (hyperchromic)
  • Hypersegmented neutrophils
  • ↓ Reticulocytes
• Frequently thrombocytopenia and leukopenia (possibly pancytopenia)

Approach

• If vitamin B₁₂ serum levels are decreased, determine the underlying cause
  1. Test for autoantibodies
     • Anti-IF antibodies
     • Antiparietal cell antibodies
  2. If autoantibodies are negative, perform:
     • Schilling test: a test to determine the cause of vitamin B₁₂ deficiency ^[4]
       • Consists of four stages that assess impaired vitamin B₁₂ uptake (see table below)
       • Patients ingest radiolabelled vitamin B₁₂ and then its urinary excretion is measured.
       • Urinary excretion of 8–40% (varies between laboratories) of radioactive vitamin B₁₂ within 24 hours is considered normal. ^[5]
       • No longer routinely used in clinical practice
     • Gastroscopy
     • D-xylose absorption test

• If vitamin B₁₂ serum levels are normal: ^[3]
  • Measure homocysteine: elevated in both vitamin B₁₂ deficiency and folate deficiency
  • Measure methylmalonic acid (MMA); to help rule out folate deficiency (MMA is normal in folate deficiency and elevated in vitamin B₁₂ deficiency )

Differential diagnoses of vitamin B₁₂ deficiency

Starting folate treatment before excluding vitamin B₁₂ deficiency may correct anemia, but it can worsen neuropathy!

In contrast to vitamin B₁₂ deficiency, folic acid deficiency is generally not associated with neurological symptoms.

Other causes of macrocytic anemia

• Myelodysplastic syndrome
  • CBC may show macrocytic (but not megaloblastic) anemia
  • Most patients have other concurrent hematological alterations (e.g., neutropenia, thrombocytopenia).

Other causes of neuropathy

• Multiple sclerosis
• Friedreich ataxia
• Charcot-Marie-Tooth disease
• Infectious diseases (e.g., syphilis/tabes dorsalis, HIV myelopathy, herpes virus myelitis)
• Other deficiencies (e.g., copper deficiency, vitamin E deficiency)
• Neoplasms (e.g., astrocytoma, ependymoma)

The differential diagnoses listed here are not exhaustive.

• IM supplementation of vitamin B₁₂ (e.g., cyanocobalamin, hydroxycobalamin)
• Prevent future vitamin B₁₂ deficiency by treating the underlying disease if possible (causative therapy): e.g., in patients with fish tapeworm infestation, intestinal bacterial overgrowth, or chronic pancreatitis. ^[3]
• If causative therapy is not possible, lifelong IM supplementation may be necessary.
• Secondary folate deficiency: Folate supplementation can improve and mask hematologic symptoms but has no influence on neurologic symptoms of vitamin B₁₂ deficiency.

References:^[6]