Cobalamin (Vitamin B12) Deficiency

Physiological Role and Metabolism

• Forms and Coenzymes:
  • Vitamin B12 (cobalamin) exists in two metabolically active coenzyme forms: methylcobalamin and 5-deoxyadenosylcobalamin.
  • These coenzymes are essential for reactions in lipid and carbohydrate metabolism, as well as protein and nucleic acid biosynthesis.
• Absorption Mechanisms:
  • Dietary vitamin B12, found almost exclusively in animal foods (meat, milk, eggs), binds to intrinsic factor (IF), a glycoprotein secreted by gastric parietal cells,.
  • The B12-IF complex is absorbed via specific receptors in the terminal ileum,.
  • Absorbed B12 is transported in the plasma bound to transcobalamin II (the active fraction) and stored primarily in the liver,.

Pathophysiology of Deficiency

The pathophysiology of vitamin B12 deficiency stems from the failure of two specific enzymatic reactions dependent on cobalamin.

1. Hematological Pathophysiology (The Folate Trap)

• Mechanism:
  • Methylcobalamin is required for the conversion of homocysteine to methionine by the enzyme methionine synthase,.
  • This reaction is coupled with the regeneration of tetrahydrofolate (THF) from 5-methyltetrahydrofolate.
  • In the absence of vitamin B12, folate becomes "trapped" as 5-methyltetrahydrofolate, which cannot be utilized for DNA synthesis.
• Consequences:
  • This "functional folate deficiency" impairs thymidine synthesis, which is critical for DNA replication,.
  • RNA synthesis remains unaffected, leading to cytoplasmic maturation while the nucleus remains immature (nuclear-cytoplasmic asynchrony).
  • This results in megaloblastic anemia characterized by large, immature red blood cells (macrocytes) and hypersegmented neutrophils,.
  • Ineffective erythropoiesis occurs, leading to intramedullary hemolysis and potentially pancytopenia,.

2. Neurological Pathophysiology

• Mechanism:
  • 5-deoxyadenosylcobalamin is a cofactor for the enzyme methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA,.
  • Deficiency leads to the accumulation of methylmalonyl-CoA and its precursor, propionyl-CoA.
• Consequences:
  • Excessive accumulation of methylmalonic acid (MMA) leads to defects in fatty acid synthesis within neural tissues.
  • This results in the incorporation of abnormal fatty acids into neuronal lipids, causing destabilization and degeneration of the myelin sheath,.
  • Demyelination affects the posterior and lateral columns of the spinal cord (subacute combined degeneration) and peripheral nerves.
  • This manifests as hypotonia, developmental regression, paresthesias, sensory deficits, and infantile tremor syndrome,.

3. Biochemical Alterations

• Homocysteine Accumulation: Due to the failure of methionine synthase, homocysteine levels rise in the blood, which is a sensitive but non-specific marker for B12 deficiency (also elevated in folate deficiency).
• Methylmalonic Acid (MMA) Accumulation: Due to the failure of methylmalonyl-CoA mutase, MMA accumulates. Elevated serum MMA is specific to vitamin B12 deficiency and distinguishes it from folate deficiency.

Clinical Features

The clinical presentation varies by age and severity. Symptoms affect the hematologic, gastrointestinal, and nervous systems.

Hematological and General Manifestations

• Anemia: Pallor, fatigue, lethargy, and anorexia.
• Pancytopenia: In severe cases, children may present with bleeding manifestations (thrombocytopenia) or recurrent infections (leukopenia).
• Jaundice: Mild icterus due to intramedullary hemolysis (ineffective erythropoiesis).
• Growth: Failure to thrive, stunting, and weight loss.

Dermatological Manifestations

• Hyperpigmentation: Characteristically affects the knuckles, dorsal aspect of fingers, toes, and nail beds.
• Mucosal Changes: Glossitis (smooth, beefy red tongue), angular stomatitis, and cheilosis.
• Skin: Generalized dry skin or brownish pigmentation.

Neurological Manifestations

Neurological damage may precede hematological changes or occur in their absence.

• Infants (6-18 months):
  • Infantile Tremor Syndrome (ITS): Characterized by pallor, regression of milestones, hypotonia, and coarse tremors.
  • Developmental delay or regression.
  • Apathy, irritability, and staring look.
  • Seizures and hypotonia.
  • Brain atrophy on imaging.
• Older Children and Adolescents:
  • Subacute Combined Degeneration of the Cord: Demyelination of dorsal and lateral columns leading to ataxia, loss of vibration and position sense, and spastic paresis.
  • Paresthesias (tingling/numbness) and peripheral neuropathy.
  • Behavioral changes, psychosis, or cognitive decline.
  • Extrapyramidal signs like chorea or dystonia.

Diagnosis

Hematological Investigations

• Complete Blood Count (CBC):
  • Macrocytic Anemia: Mean Corpuscular Volume (MCV) is elevated.
    • Cut-off for Macrocytosis: >70 fL + age (in years) for children 2-10 years; >90-100 fL for adolescents.
  • Pancytopenia: Thrombocytopenia and leukopenia may be present in severe cases.
  • Reticulocyte Count: Usually low (reticulocytopenia).
• Peripheral Blood Smear:
  • Macro-ovalocytes: Large, oval-shaped red blood cells.
  • Hypersegmented Neutrophils: Defined as >5% of neutrophils having $\ge$5 lobes or any neutrophil having $\ge$6 lobes. This is a hallmark of megaloblastic anemia.
  • Cabot rings and Howell-Jolly bodies may be seen.

Biochemical Investigations

• Serum Vitamin B12 Levels:
  • Deficiency: <200 pg/mL (<148 pmol/L).
  • Borderline: 200–300 pg/mL.
  • Note: Levels may be falsely normal in liver disease or myeloproliferative disorders.
• Metabolite Assays (Functional Deficiency): Useful when B12 levels are borderline or clinical suspicion is high.
  • Serum Methylmalonic Acid (MMA): Elevated in B12 deficiency (>750 nmol/L). This is specific for B12 deficiency.
  • Plasma Homocysteine: Elevated in both B12 and Folate deficiency (>15 µmol/L).
  • Holo-transcobalamin (HoloTC): Measures the active fraction of B12. More sensitive and specific; levels <35 pmol/L indicate deficiency.

Ancillary Tests

• Bone Marrow Aspiration: Shows megaloblastic erythropoiesis (asynchrony between nuclear and cytoplasmic maturation), giant metamyelocytes. Not routinely required for diagnosis if serology is clear.
• Lactate Dehydrogenase (LDH): Markedly elevated due to ineffective erythropoiesis (intramedullary hemolysis).
• Workup for Cause:
  • Anti-Intrinsic Factor antibodies (for Pernicious Anemia).
  • Stool examination for parasites.
  • Celiac serology (tTG).

Management

Principles of Treatment

• Correct the deficiency and replenish stores.
• Treat the underlying cause (e.g., diet modification, malabsorption).
• Warning: Folate supplementation alone in B12 deficiency can correct anemia but precipitate or worsen irreversible neurological damage (Subacute combined degeneration). Always exclude B12 deficiency before treating with folate, or treat with both.

Vitamin B12 Supplementation Regimens (IAP Guidelines 2022)

Treatment can be Parenteral or Oral. Parenteral is preferred for severe cases, neurological symptoms, or compliance issues.

1. Parenteral Therapy (Intramuscular/Deep Subcutaneous)

• Indications: Neurological manifestations, pancytopenia, severe anemia, malabsorption, or doubtful compliance.
• Dose & Schedule:
  • Initial: 25-50 µg daily for infants/young children (to avoid hypokalemia/tremors). For older children/adults: 100 µg daily.
  • Loading Phase: 100 µg daily for 7 days (extend to 3 weeks if neuro signs present).
  • Consolidation: 100 µg on alternate days for 1 week.
  • Maintenance: 1000 µg weekly for 1 month, then 1000 µg monthly.
• Note: In cases of thrombocytopenia, use the IV route or oral route to avoid hematomas.

2. Oral Therapy

• Indications: Mild to moderate anemia, nutritional cause, compliant parents.
• Dose: High doses are required to ensure absorption via passive diffusion (1-5% absorbed even without IF).
  • Infants: 500 µg daily.
  • Older Children: 1000 µg daily.
• Administration: Should be given in a fasting state for better absorption.

Duration of Therapy

• Hematological presentation: At least 3 months or until parameters normalize.
• Neurological presentation: At least 6 months.
• Irreversible causes (e.g., Pernicious Anemia): Lifelong therapy (1000 µg IM monthly).

Adjunctive Therapy

• Folic Acid: 1–5 mg/day orally is usually added to replenish folate stores, but only after initiating B12.
• Iron: Prophylactic iron may be needed as rapid erythropoiesis following B12 treatment can deplete iron stores.
• Potassium: Monitor for hypokalemia in severe anemia during the first few days of treatment due to rapid cellular uptake.
• Diet: Dietary counseling to include animal-source foods or fortified cereals. Breastfeeding mothers should be supplemented if deficient.

Response to Therapy

• Subjective: Improved well-being/appetite within 24–48 hours.
• Reticulocytosis: Peaks in 5–7 days.
• Hematology: Anemia corrects in 4–8 weeks; Hypersegmentation disappears in 10–14 days.
• Neurology: Improvement begins within 1 week but may take months to plateau; some deficits may be permanent. Note: Tremors in ITS may transiently worsen during recovery.