Vitamin B12 Deficiency
Definition and Pathophysiology
- Megaloblastic anemia resulting from impaired DNA synthesis.
- Characterized by macrocytosis, dyssynchrony between nuclear and cytoplasmic maturation, and hypersegmented neutrophils.
- Dietary Sources: Synthesized exclusively by microorganisms; humans rely on animal products (meat, eggs, fish, milk).
- Absorption Pathway:
- Dietary cobalamin (Cbl) released by gastric proteases.
- Binds Intrinsic Factor (IF) secreted by gastric parietal cells.
- IF-Cbl complex absorbed via cubam receptors in proximal ileum.
- Transport: Released into portal circulation bound to Transcobalamin II (TC II).
- Intracellular Metabolism: Converted to active coenzymes.
- Adenosylcobalamin (AdoCbl): Cofactor for methylmalonyl-CoA mutase. Deficiency causes elevated methylmalonic acid (MMA).
- Methylcobalamin (MeCbl): Cofactor for methionine synthase. Deficiency causes elevated homocysteine.
Etiologic Classification
| Category | Specific Disorders & Mechanisms |
|---|---|
| Inadequate Intake | - Maternal deficiency: Exclusively breastfed infants of B12-deficient/vegan mothers.- Strict vegetarian or vegan diet.- Malnutrition or poorly controlled PKU diet. |
| Defective Absorption (Gastric) | - Hereditary Intrinsic Factor Deficiency (HIFD): Autosomal recessive; absent/defective IF; normal gastric mucosa and acid secretion.- Juvenile Pernicious Anemia: Autoimmune destruction of parietal cells; antibodies against IF/parietal cells; associated endocrinopathies.- Gastric surgery/bypass, Helicobacter pylori infection. |
| Defective Absorption (Intestinal) | - Imerslund-Gräsbeck Syndrome: Defective ileal receptor (CUBN or AMN mutations); associated with benign proteinuria.- Celiac disease, Crohn disease, terminal ileum resection.- Bacterial Overgrowth: Intestinal diverticula, blind loops.- Parasitic Infection: Diphyllobothrium latum (fish tapeworm). |
| Defective Transport | - Transcobalamin II (TC II) Deficiency: Autosomal recessive; failure to transport B12 to tissues. |
| Disorders of Metabolism | - Intracellular Defects: Complementation groups cblA through cblX.- Inadequate synthesis of AdoCbl, MeCbl, or both. |
| Inactivation | - Nitrous Oxide Abuse: Inactivates cobalamin (recreational "Whippets" or anesthesia). |
Clinical Manifestations
General & Gastrointestinal
- Insidious onset.
- Weakness, lethargy, fatigue, anorexia, failure to thrive.
- Sore red tongue, glossitis, stomatitis.
- Vomiting, diarrhea, intermittent icterus.
- Skin hyperpigmentation (mimicking Addison disease).
Neurologic
- Neurologic problems may precede hematologic abnormalities or occur in isolation.
- Infants: Developmental delay, loss of motor milestones (head control, sitting), hypotonia, athetoid movements, seizures, brain atrophy.
- Older Children/Adolescents: Subacute combined degeneration of spinal cord (posterior and lateral columns).
- Loss of vibration and position sense, ataxic gait, positive Romberg sign.
- Peripheral neuropathy: Paresthesia, hyporeflexia, clonus, Babinski responses.
- Neuropsychiatric changes, dementia.
Laboratory Investigations
Hematologic Evaluation
- Complete Blood Count: Macrocytic anemia (MCV >100 fL, often 110-140 fL).
- Elevated Red Cell Distribution Width (RDW).
- Leukopenia and thrombocytopenia (simulating aplastic anemia or leukemia in advanced cases).
- Inappropriately low absolute reticulocyte count.
- Peripheral Smear:
- Macro-ovalocytes.
- Hypersegmented neutrophils (>5 lobes in >5% of cells).
- Marked anisocytosis, poikilocytosis, teardrop cells.
- Cabot rings, Howell-Jolly bodies, punctate basophilia.
- Bone Marrow Aspirate:
- Hypercellular with erythroid predominance (reversed myeloid:erythroid ratio).
- Megaloblastic changes: Nuclear-cytoplasmic dyssynchrony (retarded nuclear condensation).
- Giant metamyelocytes with horseshoe-shaped nuclei.
Biochemical Evaluation
- Serum Vitamin B12: Typically <80 pg/mL (normal 200-800 pg/mL). May be falsely normal in TC II deficiency or metabolic defects.
- Metabolites (Confirmatory):
- Methylmalonic Acid (MMA): Markedly elevated (>280 nmol/L); specific for B12 deficiency. Excessive urinary MMA excretion.
- Homocysteine: Elevated (non-specific; also raised in folate deficiency).
- Hemolysis Markers: Markedly elevated Lactate Dehydrogenase (LDH), moderate indirect hyperbilirubinemia (2-3 mg/dL), decreased haptoglobin (reflecting ineffective erythropoiesis and apoptosis of megaloblastic cells).
- Iron/Folate Profile: Serum iron and serum folic acid typically normal or elevated.
Differential Diagnosis
| Disorder | Differentiating Features |
|---|---|
| Folic Acid Deficiency | Elevated homocysteine, but normal methylmalonic acid (MMA). Low serum/RBC folate. |
| Thiamine-Responsive Megaloblastic Anemia | Autosomal recessive (SLC19A2). Triad: Megaloblastic anemia, sensorineural deafness, diabetes mellitus. Marrow shows ringed sideroblasts. |
| Orotic Aciduria | Defect in pyrimidine synthesis. Normal B12/folate. Orotic acid in urine. Uridine-responsive. |
| Lesch-Nyhan Syndrome | Hypoxanthine phosphoribosyltransferase deficiency. Mental retardation, choreoathetosis, self-mutilation. Adenine-responsive. |
| Drug-Induced Macrocytosis | Use of purine/pyrimidine analogs (methotrexate, 6-mercaptopurine, azathioprine, 5-fluorouracil) or ribonucleotide reductase inhibitors (hydroxyurea). |
| Bone Marrow Failure / MDS | Aplastic anemia, Fanconi anemia, or Myelodysplastic syndromes. Distinguished by marrow cellularity, cytogenetics, and absence of hypersegmented neutrophils. |
Management and Treatment
Pharmacologic Therapy
- Formulations: Cyanocobalamin (CNCbl) or Hydroxocobalamin (OHCbl). OHCbl preferred for HIFD and metabolic defects.
- Standard Dosing:
- Initial: 25-100 mcg daily (oral, intramuscular, or deep subcutaneous) for 1 week.
- Taper: Weekly doses, followed by monthly maintenance (200-1000 mcg IM).
- Neurologic Complications: Higher dosing required. 1000 mcg daily for 2 weeks, then every 2 weeks for 6 months, followed by lifelong monthly administration.
- Disease-Specific Dosing:
- TC II Deficiency: Requires massive doses (1000 mcg 2-3 times weekly) to force Cbl into cells.
- Intracellular Defects (cblA-X): High-dose OHCbl, betaine, and specific metabolic management.
- Imerslund-Gräsbeck / HIFD: Lifelong IM or intranasal Cbl bypasses the absorption defect.
Clinical Course and Response
- Contraindication: Folic acid supplementation without B12 is strictly contraindicated. It may partially correct anemia but rapidly accelerates irreversible neurologic degeneration.
- Hematologic Response:
- Bone marrow megaloblastosis reverses to normoblastic within 3 days.
- Reticulocytosis begins by day 3-4, peaks at days 6-8, and normalizes by 3 weeks.
- Neurologic Response: Alertness improves within 48 hours. However, long-term cognitive deficits and severe neurologic sequelae often remain irreversible despite therapy.