Overview
Definition
A lack of Vitamin B12, essential for DNA synthesis and myelin formation, leading to macrocytic anemia and neurological damage.
Epidemiology
Common in the elderly, strict vegans, and patients with malabsorptive diseases.
Etiology & Risk Factors
- Pernicious Anemia (Most common cause): Autoimmune destruction of parietal cells in the stomach, leading to a lack of Intrinsic Factor (required for B12 absorption in the terminal ileum).
- Dietary lack (Strict vegans, as B12 is only found in animal products).
- Malabsorption (Crohn's disease affecting the terminal ileum, gastric bypass surgery).
Clinical Symptoms
- Hematologic: Symptoms of anemia (fatigue, pallor)
- Neurologic (CLASSIC for B12): Subacute combined degeneration of the spinal cord. Leads to loss of vibration and proprioception (dorsal columns), spastic weakness (corticospinal tracts), and paresthesias.
- Glossitis (smooth, beefy red tongue)
Clinical Approach
Diagnosis
- CBC: Macrocytic anemia (MCV > 100) with hypersegmented neutrophils on peripheral smear
- Serum B12 level: Decreased
- Methylmalonic Acid (MMA) and Homocysteine levels: BOTH are elevated (Folate deficiency only has elevated Homocysteine, normal MMA).
- Anti-Intrinsic Factor antibodies (to diagnose Pernicious Anemia)
Management
- Intramuscular (IM) Vitamin B12 injections: Required for Pernicious Anemia or severe malabsorption
- High-dose Oral B12: Can be used for dietary deficiency
- Neurological symptoms may be irreversible if treatment is delayed
Complications
- Irreversible neurological damage (dementia, severe neuropathy)