Most Common Cause of Megaloblastic Anemia Globally
Key Point
Vitamin B12 deficiency is the most common cause of megaloblastic anemia worldwide, and pernicious anemia (autoimmune gastritis with anti-intrinsic factor antibodies) is the most common cause of B12 deficiency in developed countries. In India, dietary B12 deficiency is also prevalent in vegetarians.
High-YieldNEET PG
The clinical presentation of progressive dyspnea (anemia) + paresthesias (B12 neuropathy) + macrocytic anemia + hypersegmented neutrophils + low serum B12 is pathognomonic for B12 deficiency.
Comparison of Megaloblastic Anemia Causes
| Cause | Mechanism | Prevalence | Key Feature |
|---|
| B12 deficiency (pernicious anemia) | Autoimmune gastritis → ↓ intrinsic factor | Most common globally | Neurologic symptoms (paresthesias, ataxia) |
| Folate deficiency | Dietary inadequacy, malabsorption, drugs | Common in developing countries | No neurologic symptoms; pregnancy-related |
| Methotrexate | Inhibits dihydrofolate reductase | Iatrogenic; less common | Reversible; associated with chemotherapy |
| Nitrous oxide | Inactivates B12 cofactor | Rare; abuse-related | Acute presentation; reversible if caught early |
Clinical Pearl
B12 deficiency causes both megaloblastic anemia AND subacute combined degeneration (SCD) of the spinal cord, presenting with paresthesias, ataxia, and loss of vibration/proprioception. Folate deficiency causes anemia alone without neurologic manifestations.
Mnemonic: B12 NEURO — B12 deficiency → Neuropathy, Encephalopathy, Unsteadiness, Reflexes (hyperactive), Optic atrophy, Orthostatic hypotension
Why B12 Deficiency Is Most Common
- 1.
Pernicious anemia accounts for ~60% of B12-deficient megaloblastic anemia in Western populations
- 2.
Dietary B12 deficiency is prevalent in vegetarians and vegans (no animal products)
- 3.
Malabsorption (post-gastrectomy, Crohn's disease, tropical sprue) is common in developing countries
- 4.
Folate deficiency, while common, is less frequent as a sole cause of megaloblastic anemia in clinical practice
Warning
Do not confuse folate deficiency (common in pregnancy, alcoholism, malnutrition) with B12 deficiency. Folate deficiency does NOT cause neurologic symptoms; B12 deficiency does.
