Megaloblastic Anemias¶
Chapter 104 | Part 4: Oncology and Hematology
KEY CLINICAL POINTS¶
- Megaloblastic anemias result from defects in DNA synthesis due to cobalamin (vitamin B12) or folate deficiency, or genetic/metabolic disorders.
- Clinical features include macrocytic anemia, neurological symptoms (neuropathy, cognitive decline), and hematologic abnormalities (hypersegmented neutrophils).
- Diagnosis involves measuring serum cobalamin, folate, MMA, homocysteine, and assessing dietary intake/absorption (e.g., intrinsic factor antibodies, Schilling test).
- Treatment includes cobalamin injections (hydroxocobalamin/cyanocobalamin), folic acid supplementation, and addressing underlying causes (e.g., Pernicious Anemia, malabsorption).
- Special considerations: Pregnancy, elderly patients, and drug interactions (e.g., antifolates, proton pump inhibitors).
1. DEFINITION & OVERVIEW¶
Megaloblastic anemias are characterized by impaired DNA synthesis in rapidly dividing cells, leading to macrocytic anemia and megaloblastic changes in bone marrow. The primary causes are cobalamin (vitamin B12) deficiency, folate deficiency, or genetic/metabolic disorders affecting folate metabolism.
Table 104-1 Causes of Megaloblastic Anemia¶
| Cause | Description |
|---|---|
| Cobalamin deficiency | Malabsorption (Pernicious Anemia, total gastrectomy) or dietary deficiency |
| Folate deficiency | Malabsorption (gluten enteropathy, tropical sprue) or dietary deficiency |
| Antifolate drugs | Methotrexate, trimethoprim, sulfonamides |
| Genetic disorders | Intrinsic factor deficiency, transcobalamin II deficiency |
| Other causes | Alcoholism, malnutrition, drug-induced (e.g., metformin) |
1.1 Pathophysiology¶
Defects in DNA synthesis due to impaired folate or cobalamin metabolism disrupt erythropoiesis. Cobalamin deficiency impairs methionine synthase, leading to homocysteine and methylmalonic acid accumulation. Folate deficiency blocks thymidylate synthesis, causing DNA strand breaks.
1.2 Clinical Spectrum¶
Ranges from asymptomatic macrocytosis to severe anemia, neurological complications (neuropathy, cognitive decline), and hematologic abnormalities (hypersegmented neutrophils, anemia).
2. EPIDEMIOLOGY¶
Cobalamin deficiency is more common in older adults, vegans, and those with malabsorption (e.g., Pernicious Anemia). Folate deficiency is prevalent in malnourished populations, alcoholics, and pregnant women. Pernicious Anemia accounts for 30-50% of cobalamin deficiency cases in Western countries.
2.1 Risk Factors¶
Age >60 years, vegetarian diets, gastric surgery, chronic gastritis, alcoholism, pregnancy, and drug use (e.g., proton pump inhibitors).
2.2 Demographics¶
Pernicious Anemia is more common in northern Europeans; folate deficiency is prevalent in low-income populations and regions with poor dietary fortification.
3. ETIOLOGY & PATHOPHYSIOLOGY¶
Cobalamin deficiency arises from malabsorption (e.g., Pernicious Anemia, total gastrectomy) or dietary insufficiency. Folate deficiency is due to malabsorption (e.g., gluten enteropathy, tropical sprue) or poor diet. Genetic defects (e.g., transcobalamin II deficiency) or drug interactions (e.g., antifolates) also contribute.
Table 104-2 Biochemical Reactions of Folate Coenzymes¶
| Reaction | Coenzyme Form | Single Carbon Unit | Importance |
|---|---|---|---|
| Formate activation | THF | -CHO | Generation of 10-formyl-THF |
| Methylation of dUMP to dTMP | 5,10-Methylene-THF | -CH3 | Rate-limiting in DNA synthesis |
| Homocysteine to methionine | 5-Methyl-THF | -CH3 | Requires cobalamin and 5-MTHF |
3.1 Cobalamin Metabolism¶
Cobalamin requires intrinsic factor (IF) for absorption. Deficiency leads to impaired methionine synthase, causing homocysteine and methylmalonic acid accumulation. Neurological complications arise from myelin dysfunction.
3.2 Folate Metabolism¶
Folate is converted to THF, which is essential for DNA synthesis. Deficiency disrupts thymidylate synthesis, leading to DNA strand breaks and ineffective erythropoiesis.
4. CLINICAL FEATURES¶
Symptoms include macrocytic anemia, fatigue, weakness, neurological deficits (peripheral neuropathy, cognitive decline), and hematologic abnormalities (hypersegmented neutrophils). Severe cases may present with jaundice, glossitis, and neurological complications.
4.1 Neurological Manifestations¶
Cobalamin deficiency causes demyelination, leading to peripheral neuropathy, ataxia, and cognitive decline. Folate deficiency may cause megaloblastic anemia without neurological symptoms.
5. DIFFERENTIAL DIAGNOSIS¶
Differentiate from other macrocytic anemias (e.g., alcohol-induced, liver disease) and non-megaloblastic anemias (e.g., iron deficiency, hemolytic anemia). Hypersegmented neutrophils and megaloblastic marrow changes are key diagnostic features.
6. INVESTIGATIONS & DIAGNOSIS¶
Diagnostic tests include serum cobalamin, folate, MMA, homocysteine, and red cell folate. Bone marrow biopsy shows megaloblastic changes. Serological tests for intrinsic factor antibodies and parietal cell antibodies aid in Pernicious Anemia diagnosis.
Table 104-3 Causes of Cobalamin Deficiency¶
| Cause | Description |
|---|---|
| Pernicious Anemia | Autoimmune gastritis with IF deficiency |
| Total gastrectomy | Absence of IF and intrinsic factor |
| Intestinal malabsorption | Jejunal resection, Crohn’s disease |
| Genetic defects | Transcobalamin II deficiency, AMN mutations |
| Dietary deficiency | Vegans, strict vegetarians |
6.1 Laboratory Tests¶
Serum cobalamin <74 pmol/L, folate <11 nmol/L, elevated MMA (>500 µmol/L), and homocysteine (>15 µmol/L) indicate deficiency. Red cell folate <880 µmol/L confirms folate deficiency.
6.2 Imaging and Biopsy¶
MRI may show spinal cord demyelination. Bone marrow biopsy reveals megaloblastic erythropoiesis with hypersegmented neutrophils.
7. MANAGEMENT & TREATMENT¶
Treatment includes cobalamin injections (1000 µg IM weekly), folic acid (5-15 mg/day), and addressing underlying causes (e.g., Pernicious Anemia, malabsorption). Lifelong therapy is required for irreversible neurological damage.
Table 104-4 Malabsorption of Cobalamin¶
7.1 Pharmacologic Therapy¶
Hydroxocobalamin (1000 µg IM weekly) or cyanocobalamin (1000 µg IM monthly) for cobalamin deficiency. Folic acid (5-15 mg/day) for folate deficiency.
7.2 Non-Pharmacologic¶
Dietary supplementation, correction of malabsorption (e.g., treating Pernicious Anemia), and avoiding antifolate drugs.
8. PROGNOSIS & COMPLICATIONS¶
Early treatment prevents neurological damage. Untreated cobalamin deficiency may lead to irreversible neuropathy, dementia, and cardiovascular complications. Folate deficiency is associated with neural tube defects and cognitive decline.
8.1 Long-Term Outcomes¶
Prognosis is favorable with prompt treatment. Chronic deficiency may cause irreversible neurological damage, especially in elderly patients.
8.2 Complications¶
Neurological deficits, cardiovascular disease, and increased risk of malignancies (e.g., leukemia) in untreated cases.
9. SPECIAL CONSIDERATIONS¶
Pregnancy: Folic acid supplementation (400 µg/day) prevents neural tube defects. Elderly: Cobalamin deficiency may mimic dementia. Pediatrics: Premature infants require folic acid prophylaxis. Drug interactions: Antifolates (e.g., methotrexate) and proton pump inhibitors may exacerbate deficiency.
10. KEY POINTS & CLINICAL PEARLS¶
- Cobalamin deficiency is more common in older adults and vegans.
- Folate deficiency is prevalent in malnourished populations.
- Hypersegmented neutrophils and megaloblastic marrow are diagnostic.
- Lifelong cobalamin therapy is required for irreversible neurological damage.
- Folic acid supplementation prevents neural tube defects in pregnancy.