Megaloblastic (B12/Folate deficiency)

Summary

Megaloblastic anemia is a type of macrocytic anemia (MCV > 100 fL) caused by impaired DNA synthesis in red blood cell precursors, most commonly due to vitamin B12 (cobalamin) or folate (B9) deficiency.

• Both vitamins are needed to make thymidine (a DNA building block). When deficient, the cell nucleus matures slowly while the cytoplasm keeps growing → large (megaloblastic) cells.
• Hallmark blood findings: macro-ovalocytes + hypersegmented neutrophils (> 5 lobes).
• B12 deficiency → anemia plus neurologic symptoms (subacute combined degeneration, peripheral neuropathy).
• Folate deficiency → anemia only, no neurologic symptoms; common in pregnancy, alcoholics, and with certain drugs (methotrexate, phenytoin, trimethoprim).
• Treatment: replace the missing vitamin. Never give folate alone if B12 deficiency is possible — it corrects the anemia but worsens the neurologic damage.

Definition

Key concept

• RBC precursors need thymidine (dTMP) to build DNA.
• Both B12 and folate are needed to make dTMP.
• If either is deficient → DNA synthesis slows → nucleus lags behind cytoplasm → cell becomes large and immature → destroyed in marrow (ineffective erythropoiesis) → anemia.
• The same defect affects all rapidly dividing cells — explains the pancytopenia and GI mucosal changes (glossitis).

Pathophysiology

The biochemical link

Both vitamins converge on one reaction: making dTMP (thymidine) for DNA.

• Folate (as 5,10-methylene-THF) donates a methyl group to dUMP → dTMP via thymidylate synthase.
• B12 is needed to regenerate THF from 5-methyl-THF (via the homocysteine → methionine reaction, enzyme = methionine synthase).
• So in B12 deficiency, folate gets "trapped" as 5-methyl-THF → functional folate deficiency → same megaloblastic picture.

📊 Click here: Folate & Methionine cycle diagram

Why only B12 causes neurologic symptoms

• B12 has a second job: it is a cofactor for methylmalonyl-CoA mutase (converts methylmalonyl-CoA → succinyl-CoA).
• In B12 deficiency → methylmalonic acid (MMA) accumulates → abnormal odd-chain fatty acids incorporated into myelin → demyelination of dorsal columns, lateral corticospinal tracts, and peripheral nerves.
• Folate has no role in myelin → no neurologic symptoms in folate deficiency.

Etiology

The key is knowing which patient gets which deficiency:

Site of absorption – clinical clue

📍 Click here: GI absorption sites map

• B12 → terminal ileum (needs intrinsic factor). Disease of terminal ileum = B12 deficiency.
• Folate → jejunum. Disease of jejunum (celiac, tropical sprue) = folate deficiency first, then B12 if disease spreads.

Clinical Features

1. Anemia (both B12 and folate)

• Fatigue, weakness, pallor, dyspnea on exertion
• May be mild jaundice (from ineffective erythropoiesis → hemolysis of precursors in marrow) → mild ↑ indirect bilirubin, ↑ LDH

2. Epithelial / GI (both)

• Glossitis – smooth, beefy-red, painful tongue
• Angular cheilitis, oral ulcers
• Anorexia, diarrhea, weight loss

3. Neurologic findings — B12 ONLY 🚨

Develops before or independent of anemia. Often irreversible if treated late.

🧠 Click here: Subacute combined degeneration – spinal cord

• Peripheral neuropathy – symmetrical numbness, tingling, paresthesia of feet/hands (first symptom)
• Subacute combined degeneration – demyelination of:
  • Dorsal columns → loss of vibration, proprioception → + Romberg sign, sensory ataxia
  • Lateral corticospinal tracts → spastic paresis, hyperreflexia, + Babinski
  • Spinocerebellar tracts → ataxia
• Cognitive symptoms – memory loss, dementia, depression, psychosis ("megaloblastic madness")
• Romberg sign is positive

Diagnosis

Step 1 – CBC + smear

• ↑ MCV (> 100 fL; often > 110 in megaloblastic)
• Pancytopenia may be present in severe cases (all marrow lineages affected)
• Peripheral smear:
  • Macro-ovalocytes (large, oval RBCs)
  • Hypersegmented neutrophils (> 5 lobes) — the earliest and most specific sign

🔬 Click here: Smear findings (macro-ovalocyte + hypersegmented neutrophil)

Step 2 – Confirm with vitamin levels

• Check serum B12 and serum (or RBC) folate
• If results are equivocal or borderline → check MMA and homocysteine:
  • ↑ MMA + ↑ homocysteine → B12 deficiency
  • Normal MMA + ↑ homocysteine → folate deficiency

Step 3 – Find the cause of B12 deficiency

• Anti-intrinsic factor antibodies – highly specific for pernicious anemia (test of choice)
• Anti-parietal cell antibodies – sensitive but less specific
• Serum gastrin – elevated in pernicious anemia (no acid feedback)
• Endoscopy – if atrophic gastritis or malignancy suspected
• Schilling test – historical, no longer used

Differential Diagnosis

Other causes of macrocytic anemia (MCV > 100)

Not every high MCV is megaloblastic! The smear is key.

📋 Click here: Full table – Causes of macrocytic anemia

Megaloblastic causes (hypersegmented PMNs present)

• B12 deficiency
• Folate deficiency
• Drug-induced: methotrexate, hydroxyurea, 5-FU, AZT, phenytoin, trimethoprim
• Inherited: orotic aciduria, Lesch-Nyhan

Non-megaloblastic causes (NO hypersegmented PMNs)

• Chronic alcoholism (direct marrow toxicity ± associated folate deficiency)
• Liver disease – increased cholesterol in RBC membrane → target cells, round macrocytes
• Hypothyroidism
• Reticulocytosis (hemolysis, recent blood loss) – reticulocytes are large
• Myelodysplastic syndrome (MDS) – elderly, dysplastic cells, often progresses to AML

B12 deficiency vs other neurologic conditions

• Diabetic neuropathy – distal symmetric, sensory; no anemia/glossitis
• Multiple sclerosis – relapsing-remitting, younger pts, MRI lesions
• Tabes dorsalis (syphilis) – dorsal column involvement but RPR/VDRL positive
• Alcoholic neuropathy – often co-exists; check B12, folate, thiamine

Management

Vitamin B12 deficiency

• Pernicious anemia / malabsorption → IM cyanocobalamin (or hydroxocobalamin):
  • 1000 μg IM daily × 1 week → weekly × 4 weeks → monthly for life
• Dietary deficiency or mild cases → oral B12 (1000–2000 μg/day) can be effective even in pernicious anemia (passive absorption ~1%)
• Treat the underlying cause (stop PPIs, treat H. pylori, etc.)

Folate deficiency

• Oral folic acid 1–5 mg/day for 1–4 months
• Treat / remove the cause: stop alcohol, change medication, treat celiac disease
• Pregnancy: 400 μg/day for all women of reproductive age; 4 mg/day if previous NTD child

Response to treatment

• Day 2–3: patient feels better; ↓ MMA and homocysteine
• Day 5–7: reticulocytosis peaks → this is the marker that treatment is working
• Watch for hypokalemia during recovery — rapid hematopoiesis consumes K⁺
• Hgb normalizes in 6–8 weeks
• Neurologic recovery: weeks to months; may be incomplete if > 6 months of damage

Special situations

• Methotrexate toxicity → rescue with folinic acid (leucovorin), NOT folic acid (MTX blocks DHFR; leucovorin bypasses it)
• Trimethoprim-induced → can be reversed by leucovorin

Complications

Untreated B12 deficiency

• Permanent neurologic deficits – the most feared complication (subacute combined degeneration, dementia, optic atrophy)
• Severe anemia → high-output heart failure
• Pernicious anemia → ↑ risk of gastric adenocarcinoma and gastric carcinoid tumors (chronic atrophic gastritis, hypergastrinemia)
• ↑ homocysteine → ↑ risk of atherosclerosis, thrombosis, MI, stroke

Untreated folate deficiency

• In pregnancy: neural tube defects (spina bifida, anencephaly) – occurs in the first 4 weeks, often before pregnancy is recognized
• ↑ homocysteine → ↑ cardiovascular risk
• Severe anemia, especially in pregnancy → ↑ maternal/fetal morbidity

Treatment complications

• Hypokalemia during reticulocyte response (newly forming cells take up K⁺) – may cause arrhythmia in elderly
• Allergic reactions to IM B12 (rare)

Mnemonics