Megaloblastic anemia is a macrocytic anemia characterized by ineffective erythropoiesis and the presence of large, structurally abnormal, immature red blood cell precursors (megaloblasts) in the bone marrow.

It is most commonly caused by:

• Vitamin B12 (cobalamin) deficiency

• Folate (vitamin B9) deficiency

Both vitamins are essential for DNA synthesis, and their deficiency results in defective nuclear maturation with preserved cytoplasmic development (nuclear–cytoplasmic asynchrony).

Pathophysiology

• Impaired thymidine synthesis → defective DNA replication

• Delayed nuclear maturation

• Ineffective hematopoiesis

• Intramedullary hemolysis

• Pancytopenia (in severe cases)

Vitamin B12 deficiency uniquely affects the nervous system, leading to demyelination (subacute combined degeneration).

Risk Factors

Vitamin B12 Deficiency

• Pernicious anemia (autoimmune intrinsic factor deficiency)

• Gastrectomy or bariatric surgery

• Chronic gastritis

• Malabsorption (Crohn’s disease, celiac disease)

• Long-term metformin use

• Strict vegan diet

• Elderly age

• Fish tapeworm (Diphyllobothrium latum)

Folate Deficiency

• Poor dietary intake

• Chronic alcoholism

• Pregnancy

• Hemolytic anemia (increased demand)

• Malabsorption

• Methotrexate or anticonvulsant therapy

Signs and Symptoms

General Anemia Features

• Pallor

• Fatigue

• Dyspnea

• Tachycardia

Characteristic Features

• Mild jaundice (lemon-yellow tint)

• Glossitis (beefy red tongue)

• Hyperpigmentation of palms

• Hair loss

• Depression

• Weight loss

Neurological Features (Vitamin B12 Deficiency)

• Paresthesia (pins and needles)

• Numbness (hands and feet)

• Ataxic gait

• Loss of vibration and position sense

• Tremors

• Peripheral neuropathy

• Cognitive changes

• Psychosis (rare)

Neurological symptoms may occur even without severe anemia.

Diagnostic Criteria

Diagnosis is based on:

1. Macrocytic anemia

2. Laboratory evidence of B12 or folate deficiency

3. Characteristic peripheral smear findings

4. Clinical features

Investigations

Hematological Tests

• FBC:

  • Low hemoglobin

  • Elevated MCV (>100 fL)

  • Possible pancytopenia

• Reticulocyte count (usually low initially)

• Peripheral smear:

  • Oval macrocytes

  • Hypersegmented neutrophils (>5 lobes)

Biochemical Tests

• Serum vitamin B12

• Serum folate

• Red cell folate (more reliable)

• Elevated LDH

• Elevated indirect bilirubin

• Homocysteine (↑ in both B12 and folate deficiency)

• Methylmalonic acid (↑ only in B12 deficiency)

Additional Tests

• TSH (exclude hypothyroidism)

• Urea and electrolytes

• Liver function tests

• Anti-intrinsic factor antibodies (pernicious anemia)

• Bone marrow biopsy (if diagnosis unclear)

Differential Diagnosis

• Liver disease

• Hypothyroidism

• Alcoholism

• Myelodysplastic syndrome

• Aplastic anemia

• Drug-induced macrocytosis

Treatment

Treatment depends on the underlying deficiency.

Non-Pharmacological Management

• Dietary counseling

• Alcohol cessation

• Management of malabsorption

• Treat underlying gastrointestinal pathology

Pharmacological Management

1. Vitamin B12 Deficiency (Without Neurological Signs)

Hydroxocobalamin

• 1 mg IM three times weekly for 2 weeks

• Then 1 mg IM every 3 months (maintenance)

Review every 2 months initially, adjust based on clinical response.

2. Pernicious Anemia or B12 Deficiency with Neurological Symptoms

• 1 mg IM on alternate days

• Continue until maximum neurological improvement

• Then 1 mg IM every 2 months (lifelong)

Neurological recovery may be incomplete if treatment is delayed.

3. Folate Deficiency

• Folic acid 5 mg PO daily for at least 4 months

• Treat underlying cause

Important: Always exclude or treat B12 deficiency before giving folate alone, as folate may correct anemia but worsen neurological damage.

4. Combined Therapy (When Indicated)

• Folic acid 5 mg PO daily (at least 2 months)

• Ferrous sulfate 200 mg three times daily (if iron deficiency coexists)

Monitoring Response

• Reticulocyte rise within 5–7 days

• Hemoglobin improves within 2–3 weeks

• Full correction in ~2 months

• Neurological recovery may take months

Complications

• Irreversible neuropathy (untreated B12 deficiency)

• Cognitive impairment

• Subacute combined degeneration

• Heart failure (severe anemia)

• Infertility

• Increased thrombotic risk (elevated homocysteine)

Prevention

• Balanced diet rich in:

  • Meat, fish, dairy (B12)

  • Leafy greens (folate)

• Supplementation in:

  • Pregnancy

  • Chronic hemolytic anemia

  • Malabsorption syndromes

• Lifelong B12 in pernicious anemia

• Screening high-risk elderly patients

Prognosis

• Excellent with early treatment

• Neurological complications reversible if treated promptly

• Pernicious anemia requires lifelong therapy