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Megaloblastic Anemia

Megaloblastic Anemia is a type of anemia caused by impaired DNA synthesis, resulting in abnormal maturation of red blood cell precursors in the bone marrow. The most common causes are vitamin B12 deficiency and folate deficiency.

Also called

Megalocytic Anemia

ICD-10

D51.9

Specialty

Hematology

Onset

Chronic

Reviewed

August 2026

On This Page

Overview

Megaloblastic Anemia occurs when impaired DNA synthesis prevents normal cell division in the bone marrow. This produces large, immature red blood cell precursors (megaloblasts) and large circulating red blood cells (macrocytosis). Vitamin B12 and folate deficiencies are the most common causes.

It may also cause leukopenia and thrombocytopenia, resulting in pancytopenia in severe cases. Vitamin B12 deficiency can additionally cause neurological complications, which are not typically seen in folate deficiency.

Etiology & Risk Factors

-Etiology

Vitamin B12 Deficiency

  • Poor dietary intake

  • Strict vegan diet

  • Pernicious anemia

  • Atrophic gastritis

  • H. pylori infection

  • Gastrectomy

  • Bariatric surgery

  • Terminal ileal disease

  • Crohn’s disease

  • Ileal resection

  • Malabsorption

  • Long-term metformin use

  • Long-term proton pump inhibitor use

  • Nitrous oxide exposure

Folate Deficiency

  • Poor dietary intake

  • Alcohol use disorder

  • Malnutrition

  • Malabsorption

  • Celiac disease

  • Increased requirements during pregnancy

  • Hemolytic anemia

  • Chronic inflammatory disease

Drugs

  • Methotrexate

  • Trimethoprim

  • Phenytoin

  • Valproate

  • Sulfasalazine

  • Hydroxyurea

Pathophysiology

Vitamin B12 or folate deficiency → impaired DNA synthesis → delayed nuclear maturation and cell division → cytoplasmic maturation continues → nuclear-cytoplasmic asynchrony → formation of large abnormal megaloblasts in bone marrow → ineffective erythropoiesis → intramedullary destruction of abnormal precursors → macrocytic anemia → reduced oxygen delivery to tissues → fatigue and other symptoms of anemia.

Vitamin B12 deficiency → impaired DNA synthesis + accumulation of methylmalonic acid → demyelination and neurological injury.

Clinical Presentation

-Symptoms

  • Fatigue

  • Weakness

  • Pallor

  • Shortness of breath

  • Dizziness

  • Headache

  • Palpitations

  • Reduced exercise tolerance

-Gastrointestinal Features

  • Glossitis

  • Sore tongue

  • Angular cheilitis

  • Loss of appetite

  • Weight loss

  • Diarrhea

-Neurological Features of Vitamin B12 Deficiency

  • Peripheral paresthesia

  • Numbness

  • Loss of vibration sense

  • Loss of proprioception

  • Ataxia

  • Muscle weakness

  • Cognitive impairment

  • Memory problems

-Severe Disease

  • Pancytopenia

  • Recurrent infections

  • Easy bruising

  • Bleeding

History Taking

Ask about:

  • Fatigue and weakness
  • Dietary habits
  • Vegan or vegetarian diet
  • Weight loss
  • Gastrointestinal symptoms
  • Chronic diarrhea
  • Previous gastric surgery
  • Bariatric surgery
  • Ileal resection
  • Crohn’s disease
  • Celiac disease
  • Alcohol intake
  • Pregnancy
  • Medication history
  • Metformin use

Physical Examination

  • General Examination

    Look for:

    • Pallor

    • Jaundice

    • Tachycardia

    • Weight loss

    • Signs of malnutrition

    Oral Examination

    Look for:

    • Glossitis

    • Smooth, red tongue

    • Angular cheilitis

    Neurological Examination

    Particularly in suspected Vitamin B12 deficiency:

    • Sensory impairment

    • Reduced vibration sense

    • Impaired proprioception

    • Peripheral neuropathy

    • Ataxia

    • Muscle weakness

    • Cognitive changes

Investigations

-Complete Blood Count (CBC)

Typical findings:

  • Low hemoglobin

  • Increased MCV

  • Increased MCH

  • Normal or low MCHC

  • Increased RDW

Severe disease may cause:

  • Leukopenia

  • Thrombocytopenia

  • Pancytopenia

-Peripheral Blood Film

Characteristic findings:

  • Macro-ovalocytes

  • Hypersegmented neutrophils

  • Anisocytosis

  • Poikilocytosis

-Reticulocyte Count

  • Low or inappropriately normal due to ineffective erythropoiesis

-Vitamin Levels

  • Serum vitamin B12

  • Serum folate

Additional Tests

-If vitamin B12 deficiency is suspected:

  • Methylmalonic acid (MMA)

  • Homocysteine

Typical pattern:

  • B12 deficiency: ↑ MMA + ↑ homocysteine

  • Folate deficiency: Normal MMA + ↑ homocysteine

Tests for Underlying Cause

Depending on the clinical context:

  • Anti-intrinsic factor antibodies

  • Anti-parietal cell antibodies

  • Celiac disease screening

  • H. pylori testing

  • Gastrointestinal evaluation

  • Thyroid function tests

  • Liver function tests

Bone Marrow Examination

Not routinely required.

May show:

    • Megaloblastic erythropoiesis

    • Nuclear-cytoplasmic asynchrony

    • Giant granulocyte precursors

Diagnosis

Diagnosis is based on:

  • Macrocytic anemia
  • Peripheral blood film showing macro-ovalocytes and hypersegmented neutrophils
  • Low vitamin B12 and/or folate levels
  • Elevated homocysteine
  • Elevated methylmalonic acid in vitamin B12 deficiency

The underlying cause should always be identified.

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Management

Vitamin B12 Deficiency

Replace vitamin B12 with:

  • Intramuscular hydroxocobalamin or cyanocobalamin

  • High-dose oral vitamin B12 in selected patients

Long-term or lifelong replacement may be required in:

  • Pernicious anemia

  • Irreversible malabsorption

  • Previous gastric or ileal surgery

Folate Deficiency

  • Oral folic acid

  • Improve dietary folate intake

  • Treat the underlying cause

  • Correct associated malnutrition

Important Management Rule

Do not treat suspected megaloblastic anemia with folic acid alone until vitamin B12 deficiency has been excluded or adequately treated.

Supportive Management

  • Treat severe anemia appropriately

  • Correct nutritional deficiencies

  • Stop or modify causative medications when possible

  • Treat underlying malabsorption

  • Manage associated neurological complications

Complications

  • Severe anemia
  • Pancytopenia
  • Infection due to leukopenia
  • Bleeding due to thrombocytopenia
  • Glossitis
  • Neurological damage
  • Peripheral neuropathy
  • Subacute combined degeneration of the spinal cord
  • Cognitive impairment
  • Irreversible neurological damage from untreated B12 deficiency

Prognosis

The prognosis is generally excellent when the underlying deficiency is recognized and treated early. Hematological abnormalities usually improve with appropriate replacement therapy. Neurological complications caused by vitamin B12 deficiency may be partially or completely reversible if treated early but can become permanent with prolonged deficiency.

Key Points / Clinical Pearls

  • Megaloblastic Anemia is caused by impaired DNA synthesis.
    The two main causes are vitamin B12 and folate deficiency.
  • It typically causes macrocytic anemia.
    Peripheral blood film shows macro-ovalocytes and hypersegmented neutrophils.
  • B12 deficiency causes increased methylmalonic acid and homocysteine.
    Folate deficiency causes increased homocysteine with normal methylmalonic acid.
  • Vitamin B12 deficiency can cause neurological damage.
    Always exclude B12 deficiency before giving folic acid alone.
  • Treatment depends on the underlying cause and involves vitamin replacement.
  • Early treatment usually results in an excellent prognosis.