Definition
Vitamin B12 (cobalamin) and folate (vitamin B9) deficiency anaemias are megaloblastic anaemias, characterised by impaired DNA synthesis in red blood cell precursors, leading to production of large, immature red blood cells (megaloblasts) in the bone marrow and macrocytic red cells in the blood.
Causes
A. Vitamin B12 deficiency
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Dietary: Rare, except in strict vegans without supplementation
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Malabsorption:
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Pernicious anaemia (autoimmune destruction of gastric parietal cells → intrinsic factor deficiency)
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Gastrectomy or gastric bypass surgery
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Chronic gastritis
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Crohn’s disease, ileal resection
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Infection with Diphyllobothrium latum (fish tapeworm)
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Other: Chronic metformin use, long-term proton pump inhibitors or H2 blockers
B. Folate deficiency
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Dietary: Poor intake (elderly, alcohol dependence, unbalanced diet)
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Increased demand: Pregnancy, lactation, haemolysis, severe skin disease (psoriasis)
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Malabsorption: Coeliac disease, Crohn’s disease
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Drugs: Methotrexate, phenytoin, trimethoprim (folate antagonists)
Symptoms and Signs
General anaemia symptoms
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Fatigue, weakness
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Dyspnoea on exertion
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Pallor
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Palpitations
Specific to B12 deficiency
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Neurological symptoms (due to demyelination):
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Peripheral neuropathy (numbness, tingling)
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Loss of vibration and position sense
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Ataxia
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Cognitive changes, depression, psychosis
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Glossitis (red, sore tongue)
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Angular cheilitis
Specific to folate deficiency
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Similar anaemia symptoms but no neurological signs
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Glossitis, mouth ulcers
Diagnosis
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Full blood count (FBC): Macrocytic anaemia (MCV >100 fL), low haemoglobin
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Peripheral blood smear: Macro-ovalocytes, hypersegmented neutrophils
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Serum vitamin levels: Low serum B12 or folate
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Other tests:
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Serum methylmalonic acid ↑ in B12 deficiency (normal in folate deficiency)
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Serum homocysteine ↑ in both deficiencies
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Anti–intrinsic factor and anti–parietal cell antibodies for pernicious anaemia
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Bone marrow (rarely needed): Megaloblastic erythropoiesis
Treatment
A. Vitamin B12 deficiency
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Hydroxocobalamin (UK) or cyanocobalamin (US)
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Initial: 1 mg intramuscularly three times a week for 2 weeks
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Maintenance: 1 mg IM every 3 months for life if irreversible cause (e.g., pernicious anaemia)
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Oral B12 may be used for dietary deficiency if absorption is intact
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Important: Replace B12 before folate in combined deficiency to avoid precipitating or worsening neurological damage
B. Folate deficiency
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Folic acid: 5 mg orally once daily for at least 4 months (longer if ongoing cause)
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Treat underlying cause (e.g., improve diet, stop causative drugs)
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Supplement all pregnant women with 400 micrograms folic acid daily to prevent neural tube defects
Prognosis
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Anaemia usually improves within weeks of treatment
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Neurological symptoms of B12 deficiency may be irreversible if treatment delayed
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Lifelong monitoring may be required for chronic or irreversible causes
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