Megaloblastic Anaemia
Megaloblastic anaemia is a type of macrocytic anaemia caused by impaired DNA synthesis, most commonly due to vitamin B12 or folate deficiency, resulting in large, abnormal erythroid precursors (megaloblasts) in the bone marrow.
Megaloblastic Anaemia
Megaloblastic anaemia is a macrocytic anaemia characterised by a distinctive morphological abnormality in rapidly dividing cells: nuclear-cytoplasmic maturation asynchrony (also called nuclear-cytoplasmic dissociation). The cytoplasm matures normally (haemoglobin synthesis proceeds), but the nucleus lags behind because of impaired DNA synthesis, resulting in cells that are abnormally large ("megalo-" = large, "-blast" = immature precursor cell). This affects all rapidly dividing cell lines — erythroid, myeloid, and megakaryocytic — which is why you see pancytopenia, not just anaemia [1][2].
The name literally tells you the pathology: megalo (large) + blast (precursor/immature cell). The bone marrow is full of large, immature-looking precursors that cannot complete nuclear division properly.
Key Distinction: Megaloblastic vs Non-Megaloblastic Macrocytic Anaemia
Not all macrocytic anaemias are megaloblastic. Megaloblastic anaemia shows oval macrocytes and hypersegmented neutrophils on peripheral blood smear (PBS), reflecting impaired DNA synthesis. Non-megaloblastic macrocytic anaemia shows round macrocytes with normal neutrophils, due to membrane abnormalities with abnormal cholesterol metabolism [2][3].
| Feature | Megaloblastic | Non-Megaloblastic |
|---|---|---|
| Pathophysiology | Nuclear-cytoplasmic maturation asynchrony | Membrane abnormalities with abnormal cholesterol metabolism |
| PBS | Oval macrocytes, hypersegmented neutrophils | Round macrocytes, normal neutrophils |
| Aetiology | B12 deficiency, folate deficiency, drugs (immunosuppressants: MTX/AZA, TKI: imatinib, sunitinib) | Alcoholism (most common), liver disease, hypothyroidism, haemolytic anaemia (reticulocytosis), aplastic anaemia, myelodysplastic syndrome |
| Investigations | Active B12 and folate levels ± RBC folate | Haemolytic screen (LDH, haptoglobin, bilirubin, reticulocyte), LFT, TFT |
2. Epidemiology
- Megaloblastic anaemia is common worldwide, particularly in populations with dietary deficiencies or high prevalence of autoimmune disease.
- B12 deficiency is the predominant cause in developed countries; folate deficiency is more common in developing countries and in the context of malnutrition/alcoholism.
- Pernicious anaemia (the most common medical cause of B12 deficiency) has a prevalence of approximately 0.1% in the general population and up to 2% in those over 60, with a female predominance.
- In Hong Kong, B12 deficiency is the primary cause of megaloblastic anaemia. Folate deficiency causing megaloblastic anaemia is exceedingly rare — Professor Kwong's paper on HK subjects found zero cases of folate-deficiency megaloblastic anaemia [1][4].
- Dietary B12 deficiency is uncommon given the Cantonese diet (rich in meat, fish, shellfish), so most cases in HK are due to pernicious anaemia, gastric surgery, or terminal ileal disease.
- Strict veganism (increasingly popular) is an emerging risk factor.
HKUMed Exam Pearl
In front of Professor Kwong, never say folate deficiency as a cause of megaloblastic anaemia — since in his paper on HK subjects there were zero cases [1]. In the HK exam context, always lead with B12 deficiency.
| Category | Risk Factors | Mechanism |
|---|---|---|
| Dietary | Strict veganism/vegetarianism, alcoholism, elderly with poor nutrition, poverty | Inadequate intake of B12 (animal products) or folate (green vegetables) |
| Gastric | Pernicious anaemia, total/partial gastrectomy, gastric bypass surgery, chronic PPI/H2-blocker use, H. pylori atrophic gastritis | Loss of intrinsic factor (IF) or acid → impaired B12 liberation/absorption |
| Intestinal | Crohn's disease (terminal ileum), ileal resection, coeliac disease, tropical sprue, blind loop syndrome (bacterial overgrowth), fish tapeworm (Diphyllobothrium latum) | B12 absorbed in terminal ileum; folate in upper small intestine |
| Increased demand | Pregnancy, lactation, haemolytic anaemia, exfoliative skin disease (psoriasis), rapid cell turnover | Folate requirements increase dramatically |
| Drugs | Methotrexate (MTX), azathioprine (AZA), TKI (imatinib, sunitinib), trimethoprim, phenytoin, metformin, PPIs | Various: anti-folate, impaired DNA synthesis, reduced absorption |
| Autoimmune | Other autoimmune diseases (vitiligo, autoimmune thyroid disease, type 1 DM, Addison's) | Pernicious anaemia clusters with other autoimmune conditions |
| Age/Sex | Older age, female sex | Pernicious anaemia is more common in elderly females |
4. Anatomy and Physiology of B12 and Folate Metabolism
Understanding megaloblastic anaemia requires understanding the normal absorption, transport, storage, and biochemical function of vitamin B12 (cobalamin) and folate (vitamin B9).
4.1 Vitamin B12 (Cobalamin)
- Diet: meat, fish, egg, dairy products [2].
- Also produced by gut microbes (though not significantly absorbed in humans).
- Not found in plant-based foods — hence strict vegans are at risk.
- Stomach: B12 in food is bound to proteins. Gastric acid and pepsin liberate B12 from food proteins. Free B12 then binds to haptocorrin (also called R-protein or transcobalamin I/III), a salivary/gastric glycoprotein. Simultaneously, parietal cells in the gastric body/fundus secrete intrinsic factor (IF).
- Duodenum: Pancreatic proteases digest haptocorrin, releasing B12, which then binds to IF (IF is resistant to pancreatic proteases).
- Terminal ileum: The B12–IF complex binds to specific receptors (cubilin) on the enterocytes of the terminal ileum → absorbed by receptor-mediated endocytosis.
This is why diseases of the terminal ileum (Crohn's, resection) specifically cause B12 deficiency, not folate deficiency.
- Once absorbed, B12 enters the portal circulation and binds to transport proteins:
- Transcobalamin II (TCII): carries the active fraction of B12 (only 10–30% of total serum B12) → this is what is delivered to tissues. This is measured as holotranscobalamin [4].
- Haptocorrin (TCI/TCIII): carries the inactive fraction (70–90% of total serum B12) → acts as a storage/buffer pool.
Why Measure Holotranscobalamin?
Serum holotranscobalamin (TCII-bound B12) is the active fraction and reflects true B12 available to tissues. Total serum B12 includes the large inactive haptocorrin-bound pool and can be misleadingly normal even when tissue B12 is low. Modern practice measures holotranscobalamin [4].
- Liver stores sufficient B12 for approximately 3 years of use [2].
- There is also significant enterohepatic circulation of B12 (bile → gut → reabsorbed in terminal ileum).
- This means it takes years for B12 deficiency to manifest after cessation of intake (e.g., after going vegan or after gastrectomy).
B12 has two active coenzyme forms, each critical for a different reaction:
-
Methylcobalamin (Methyl-B12): cofactor for methionine synthase
- Reaction: Homocysteine → Methionine (with concurrent conversion of methyl-THF → THF)
- This reaction is critical because it regenerates tetrahydrofolate (THF), which is then available for thymidylate synthesis (needed for DNA synthesis).
- This is the link between B12 and folate metabolism — B12 deficiency "traps" folate as methyl-THF (the "methylfolate trap"), making it unavailable for DNA synthesis even if total folate levels are adequate.
-
Adenosylcobalamin (Ado-B12): cofactor for methylmalonyl-CoA mutase
- Reaction: Methylmalonyl-CoA → Succinyl-CoA (feeds into Krebs cycle)
- Deficiency causes accumulation of methylmalonic acid (MMA) → toxic to myelin → this is the basis for the neurological complications of B12 deficiency (subacute combined degeneration of the cord).
4.2 Folate (Vitamin B9)
- Diet only: green vegetables, liver, nuts [2].
- Destroyed by prolonged cooking — hence overcooking reduces folate content.
- Dietary folate (polyglutamate form) is hydrolysed to monoglutamate by jejunal brush border enzymes → converted to methyl-THF → absorbed in the upper small intestine [2].
- Folate (as THF derivatives) is essential for DNA synthesis — specifically for thymidylate synthesis (dUMP → dTMP) and purine synthesis.
- B12 traps folate into cells [2] — in B12 deficiency, folate is trapped as methyl-THF and cannot be converted to the active THF forms needed for DNA synthesis. This explains why B12 deficiency can cause a falsely elevated serum folate level (folate is trapped in plasma as methyl-THF and not utilized).
| Feature | B12 | Folate |
|---|---|---|
| Source | Meat, fish, egg, dairy + microbes | Green vegetables, liver, nuts (destroyed by cooking) |
| Absorption site | Terminal ileum (requires IF) | Upper small intestine |
| Transport | Transcobalamin (active, 10-30%), Haptocorrin (inactive) | Weakly to albumin |
| Storage | Liver; sufficient for ~3 years | Body stores sufficient for ~3 months |
| Biochemical role | Methyl-B12: cofactor of methionine synthase (homocysteine → methionine) for DNA synthesis; Ado-B12: MMA → succinyl-CoA for Krebs cycle | DNA synthesis (B12 traps folate into cells) |
| Time to deficiency | Years | Months |
5. Aetiology and Pathophysiology
The most common medical cause of vitamin B12 deficiency is pernicious anaemia (95% of cases) [4].
Aetiological Classification (High-Yield for SAQ)
| Category | Cause | Mechanism |
|---|---|---|
| Dietary | Strict vegetarian/vegan, alcoholic | Inadequate intake |
| Gastric | Pernicious anaemia (anti-parietal cell / anti-IF antibodies → impaired IF secretion and achlorhydria; a/w other autoimmune diseases e.g. vitiligo, thyroid) | Autoimmune destruction → ↓IF |
| Post-total gastrectomy | No parietal cells → no IF | |
| Intestinal | Ileal resection (e.g. Crohn's, TB) | Loss of absorption site |
| Blind loop syndrome (bacterial overgrowth) | Bacteria consume B12 | |
| Malabsorption (e.g. coeliac disease) | Generalized malabsorption | |
| Fish tapeworm (Diphyllobothrium latum) | Parasite consumes B12 | |
| Drugs | PPI, metformin | PPI: ↓acid → ↓B12 liberation from food; Metformin: interferes with calcium-dependent IF-B12 absorption in ileum |
SAQ Favourite: Causes of B12 Deficiency
Remember the pathway: Mouth → Stomach → Duodenum → Terminal Ileum. At each step, something can go wrong:
- Mouth/Diet: Not eating B12 (vegan)
- Stomach: No IF (pernicious anaemia, gastrectomy), no acid (PPI)
- Duodenum: No pancreatic enzymes (rare cause)
- Terminal ileum: No absorption (Crohn's, resection, coeliac, tapeworm, overgrowth)
| Category | Cause | Mechanism |
|---|---|---|
| Dietary | Poor nutrition (elderly, alcoholic) | Inadequate intake; alcohol also directly impairs folate metabolism |
| Increased demand | Pregnancy, lactation, haemolytic anaemia, psoriasis, exfoliative dermatitis | Rapid cell turnover demands more folate for DNA synthesis |
| Malabsorption | Coeliac disease, tropical sprue, Crohn's (proximal) | Folate absorbed in upper small intestine |
| Drugs | Methotrexate (dihydrofolate reductase inhibitor), trimethoprim, phenytoin, sulfasalazine | Anti-folate drugs block conversion of dihydrofolate → THF |
Folate deficiency causing megaloblastic anaemia is extremely rare in Hong Kong [1]. In clinical practice and HKUMed exams, B12 deficiency (especially pernicious anaemia) is the primary focus.
Drugs causing megaloblastic change include immunosuppressants (MTX, AZA) and TKI (imatinib, sunitinib) [2]. These directly impair DNA synthesis:
- Methotrexate (MTX): Inhibits dihydrofolate reductase → blocks regeneration of THF → impaired thymidylate synthesis.
- Azathioprine (AZA): Converted to 6-mercaptopurine → inhibits purine synthesis.
- TKIs: Mechanism less clear but involve impaired haematopoiesis.
The fundamental defect is impaired DNA synthesis with relatively preserved RNA and protein (including haemoglobin) synthesis.
Why does this happen?
- B12 and folate are both required for thymidylate synthesis (converting dUMP → dTMP), which is essential for DNA replication.
- Without adequate thymidylate, cells cannot replicate their DNA properly.
- However, RNA translation and protein synthesis continue normally → the cytoplasm matures and enlarges normally while the nucleus remains immature.
- The result: large cells with immature nuclei (megaloblasts in the bone marrow).
Consequences in each cell line:
- Erythroid: Megaloblastic erythroid precursors → large RBCs (macrocytes, specifically oval macrocytes). Many of these abnormal precursors die within the bone marrow before reaching maturity → this is called intramedullary haemolysis (also termed "ineffective erythropoiesis") [4].
- Myeloid: Giant metamyelocytes and band forms → hypersegmented neutrophils (≥ 5 lobes in one neutrophil or ≥ 6 lobes = diagnostic) [1]. The mechanism is that the nucleus keeps attempting division but fails to complete it, creating extra lobes.
- Megakaryocytic: Impaired platelet production → thrombocytopenia.
The net effect is pancytopenia — all three cell lines are affected because all rapidly dividing cells need DNA synthesis [4].
Why Intramedullary Haemolysis?
Haemolysis occurs inside the bone marrow in megaloblastic anaemia. The megaloblastic precursors are so abnormal that many undergo apoptosis/destruction before ever leaving the marrow. This means:
- You may NOT see reticulocytosis or polychromasia (unlike extravascular haemolysis where the marrow is trying to compensate) [4]
- You WILL see elevated unconjugated bilirubin and LDH (from cell destruction)
- The reticulocyte count is inappropriately low for the degree of anaemia
Other conditions with intramedullary haemolysis include thalassaemia major and MDS [4].
6. Classification
| Type | Cause |
|---|---|
| B12 deficiency | Dietary, pernicious anaemia, gastrectomy, ileal disease, drugs |
| Folate deficiency | Dietary, increased demand, malabsorption, anti-folate drugs |
| Drug-induced | MTX, AZA, TKIs, hydroxyurea, cytarabine |
| Other rare causes | Congenital disorders of DNA synthesis (e.g., orotic aciduria, Lesch-Nyhan syndrome) |
Pernicious anaemia is the most common cause of Vitamin B12 deficiency but it is not the only reason for patients becoming deficient in this vitally important vitamin [4].
Pathophysiology:
- Autoimmune disease targeting the parietal cells of the stomach and/or intrinsic factor → less production/function of intrinsic factor [4].
- Autoantibodies against parietal cells / intrinsic factor [4].
- T-cell directed destruction of oxyntic (parietal) cells → loss of acid secretion (achlorhydria) + loss of IF secretion [7].
- The resulting chronic autoimmune gastritis is confined to the body and fundus (where parietal cells reside) [7].
Associations:
- Associated with other autoimmune diseases: vitiligo, autoimmune thyroid disease, type 1 DM, Addison's disease [2].
- Epidemiology: F > M, associated with other autoimmune diseases especially thyroid disease [7].
7. Clinical Features
The clinical features of megaloblastic anaemia reflect three domains: (1) general anaemia symptoms, (2) specific features of B12/folate deficiency, and (3) features of the underlying cause.
| Symptom | Pathophysiological Basis |
|---|---|
| Fatigue, weakness, exercise intolerance | Reduced oxygen-carrying capacity due to anaemia |
| Dyspnoea on exertion | Tissue hypoxia triggers increased respiratory rate |
| Palpitations | Compensatory increased cardiac output; high-output cardiac state |
| Dizziness, light-headedness | Cerebral hypoperfusion due to anaemia |
| Anorexia, weight loss | GI mucosal atrophy from B12/folate deficiency affecting rapidly dividing GI epithelium |
| Sore tongue (glossitis) | Atrophy of tongue papillae due to impaired DNA synthesis in rapidly dividing lingual epithelium |
| Altered taste | Related to glossitis |
| Paraesthesiae (tingling, numbness) in hands/feet | B12 deficiency → accumulation of MMA → toxic to myelin → peripheral neuropathy (dorsal columns and peripheral nerves) |
| Difficulty walking, unsteadiness (ataxia) | Subacute combined degeneration of the cord — demyelination of posterior columns (proprioception/vibration) and lateral corticospinal tracts (motor) |
| Cognitive impairment, irritability, depression, dementia | B12 deficiency directly affects CNS — impaired myelination and neurotransmitter synthesis |
| Visual disturbance | Optic neuropathy (rare) |
| Symptoms of underlying cause | E.g., diarrhoea (Crohn's, coeliac), post-surgical history, dietary history |
Subacute Combined Degeneration of the Cord (SACD)
Subacute combined degeneration of the cord is caused by pernicious anaemia [4]. The "combined" refers to involvement of multiple spinal cord tracts:
- Posterior (dorsal) columns: Loss of proprioception, vibration sense → sensory ataxia, positive Romberg's sign
- Lateral corticospinal tracts: Upper motor neuron signs (spasticity, hyperreflexia, upgoing plantars)
- Peripheral nerves: Lower motor neuron signs (absent ankle jerks)
Can cause both upper AND lower motor neuron lesions → all cells require B12 [4]. The classic examination finding is absent ankle jerks with upgoing plantars — a combination of LMN (peripheral neuropathy) and UMN (corticospinal tract) damage.
The mechanism is accumulation of methylmalonic acid (from failure of Ado-B12-dependent methylmalonyl-CoA mutase) → abnormal fatty acids incorporated into myelin → demyelination.
| Sign | Pathophysiological Basis |
|---|---|
| Pallor (conjunctival, palmar crease, nail bed) | Low haemoglobin → reduced red colouring of perfused tissues |
| Mild jaundice (lemon-yellow tinge) | Intramedullary haemolysis → elevated unconjugated bilirubin [4]. The combination of pallor + mild jaundice gives the classic "lemon-yellow" complexion |
| Early greying of hair | Due to the autoimmunity (in pernicious anaemia) — vitiligo-like depigmentation process [4] |
| Glossitis: beefy-red tongue, loss of roughness | Atrophy of lingual papillae → smooth, shiny, red tongue. Rapidly dividing epithelial cells of the tongue are particularly vulnerable [4] |
| Angular stomatitis (cheilitis) | Mucosal epithelial atrophy at the corners of the mouth [4] |
| Tachycardia | Compensatory increased cardiac output due to anaemia |
| Flow murmur | Hyperdynamic circulation in severe anaemia |
| Hepatosplenomegaly (mild) | Extramedullary haematopoiesis (in severe, chronic cases); also from intramedullary haemolysis |
| Peripheral neuropathy signs | Glove-and-stocking sensory loss, reduced vibration/proprioception, absent ankle jerks |
| Posterior column signs | Loss of vibration and joint position sense, sensory ataxia, positive Romberg's sign |
| Upper motor neuron signs | Spasticity, hyperreflexia, extensor plantar responses (Babinski sign) — due to lateral corticospinal tract demyelination |
| Cognitive/psychiatric signs | Confusion, dementia, psychosis ("megaloblastic madness") |
| Signs of associated autoimmune disease | Vitiligo (depigmented skin patches), thyroid goitre/signs of hypothyroidism |
| Feature | B12 Deficiency | Folate Deficiency |
|---|---|---|
| Neurological involvement | Yes — SACD, peripheral neuropathy, cognitive changes | No (or very rare and mild) |
| Glossitis | Yes | Yes |
| Angular stomatitis | Yes | Yes |
| Time to develop | Years (large liver stores) | Months (small body stores) |
| Associations | Autoimmune diseases, gastric pathology | Pregnancy, alcoholism, rapid cell turnover |
Critical Exam Point
Never give folate alone to a patient with undiagnosed B12 deficiency. Folate supplementation can improve the haematological abnormalities (anaemia), giving a false sense of improvement, while the neurological damage from B12 deficiency continues to progress silently and irreversibly. This is because folate bypasses the methylfolate trap for DNA synthesis but does NOT address the MMA accumulation causing demyelination. Always check and correct B12 status before or alongside folate replacement.
The PBS is arguably the most important initial investigation. The findings are pathognomonic:
State the peripheral blood smear finding results for megaloblastic anaemia: [1]
- Red cells are macrocytic (generally > 100 fL as well)
- Red cells are oval in shape → oval macrocytes
- Hypersegmented neutrophils → diagnostic, more than 6 segments
- Have some small degree of pancytopenia
| Finding | Significance |
|---|---|
| Oval macrocytes | Large, oval-shaped RBCs — the hallmark of megaloblastic change (cf. round macrocytes in liver disease/alcohol) |
| Hypersegmented neutrophils | Diagnostic — defined as ≥ 1 neutrophil with ≥ 6 lobes, or ≥ 5% of neutrophils with ≥ 5 lobes. Reflects failed nuclear division |
| Anisopoikilocytosis | Variation in size and shape |
| Howell-Jolly bodies | Nuclear remnants in RBCs (normally removed by spleen; here there is so much nuclear material the spleen cannot clear it all) |
| Cabot rings | Remnants of mitotic spindle |
| Pancytopenia | All cell lines affected |
| Low reticulocyte count | Inappropriately low due to intramedullary haemolysis / ineffective erythropoiesis |
MCV > 120 fL: Only Two Diagnoses
A MCV of greater than 120 fL generally only has 2 differential diagnoses: [4]
- Chemotherapy → very easy to exclude, just ask the patient
- Pernicious anaemia
All other DDx for macrocytosis generally will not be to this magnitude [4]. This is a classic exam pearl — if MCV is massively elevated (> 120 fL) and the patient is not on chemo, think pernicious anaemia.
Name the laboratory abnormalities seen in patients with pernicious anaemia: [4]
| Investigation | Finding | Explanation |
|---|---|---|
| CBC | Pancytopenia | All cells require B12 |
| MCV | Macrocytic anaemia (often > 100–120 fL) | Nuclear-cytoplasmic dissociation → large cells |
| Bilirubin | Elevated unconjugated bilirubin | Intramedullary haemolysis |
| LDH | Elevated LDH | Intramedullary haemolysis — LDH released from destroyed precursors |
| Reticulocytes | Low / inappropriately normal | Haemolysis occurs inside bone marrow → may not see reticulocytes/polychromasia since the problem is within the bone marrow [4] |
| PBS | Hypersegmented neutrophils and macroovalocytes | Megaloblastic change |
| Haptoglobin | May be mildly reduced | Haemolysis (though primarily intramedullary) |
| Serum B12 / Holotranscobalamin | Low | Deficient B12 |
| Serum folate | May be paradoxically elevated | B12 traps folate as methyl-THF in plasma (methylfolate trap) |
| Homocysteine | Elevated | Both B12 and folate deficiency cause ↑homocysteine (both needed for homocysteine → methionine) |
| Methylmalonic acid (MMA) | Elevated | Specific to B12 deficiency (Ado-B12 required for MMA → succinyl-CoA) |
Distinguishing B12 from folate deficiency biochemically:
- Both cause ↑homocysteine
- Only B12 deficiency causes ↑MMA
- If homocysteine is ↑ but MMA is normal → folate deficiency
- If both are ↑ → B12 deficiency (or combined)
Name the laboratory investigations that can be ordered in a patient with pernicious anaemia: [4]
| Investigation | Details |
|---|---|
| Serum Vitamin B12 level | Old days, don't measure total level since it does not reflect the active levels |
| Serum holotranscobalamin | The TCII bound to Vit B12, active fraction, what we use nowadays |
| Serum and red cell folate level | But in real life folate deficiency anaemia is rare. Folate deficiency occurs in alcoholic patients, rapid cell turnover in psoriasis, but it will not be so low to cause folate deficiency anaemia |
| Anti-parietal cell antibody | Sensitive but less specific, also detectable in small proportion of normal individuals |
| Anti-intrinsic factor antibody | Specific but less sensitive, some patients with PA may be negative |
| Upper endoscopy | Atrophic gastritis → common complication of PA, autoimmune attack to stomach |
| Carcinoma of stomach → notably associated with PA, due to autoantibody attack to stomach | |
| Schilling test | Historical interest, obsolete in HK. Old test, test how good body absorbs B12, radioactive label B12 then measure urine → don't use anymore since we have the antibodies |
| Bone marrow examination | Not routinely needed, except when laboratory findings incompatible with PA → particularly when you are thinking of MDS (can also cause pancytopenia with macrocytosis), or when patient does not improve when you replace patient with parenteral B12 |
Two Gastric Diseases Associated with Pernicious Anaemia
- Atrophic gastritis — autoimmune destruction of parietal cells leads to chronic inflammation and glandular atrophy in the body/fundus
- Gastric carcinoma (including adenocarcinoma and neuroendocrine tumours) — 2-3× increased risk of gastric cancer [7] due to chronic inflammation, achlorhydria, and intestinal metaplasia
This is why upper endoscopy with biopsy is recommended in pernicious anaemia — both for confirmation and cancer surveillance.
This concept is a favourite exam topic because it elegantly links B12 and folate metabolism:
- Dietary folate enters cells primarily as methyl-THF.
- To become usable for DNA synthesis, methyl-THF must donate its methyl group to homocysteine (forming methionine), a reaction catalysed by methionine synthase which requires methyl-B12 as a cofactor.
- This reaction converts methyl-THF → THF, which can then be converted to 5,10-methylene-THF for thymidylate synthesis.
- In B12 deficiency, methionine synthase cannot function → methyl-THF accumulates ("trapped") and cannot be converted to THF → functional folate deficiency despite adequate total folate.
- This is why B12 deficiency mimics folate deficiency at the biochemical level — and why giving folate can partially correct the anaemia (it provides additional substrate) but does NOT fix the neurological damage (which is due to MMA accumulation, independent of folate).
Note: B12 deficiency falsely elevates serum folate levels [2] — because folate is trapped in its methyl-THF form in plasma and cannot enter cells for utilization. RBC folate (which reflects intracellular stores) will be low.
12. Special Considerations
B12 deficiency in pregnancy can cause neural tube defects (similar to folate deficiency) and megaloblastic anaemia in the mother. All pregnant women receive folate supplementation, but B12 status should also be checked, especially in vegetarian/vegan mothers [8].
In the old textbooks, CML/MPN diagnosis was based on an increase in serum B12 → because these conditions caused an increase in haptocorrin/TCIII, causing greater B12 [4]. This is a historical pearl — elevated B12 without deficiency symptoms should prompt consideration of myeloproliferative disease.
Metformin (widely used in type 2 DM) interferes with calcium-dependent B12-IF complex absorption in the terminal ileum. Long-term metformin use (> 4 years) can cause subclinical B12 deficiency. Guidelines recommend periodic B12 monitoring in patients on long-term metformin.
PPIs reduce gastric acid → impaired liberation of B12 from food proteins. Long-term PPI use (> 2 years) is associated with increased risk of B12 deficiency, though clinically significant deficiency is relatively uncommon.
High Yield Summary
-
Megaloblastic anaemia = macrocytic anaemia due to impaired DNA synthesis → nuclear-cytoplasmic dissociation → oval macrocytes + hypersegmented neutrophils on PBS
-
In HK, B12 deficiency is the primary cause. Folate deficiency causing megaloblastic anaemia is exceedingly rare (zero cases in Prof Kwong's HK study)
-
Pernicious anaemia is the most common medical cause of B12 deficiency (95% of cases) — autoimmune destruction of parietal cells / anti-IF antibodies
-
B12 absorbed in terminal ileum (requires IF); folate absorbed in upper small intestine. B12 stores last ~3 years; folate stores ~3 months
-
B12 has two roles: Methyl-B12 (methionine synthase → DNA synthesis via THF); Ado-B12 (MMA → succinyl-CoA → Krebs). B12 deficiency causes BOTH haematological (methylfolate trap) AND neurological (MMA accumulation) disease
-
PBS: oval macrocytes + hypersegmented neutrophils (≥ 6 lobes = diagnostic). MCV > 120 fL → think pernicious anaemia or chemotherapy
-
Intramedullary haemolysis → ↑LDH, ↑unconjugated bilirubin, ↓reticulocytes (not extravascular, so no reticulocytosis!)
-
Pernicious anaemia clinical features: macrocytic anaemia, mild jaundice (lemon-yellow), early greying, glossitis (beefy-red tongue), angular stomatitis, subacute combined degeneration of the cord
-
SACD = posterior columns + lateral corticospinal tracts + peripheral nerves → absent ankle jerks + upgoing plantars
-
Never give folate alone without checking B12 — will correct anaemia but allow irreversible neurological damage to progress
-
Investigations: holotranscobalamin (active B12), anti-parietal cell Ab (sensitive), anti-IF Ab (specific), upper endoscopy (atrophic gastritis, gastric cancer), MMA + homocysteine
-
Associated with gastric carcinoma (2-3× increased risk) — endoscopic surveillance needed
Active Recall - Megaloblastic Anaemia
[1] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (Megaloblastic anemia section) [2] Senior notes: Maksim Medicine Notes.pdf (Haematology, p.158, Macrocytic anaemia section) [3] Senior notes: Ryan Ho Haemtology.pdf [4] Senior notes: Block A - Pallor_ diagnosis of anaemia; nutritional anaemia; anaemia of systemic diseases.pdf (Pernicious anaemia sections, pp.18-19) [5] Lecture slides: GC 097. Many members of the family have anaemia (File 2).pdf (p.6, Clinical Classification of Anaemia) [6] Lecture slides: GC 097. Many members of the family have anaemia (PATH).pdf (p.6, Clinical Classification of Anaemia) [7] Senior notes: Ryan Ho GI.pdf (Autoimmune chronic gastritis, pp.75, 126) [8] Lecture slides: GC 115. I am pregnant medical problems complicating pregnancy.pdf
Differential Diagnosis of Megaloblastic Anaemia
When you encounter a patient with macrocytic anaemia — particularly with oval macrocytes and hypersegmented neutrophils on peripheral blood smear — your primary consideration is megaloblastic anaemia. However, you must systematically work through the differential diagnosis at two levels:
- Level 1: Is this truly megaloblastic macrocytic anaemia, or is it non-megaloblastic macrocytic anaemia? (The PBS tells you.)
- Level 2: If megaloblastic, what is the underlying cause (B12 vs folate vs drugs vs rare)?
- Level 3: If the presentation is pancytopenia ± macrocytosis, what other serious diagnoses must be excluded?
This is the first and most critical branch point. The PBS is your best friend here.
Clinical classification of anaemia by red cell size (MCV): [5][6]
| Microcytic | Normocytic | Macrocytic |
|---|---|---|
| Iron deficiency | Haemolysis | Megaloblastic anaemia |
| Thalassaemia | Anaemia of chronic disease | Aplastic anaemia |
| Renal failure | Myelodysplasia | |
| Liver disease |
Within macrocytic anaemia, the key distinction:
| Feature | Megaloblastic | Non-Megaloblastic |
|---|---|---|
| PBS | Oval macrocytes + hypersegmented neutrophils | Round macrocytes + normal neutrophils |
| Pathophysiology | Impaired DNA synthesis → nuclear-cytoplasmic dissociation | Membrane lipid abnormalities / other mechanisms |
| Causes | B12 deficiency, folate deficiency, anti-metabolite drugs | Alcohol, liver disease, hypothyroidism, reticulocytosis, MDS, aplastic anaemia |
Why is this distinction important? Because the causes, investigations, and management are completely different. A patient with round macrocytes and normal neutrophils needs LFTs and TFTs, not B12 levels. A patient with oval macrocytes and hypersegmented neutrophils needs B12/folate workup.
Comprehensive Causes of Macrocytic Anaemia (Expanded DDx Table)
| Category | Condition | Mechanism | Key Distinguishing Features |
|---|---|---|---|
| Megaloblastic | B12 deficiency | Impaired DNA synthesis via methylfolate trap + ↓Ado-B12 | Oval macrocytes, hypersegmented neutrophils, ↑MMA, ↑homocysteine, neurological signs (SACD) |
| Folate deficiency | Impaired DNA synthesis (↓THF for thymidylate) | Same PBS as B12 deficiency but NO neurological signs, ↑homocysteine, normal MMA | |
| Drugs: immunosuppressants (MTX, AZA), TKI (imatinib, sunitinib) [2] | Direct impairment of DNA synthesis | Drug history! May see megaloblastic features on PBS | |
| Drugs: antiretrovirals (e.g. zidovudine), cytotoxic agents (6-MP, capecitabine, cytosine arabinoside, hydroxyurea) [9] | Direct impairment of DNA synthesis / nucleotide metabolism | Drug history | |
| Copper deficiency | Cofactor for enzymes in haematopoiesis | Rare; post-gastric bypass, excessive zinc supplementation | |
| Non-Megaloblastic | Alcohol abuse (most common cause of non-megaloblastic macrocytosis) [2] | Direct toxic effect on erythroid precursors + membrane lipid abnormalities | Round macrocytes, target cells, ↑GGT, social history |
| Liver disease [5][6] | Abnormal cholesterol/phospholipid metabolism → excess lipid deposition on RBC membrane → larger cells | Round macrocytes, target cells, abnormal LFTs, stigmata of chronic liver disease | |
| Hypothyroidism | Unclear; possibly ↓erythropoiesis rate, membrane changes | ↑TSH, fatigue, weight gain, constipation | |
| Haemolytic anaemia | Reticulocytosis — reticulocytes are larger than mature RBCs (still contain RNA, not yet fully condensed) → falsely elevates MCV [9] | ↑reticulocyte count, ↑LDH, ↑unconjugated bilirubin, ↓haptoglobin, polychromasia on PBS | |
| Recovering bone marrow | Same reticulocytosis mechanism — marrow recovering from any insult (chemo, infection, B12 replacement) releases young, large RBCs | Recent history of marrow insult, ↑reticulocyte count | |
| Aplastic anaemia [5][6] | Primary marrow failure; macrocytosis often mild-moderate | Pancytopenia, reticulocytopenia, hypocellular marrow on biopsy, NO megaloblastic haematopoiesis, NO dysplasia | |
| Myelodysplastic syndrome (MDS) [5][6] | Clonal haematopoietic stem cell disorder → ineffective/dysplastic haematopoiesis | Pancytopenia, dysplastic features on PBS (hypolobulated/hypogranular neutrophils, pseudo-Pelger-Huët cells), dysplastic marrow, cytogenetic abnormalities | |
| Multiple myeloma / plasma cell dyscrasia [9] | Unknown mechanism; possibly related to paraprotein interference or marrow replacement | Bone pain, ↑total protein, M-spike on SPEP, Bence Jones protein | |
| Primary marrow disorders | Congenital dyserythropoietic anaemia | Inherited defect in erythropoiesis | Rare; typically younger patients, specific marrow findings |
| Sideroblastic anaemia (some forms) | Defective haem synthesis → iron accumulates in mitochondria as ring sideroblasts | Ring sideroblasts on BM; can be micro-, normo-, or macrocytic | |
| LGL leukaemia | Large granular lymphocyte proliferation | Splenomegaly, neutropenia, characteristic LGL morphology | |
| Factitious | Artefactual macrocytosis | RBC clumping, hyperglycaemia (osmotic swelling), prolonged storage in EDTA tube | No clinical anaemia, repeat with fresh sample |
High-Yield Approach to Macrocytic Anaemia from Lecture Slides
Approach to evaluation of macrocytosis: [9]
- Consider factitious cause: RBC clumping, osmotic swelling with hyperglycaemia, prolonged storage, EDTA tube
- Clinical: age, PMHx, drugs, diet, alcohol
- CBC:
- Severe macrocytosis (MCV > 110–115) → almost exclusively associated with megaloblastic anaemia
- ≥1 additional cytopenias → primary BM problem, e.g. megaloblastic anaemia, MDS
- PBS:
- Megaloblastic anaemia: macro-ovalocytes, hypersegmented neutrophils (> 5 lobes)
- Liver disease: target cells
- Myelodysplastic syndrome: hypolobulated or hypogranular, dysplastic neutrophils
- Investigations for cause:
- Reticulocyte count: ↑ in haemolysis, ↓ in BM disorders
- Serum B12 and folate (± copper if gastric bypass) for megaloblastic anaemia
- TFT for hypothyroidism
- LFT for liver disease
- BM examination: only when uncertain diagnosis or pancytopenia
Once you have confirmed megaloblastic change on PBS, the next step is to determine: Is this B12 deficiency, folate deficiency, or drug-induced?
| Investigation | B12 Deficiency | Folate Deficiency | Drug-Induced |
|---|---|---|---|
| Serum holotranscobalamin | ↓ | Normal | Normal |
| Serum/RBC folate | Normal or paradoxically ↑ (methylfolate trap) | ↓ | Variable |
| Methylmalonic acid (MMA) | ↑ | Normal | Normal |
| Homocysteine | ↑ | ↑ | Variable |
| Neurological signs | Yes (SACD) | No | Unlikely |
| Drug history | — | — | +ve |
Then determine the cause of B12 or folate deficiency (see Aetiology section from Part 1 — dietary, gastric, intestinal, drugs, increased demand).
3. Level 3: The Pancytopenia Differential — The Most Critical DDx
Megaloblastic anaemia frequently presents with pancytopenia (anaemia + neutropenia + thrombocytopenia). This is a high-stakes presentation because the differential includes several serious/life-threatening conditions. You must exclude these before concluding it is "just B12 deficiency."
The Pancytopenia Differential — Do Not Miss These
When you see pancytopenia with macrocytosis, the differential includes:
- Megaloblastic anaemia (B12/folate deficiency)
- Myelodysplastic syndrome (MDS)
- Aplastic anaemia
- Acute leukaemia (especially AML)
- Bone marrow infiltration (myelofibrosis, metastatic cancer, lymphoma)
- HIV infection
- Hypersplenism
The key investigations to differentiate these are PBS morphology and bone marrow examination when needed.
This is the most commonly tested and clinically important distinction. Both can present with pancytopenia + macrocytosis in an elderly patient.
| Feature | Megaloblastic Anaemia | Myelodysplastic Syndrome |
|---|---|---|
| Age | Any (but PA peaks in elderly) | Typically elderly |
| PBS | Oval macrocytes, hypersegmented neutrophils | Hypolobulated or hypogranular, dysplastic neutrophils (pseudo-Pelger-Huët cells), dysplastic RBCs [9] |
| Bone marrow | Megaloblastic erythropoiesis, giant metamyelocytes; morphologically megaloblastic but NOT dysplastic | Dysplastic haematopoiesis in ≥1 lineages, ring sideroblasts (some subtypes), may have increased blasts |
| Cytogenetics | Normal | Characteristic cytogenetic abnormalities (e.g. del(5q), del(7q), monosomy 5/7) [10] |
| B12/folate | Low | Normal |
| Response to B12/folate | Dramatic improvement | No improvement |
Bone marrow examination is not routinely needed in megaloblastic anaemia, except when laboratory findings are incompatible with PA → particularly when you are thinking of MDS (can also cause pancytopenia with macrocytosis), or when patient does not improve when you replace patient with parenteral B12 [4].
Should check vitamin B12, folate ± zinc, copper in all patients with suspected MDS [10] — because you don't want to miss a treatable nutritional deficiency masquerading as MDS.
| Feature | Megaloblastic Anaemia | Aplastic Anaemia |
|---|---|---|
| PBS | Oval macrocytes, hypersegmented neutrophils | Macrocytosis and anisopoikilocytosis common but NO megaloblastic features, NO abnormal cells [11][12] |
| Bone marrow | Hypercellular with megaloblastic change | Profoundly hypocellular with decrease in all elements and replacement by fat/stromal cells [11] |
| Residual cells | Megaloblastic | Morphologically normal, haematopoiesis is NOT megaloblastic [11] |
| Reticulocytes | Low (intramedullary haemolysis) | Low (reticulocytopenia due to BM failure) [11] |
| Distinguishing | B12/folate low, responds to replacement | B12/folate normal; BM biopsy diagnostic |
In the workup of aplastic anaemia, serum vitamin B12 and folate levels are measured specifically for exclusion of megaloblastic anaemia [11][12].
| Feature | Megaloblastic Anaemia | Acute Leukaemia |
|---|---|---|
| PBS | Oval macrocytes, hypersegmented neutrophils, NO blasts | Circulating blasts (≥ 20% diagnostic); may see Auer rods in AML [13] |
| BM | Megaloblastic change, no increased blasts | Hypercellular with replacement by blasts |
| Clinical | Insidious onset, glossitis, SACD | Bone marrow failure (anaemia, infection, bleeding) + extramedullary infiltration (hepatosplenomegaly, lymphadenopathy, gum hypertrophy) [14] |
| Special note | B12/folate deficiency with prominent erythroid elements may mimic erythroleukaemia [13] | Must exclude with BM morphology + immunophenotyping |
Megaloblastic Anaemia Mimicking Erythroleukaemia
Vitamin B12/folate deficiency with pancytopenia and prominent erythroid elements may mimic erythroleukaemia (a subtype of AML) [13]. The key differentiator is the PBS (hypersegmented neutrophils are NOT a feature of AML), B12/folate levels, and definitive BM examination if in doubt.
Both can present with anaemia + raised LDH + raised unconjugated bilirubin. But the mechanism is different:
| Feature | Megaloblastic Anaemia | Haemolytic Anaemia |
|---|---|---|
| Site of RBC destruction | Intramedullary (ineffective erythropoiesis) | Extravascular (spleen) or intravascular |
| Reticulocyte count | Low / inappropriately normal (marrow cannot release cells) [4] | Elevated (compensatory marrow response) |
| PBS | Oval macrocytes, hypersegmented neutrophils | Polychromasia, spherocytes, RBC fragmentation, or agglutination depending on cause [15] |
| Haptoglobin | Mildly ↓ or normal | Reduced (especially intravascular) [15] |
| MCV | Elevated (macro) | Mildly macrocytic (due to reticulocytosis — reticulocytes are larger) or normocytic |
| DAT | Negative | Positive in immune haemolytic anaemia [15] |
The critical discriminator is the reticulocyte count: if it is appropriately elevated with anaemia + ↑LDH + ↑bilirubin, think haemolysis. If it is paradoxically low despite those markers, think intramedullary haemolysis (megaloblastic anaemia, thalassaemia major, or MDS) [4].
Intramedullary haemolysis differential diagnosis: pernicious anaemia, thalassaemia major, MDS [4].
A MCV of greater than 120 fL generally only has 2 differential diagnoses: [4]
- Chemotherapy → very easy to exclude, just ask the patient
- Pernicious anaemia
All other DDx for macrocytosis generally will not be to this magnitude [4].
Why? Because the degree of nuclear-cytoplasmic dissociation in severe B12 deficiency/chemotherapy-related DNA synthesis impairment is so profound that cells become massively enlarged. Other causes of macrocytosis (alcohol, liver disease, hypothyroidism, MDS) typically cause only mild-to-moderate MCV elevation (100–115 fL).
| Condition | MCV | PBS | Reticulocytes | BM | Key Investigation |
|---|---|---|---|---|---|
| Megaloblastic anaemia | ↑↑ (often > 110, can > 120) | Oval macrocytes, hypersegmented neutrophils | ↓ | Hypercellular, megaloblastic | B12, folate, MMA |
| MDS | ↑ (usually 100–115) | Dysplastic neutrophils, monocytosis | ↓ | Dysplastic, ± ring sideroblasts, cytogenetic abnormalities | BM biopsy + cytogenetics |
| Aplastic anaemia | ↑ (mild) | No megaloblastic features, no blasts | ↓↓ | Hypocellular, fat replacement | BM biopsy |
| AML | Variable | Blasts ± Auer rods | ↓ | ≥ 20% blasts | BM + immunophenotyping |
| Haemolytic anaemia | Mildly ↑ or normal | Polychromasia, spherocytes / fragments | ↑↑ | Erythroid hyperplasia | DAT, haptoglobin, LDH |
| Liver disease | ↑ (usually < 110) | Round macrocytes, target cells | Normal | Normal | LFTs |
| Hypothyroidism | ↑ (mild) | Round macrocytes | Normal | Normal | TSH |
| Alcohol | ↑ (usually < 110) | Round macrocytes, target cells | Normal | Normal | History, GGT |
High-Yield DDx Summary
When you see macrocytic anaemia + pancytopenia, your top 3 differential diagnoses to exclude are:
- Megaloblastic anaemia — confirm with PBS (oval macrocytes + hypersegmented neutrophils), B12/folate/MMA
- MDS — suspect if dysplastic neutrophils, no response to B12, elderly patient; confirm with BM biopsy + cytogenetics
- Aplastic anaemia — suspect if pancytopenia + reticulocytopenia without megaloblastic features; confirm with BM biopsy showing hypocellularity
Always check B12 and folate before diagnosing MDS or aplastic anaemia — you don't want to miss a treatable cause! [10][11]
Active Recall - DDx of Megaloblastic Anaemia
References
[2] Senior notes: Maksim Medicine Notes.pdf (Haematology, p.158, Macrocytic anaemia section) [4] Senior notes: Block A - Pallor_ diagnosis of anaemia; nutritional anaemia; anaemia of systemic diseases.pdf (Pernicious anaemia sections, pp.18-19) [5] Lecture slides: GC 097. Many members of the family have anaemia (File 2).pdf (p.6, Clinical Classification of Anaemia) [6] Lecture slides: GC 097. Many members of the family have anaemia (PATH).pdf (p.6, Clinical Classification of Anaemia) [9] Senior notes: Ryan Ho Haemtology.pdf (p.27, Macrocytic and Megaloblastic Anaemia); Senior notes: Ryan Ho Fundamentals.pdf (p.386, Macrocytosis) [10] Senior notes: Ryan Ho Haemtology.pdf (p.83, MDS differential diagnoses) [11] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.369, Aplastic anaemia); Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.1468, AA diagnosis) [12] Senior notes: MBBS Final MB (Pediatrics) (Felix PY Lai).pdf (p.624, AA diagnosis) [13] Senior notes: Ryan Ho Haemtology.pdf (p.54, AML differential diagnoses) [14] Senior notes: Block A - High white cell count_ acute and chronic leukaemia; bone marrow transplantation; immunogenetics.pdf (p.3, Acute leukaemia clinical features) [15] Lecture slides: Haematology Introduction to Haematological investigations (CBP, Clotting).pdf (p.32, Haemolytic anaemia laboratory features)
Diagnostic Criteria, Algorithm and Investigations for Megaloblastic Anaemia
Megaloblastic anaemia does not have a single, consensus-panel "diagnostic criteria" checklist the way aplastic anaemia or MDS does. Instead, the diagnosis is made by synthesising clinical, morphological and biochemical evidence in a stepwise manner. Think of it as a three-layered confirmation:
| Layer | What You Confirm | How |
|---|---|---|
| Layer 1 — Morphological confirmation | The anaemia is megaloblastic (not just macrocytic) | PBS showing oval macrocytes + hypersegmented neutrophils [1][9] |
| Layer 2 — Biochemical confirmation | There is B12 and/or folate deficiency (or drug-induced) | Serum holotranscobalamin (active B12), serum/RBC folate, ± MMA and homocysteine [2][4][16] |
| Layer 3 — Aetiological confirmation | The underlying cause of deficiency is identified | Autoantibodies (anti-IF, anti-parietal cell), upper endoscopy, dietary/drug/surgical history [4][16] |
A patient meets the diagnosis of megaloblastic anaemia when all three layers converge:
- Macrocytic anaemia on CBC (MCV typically > 100 fL, often > 110–115 fL)
- Megaloblastic morphology on PBS — oval macrocytes + hypersegmented neutrophils (> 5% with ≥ 5 lobes, or ≥ 1 neutrophil with ≥ 6 lobes) [1][9]
- Confirmed deficiency of B12 and/or folate (or an identified offending drug)
- Exclusion of mimics — particularly MDS and aplastic anaemia (by BM biopsy if findings are atypical or if the patient fails to respond to replacement)
Why No Formal 'Diagnostic Criteria'?
Unlike conditions like aplastic anaemia (which has the Camitta/modified criteria with specific cell count thresholds) or MDS (which requires BM blast percentage and cytogenetics), megaloblastic anaemia is a morphological-biochemical diagnosis. The PBS findings are essentially pathognomonic when combined with low B12/folate levels. The "criteria" is really the pattern recognition: oval macrocytes + hypersegmented neutrophils + low B12/folate = megaloblastic anaemia. The remaining investigation is to determine why the patient is deficient.
Specific Diagnostic Criteria for Pernicious Anaemia
For pernicious anaemia specifically (the most common medical cause), the diagnosis rests on:
- Evidence of B12 deficiency (low holotranscobalamin and/or elevated MMA)
- Positive auto-antibodies: [4][9][16]
- Upper endoscopy showing atrophic gastritis (body/fundus) ± intestinal metaplasia [4][16]
- Exclusion of other causes of B12 deficiency (dietary, surgical, intestinal)
Diagnosis of pernicious anaemia is made by the presence of auto-antibodies: Anti-IF is insensitive (only seen in ~50-70%) but specific (diagnostic if associated with B12 deficiency). Anti-parietal cell is non-specific (also present in 20% of females > 60 years) but sensitive (found in 85-90%) [9]
The clinical approach to a patient with suspected megaloblastic anaemia follows a systematic, stepwise pathway. Let me walk through it as if you are seeing this patient on the ward.
Step 1: Identify macrocytic anaemia — CBC shows Hb below reference range with MCV > 100 fL.
Step 2: Examine the PBS — This is the most critical step. You are looking for the hallmark duo:
- Oval macrocytes (not round — round macrocytes suggest alcohol/liver disease)
- Hypersegmented neutrophils (≥ 5 lobes in > 5% of neutrophils, or any neutrophil with ≥ 6 lobes) [1]
- Also note pancytopenia, anisopoikilocytosis, Howell-Jolly bodies
If these are present → megaloblastic anaemia is confirmed morphologically. If absent → consider non-megaloblastic causes.
Step 3: Confirm deficiency — Measure:
- Serum holotranscobalamin (active B12; preferred over total serum B12) [4]
- Serum and RBC folate [2][16]
- If equivocal: MMA and homocysteine to distinguish B12 from folate deficiency [2][9]
Step 4: Determine the cause — Based on which nutrient is deficient:
- If B12 deficient: Anti-IF Ab, anti-parietal cell Ab, dietary history, surgical history, drug history (PPI, metformin), GI history (Crohn's, coeliac), upper endoscopy
- If folate deficient: Dietary history, alcohol history, drug history (MTX, phenytoin), assess for increased demand (pregnancy, haemolysis, psoriasis)
- If both normal but megaloblastic features present: Consider drug-induced (ask about MTX, AZA, TKIs, hydroxyurea, zidovudine)
Step 5: Exclude mimics if needed — Bone marrow examination is not routinely needed, except when laboratory findings are incompatible with PA → particularly when you are thinking of MDS (can also cause pancytopenia with macrocytosis), or when patient does not improve when you replace patient with parenteral B12 [4][16]
When to Perform Bone Marrow Biopsy
Bone marrow examination is not routinely needed in megaloblastic anaemia [4][16]. You perform it only when:
- Laboratory findings are incompatible with pernicious anaemia — e.g., B12/folate levels are normal despite megaloblastic morphology
- You are thinking of MDS — which can also cause pancytopenia with macrocytosis
- Patient does not improve when you replace with parenteral B12 — if no reticulocyte response by day 5–7, something else is going on
- Pancytopenia is severe and you need to exclude aplastic anaemia or acute leukaemia
3. Investigation Modalities — Detailed Interpretation
| Parameter | Expected Finding | Interpretation |
|---|---|---|
| Haemoglobin | Low (can be profoundly low: 40–80 g/L in severe cases) | Anaemia; degree depends on chronicity (B12 deficiency is slow, so patients can tolerate very low Hb) |
| MCV | > 100 fL; typically > 110–115 fL; can exceed 120 fL [1][4][9] | Severe macrocytosis (MCV > 110–115 fL) is almost exclusively associated with megaloblastic anaemia [9]. MCV > 120 fL → only pernicious anaemia or chemotherapy [4] |
| MCH | Elevated | Larger cells carry more haemoglobin per cell |
| MCHC | Normal | Haemoglobin concentration per unit volume of RBC is maintained |
| RBC count | Low | Reduced production due to ineffective erythropoiesis |
| WBC | Mild leukopenia [9] | All rapidly dividing cells affected → neutropenia |
| Platelets | Mild thrombocytopenia [9] | Megakaryocytes also affected |
| Reticulocyte count | Decreased [9] | Intramedullary haemolysis — precursors die in marrow before release; this is a critical distinguishing feature from extravascular haemolysis where reticulocytes would be elevated |
| RDW | Elevated | Variation in RBC sizes reflects ongoing production of abnormal cells |
Why is the reticulocyte count so important? It tells you WHERE the problem is. Low reticulocytes = problem in the bone marrow (production failure or intramedullary destruction). High reticulocytes = marrow is working overtime, so the problem is peripheral (haemolysis, bleeding). In megaloblastic anaemia, despite markers of haemolysis (↑LDH, ↑bilirubin), the reticulocyte count is paradoxically low because the defective cells are destroyed before leaving the marrow.
State the peripheral blood smear finding results for megaloblastic anaemia: [1]
| Finding | Description | Significance |
|---|---|---|
| Oval macrocytes | Large, oval-shaped RBCs (macro-ovalocytes) | Hallmark of megaloblastic change — nucleus fails to divide while cytoplasm continues to mature and enlarge. "Oval" distinguishes from "round" macrocytes of liver disease/alcohol |
| Hypersegmented neutrophils | Diagnostic; more than 6 segments [1]. Formally: > 5% with ≥ 5 lobes, or ≥ 1% with ≥ 6 lobes [9] | The nucleus keeps attempting division but cannot complete it, creating extra lobes. This is considered pathognomonic for megaloblastic anaemia |
| Pancytopenia (some small degree) [1] | Reduction in all three lineages | All rapidly dividing cells are affected by impaired DNA synthesis |
| Anisopoikilocytosis | Variation in size and shape of RBCs | Reflects disordered erythropoiesis |
| Howell-Jolly bodies | Nuclear remnants within RBCs | Excess nuclear material from impaired nuclear maturation |
| Cabot rings | Remnants of mitotic spindle | Same mechanism as Howell-Jolly bodies |
| Basophilic stippling | Ribosomal RNA aggregates | Reflects accelerated but abnormal erythropoiesis |
| Teardrop cells | Occasionally seen | Can occur with marrow stress |
Hypersegmented Neutrophils — The 'Smoking Gun'
Hypersegmented neutrophils are considered the earliest and most specific morphological change in megaloblastic anaemia. They can appear even before macrocytosis develops (because neutrophils have a shorter lifespan than RBCs, so the morphological change becomes apparent sooner). If you see hypersegmented neutrophils on a PBS, think megaloblastic anaemia until proven otherwise.
Markers of haemolysis: ↑LDH, ↑unconjugated bilirubin, ↓haptoglobin due to decreased survival and ineffective erythropoiesis [9]
| Marker | Expected | Why |
|---|---|---|
| LDH | Elevated (often markedly) | Released from destroyed megaloblastic precursors within the bone marrow. LDH can be very high — sometimes mimicking levels seen in lymphoma or tumour lysis |
| Unconjugated bilirubin | Elevated | Haem from destroyed precursors is metabolised to unconjugated bilirubin → mild jaundice ("lemon-yellow") |
| Haptoglobin | Decreased | Binds free haemoglobin released from destroyed cells. However, the decrease may be less dramatic than in frank intravascular haemolysis |
| Reticulocyte count | Decreased | Unlike peripheral haemolysis; this is the critical discriminator |
The combination of ↑LDH + ↑bilirubin + ↓reticulocytes is the biochemical signature of intramedullary haemolysis (ineffective erythropoiesis). This pattern is seen in only a few conditions: pernicious anaemia, thalassaemia major, and MDS [4].
Investigations for megaloblastic anaemia: active B12 and folate levels ± RBC folate [2]
| Test | Details | Interpretation |
|---|---|---|
| Serum holotranscobalamin | TCII-bound B12 = the active fraction; what we use nowadays [4][16] | This is the preferred test. It reflects the B12 actually available to tissues (only 10–30% of total serum B12 is in this active form). Total serum B12 includes the large inactive haptocorrin-bound pool and can be misleadingly normal |
| Total serum B12 | Still widely available | Old days, don't measure total level since it does not reflect the active levels [4]. However, still used as initial screen in many labs. Normal > 300 pg/mL; borderline 200–300 pg/mL; deficient < 200 pg/mL [9] |
| Serum folate | Reflects recent dietary intake | Can be affected by recent meals, alcohol, anticonvulsants. B12 deficiency can falsely elevate serum folate (methylfolate trap → folate trapped in plasma as methyl-THF) [2] |
| RBC folate | Reflects intracellular folate stores over the preceding 120 days (RBC lifespan) | More reliable than serum folate for assessing true folate status. < 150 ng/mL indicates prolonged deficiency [9]. In B12 deficiency, RBC folate may be low (because folate cannot be retained intracellularly without B12 to convert methyl-THF → THF) |
Why Serum Folate Can Be Misleading in B12 Deficiency
B12 deficiency falsely elevates serum folate levels [2]. Here's why: without B12, methionine synthase cannot function, so methyl-THF cannot donate its methyl group and be converted to THF. Methyl-THF therefore accumulates in the plasma (it cannot be used intracellularly), leading to a high serum folate that masks a functional intracellular folate deficiency. This is the "methylfolate trap." To get the true picture, check RBC folate (which will be low, reflecting the intracellular deficit) [2].
Further testing of metabolites if cannot tell B12 vs folate deficiency: [2]
| Pattern | MMA | Homocysteine | Interpretation |
|---|---|---|---|
| Normal MMA, normal homocysteine | Normal | Normal | No B12 or folate deficiency |
| Normal MMA, ↑ homocysteine | Normal | ↑ | Folate deficiency (folate is needed for homocysteine → methionine, but MMA → succinyl-CoA does not require folate) |
| ↑ MMA, ↑ homocysteine | ↑ | ↑ | B12 deficiency ± concomitant folate deficiency (B12 is needed for BOTH reactions) |
Why does this work?
- Homocysteine accumulates when either B12 OR folate is deficient, because the methionine synthase reaction (homocysteine → methionine) requires BOTH methyl-B12 and methyl-THF as cofactor/substrate.
- MMA accumulates ONLY in B12 deficiency, because the methylmalonyl-CoA mutase reaction (MMA → succinyl-CoA) requires Ado-B12 but NOT folate.
- Therefore, MMA is the specific discriminator: if MMA is elevated, it must be B12 deficiency.
Metabolite testing (MMA, homocysteine) is used when serum B12 and RBC folate are equivocal [9]. In practice, this is helpful when serum B12 is in the borderline range (200–300 pg/mL).
Laboratory investigations for patients with suspected pernicious anaemia: [16]
| Antibody | Sensitivity | Specificity | Clinical Utility |
|---|---|---|---|
| Anti-parietal cell antibody | High (85–90%) | Less specific — also detectable in small proportion of normal individuals (up to 20% of females > 60 years) [4][9][16] | Good screening test but positive result alone does not confirm PA. A negative result makes PA less likely |
| Anti-intrinsic factor antibody | Less sensitive (~50–70%) | Highly specific — diagnostic if associated with B12 deficiency [4][9][16] | A positive result in the context of B12 deficiency is essentially diagnostic of PA. However, a negative result does NOT exclude PA |
Clinical approach: Order both antibodies together. If anti-IF is positive with confirmed B12 deficiency, the diagnosis of PA is secure. If anti-IF is negative but anti-parietal cell is positive with compatible clinical features, PA remains the most likely diagnosis but is not definitively confirmed — proceed to upper endoscopy for supportive evidence.
Upper endoscopy — atrophic gastritis and carcinoma of stomach [4][16]
| Finding | Significance |
|---|---|
| Atrophic gastritis (body and fundus) | Common complication of PA — autoimmune attack to stomach [4]. Histology shows loss of parietal/chief cells, lymphocytic infiltration, intestinal metaplasia. Confirms the gastric pathology underlying PA |
| Carcinoma of stomach | Notably associated with PA, due to autoantibody attack to stomach [4]. 2–3× increased risk of gastric adenocarcinoma and neuroendocrine (carcinoid) tumours. This is why endoscopic surveillance is warranted |
| Achlorhydria | Absent acid secretion due to loss of parietal cells — can be confirmed by gastric pH testing |
| Intestinal metaplasia / ECL cell hyperplasia | Pre-malignant changes resulting from chronic achlorhydria and compensatory hypergastrinaemia |
Why does PA increase gastric cancer risk? Chronic achlorhydria → bacterial overgrowth in the stomach → production of carcinogenic N-nitroso compounds. Additionally, compensatory hypergastrinaemia (the body tries to stimulate acid production by releasing more gastrin) → trophic effect on gastric mucosa → ECL cell hyperplasia → potential carcinoid tumour development.
Schilling test — historical interests, obsolete in HK [4][16]
Old test, test how good body absorbs B12, radioactive label B12 then measure urine → don't use anymore since we have the antibodies [4]
Principle: Patient given oral radiolabelled B12, then a "flushing" dose of parenteral unlabelled B12 (to saturate body stores so that absorbed radioactive B12 would spill into urine). 24-hour urine collected and radioactivity measured. If low urinary excretion → malabsorption. Then repeat with added IF: if excretion normalises → IF deficiency (PA); if still low → intestinal cause.
This test is no longer available in most centres because radiolabelled B12 is no longer manufactured and serology (anti-IF/anti-parietal cell Ab) provides equivalent diagnostic information without radiation exposure.
Bone marrow examination — not routinely needed, except when laboratory findings incompatible with PA [4][16]
| Indication | Rationale |
|---|---|
| Laboratory findings incompatible with PA | Need to look for alternative diagnosis |
| Thinking of MDS | MDS can also cause pancytopenia with macrocytosis [4] — BM shows dysplasia and cytogenetic abnormalities |
| Patient does not improve with parenteral B12 | Failure to respond suggests alternative diagnosis (MDS, aplastic anaemia, occult malignancy) |
| Severe/unexplained pancytopenia | Need to exclude aplastic anaemia (hypocellular marrow), acute leukaemia (blasts), or marrow infiltration |
Megaloblastic BM features (when performed):
- Hypercellular marrow (in contrast to aplastic anaemia which is hypocellular)
- Megaloblastic erythroid precursors: large cells with open, lacy, immature-looking chromatin despite mature cytoplasm
- Giant metamyelocytes and band forms: myeloid precursors also affected
- Nuclear-cytoplasmic dissociation in all lineages
- NO dysplastic features (distinguishing from MDS)
- NO increased blasts (distinguishing from AML)
| Investigation | When to Order | What You're Looking For |
|---|---|---|
| Iron studies | Concurrent microcytosis ("dimorphic" picture) | Combined B12 + iron deficiency (e.g., in coeliac disease, gastrectomy) — MCV may be falsely normal due to opposing effects |
| LFT | Always | Exclude liver disease as cause of macrocytosis; assess for coexistent hepatitis |
| TFT | Always | Hypothyroidism causes non-megaloblastic macrocytosis and is a common autoimmune comorbidity of PA |
| Coeliac serology | If malabsorption suspected | Anti-tTG IgA — coeliac disease can cause both B12 and folate malabsorption |
| HIV serology | If risk factors present | HIV can cause macrocytosis, cytopenias, and dysplastic haematopoiesis |
| Serum gastrin | Suspected PA with equivocal antibodies | Markedly elevated gastrin (compensatory, due to achlorhydria) supports PA |
| Serum pepsinogen I | Suspected PA | Low pepsinogen I reflects loss of chief cells (body/fundal atrophy) |
| MRI spine | Neurological symptoms | SACD: T2 signal changes in posterior and lateral columns of the spinal cord |
| Nerve conduction studies | Peripheral neuropathy | Confirms peripheral neuropathy; axonal degeneration pattern |
One of the most satisfying and confirmatory tests in haematology is the reticulocyte response to B12/folate replacement. This serves as a therapeutic trial and diagnostic confirmation simultaneously:
| Timepoint | Expected Response |
|---|---|
| Day 2–3 | Subjective improvement in well-being |
| Day 3–5 | Reticulocyte count begins to rise |
| Day 5–7 | Peak reticulocytosis — dramatic increase (reticulocyte crisis) |
| Week 2–4 | Haemoglobin progressively rises |
| Week 6–8 | Haemoglobin normalises (though MCV may take longer) |
| Weeks–months | Neurological symptoms may improve (but irreversible if severe/prolonged) |
If there is no reticulocyte response by day 7: Re-evaluate the diagnosis. Consider MDS, aplastic anaemia, concurrent iron/folate deficiency (in B12-treated patients), infection, or occult malignancy. This is the indication for bone marrow examination [4].
The 'Dimorphic' Blood Film — A Common Trap
If a patient has combined B12 and iron deficiency (e.g., post-gastrectomy, coeliac disease), the opposing effects on MCV may result in a normal MCV — the macrocytosis from B12 deficiency and microcytosis from iron deficiency cancel each other out. The PBS will show a dimorphic picture (two populations: small hypochromic cells and large oval cells). The RDW will be markedly elevated. Always check iron studies alongside B12/folate in a patient with unexplained anaemia, even if MCV is normal.
Laboratory investigations for patients with suspected pernicious anaemia: [16]
| Investigation | Purpose |
|---|---|
| Serum Vitamin B12 level | Screening (but limited — does not reflect active levels) |
| Serum holotranscobalamin | Active B12 fraction — modern preferred test |
| Serum and red cell folate level | Exclude folate deficiency (rare in HK) |
| Anti-parietal cell antibody | Sensitive but less specific |
| Anti-intrinsic factor antibody | Specific but less sensitive |
| Upper endoscopy | Atrophic gastritis and carcinoma of stomach |
| Schilling test | Historical interests, obsolete in HK |
| Bone marrow examination | Not routinely needed, except when laboratory findings incompatible with PA |
High Yield Summary — Diagnosis of Megaloblastic Anaemia
- Diagnosis is morphological + biochemical + aetiological — no single "gold standard" test
- PBS is the pivotal investigation: oval macrocytes + hypersegmented neutrophils (≥ 6 lobes = diagnostic) [1]
- Severe macrocytosis (MCV > 110–115 fL) is almost exclusively megaloblastic anaemia [9]
- Holotranscobalamin (active B12) is the preferred modern test over total serum B12 [4]
- MMA and homocysteine distinguish B12 from folate deficiency: ↑MMA = B12 deficiency; normal MMA + ↑Hcy = folate deficiency [2]
- Anti-IF Ab is specific but insensitive; anti-parietal cell Ab is sensitive but non-specific [4][16]
- Upper endoscopy for atrophic gastritis + gastric carcinoma surveillance [4][16]
- BM biopsy is NOT routine — only when findings are incompatible with PA or patient fails to respond to replacement [4][16]
- Schilling test is obsolete in HK [4][16]
- Reticulocyte response at day 5–7 after B12 replacement confirms the diagnosis; failure to respond mandates BM biopsy
- Always check iron studies alongside B12/folate — dimorphic picture can mask the MCV abnormality
Active Recall - Diagnosis of Megaloblastic Anaemia
References
[1] Senior notes: Block A - Introduction to Haematological investigations (CBP, Clotting).pdf (Megaloblastic anemia section) [2] Senior notes: Maksim Medicine Notes.pdf (Haematology, pp.158–159, Macrocytic anaemia and metabolite testing) [4] Senior notes: Block A - Pallor_ diagnosis of anaemia; nutritional anaemia; anaemia of systemic diseases.pdf (Pernicious anaemia sections, pp.18–19) [9] Senior notes: Ryan Ho Haemtology.pdf (pp.27–29, Megaloblastic Anaemia and Pernicious Anaemia) [16] Lecture slides: GC 076. Pallor_diagnosis of anaemia; nutritional anaemia; anaemia of systemic diseases.pdf (p.27, Laboratory investigations for suspected pernicious anaemia)
Management of Megaloblastic Anaemia
The management of megaloblastic anaemia is conceptually straightforward — replace what is missing — but there are critical nuances around route, urgency, duration, monitoring, and the all-important rule about B12-before-folate that can make or break a patient's neurological outcome.
1. General Principles of Management
Management is by replacement of B12 and/or folate [9]. Before diving into specifics, let's establish the overarching principles:
Usually replace over weeks, but urgent replacement is indicated in: [9]
| Urgency | Situation | Rationale |
|---|---|---|
| Urgent | Symptomatic: severe anaemia or neuropsychiatric symptoms | Risk of adverse events and irreversible neurological deficits [9]. Severe anaemia (Hb < 70 g/L) can cause cardiac decompensation. Neurological damage from B12 deficiency can become permanent if untreated |
| Urgent | Pregnancy and neonate/infant | Development may be affected [9]. B12 is critical for fetal neural tube formation and brain development. Delay risks irreversible harm to the developing nervous system |
| Non-urgent | Mild anaemia found incidentally, no neurological symptoms | Can replace over weeks with monitoring |
Why is urgency stratified? Because megaloblastic anaemia is typically chronic and slowly progressive (B12 stores last ~3 years), patients often compensate remarkably well even with very low haemoglobin levels (you may see patients walking around with Hb of 50 g/L). The danger points are: (1) cardiac decompensation from severe anaemia, and (2) irreversible SACD from prolonged B12 deficiency.
Folate with B12 if B12 status unknown, e.g. urgent treatment needed while awaiting B12/folate result [9]
Use of folate alone in presence of vitamin B12 deficiency may cause worsening of neurological deficit despite partially masking its haematological deficits [9]
CRITICAL: The Folate Trap — Why You Must Check B12 First
This is perhaps the single most important management principle in megaloblastic anaemia:
Folate supplementation alone in a B12-deficient patient will:
- Partially correct the anaemia — folate provides substrate for thymidylate synthesis, partially bypassing the methylfolate trap, so the blood counts improve
- Mask the haematological deficit — the improving blood picture gives a false sense that the problem is solved
- Worsen the neurological deficit [9] — the MMA accumulation causing demyelination is entirely independent of folate; without B12 replacement, SACD continues to progress silently
The mechanism: Folate addresses the DNA synthesis arm (via alternative pathways for thymidylate production), but it cannot substitute for adenosylcobalamin in the methylmalonyl-CoA mutase reaction. MMA continues to accumulate → abnormal fatty acids continue to be incorporated into myelin → progressive, potentially irreversible demyelination.
The practical rule: If you need to treat urgently and don't yet have B12/folate results back:
- Give BOTH B12 AND folate simultaneously
- Never give folate alone
Route of administration: parenteral if urgent or impaired absorption, oral if dietary deficiency [9]
| Route | When to Use | Rationale |
|---|---|---|
| Parenteral (IM) | Pernicious anaemia, gastrectomy, ileal disease, severe/urgent cases | Absorption is impaired → oral B12 won't work via the normal IF-dependent pathway |
| Oral | Dietary deficiency (vegans), mild cases | Absorption pathway intact; oral is cheaper and more convenient |
| High-dose oral | When parenteral is impractical/refused | Even with impaired absorption, very-high dose oral B12 can be absorbed via non-IF/terminal ileum-dependent mechanisms, e.g. diffusion [9]. ~1% of oral B12 is absorbed by passive diffusion regardless of IF status → if you give 1000 μg, ~10 μg gets absorbed (daily requirement is only 1–2 μg) |
Duration: lifelong if underlying condition irreversible (e.g. gastrectomy, pernicious anaemia), otherwise until deficiency corrected [9]
| Situation | Duration |
|---|---|
| Pernicious anaemia | Lifelong — the autoimmune process is permanent |
| Post-gastrectomy | Lifelong — no parietal cells to make IF |
| Ileal resection | Lifelong — no absorption site |
| Vegan diet | Lifelong replacement (no plant source) [9] — unless patient changes diet |
| Dietary deficiency (non-vegan) | Until deficiency corrected + dietary counselling |
| Drug-induced (PPI, metformin) | Until deficiency corrected; consider ongoing if drug continued |
| Folate deficiency from increased demand | Until the demand state resolves (e.g., after delivery) |
2. Specific Treatment Modalities
2.1 Vitamin B12 Replacement
B12 deficiency: vegan diet require lifelong replacement (no plant source) [9]
Route: prefer parenteral if impaired absorption, oral otherwise [9]
| Route | Regimen | Notes |
|---|---|---|
| Intramuscular (IM) | 1000 μg IM every 1 week until normalised, then 1000 μg IM every 1–2 months [9] | Standard for pernicious anaemia, post-gastrectomy, ileal disease. Loading doses replete stores rapidly; maintenance injections prevent recurrence |
| Oral (PO) | 1000 μg PO daily [9] | For dietary deficiency with intact absorption. Also can be used in impaired absorption states: can try very-high dose PO even if impaired absorption → will be absorbed via non-IF/terminal ileum-dependent mechanisms, e.g. diffusion [9] |
| Sublingual | 1000–2000 μg SL daily | Alternative to IM; some evidence suggests comparable efficacy to IM; useful for patients who refuse injections |
Why 1000 μg? The daily requirement for B12 is only ~1–2 μg. But when you give 1000 μg IM, you are flooding the body with B12 to rapidly replete the depleted stores (liver stores can hold 2–5 mg total). When given orally at 1000 μg, only about 1% (10 μg) is absorbed passively — still 5–10× the daily requirement, which is sufficient.
- Hydroxocobalamin (preferred in many countries including UK/HK): longer-acting, binds more tightly to TCII, requires less frequent injections
- Cyanocobalamin: shorter-acting, more commonly available in some countries; must be converted to active forms in the body
Effect: [9]
| Timepoint | Response | Notes |
|---|---|---|
| Day 2–3 | Subjective improvement in energy and well-being | Patient "feels better" before blood counts change |
| Day 5–10 | Reticulocyte count peaks [9] | The "reticulocyte crisis" — a dramatic surge of new RBCs from the now-functioning marrow. This is a confirmatory sign that your diagnosis was correct |
| Weekly | Haemoglobin increases by ~1 g/dL per week until normalised [9] | Steady, predictable rise |
| 6–8 weeks | Haemoglobin normalises | MCV may take longer to normalise (existing large RBCs need to be replaced) |
| 6–12 months | Sensory neuropathy takes 6–12 months to correct [9] | Peripheral nerves can regenerate if damage is not too severe |
| Variable | Others may not improve [9] | Spinal cord damage (SACD) may be irreversible if severe or prolonged (> 6–12 months). This is why early treatment is critical |
Complications of B12 Replacement — Hypokalaemia and Iron Depletion
B12 replacement is associated with decreased serum potassium and rapid iron depletion [9]:
-
Hypokalaemia: When B12 is replaced, the bone marrow suddenly "wakes up" and begins producing RBCs rapidly. This massive erythropoiesis consumes potassium (K⁺ is taken up by the proliferating cells) → serum K⁺ drops → risk of cardiac arrhythmias. Monitor potassium during the first 48–72 hours and replace as needed.
-
Rapid iron depletion → may develop iron deficiency (as evidenced by dimorphic red cells on PBS) → require iron replacement if necessary [9]. The surging erythropoiesis consumes iron stores rapidly. If iron stores were already marginal, the patient may develop a dimorphic blood film (large oval cells from residual B12-deficient erythropoiesis + small hypochromic cells from new iron-deficient erythropoiesis). Check ferritin and consider concurrent iron supplementation.
Side effects: generally minimal (excreted if in excess) [9]
B12 is a water-soluble vitamin — excess is excreted renally. There is no toxicity from B12 excess. Local injection site reactions (pain, erythema) may occur with IM injections.
2.2 Folate Replacement
Folate deficiency: [9]
Indications: apart from folate deficiency, also in those with decreased RBC survival (e.g. haemolytic anaemia) [9]
| Indication | Rationale |
|---|---|
| Confirmed folate deficiency | Direct replacement of the missing nutrient |
| Haemolytic anaemia | Chronic haemolysis → increased RBC turnover → increased folate demand → relative folate deficiency; prophylactic folate prevents megaloblastic change superimposed on haemolytic anaemia |
| Pregnancy | 400 μg/day folic acid recommended for all pregnant women to prevent neural tube defects; higher doses (5 mg/day) if previous NTD-affected pregnancy or on anticonvulsants |
| Psoriasis / exfoliative dermatitis | Rapid skin cell turnover increases folate demand |
| Methotrexate therapy | Folinic acid (leucovorin) "rescue" — provides reduced folate that bypasses the MTX-inhibited dihydrofolate reductase step |
Route: usually oral 1–5 mg/day → sufficient even if malabsorption (far exceeds the 0.2 mg/day daily need) [9]
| Route | Regimen | Notes |
|---|---|---|
| Oral (PO) | 1–5 mg daily | Standard treatment. Even in malabsorption, the dose is so far above daily requirements (~200 μg/day) that enough is absorbed |
| Intravenous (IV) | Only when cannot take oral medications or severe anaemia requiring urgent correction [9] | Rarely needed |
| Folinic acid (leucovorin) | 15 mg PO/IV every 6 hours for 4 doses (after MTX) | Specific for MTX rescue — provides 5-formyl-THF, bypassing the blocked dihydrofolate reductase |
Why is 1–5 mg sufficient even in malabsorption? The daily folate requirement is only ~200 μg (0.2 mg). A dose of 1 mg is 5× the daily requirement, and even if absorption is severely impaired (say only 10%), you still get 100 μg — more than enough. At 5 mg, even with 5% absorption, you get 250 μg, which exceeds the daily need.
Side effects: generally minimal (excreted if in excess), but previously reported association of high folic acid intake with increased cancer risk → caution not to exceed daily allowance excessively [9]
There is controversial epidemiological data suggesting that very high folic acid supplementation (above 1 mg/day for prolonged periods) may promote the growth of pre-existing neoplasms (particularly colorectal), possibly because folate promotes DNA synthesis in all rapidly dividing cells including cancer cells. This should not deter appropriate treatment of folate deficiency, but avoid unnecessary mega-dosing.
Replacement therapy addresses the deficiency, but you must also treat or modify the underlying cause whenever possible:
| Underlying Cause | Specific Management |
|---|---|
| Pernicious anaemia | Lifelong B12 replacement + upper endoscopy for gastric surveillance (atrophic gastritis, gastric carcinoma) [4]. Screen for associated autoimmune diseases (TFTs, glucose, vitiligo) |
| Dietary (vegan/vegetarian) | Lifelong B12 supplementation + dietary counselling. Consider fortified foods (soy milk, cereals, nutritional yeast) |
| Post-gastrectomy / ileal resection | Lifelong parenteral B12. Also consider iron supplementation (gastrectomy impairs iron absorption too) and calcium/vitamin D |
| Crohn's disease | Treat the underlying Crohn's (immunosuppression, biologics) + B12 replacement if terminal ileum involved |
| Coeliac disease | Strict gluten-free diet + replace B12/folate/iron as needed |
| Drug-induced (PPI) | Consider deprescribing or dose reduction if appropriate; supplement B12 if on long-term PPI |
| Drug-induced (metformin) | Continue metformin (benefits outweigh risks in most DM patients); monitor B12 periodically; supplement if deficient |
| Drug-induced (MTX) | Folinic acid rescue; do NOT use folic acid as it competes with MTX at the same enzyme |
| Alcoholism | Alcohol cessation counselling; folate supplementation; nutritional rehabilitation |
| Bacterial overgrowth (blind loop) | Antibiotics (e.g., rifaximin, metronidazole) to reduce bacterial overgrowth; surgical correction if anatomical cause |
Consider transfusion in symptomatic severe anaemia [9].
| Consideration | Details |
|---|---|
| Indication | Hb < 70 g/L with cardiovascular compromise (chest pain, dyspnoea at rest, heart failure) |
| Caution | Transfuse slowly and cautiously in chronic severe anaemia — the expanded plasma volume means rapid transfusion can precipitate acute heart failure and pulmonary oedema. Use 1 unit at a time with diuretic cover (frusemide 20–40 mg IV) if needed |
| General principle | Most patients with chronic megaloblastic anaemia tolerate very low Hb levels because of compensatory mechanisms (increased 2,3-DPG, increased cardiac output). B12/folate replacement will correct the anaemia — transfusion is rarely needed if you start replacement early |
| Parameter | Timing | What to Look For |
|---|---|---|
| Reticulocyte count | Day 5–7 after starting replacement | Peak reticulocytosis confirms correct diagnosis and response |
| CBC | Weekly × 4–6 weeks, then monthly | Hb should rise ~1 g/dL/week; MCV gradually normalises |
| Serum potassium | First 48–72 hours | Hypokalaemia from rapid erythropoiesis — replace if needed |
| Iron studies | After 1–2 weeks | Iron depletion from rapid erythropoiesis — supplement if ferritin drops |
| Holotranscobalamin / serum B12 | After loading course | Confirm repletion of B12 stores |
| Neurological examination | At baseline, then every 3–6 months | Sensory neuropathy may take 6–12 months to improve; motor/SACD may be irreversible |
| Upper endoscopy | At diagnosis (for PA), then every 3–5 years | Surveillance for atrophic gastritis progression and gastric carcinoma |
| Autoimmune screening | At diagnosis (for PA) | TFTs, fasting glucose, clinical assessment for vitiligo, Addison's |
5. Special Situations
Folinic acid (leucovorin = 5-formyl-THF) is used, NOT folic acid, because:
- MTX inhibits dihydrofolate reductase (DHFR), blocking the conversion of dihydrofolate → THF
- Folic acid enters the folate cycle above this block (as dihydrofolate) and is therefore also blocked by MTX
- Folinic acid enters the cycle below the block (it is already a reduced folate — 5-formyl-THF) and can be directly converted to 5,10-methylene-THF for DNA synthesis, bypassing DHFR entirely
- All pregnant women should receive folic acid 400 μg/day (or 5 mg/day if high-risk for NTDs)
- B12 should be checked and supplemented in vegan/vegetarian mothers
- If megaloblastic anaemia is diagnosed in pregnancy, treat urgently — both B12 and folate if status unknown
Post-gastrectomy patients and those with coeliac disease commonly have combined B12 + iron deficiency. The MCV may be paradoxically normal (opposing effects). After B12 replacement, the MCV may initially rise (as the microcytic iron-deficient population is diluted by new macrocytic cells), then normalise with concurrent iron supplementation. Monitor iron studies and supplement accordingly.
High-dose oral B12 (1000–2000 μg daily) is an acceptable alternative even in pernicious anaemia, based on the principle that ~1% is absorbed passively. Multiple studies have shown comparable efficacy to IM injections at these doses. Sublingual preparations are also available.
| Treatment | Contraindications / Cautions |
|---|---|
| B12 (any form) | No absolute contraindications. Rare hypersensitivity to cobalt or hydroxocobalamin. Caution: monitor K⁺ and iron during replacement |
| Folate | Absolute contraindication: do not give folate ALONE if B12 deficiency is not excluded [9] — risk of worsening neurological deficit. Relative caution: very high doses may promote growth of pre-existing neoplasms |
| Folinic acid | Should NOT be given concurrently with MTX (would negate the anti-neoplastic/anti-inflammatory effect); give as "rescue" at the appropriate time interval after MTX |
| Blood transfusion | Caution in chronic anaemia: risk of fluid overload and heart failure. Transfuse slowly, 1 unit at a time, with diuretic cover |
High Yield Summary — Management of Megaloblastic Anaemia
- Management is by replacement of B12 and/or folate [9]
- NEVER give folate alone without confirming B12 status — folate alone may worsen neurological deficits while masking haematological improvement [9]
- If B12 status unknown and urgent treatment needed → give BOTH B12 and folate [9]
- B12 replacement route: parenteral (IM) if impaired absorption; oral if dietary deficiency. High-dose oral (1000 μg) can work even in PA via passive diffusion [9]
- IM B12 regimen: 1000 μg IM weekly until normalised, then 1000 μg IM every 1–2 months [9]
- Duration: lifelong if irreversible cause (PA, gastrectomy, ileal resection, vegan diet) [9]
- Monitor for hypokalaemia (first 48–72 hours) and iron depletion (first 1–2 weeks) during B12 replacement [9]
- Reticulocyte count peaks day 5–10; Hb rises ~1 g/dL/week. No reticulocyte response → reconsider diagnosis, BM biopsy [9]
- Neurological recovery: sensory neuropathy takes 6–12 months; SACD may be irreversible if severe/prolonged [9]
- Folate replacement: 1–5 mg PO daily; sufficient even in malabsorption [9]
- PA requires lifelong B12 + endoscopic surveillance for gastric carcinoma + screening for associated autoimmune diseases
Active Recall - Management of Megaloblastic Anaemia
References
[4] Senior notes: Block A - Pallor_ diagnosis of anaemia; nutritional anaemia; anaemia of systemic diseases.pdf (Pernicious anaemia sections, pp.18–19) [9] Senior notes: Ryan Ho Haemtology.pdf (p.30, Management of megaloblastic anaemia)
Complications of Megaloblastic Anaemia
The complications of megaloblastic anaemia can be organised into three broad categories: (1) complications of the anaemia itself, (2) complications of the underlying B12/folate deficiency (beyond just anaemia), and (3) complications of the underlying cause (particularly pernicious anaemia). Additionally, there are (4) complications of treatment. Let's work through each systematically, explaining the "why" from first principles.
These complications apply to any severe anaemia regardless of cause, but are relevant because megaloblastic anaemia can present with profoundly low haemoglobin (patients may compensate for years as B12 stores deplete slowly, so Hb can drop to 40–60 g/L before presentation).
Complications of anaemia include: cardiac ischaemia, increased thrombocytopenic bleeding, increased mortality [9][17]
| Complication | Mechanism | Clinical Significance |
|---|---|---|
| High-output cardiac failure | Severe anaemia → reduced oxygen-carrying capacity → compensatory increase in cardiac output (↑heart rate, ↑stroke volume) → if prolonged, the heart cannot sustain this and decompensates | Presents with dyspnoea, peripheral oedema, pulmonary oedema. Particularly dangerous in elderly patients with pre-existing cardiac disease |
| Cardiac ischaemia [9][17] | Reduced oxygen delivery to the myocardium → supply-demand mismatch → angina or myocardial infarction (even without coronary artery disease, i.e. MINOCA — myocardial infarction with non-obstructive coronary arteries) | Can be the presenting complaint in elderly patients; always check for anaemia in unexplained ACS |
| Cerebral hypoperfusion | Reduced oxygen delivery to the brain | Dizziness, syncope, confusion; may precipitate stroke in patients with cerebrovascular disease |
| Increased thrombocytopenic bleeding [9][17] | Megaloblastic anaemia causes pancytopenia — the thrombocytopenia increases bleeding risk, and the anaemia exacerbates its clinical impact (bleeding is less well tolerated when Hb is already low) | Mucocutaneous bleeding (epistaxis, petechiae, bruising, menorrhagia) |
| Increased susceptibility to infection | Neutropenia from pancytopenia → reduced immunity | Risk of bacterial infections; typically not as severe as in aplastic anaemia or post-chemotherapy neutropenia, because the neutropenia is usually mild |
| Increased mortality [9][17] | All of the above contribute | Untreated severe megaloblastic anaemia can be fatal, particularly in the elderly from cardiac failure or infection |
Why do patients tolerate such low Hb levels in megaloblastic anaemia? Because the anaemia develops very slowly (over months to years), the body has time to compensate: increased 2,3-DPG in RBCs (shifts the oxygen-dissociation curve rightward, improving oxygen release to tissues), increased cardiac output, and peripheral vasodilation. A patient with acute blood loss to Hb 50 g/L would be in shock; a patient with chronic megaloblastic anaemia at Hb 50 g/L may be walking around, just very tired.
2. Complications of B12 Deficiency (Tissue-Specific)
These are the complications unique to B12 deficiency, arising from its dual biochemical roles. They are among the most important complications because some are irreversible if not treated promptly.
The neurological complications are the most feared and clinically significant. They occur because adenosylcobalamin deficiency leads to MMA accumulation → abnormal fatty acid incorporation into myelin → demyelination. They can also relate to impaired methylation of myelin proteins (from the methionine synthase pathway).
| Complication | Mechanism | Clinical Features | Prognosis |
|---|---|---|---|
| Subacute combined degeneration of the cord (SACD) | MMA accumulation → abnormal odd-chain fatty acids incorporated into myelin of the spinal cord → demyelination of posterior columns (dorsal column-medial lemniscal pathway) and lateral columns (corticospinal tracts) | Loss of proprioception/vibration sense (posterior columns), spastic paraparesis with upgoing plantars (lateral corticospinal tracts), sensory ataxia, positive Romberg sign. Classical exam finding: absent ankle jerks + upgoing plantars (combined LMN peripheral neuropathy + UMN cord disease) | Sensory neuropathy takes 6–12 months to correct; others may not improve [9]. Motor deficits and incontinence from severe SACD may be permanent |
| Peripheral neuropathy | MMA toxicity to peripheral nerve myelin → axonal degeneration + demyelination | Glove-and-stocking paraesthesiae, numbness, reduced ankle jerks | Usually reversible with early treatment |
| Optic neuropathy | Demyelination of optic nerves | Bilateral visual loss, centrocaecal scotomata | Rare; may partially recover |
| Cognitive impairment / Dementia | Impaired CNS myelination + reduced methyl group availability for neurotransmitter synthesis | Memory loss, confusion, cognitive slowing | May be partially reversible if treated early |
| Psychiatric manifestations ("Megaloblastic madness") | Same as above + possible effects on monoamine metabolism | Depression, irritability, psychosis, personality change | May improve with B12 replacement |
Neurological Damage Can Occur WITHOUT Anaemia
Neurological complications can occur in the absence of haematological changes, as neurones require a higher B12 level to function [9]. This means a patient can present with SACD, peripheral neuropathy, or cognitive decline with a completely normal blood count and MCV. The B12 level needed for adequate myelin maintenance is higher than that needed for erythropoiesis. Always check B12 in unexplained neuropathy or dementia, regardless of whether there is anaemia.
| Complication | Mechanism | Clinical Relevance |
|---|---|---|
| Pancytopenia | Impaired DNA synthesis affects all rapidly dividing cell lines — erythroid, myeloid, megakaryocytic | Anaemia (reduced O₂ carrying capacity), neutropenia (infection risk), thrombocytopenia (bleeding risk) |
| Intramedullary haemolysis (ineffective erythropoiesis) | Megaloblastic precursors are so abnormal they are destroyed within the bone marrow before release | ↑LDH, ↑unconjugated bilirubin; paradoxically low reticulocytes despite haemolysis markers. Can cause mild jaundice (lemon-yellow tinge) |
| Hyperhomocysteinaemia | Both B12 and folate deficiency impair the methionine synthase reaction → homocysteine accumulates | Elevated homocysteine is an independent risk factor for atherosclerosis, venous thromboembolism, and arterial events (MI, stroke). Patients with untreated B12/folate deficiency have increased cardiovascular risk |
Why does hyperhomocysteinaemia cause cardiovascular disease? Homocysteine at high concentrations is directly toxic to vascular endothelium → endothelial dysfunction → promotes atherosclerosis, platelet activation, and coagulation cascade activation. It also impairs nitric oxide-mediated vasodilation and increases oxidative stress. This is why B12/folate deficiency is now recognised as a cardiovascular risk factor.
B12 and folate are essential for DNA synthesis in all rapidly dividing cells, not just haematopoietic cells. The GI epithelium turns over every 3–5 days — making it particularly vulnerable.
| Complication | Mechanism | Clinical Features |
|---|---|---|
| Glossitis (atrophic) | Papillae of the tongue are composed of rapidly dividing epithelial cells → impaired DNA synthesis → atrophy | Beefy-red, smooth, painful tongue; loss of papillae ("loss of roughness") |
| Angular stomatitis / cheilitis | Same mechanism at the corners of the mouth | Painful cracks and fissures at oral commissures |
| GI mucosal atrophy | Atrophy of GI epithelium throughout the tract | Malabsorption (can paradoxically worsen the deficiency — a vicious cycle), anorexia, weight loss |
| Infertility | Impaired DNA synthesis in rapidly dividing germ cells | Reversible subfertility in both sexes; in women, may also contribute to recurrent miscarriage |
| Complication | Mechanism | Clinical Significance |
|---|---|---|
| Neural tube defects (NTDs) | Folate (and B12) are essential for neural tube closure in the first 28 days of embryogenesis. Deficiency → failed closure → spina bifida, anencephaly | This is why periconceptional folic acid supplementation (400 μg/day) is recommended for all women planning pregnancy |
| Fetal growth restriction / low birth weight | Impaired DNA synthesis in rapidly dividing fetal cells | Particularly relevant in developing countries with endemic folate deficiency |
| Pregnancy megaloblastic anaemia | Increased folate demand (fetal growth) in the context of marginal stores | More common with folate deficiency (stores only 3 months) than B12 deficiency (stores 3 years) |
3. Complications Specific to Pernicious Anaemia (The Underlying Cause)
Pernicious anaemia is an autoimmune disease, and its complications extend beyond B12 deficiency itself.
Two gastric diseases associated with pernicious anaemia: [4]
- Atrophic gastritis → common complication of PA, autoimmune attack to stomach [4]
- Carcinoma of stomach → notably associated with PA, due to autoantibody attack to stomach [4]
| Complication | Mechanism | Clinical Significance |
|---|---|---|
| Chronic autoimmune atrophic gastritis | Anti-parietal cell antibodies → T-cell mediated destruction of oxyntic (parietal) cells in gastric body/fundus → glandular atrophy, intestinal metaplasia | Permanent loss of acid and IF secretion; the pathological basis of PA |
| Achlorhydria | Loss of parietal cells → no HCl production | ↑Risk of GI infections (acid is a barrier to enteric pathogens), impaired iron absorption (acid needed to reduce Fe³⁺ to Fe²⁺), bacterial overgrowth |
| Iron deficiency anaemia | Achlorhydria → impaired iron absorption (gastric acid facilitates conversion of dietary non-haem iron from Fe³⁺ to the absorbable Fe²⁺ form) | Can coexist with B12 deficiency → dimorphic blood film |
| Gastric adenocarcinoma | Chronic atrophic gastritis → intestinal metaplasia → dysplasia → carcinoma sequence. Also: achlorhydria → bacterial overgrowth → production of carcinogenic N-nitroso compounds | 2–3× increased risk of gastric adenocarcinoma. Requires endoscopic surveillance |
| Gastric neuroendocrine (carcinoid) tumours | Achlorhydria → loss of negative feedback on gastrin → compensatory hypergastrinaemia → trophic effect on enterochromaffin-like (ECL) cells in gastric fundus → ECL cell hyperplasia → carcinoid tumour | Type I gastric carcinoid; usually low-grade and indolent but requires monitoring |
| Gastric polyps | Hypergastrinaemia → fundic gland polyps | Usually benign; endoscopic surveillance |
Why does achlorhydria lead to cancer? The stomach needs acid for more than just digestion. Without acid: (1) bacteria colonise the stomach (normally sterile due to low pH) → they convert dietary nitrates to carcinogenic N-nitroso compounds; (2) the body tries to produce more acid by releasing more gastrin (negative feedback lost) → chronic hypergastrinaemia → trophic stimulation of ECL cells → hyperplasia → neoplasia.
Pernicious anaemia is associated with other autoimmune diseases: vitiligo, autoimmune thyroid disease [2][9]
| Associated Condition | Frequency | Clinical Relevance |
|---|---|---|
| Autoimmune thyroid disease (Hashimoto's, Graves') | Most common association | Screen with TFTs at diagnosis. Hypothyroidism can independently cause macrocytosis → confounds the picture |
| Vitiligo | Common | Depigmented skin patches; explains the "early greying of hair" |
| Type 1 Diabetes Mellitus | Less common | Screen with fasting glucose |
| Addison's disease | Rare | Adrenal insufficiency; screen if suspicious |
| Hypoparathyroidism | Rare | May cause hypocalcaemia |
| Myasthenia gravis | Rare | Autoimmune neuromuscular junction disorder |
These associations follow the pattern of polyglandular autoimmune syndromes (particularly Type II / APS-2: Addison's + autoimmune thyroid ± type 1 DM). PA clusters with these because they share common HLA susceptibility alleles and autoimmune predisposition.
Even treatment itself can cause complications if not properly monitored (as discussed in the Management section, but worth consolidating here):
| Complication | Mechanism | Prevention |
|---|---|---|
| Hypokalaemia | Rapid erythropoiesis after B12 replacement → proliferating cells take up K⁺ → serum K⁺ drops | Monitor K⁺ in first 48–72 hours; replace if < 3.5 mmol/L |
| Iron depletion → iron deficiency anaemia | Rapid erythropoiesis consumes iron stores → may develop iron deficiency as evidenced by dimorphic red cells on PBS [9] | Check ferritin after 1–2 weeks; supplement iron if needed |
| Fluid overload from transfusion | Chronic severe anaemia → expanded plasma volume → rapid transfusion can precipitate pulmonary oedema / heart failure | Transfuse slowly (1 unit at a time); use diuretic cover (frusemide) |
| Folate given alone → worsening neurological deficit | Folate partially corrects anaemia but does not address MMA accumulation → SACD progresses silently | Never give folate alone without confirming B12 status [9] |
| Rebound thrombocytosis | After B12 replacement, the recovering marrow may overshoot platelet production transiently | Usually self-limiting; rarely requires intervention |
While most clinical attention focuses on B12 deficiency (especially in HK), folate deficiency has its own complication profile:
| Complication | Mechanism | Notes |
|---|---|---|
| Neural tube defects (in pregnancy) | Folate essential for neural tube closure | The single most important complication of folate deficiency from a public health perspective. Periconceptional folic acid supplementation has dramatically reduced NTD incidence worldwide |
| Megaloblastic anaemia | Same mechanism as B12 deficiency (impaired DNA synthesis) | Haematologically identical to B12 deficiency |
| Hyperhomocysteinaemia | Folate needed for methionine synthase reaction | Cardiovascular risk factor |
| NO neurological complications (or very rare) | Folate deficiency does not cause MMA accumulation (this is B12-specific) | Key differentiator from B12 deficiency. SACD does NOT occur in pure folate deficiency |
| Malabsorption syndromes (in conditions causing folate deficiency) | Coeliac disease, tropical sprue → folate deficiency + associated malabsorption of other nutrients | Vitamin B deficiency: megaloblastic anaemia, peripheral neuropathy, subacute cord degeneration (in the context of combined deficiencies from short bowel syndrome or malabsorption) [18] |
| Scenario | Outcome |
|---|---|
| Megaloblastic anaemia treated promptly | Excellent — complete haematological recovery within 6–8 weeks |
| Neurological complications treated early | Good — sensory neuropathy usually recovers over 6–12 months |
| SACD with severe/prolonged deficiency | Poor — motor deficits and incontinence may be irreversible |
| Pernicious anaemia with gastric surveillance | Good long-term outcome with lifelong B12 and endoscopic monitoring |
| Untreated megaloblastic anaemia | Fatal — from cardiac failure, infection, or progressive neurological deterioration |
High Yield Summary — Complications of Megaloblastic Anaemia
-
Complications of anaemia: cardiac ischaemia, increased thrombocytopenic bleeding, increased mortality [9][17] — particularly dangerous in elderly with pre-existing cardiac disease
-
Neurological complications (B12-specific): SACD (posterior + lateral columns), peripheral neuropathy, optic neuropathy, cognitive decline, "megaloblastic madness" — can occur without anaemia [9]
-
SACD: sensory neuropathy takes 6–12 months to correct; motor deficits may be irreversible [9]
-
Hyperhomocysteinaemia: independent cardiovascular risk factor (atherosclerosis, VTE) — present in both B12 and folate deficiency
-
Pernicious anaemia gastric complications: atrophic gastritis + gastric carcinoma (2–3× risk) + gastric carcinoid tumours [4] — endoscopic surveillance required
-
PA associated with other autoimmune diseases: thyroid disease, vitiligo, T1DM, Addison's [2][9] — screen at diagnosis
-
Treatment complications: hypokalaemia (first 48–72 hours), iron depletion (dimorphic RBCs) [9], folate-alone worsening neurological deficits
-
Folate deficiency: neural tube defects in pregnancy (major public health impact); NO SACD
-
Key teaching point: neurological damage from B12 deficiency may be irreversible if treatment is delayed — this is the most important reason for early diagnosis and prompt replacement
Active Recall - Complications of Megaloblastic Anaemia
References
[2] Senior notes: Maksim Medicine Notes.pdf (Haematology, p.158, B12/folate deficiency associations) [4] Senior notes: Block A - Pallor_ diagnosis of anaemia; nutritional anaemia; anaemia of systemic diseases.pdf (Pernicious anaemia sections, pp.18–19) [9] Senior notes: Ryan Ho Haemtology.pdf (pp.10, 29–30, Approach to anaemia, Pernicious anaemia clinical features and management) [17] Senior notes: Ryan Ho Fundamentals.pdf (p.380, Complications of anaemia) [18] Senior notes: Ryan Ho GI.pdf (p.127, Short bowel syndrome and malabsorption complications)
High Yield Summary
-
Megaloblastic anaemia = macrocytic anaemia due to impaired DNA synthesis → nuclear-cytoplasmic dissociation → oval macrocytes + hypersegmented neutrophils on PBS
-
In HK, B12 deficiency is the primary cause. Folate deficiency causing megaloblastic anaemia is exceedingly rare (zero cases in Prof Kwong's HK study)
-
Pernicious anaemia is the most common medical cause of B12 deficiency (95% of cases) — autoimmune destruction of parietal cells / anti-IF antibodies
-
B12 absorbed in terminal ileum (requires IF); folate absorbed in upper small intestine. B12 stores last ~3 years; folate stores ~3 months
-
B12 has two roles: Methyl-B12 (methionine synthase → DNA synthesis via THF); Ado-B12 (MMA → succinyl-CoA → Krebs). B12 deficiency causes BOTH haematological (methylfolate trap) AND neurological (MMA accumulation) disease
-
PBS: oval macrocytes + hypersegmented neutrophils (≥ 6 lobes = diagnostic). MCV > 120 fL → think pernicious anaemia or chemotherapy
-
Intramedullary haemolysis → ↑LDH, ↑unconjugated bilirubin, ↓reticulocytes (not extravascular, so no reticulocytosis!)
-
Pernicious anaemia clinical features: macrocytic anaemia, mild jaundice (lemon-yellow), early greying, glossitis (beefy-red tongue), angular stomatitis, subacute combined degeneration of the cord
-
SACD = posterior columns + lateral corticospinal tracts + peripheral nerves → absent ankle jerks + upgoing plantars
-
Never give folate alone without checking B12 — will correct anaemia but allow irreversible neurological damage to progress
-
Investigations: holotranscobalamin (active B12), anti-parietal cell Ab (sensitive), anti-IF Ab (specific), upper endoscopy (atrophic gastritis, gastric cancer), MMA + homocysteine
-
Associated with gastric carcinoma (2-3× increased risk) — endoscopic surveillance needed
High Yield Summary — Diagnosis of Megaloblastic Anaemia
- Diagnosis is morphological + biochemical + aetiological — no single "gold standard" test
- PBS is the pivotal investigation: oval macrocytes + hypersegmented neutrophils (≥ 6 lobes = diagnostic) [1]
- Severe macrocytosis (MCV > 110–115 fL) is almost exclusively megaloblastic anaemia [9]
- Holotranscobalamin (active B12) is the preferred modern test over total serum B12 [4]
- MMA and homocysteine distinguish B12 from folate deficiency: ↑MMA = B12 deficiency; normal MMA + ↑Hcy = folate deficiency [2]
- Anti-IF Ab is specific but insensitive; anti-parietal cell Ab is sensitive but non-specific [4][16]
- Upper endoscopy for atrophic gastritis + gastric carcinoma surveillance [4][16]
- BM biopsy is NOT routine — only when findings are incompatible with PA or patient fails to respond to replacement [4][16]
- Schilling test is obsolete in HK [4][16]
- Reticulocyte response at day 5–7 after B12 replacement confirms the diagnosis; failure to respond mandates BM biopsy
- Always check iron studies alongside B12/folate — dimorphic picture can mask the MCV abnormality
High Yield Summary — Management of Megaloblastic Anaemia
- Management is by replacement of B12 and/or folate [9]
- NEVER give folate alone without confirming B12 status — folate alone may worsen neurological deficits while masking haematological improvement [9]
- If B12 status unknown and urgent treatment needed → give BOTH B12 and folate [9]
- B12 replacement route: parenteral (IM) if impaired absorption; oral if dietary deficiency. High-dose oral (1000 μg) can work even in PA via passive diffusion [9]
- IM B12 regimen: 1000 μg IM weekly until normalised, then 1000 μg IM every 1–2 months [9]
- Duration: lifelong if irreversible cause (PA, gastrectomy, ileal resection, vegan diet) [9]
- Monitor for hypokalaemia (first 48–72 hours) and iron depletion (first 1–2 weeks) during B12 replacement [9]
- Reticulocyte count peaks day 5–10; Hb rises ~1 g/dL/week. No reticulocyte response → reconsider diagnosis, BM biopsy [9]
- Neurological recovery: sensory neuropathy takes 6–12 months; SACD may be irreversible if severe/prolonged [9]
- Folate replacement: 1–5 mg PO daily; sufficient even in malabsorption [9]
- PA requires lifelong B12 + endoscopic surveillance for gastric carcinoma + screening for associated autoimmune diseases
High Yield Summary — Complications of Megaloblastic Anaemia
-
Complications of anaemia: cardiac ischaemia, increased thrombocytopenic bleeding, increased mortality [9][17] — particularly dangerous in elderly with pre-existing cardiac disease
-
Neurological complications (B12-specific): SACD (posterior + lateral columns), peripheral neuropathy, optic neuropathy, cognitive decline, "megaloblastic madness" — can occur without anaemia [9]
-
SACD: sensory neuropathy takes 6–12 months to correct; motor deficits may be irreversible [9]
-
Hyperhomocysteinaemia: independent cardiovascular risk factor (atherosclerosis, VTE) — present in both B12 and folate deficiency
-
Pernicious anaemia gastric complications: atrophic gastritis + gastric carcinoma (2–3× risk) + gastric carcinoid tumours [4] — endoscopic surveillance required
-
PA associated with other autoimmune diseases: thyroid disease, vitiligo, T1DM, Addison's [2][9] — screen at diagnosis
-
Treatment complications: hypokalaemia (first 48–72 hours), iron depletion (dimorphic RBCs) [9], folate-alone worsening neurological deficits
-
Folate deficiency: neural tube defects in pregnancy (major public health impact); NO SACD
-
Key teaching point: neurological damage from B12 deficiency may be irreversible if treatment is delayed — this is the most important reason for early diagnosis and prompt replacement
Hemolytic Anaemia
Hemolytic anaemia is a condition characterized by the premature destruction of red blood cells at a rate exceeding the bone marrow's compensatory production capacity, leading to reduced circulating erythrocytes.
Non-megaloblastic Anaemia
Non-megaloblastic anaemia is a form of macrocytic anaemia in which large red blood cells occur without the hypersegmented neutrophils or abnormal nuclear maturation seen in megaloblastic anaemia, typically caused by conditions such as liver disease, hypothyroidism, alcoholism, or myelodysplastic syndromes.