Megaloblastic anaemia

Megaloblastic anaemia



§        Defect in DNA synthesis affecting rapidly dividing cells in bone marrow and other tissues

§        Disparity in the availability of the precursors of DNA which are required for DNA synthesis during S phase of cell cycle

§        Adenine, guanine, cytosine and thymidine

§        In B12 and folate deficiency this is due to an inability to methylate deoxyuridine monophosphate to deoxythymidine monophosphate

§        The methyl group is supplied by the folate co-enzyme, methylene tetrahydrofolate

§        Folate deficiency reduces the supply of this co-enzyme

§        B12 deficiency reduces its supply by slowing the demethylation of methyltetrahydrofolate and preventing cells receiving tetrahydrofolate for synthesis of methylene tetrahydrofolate

§        Slowed elongation of newly originated replicating segments

§        Small fragments accumulate and single stranded areas become points of weakness, where breakage may occur

§        Some cells apoptose at this stage


§        B12- folate relationship


§        In the cytoplasm, methylcobalamin serves as cofactor for methionine synthesis by allowing transfer of a methyl group from 5-methyl-tetrahydrofolate (5-methyl-THF) to homocysteine (HC), forming methionine and demethylated tetrahydrofolate (THF).

§        This results in reduction in serum homocysteine, which appears to be toxic to endothelial cells. Methionine is further metabolized to S-adenosylmethionine (SAM). (Methionine levels increase in B12 and folate deficiency)

§        THF participates in purine synthesis and the conversion of deoxyuridylate (dUTP) to deoxythymidine monophosphate (dTMP), which is then phosphorylated to deoxythymidine triphosphate (dTTP) , which is required for DNA synthesis

§        Therefore, in vitamin B-12 deficiency, formation of dTTP is inadequate and accumulation of 5-methyl-THF occurs, trapping folate in its unusable form and leading to retarded DNA synthesis.

§        RNA contains dUTP (deoxyuracil triphosphate) instead of dTTP, allowing for protein synthesis to proceed uninterrupted and resulting in macrocytosis and cytonuclear dissociation.

§        Cobalamin as well as a co-enzyme to methionin synthase is also a co-enzyme in the isomerization of methylmalonyl CoA to succinyl CoA


Causes of Megaloblastic anaemia

§        B12 and folate deficiency

§        Anti-folate drugs

§        Eg methotrexates

§        Some cases of AML and MDS

§        Drugs interfering with DNA synthesis

§        Cytarabine, HU, 6-MP, AZT

§        Orotic aciduria (AR condition UMPS deficiency – produces megaloblastic anaemia resistent to B12 and folate)

§        Lesch-Nyhan syndrome (XLR deficiency of HGPRT – hypoxanthine-guanidine phosphoribosyl transferase – results in build up of uric acid, severe gout, mental retardation and megaloblastic anaemia in some cases)


Clinical Features

§        General

§        Anorexia, weight loss, diarrhoea, constipation

§        Glossitis, angular cheilosis

§        Mild fever

§        Jaundice (unconjugated – due to death of red cells in marrow)

§        Reversible melanin skin hyperpigmentation

§        Epithelial surfaces

§        Affect all rapidly growing tissues

§        Glossitis, villous flattening with GI upset

§        Cervical smear abnormalities

§        Pregnancy

§        Infertility in men and women

§        Neural tube defects

§        Folic acid from conception to 12 weeks reduces incidence of neural tube defects by 70%

§        Cardiovascular defects

§        Homocysteinuria with high levels of homocysteine suffer with cardio and cerebrovascular disease (homocysteine toxic to endothelial cells)

§        Lesser degrees of high homocysteine have also been associated with vascular disease

§        Malignancy

§        Prophylactic folic acid during  pregnancy has been found to reduce subsequent ALL in childhood

§        Neurology

§        B12 deficiency causes peripheral neuropathy with demyelination of the posterior and pyramidal tracts

§        Optic atrophy or cerebral symptoms


§        Peripheral blood

§        Oval macrocytes with anisocytosis and poikilocytosis and fragmentation

§        Hypersegmented neutrophils (>5)

§        Leucopenia and thrombocytopenia

§        Bone marrow

§        Hypercellular with accumulation of primitive cells

§        Nuclear; cytoplasmic asynchrony in erythroblasts

§        Nucleus has an open, fine lacy appearance

§        Nuclear fragments

§        Giant metamyelocytes and hyperpolypoid megakaryocytes

§        Can be confused with erythroleukaemia


Vitamin B12

§        Only source is food of animal origin

§        Meat, fish, dairy

§        Body stores are sufficient for 3-4 years

§        Absorption

§        Passive

§        Duodenum and ileum

§        Inefficient; <1% of an oral dose is absorbed

§        Active

§        Ileum, mediated by intrinsic factor

§        Dietary B12 binds to R-protein (salivary glycoprotein)

§        B12 – R complex is digested by pancreatic trypsin

§        B12 transferred to intrinsic factor – resistant to enzyme digestion

§        Intrinsic factor is produced by gastric parietal cells in fundus and body of stomach

§        Intrinsic factor attaches to a receptor on brush border of ileal cells and B12 enters the cells, whilst intrinsic factor remains in the lumen

§        Ileum has restricted capacity, because of limited receptor sites

§        B12 then transported by transcobalamin II to all cells of the body

§        In plasma, B12 is mainly bound to transcobalamin I

§        Some B12 enters bile, binds to intrinsic factor and is reabsorbed

§        Transport

§        R-protein

§        Transcobalamin

·        Synthesised by liver, also macrophages, ileum and endothelium

·        Readily gives up B12 to marrow, placenta and others

§        Deficiency

§        Dietary deficiency

·        Vegan

§        Excess degradation

·        Nitric oxide

§        Gastric causes

·        Pernicious anaemia

§        Lack of IF due to gastric atrophy

§        Peak age of onset 60 years (younger in black, Latin Americans)

§        Association with AI disease, premature greying, blue eyes and vitiligo, Blood group A and hypogammaglobulinaemia

§        IF antibodies

·        Type 1 – prevents binding of IF to B12

·        Type 2 – prevents IF attaching to ileal mucosa

§        GPC Antibodies

·        Sensitive but not specific

·        Congenital IF deficiency

§        Present at toddler age, when stores accumulated in utero have been used up

·        Gastrectomy

§        Inevitable after total

§        10-15% after partial gastrectomy

§        Intestinal

·        Stagnant loop syndrome

§        Bacterial overgrowth of upper small intestine by faecal organisms

§        Consume cobalamin

·        Ileal resection

·        Selective malabsorption of cobalamin with proteinuria

§        AR

§        Most common cause of megaloblastic anaemia due to caolabmin deficiency in children in Western countries

§        Congenital defect of the ileum

·        Tropical sprue

§        Usually improves after antibiotic therapy

·        Fish tapeworm (Diphyllobothrium latum)

·        Gluten induced enteropathy

§        30% of untreated

·        Severe chronic pancreatitis

§        Lack of trypsin results in cobalamin –protein R complex unavailable for absorption

·        HIV

·        Zollinger-Ellison syndrome

§        High acidity inactivates trypsin

·        Radiotherapy and GVHD

·        Drugs

§        Esp long term use of H2-antagonists

§        Abnormalities of cobalamin metabolism

·        Congenital transcobalamin deficiency

§        Present within a few weeks of birth

§        Responds to massive injections of B12 which allows passive transfer of B12 into marrow cells

·        Congenital methylmalonic acidaemia and aciduria

§        Ill from birth

§        Diagnosis

§        Serum B12

·        Radioisotope dilution or ELISA

·        High levels usually due to elevated haptocorrin (cobalamin-binding protein)

§        MPD, hepatoma, increased granulocytes (IBD, liver abscess), eosinophilia

§        Methylmalonic acid level

·        Increased due to impairment of methylmalonyl CoA to succinyl CoA

·        Levels fluctuate in renal failure

§        Homocysteine levels

·        Increased, but poor specificity, therefore not used

§        Schilling test

·        Not done currently

·        Radiolabelled oral B12 and measure urinary excretion. If malabsorption, give the same but with IF. If abnormal = intestinal rather than gastric



§        Found in most foods, but easily destroyed by cooking (unlike B12)

§        Stores are only sufficient for about 4 months

§        Absorption

§        Upper small intestine – about 90% of an oral dose is absorbed

§        Enterohepatic circulation

§        Transport

§        1/3 bound to albumin, 2/3 unbound

§        Rate of uptake in cell is dependant on rate of DNA synthesis

§        Deficiency

§        Dietary

§        Tropical sprue

§        Gluten-induced enteropathy

§        Specific malabsorption of folate

§        Excess utilisation or loss

§        Pregnancy

§        Prematurity

§        Haemolysis

§        Inflammatory disease/ chronic infections

§        Homocysteinuria

§        Haemo and peritoneal dialysis

§        CCF/ liver disease

§        Release of folate from damaged liver cells

§        Anti-folate drugs

§        Diagnosis

§        Serum folate

§        Radioassay or ELISA

§        Very sensitive, can be affected by weekly dietary changes

§        Red cell folate

§        Useful test of folate stores

§        Less affected by recent diet

§        False normals if recent transfusion or raised reticulocytes


Management of megaloblastic anaemia

§        B12

§        Should treat if haematological abnormalities even in the absence of anaemia

§        Routinely to all who have undergone total gastrectomy or ileal resection

§        Uusually given IM

§        Because a small amount can be absorbed passively, large daily oral doses (1-2mg) of cyanocobalamin can be given

§        Most need to continue for life, unless treatable cause such as tropical sprue

§        Folate

§        Exclude B12 deficiency

§        Can treat for 4 months or long-term if likely to recur

§        Food is often fortified

§        Pregnency – 400mcg is an adequate dose

§        If previous NTD, then 5mg

§        Folinic acid – stable form of reduced folate

§        To overcome toxic effects of methotrexate