MEGALOBLASTIC ANEMIA

MEGALOBLASTIC ANAEMIA
This results from a deficiency of vitamin B;- or folic acid. or from disturbances in folic acid metabolism. Folate is an important substrate of, and vitamin B_: a co-factor for. the generation of the essential amino acid methiomm from homocysteine. This reaction produces tetrahydrofolate. which is converted to thymidine monophosphate for incorporation into DNA. Deficiency of either vitamin B;= or folate will therefore produce high plasma levels of homocysteine and impaired DNA synthesis.
The end result is cells with arrested nuclear maturation but normal cytoplasmic development: so-called nucleo¬cytoplasmic aasynchrony. All proliferating cells will exhibit megaloblastosis; hence changes are evident in the buccal mucosa, tongue, small intestine, cervix, vagina and uterus. The high proliferation rate of bone marrow results in striking changes in the haematopoietic system in megalo¬blastic anaemia. Cells become arrested in development and die within the marrow; this ineffective erythropoiesis results in an expanded hypercellular marrow. The megaloblastic changes are most evident in the early nucleated red cell precursors, and intramedullary haemolysis results in a raised bilirubin and lactate dehydrogenase (LDH) but no reticulo¬cytosis. Iron stores are usually raised. The mature red cells are large and oval, and sometimes contain nuclear remnants. Nuclear changes are seen in the immature granulocyte precursors and a characteristic appearance is that of `giant metamyelocytes with a large sausage-shaped nucleus. The mature neutrophils show hypersegmentation of their nuclei, with cells having six or more nuclear lobes. If severe, a pancytopenia may be present in the peripheral blood.
Vitamin Blz deficiency but not folate deficiency is associated with neurological disease in up to 40% of cases.
24.24 CLINICAL FEATURES OF 
MEGALOBLASTIC ANAEMIA 
Symptoms 
9 Malaise (90%) 9 Impotence
~ Breathlessness (50%) * Poor memory
~ Paraesthesiae (80%) 0 Depression
~ Sore mouth (20%) * Personality change
~ Weight loss 9 Hallucinations
~ Altered skin pigmentation 0 Visual disturbance
~ Grey hair 
Signs 
• Smooth tongue 0 Skin pigmentation
• Angular cheilosis 9 Heart failure
• Vitiligo 0 Pyrexia
24.25 DIAGNOSTIC FEATURES OF
MEGALOBLASTIC ANAEMIA
Investigation Result
Haemoglobin Often reduced, may be very low
MCV Usually raised, commonly > 120 fl
Erythrocyte count Low for degree of anaemia
Blood film Oval macrocytosis, poikilocytosis, red cell
fragmentation, neutrophil hypersegmentation
Reticulocyte count Low for degree of anaemia
Leucocyte count Low or normal
Plateiet count Low or normal
Bate marrow ncreased cellulanty, megaloblastic changes
~n erythroid series, giant metamyelocytes,
dysplastic megakaryocytes, increased iron in
stores, pathological non-ring sideroblasts
Serum ferritin Elevated
i Plasma LDH Elevated, often markedly
The main pathological finding is focal demyelination affecting spinal cord, peripheral nerves, optic nerves and cerebrum. The most common manifestations are sensory with peripheral paraesthesia and ataxia of gait. The clinical and diagnostic features of megaloblastic anaemia are summarised in Boxes 24.24 and 24.25, the neurological findings of B,2 deficiency in VITAMIN B12
Vitamin B12 absorption
The average daily diet contains 5-30 ~tg of vitamin B,2, mainly in meat, fish, eggs and milk-well in excess of the 1 pg daily requirement. In the stomach, gastric enzymes release vitamin B ,Z from food and at gastric pH it binds to a carrier protein termed R protein. The gastric parietal cells produce intrinsic factor, a vitamin B12-binding protein which optimally binds vitamin B;Z at pH 8. As gastric emptying occurs, pancreatic secretion raises the pH and vitamin B,2 released from the diet switches from the R protein to intrinsic factor. Bile also contains vitamin B12 which is available for reabsorption in the intestine. The vitamin 1312 intrinsic factor complex binds to specific receptors in the

 
BLOOD DISORDERS
 
24.26 NEUROLOGICAL FINDINGS IN B,z DEFICIENCY
Peripheral nerves
0 Glove and stocking paraesthesiae
Spinal cord
• Subacute combined degeneration
• Posterior columns-diminished vibration and proprioception
• Corticospinal tracts-upper motor neuron signs
Cerebrum
• Dementia
• Optic atrophy
Autonomic neuropathy
terminal ileum and vitamin B iZ is actively transported by the enterocytes to plasma, where it binds to transcobalamin II, a transport protein produced by the liver, which carries it to the tissues for utilisation. The liver stores enough vitamin B12 for 3 years and this, together with the entero¬hepatic circulation, means that vitamin B„ deficiency takes years to become manifest even if all dietary intake is stopped.
Levels of cobalamins fall in normal pregnancy. Each laboratory must validate its own normal range but levels below 150 ng/1 are common and in the last trimester 5-10% of women have levels below 100 ng/1. Similarly, parapro¬teins can interfere with vitamin B,Z assays and so myeloma may be associated with a spurious low vitamin B„.
Causes of vitamin Bt2 deficiency
Dietary deficiency
This only occurs in strict vegans but the onset of clinical features can occur at any age between 10 and 80 years. The breastfed offspring of vegan mothers are at risk of developing nutritional vitamin B,2 deficiency. Less strict vegetarians often have slightly low vitamin B ,z levels but are not tissue vitamin B12-deficient.
Gastric factors
Normal gastric acid and enzyme secretion is required for the release of vitamin B,2 from the food. Hypochlorhydria in elderly patients or following gastric surgery can impair the release of vitamin B,Z from food. Total gastrectomy invariably results in vitamin B,, deficiency within 5 years, often combined with iron deficiency. These patients need life-long 3-monthly vitamin B,= injections. After partial gastrectomy vitamin B,, deficiency only develops in 10-20% of patients by 5 years. An annual injection of vitamin B„ should prevent deficiency in this group.
Pernicious anaemia
This is an autoimmune disorder in which the gastric mucosa is atrophic with loss of parietal cells causing intrinsic factor deficiency. In the absence of intrinsic factor less than 1 % of dietary vitamin B,= is absorbed. Pernicious anaemia has an incidence of 25/100 000 population over the age of 40 years in developed countries, but an average age of onset of 60 years. It is more common in individuals with other autoimmune disease (Hashimoto s thyroiditis, Graves
disease, vitiligo, hypoparathyroidism or Addison s disease) or a family history of these or pernicious anaemia. Anti¬parietal cell antibodies are present in over 90% of cases but are also present in 20% of normal females over the age of 60 years. A negative result makes pernicious anaemia less likely but a positive result is not diagnostic. Antibodies to intrinsic factor are found in the serum of 60% of patients with pernicious anaemia and. if present, are diagnostic.
Sma116owel factors
One-third of all patients with pancreatic insufficiency fail to transfer dietary vitamin B,_ from R protein to intrinsic factor. This usually results in slightly low vitamin B12 values but no tissue evidence of vitamin B„ deficiency.
Motility disorders or hypogammaelobulinaemia can result in bacterial overgrowth and the resulting competition for free vitamin B,2 can result in deficiency. This will be corrected to some extent by a course of antibiotics.
A small number of people heavily infected with the fish tapeworm (p. 370) develop vitamin B,2 deficiency. Inflammatory disease of the terminal ileum. such as Crohn s disease, may impair the interaction of the vitamin B,2-intrinsic factor complex with its receptor, as will surgery on this part of the bowel. Both may result in vitamin B,= malabsorption.
It is possible to distinguish pernicious anaemia from intes¬tinal problems with a two-part Schilling test (Fig. 24.20). The patient must be vitamin B12-replete, have normal renal function and be able to comply with a 24-hour urine collection. This latter criterion is important as up to 25% of tests are invalidated by an incomplete urine collection. It is
Part One
Give
Part Two (if excretion I in Part One)
Give 1 pg radio-labelled B,2 plus 1 mg ordinary Btz
plus intrinsic factor p intramuscularly
Gut "  Pernicious disease anaemia
1 lrg radio-labelled Bi2 (0.5 qCi CosB) orally
Normal Normal gut intrinsic factor
B12 absorbed
> 10% Co58 excreted in urine over 24 hours
Reduced absorption
Normalised absorption
plus 1 mg ordinary B,2 intramuscularly
 
Blocks transcobalamin binding sites and ensures that any radio-labelled B12 absorbed from the gut will be directly excreted via the kidney
Blocks transcobalamin binding sites and ensures that any radio-labelled B12 absorbed from the gut will be directly excreted via the kidney
Fig. 24.20 The two-part Schilling test in the diagnosis of the
cause of vitamin B,2 deficiency.
ANAEMIAS
 CAUSES OF FOLATE DEFICIENCY
Diet
0 Poor intake of vegetables
Malabsorption
0 e.g. Coeliac disease
_
Increased demand I
• Pregnancy
• Cell proliferation, e.g. haemolysis
Drugs
• Certain anticonvulsants (e.g. phenytoin)
• Contraceptive pill
v Certain cytotoxic drugs (e.g. methotrexate)
`
Usually only a problem in patients deficient in folate from another
cause.
24.28 DIAGNOSTIC FEATURES OF FOLIC ACID ~ )
DEFICIENCY
Diagnostic findings
v Low serum folate levels (fasting blood same e
• Red cell folate levels low (but may be norrna v •olate deficiency
is of very recent onset)
Corroborative findings
• Macrocytic dysplastic blood picture
• Megaloblastic marrow
important to note that this is not a test of gut function, but simply distinguishes pernicious anaemia from the other causes of vitamin Bi2 deficiency.
FOLATE
Folate absorption
Folates are produced by plants and bacteria; hence dietary leafy vegetables (spinach, broccoli, lettuce), fruits (bananas, melons) and animal protein (liver, kidney) are a rich source. An average Western diet contains more than the minimum daily intake of 50 Itg but excess cooking for longer than 15 minutes destroys folates. Most dietary folate is present as polyglutamates; these are converted to monoglutamate in the upper small boweL and actively transported into plasma. Plasma folate is loosely bound to plasma proteins such as albumin and there is an enterohepatic circulation. Total body stores of folate are small and deficiency can occur in a matter of weeks.
Folate deficiency
The causes and diagnostic features of folate deficiency are covered in Boxes 24.27 and 24.28. The edentulous elderly or psychiatric patient is particularly susceptible to dietary deficiency and this will be exacerbated in the presence of gut disease or malignancy. Pregnancy-induced folate deficiency is the most common cause of megaloblastosis world-wide and is more likely in the context of twin pregnancies, multiparity and hyperemesis gravidarum.
Serum folate is very sensitive to dietary intake; a single meal can normalise it in a patient with true folate deficiency, and anorexia, alcohol and anticonvulsant therapy can reduce it in the absence of megaloblastosis. For this reason red cell folate levels are a more accurate indicator of folate stores and tissue folate deficiency.
~ MANAGEMENT OF MEGALOBLASTIC ANAEMIA
Where a patient with a severe megaloblastic anaemia is very ill and treatment must be started before vitamin B,2 and red cell folate results are available, always treat with both folic acid and vitamin B,2. The use of folic acid alone in the presence of vitamin 1312 deficiency may result in worsening of neurological defects.
Vitamin B12 deficiency
Vitamin B„ deficiency is treated with hydroxycobalamin 1000 lug i.m. in five doses 2 or 3 days apart followed by maintenance therapy of 1000 lug every 3 months for life. The reticulocyte count will peak by the 5th-10th day after therapy and may be as high as 50%. The haemoglobin will rise by 10 g/1 every week. The response of the man-ow is associated with a fall in plasma potassium levels and rapid depletion of iron stores. If an initial response is not maintained and the blood film is dimorphic, the patient may need additional iron therapy. A sensory neuropathy may take 6-12 months to correct; long-standing neurological damage may not improve.
Folate deficiency
Oral folic acid 5 ms daily for 3 weeks will treat acute deficiency and 5 mg once weekl_v is adequate maintenance therapy. Prophylactic folic acid in pregnancy will prevent megaloblastosis in women at risk. Folic acid supplemen¬tation may reduce the risk of neural tube defects, and in some countries all pregnant women receive routine folic acid supplementation. Prophylactic supplementation is also given in chronic haematological disease associated with reduced red cell lifespan (e.g. autoimmune haemolytic anaemia or haemoglobinopathies). There is also evidence that supraphysiological supplementation (400 pg/day) can reduce the risk of coronary and cerebrovascular disease by reducing plasma homocysteine levels. This has led the US Food and Drug Administration to introduce fortification of bread, flour and rice with folic acid.
Rarely, if severe angina or heart failure is present, transfusion can be used in megaloblastic anaemia. The cardiovascular system is adapted to the chronic anaemia present in megaloblastosis, and the volume load imposed by transfusion may result in decompensation and severe cardiac failure. In such circumstances, exchange transfusion or slow administration of 1 unit each day with diuretic cover may be cautiously used.