Chapter 4 notes

Megaloblastic Anaemia

Megaloblastic anaemias are characterized by delayed maturation of the nucleus of RBCs in the

bone marrow due to defective DNA synthesis. These cells either die in the marrow or enter the

bloodstream as enlarged, misshapen cells with a reduced survival time.

Why does deficiency of vitamin B12 or folate lead to megaloblastic anaemia?

Both folate and vitamin B12 are necessary for the synthesis of DNA

Deficiency of B12 leads to impaired conversion of homocysteine to methionine causing

folate to be ‘trapped’ in the methyl form.

Folate is needed in its tetrahydrofolate form (THF) as a cofactor in DNA synthesis

Red cells are enlarged and egg-shaped and the neutrophils hypersegmented due to retention of

surplus nuclear material

Clinical features include: jaundice, glossitis, angular stomitis, weight loss and

macrocytosis

Lab features include:

o Increased unconjugated bilirubin (formed from RBC breakdown) and

lactate dehydrogenase (cell marrow breakdown)

o oval macrocytes and hypersegmented neutrophils

o reduction in leucocyte and platelet counts (severe cases)

o Bone marrow is hypercellular, increased proportion of early cells,

o “Stippled”, large erythroblasts

o giant metamyelocytes (precursor in the granulocytic series)

o MCV >98fL or 120-140 fL(severe cases)

Other causes of megaloblastic anaemia:

Defects of B12 or folate metabolism include congenital transcobalamin (TC) II

deficiency which leads to B12 malabsorption and failure of B12 to enter cells despite a

normal (attached to TCI) serum B12 level.

N2O anaesthesia reversibly inactivates body B12, and prolonged or repeated exposure

may cause megaloblastic anaemia or B12 neuropathy.

Antifolate drugs include the inhibitors of dihydrofolate reductase (methotrexate,

pyrimethamine and trimethoprim) which have progressively less activity against the

human compared to the bacterial enzyme.

Folinic acid (5-formyl-THF) is used to overcome methotrexate or co-trimoxazole

toxicity.

cytotoxic drug therapy or acquired enzyme deficiencies (e.g., cytosine arabinoside,

alcohol, therapy with hydroxyurea)

Congenital enzyme deficiencies causing errors in DNA synthesis, e.g. orotic aciduria

Vit B12 Structure:

Consists of a small group of cobalamins with a cobalt atom at the center of a corrin

ring.

Corrin ring is attached to a nucleotide portion

Forms:

1) Methyl B12- methyl cobalamin (human plasma)

2) Ado B12- deoxyadenosylcobalamin (human tissues)

3) Hydroxo B12- hydroxocobalamin (treatment)

Causes of B12 deficiency:

1) Pernicious anaemia- an autoimmune disorder in which the immune system attack the

gastric mucosa (sometimes initiated by helicobater pylori) leading to gastric parietal cell

atrophy (decrease in size) of the stomach, achlorhydria (gastric secretions absent) and

intestinal metaplasia (transformation from one cell type to another)

o Patients have IgG autoantibodies targeted against gastric parietal cells and the

B12 transport protein intrinsic factor, IF.

o B12 deficiency ultimately arises from reduced secretion of intrinsic factor (IF) by

parietal cells and, hence, reduced availability of the B12–IF complex which is

absorbed in the terminal ileum.

o Affected patients classically have premature greying of the hair (Hashimoto’s

disease) and blue eyes (myoedema) and may develop other autoimmune disorders

including vitiligo (pigment is lost from areas of the skin; in females), thyroid

disease (hyperthyroidism, thyrotoxicosis) and Addison’s disease. Slight jaundice

(yellow skin) is caused by the haemolysis of ineffective erythropoiesis.

o Associated diseases include polyendocrine syndromes (combination of endocrine

and nonendocrine autoimmune diseases ) and carcinomas of the stomach

2) Gastric causes:

Atrophic gastritis

Malabsorption

1) Parietal cell antibodies (most common):

o present in blood serum and act against gastric H+/K= ATpase

2) IF antibodies (specific):

o Type I (blocking) - inhibit IF binding to B12

o Type II (precipitating)- inhibit IF to ileal binding site. prevents the

absorption of B12 from intrinsic factor

Inadequate intake of B12

Genetic mutation of IF-B12 receptors cubilin or amnionless

Congenital lack or abnormality of IF

Total or partial gastrectomy

3) Intestinal causes:

Stagnant loop syndrome- jejunal diverticulosis, blind loop, stricture etc.

Chronic tropical sprue

Ileal resection

Crohn’s disease

Proteinuria

Fish tapeworm

Treatment:

1000 µg hydroxocobalamin intramuscularly (IM) repeated every 2–3 days until six

injections have been given.

Then one injection every 3 months (for life if patient has total gastrectomy or ileal

resection)

Tests for causes of B12 deficiency:

Diet history

Intestinal malabsorption

tests for intrinsic factor (IF), antibody (positive in 50% of cases of PA) and

parietal cell antibodies (positive in 90% of cases of PA)

serum gastrin level (raised in PA)

Upper gastrointestinal endoscopy.

FOLATE Causes of Folate deficiency:

Dietary insufficiency

Malabsorption occurs in gluten-induced enteropathy, tropical sprue, gastrectomy

Excessive utilization: increased cell turnover and DNA synthesis causes breakdown of

folates. The most common causes include pregnancy (folate decreases with increased

DNA synthesis), haemolytic anaemia and malignant diseases (lymphoma, myeloma,

carcinoma)

Excess urinary folate loss: Folate is loosely bound to protein in plasma and is easily

removed by dialysis (chronic dialysis), congestive heart failure, active liver disease

Inflammatory diseases: Crohn disease

Treatment:

Oral folic acid 5 mg once daily

This is given for 4 months at least, the precise duration of therapy depending on the

underlying cause/disease

Life-long therapy may be needed in chronic inherited haemolytic anaemias,

myelofribrosis, renal dialysis

Tests for causes of folate deficiency:

Diet history

Intestinal malabsorption

Anti-transglutaminase and endomysial antibodies

Duodenal biopsy

Effects of Vit B12 and folate deficiency (Table 5.6)