W2 LECT 2: Megaloblastic Anaemia

Question 1

What is macrocytosis?

Answer 1

Is an increase in the size of the erythrocytes, where the red
cells are abnormally large.

Question 2

Reflects the red cell size on the Full blood count (FBC)

Answer 2

Mean Cell Volume diagnosed on the peripheral blood smear

Question 4

What is the normal shape of macrocytes IN MEGALOCYTOSIS?

Answer 4

Oval

Question 5

What is the cause of macrocytic anemias?

Answer 5

They result from abnormal
erythropoiesis, and are broadly subdivided into megaloblastic and non-megaloblastic macrocytic anemias.

Question 6

What are the causes of megaloblastic anaemia?

Answer 6

1. Vitamin B12/Cobalamin deficiency
2. Folic acid deficiency
3. Abnormalities of B12 or folate
   metabolism (eg: antifolate drugs,
   nitrous oxide, TC deficiency).

Question 7

Features of megaloblastic anemias

Answer 7

• A group of disorders characterized by characteristic abnormality in the developing
  red cells within the bone marrow:
• asynchronous maturation :
  *Maturation of the nucleus is delayedcompared to the cytoplasm
• The underlying problem is defective DNA synthesis which affects all rapidly dividing
  cells.
• Most commonly caused by a deficiency of vitamin B12 or folate.

Question 8

Describe the biochemical basis of megaloblastosis

Answer 8

• Megaloblastic anemia’s underlying cause if defective DNA synthesis
• B12 and folate deficiencies lead to failure of deoxyuridine monophosphate (dUMP)
  conversion into deoxythymidine monophosphate(dTMP)
• This results in a decreased supply of deoxythymidine triphosphate (dTTP) for DNA
  synthesis, where (dUMP) is continuosly added on a growing DNA chain followed by fragmantation of the chain as DNA polymerase breaks it in attempt to correct the errors

Question 9

What are the sources of folate?

Answer 9

liver, yeast, nuts, spinach
and other leafy greens

Question 10

What is folate daily requirement?

Answer 10

~100μg.
* Folate is easily destroyed by cooking.

Question 11

How long can folate last on the body stores upon it uptake?

Answer 11

4 months; therefore
folate deficiency can develop rapidly

Question 12

Discuss the absorption of folate from food in the stomach to the plasma

Answer 12

• Most folate is absorbed in the upper small
  intestine.
• Folate transporter proteins are located on the
  mucosal surfaces of the duodenum and to a
  lesser extent the jejunum.
• Dietary folate is converted to 5 methyl THF within the small intestinal mucosa before
  entering the plasma.
• Folate is transported in the plasma ~1/3 being
  bound to albumin and the remainder unbound. It
  is then actively transported into the cells.

Question 13

What are the causes of folate deficiency?

Answer 13

• Dietary: old age, alcoholism, institutuions, poverty, infants fed on goats milk.
• Malabsorption: Tropical sprue, gluten-induced enteropathy,
  jejunal resection, partial gastrectomy, Crohn’s disease.
• Excess utilization or loss:
• Physiological: Pregnancy, lactation, prematurity.
• Pathological: Haematological diseases (SCA, haemolytic
  anaemia), malignant diseases (lymphoma, myeloma)
  inflammatory (TB, Crohn’s)
• Excess urinary loss: CCF, liver disease.
• Long term dialysis
• Antifolate drugs: anticonvulsants, drugs which inhibit dihydrofolate reductase (methotrexate).

Question 14

The main enzyme involved in the folate metabolism pathway. Its inhibition can disrupt folate metabolism in rapidly dividing cells of the bone marrow, leading pancytopenia

Answer 14

Dihydrofolate reductase

Question 15

Drugs that intefere with utilisation or absorbtion of folate into the plasma by inhibiting dihydrofolate reductase

Answer 15

• antileukaemic drugs (e.g. methotrexate),
• antimalarials (e.g.
  pyrimethamine) and
• antibiotics (e.g. trimethoprim) may interfere with utilisation of
  folate.

NB Alcohol appears to have a direct effect preventing the use of folate by the marrow

Question 16

What is the human source of Vitamin B12?

Answer 16

foods of animal origin:
liver, meat, fish, chicken and
(eggs cheese and milk)
*Unlike folate, it is not destroyed by cooking

In animals the vitamin is made by microorganisms

Question 17

What are the daily requirements of B12?

Answer 17

Daily requirements are
about 1-5μg.

Question 18

How long can B12 last on the body stores upon it uptake?

Answer 18

Body stores are sufficient
for 3-4 years.

Question 19

Discuss the trasportation of B12/cobalamin from the stomach into the plasma

Answer 19

• B12 in the stomach is released from food by digestive proteases as well as gastric acid.
• It is made to bind protein R-binders in the stomach
• Complex is transported to the dodenum where B12 is separated from R binders (by digestive proteolysis) and made to bind to Intrinsic factor (secreted by the parietal cells)
• The B12-Intrinsic factor complex moves to the ileum and bind to the receptor on the ileal enterocytes
• Once absorbed, IF is degraded and the cobalamin is transferred to the plasma where it is bound to transcobalamin and haptocorrin.
• The biologically active fraction is that bound to transcobalamin BUT there may be MORE bound to haptocorrin
• The Vitamin B12-Transcobalamin (TCII) complex binds to a membrane receptor present on the surface of many
  cell types including bone marrow.
• The complex enters the cells, the TCII is digested and
  the Vitamin B12 released.

Question 20

Why is most B12 bound to haptocorrin?

Answer 20

for storage in the liver

Question 21

CLINICAL FINDINGS COMMON TO ALL MEGALOBLASTIC ANAEMIAS

Answer 21

• Many patients are asymptomatic as the anaemia develops slowly. These patients may be detected on a routine FBC where they are found to have a macrocytosis.
• In severe megaloblastic anaemia the symptoms are
  those of:
  1. Anaemia: weakness, fatigue, palpitations, light-headedness and shortness of breath.
  2. Thrombocytopenia: may cause bruising.
  3. Neutropenia: may predispose to infections.
  4. Mild jaundice due to increased unconjugated
  hyperbilirubinaemia (ineffective erythropoiesis)
  5. Others: glossitis (beefy red sore tongue), angular
  cheilosis, LOW.

Question 21

What are the causes of severe B12 deficiency?

Answer 21

• Nutritional:
  -Vegans, breastfed infants of vitamin B12 deficient
  mothers
• Impaired absorption
  –Gastric causes:
  -Pernicious anaemia. Gastric atrophy
  -Total or partial gastrectomy. Gastric bypass
  -Congenital deficiency of IF.
  -Medications: PPI, metformin, nitrous oxide abuse

– Intestinal causes:
-Ileal resection or dysfunction (Crone’s disease, celiac)
-Intestinal lymphoma, bacterial overgrowth from blind loop
syndrome
-Fish tapeworm (competition for host vitamin B12).
- Congenital disorders of Vit B12 transport: TC deficiency.

Question 22

Name the dividing, DNA synthesizing cells, affected by Vitamin B12 and/or folate deficiency

Answer 22

• The bone marrow is most affected followed by
• epithelial surfaces of the GIT,
• respiratory,
• urinary
  and female genital tract.

*The cells are large and fragile and there is an increase in the numbers of dying cells.

Question 23

Associated with the development of neural tube defect on foetus;
and may also cause mental changes such as depression

Answer 23

Folate deficiency

*Supplementation with folic acid from conception
for the first 12 weeks of pregnancy has decreased the incidence of NTD’s
tube defects by 70-75%.

Question 24

NEUROLOGICAL ABNORMALITIES
DUE TO VITAMIN B12 DEFICIENCY

Answer 24

• Bilateral peripheral neuropathy. symmetrical. LL >UL (more on lower limbs than upper limbs)
• Subacute combined degeneration of the spinal cord
• Demyelination of the posterior and lateral columns of the
  spinal cord.

Presentation:
* Paresthesia’s (tingling) in the feet, disturbances of vibratory sense and proprioception with difficulty walking. May fall over in the dark
* Occasionally visual impairment, dementia and psychotic disturbances
(megaloblastic madness)

Question 25

Discuss the laboratory investigations in the diagnosis of megaloblastic anaemia

Answer 25

HAEMATOLOGICAL FINDINGS:
Vitamin B12 and folate deficiency have the exact same haematological features.
1. FBC:
* RBC - Decreased
* Hb - Decreased
* HCT - Decreased
* MCV - Increased
* MCHC - Normal Ineffective haemopoiesis
* RDW - Increased (anisocytosis)
* WBC - Decreased
* PLT - Decreased

*Ineffective erythropoiesis

2. PANCYTOPENIA: anaemia, neutropenia and thrombocytopenia
3. RPI – Low for the degree of anaemia.

Question 26

Biochemical evidence of ineffective erythropoiesis includes:

Answer 26

1. Unconjugated hyperbilirubinaemia.
2. Decreased haptoglobin levels.
3. Raised urine urobilinogen.
4. Raised LDH.

These findings are the same as those noted in certain haemolytic anaemias

Question 27

FEATURES OF NORMAL MCV IN
MEGALOBLASTOSIS;WITH CO-EXISTING IRON DEFICIENCY

Answer 27

• May see a dimorphic red cell picture with both
  macrocytes and microcytic hypochromic red cells.
  – MCV may be normal
• Coexisting thalassaemia trait may also result in a normal
  MCV in the presence of megaloblastosis.
• Red cell fragments

Question 28

HOW TO DIAGNOSE FOLATE
DEFICIENCY?

Answer 28

• Serum folate:
  -Is low in all folate deficient patients.
  -It is markedly affected by diet.
  -Inadequate intake for as little as one week may result in low levels.
• Red cell folate:
  –Reflects body stores and is less affected by diet.
  –Low levels seen in folate deficiency, but, also in vitamin B12 deficiency, subsequently vitamin B12 deficiency must be
  excluded when interpreting a low red cell folate level.

*Serum folate is the only available test within the NHLS: need to write it on the form!

Question 29

HOW TO DIAGNOSE VITAMIN B12
DEFICIENCY?

Answer 29

• Serum vitamin B12:
  levels are low in most
  patients with megaloblastic anaemia due to vitamin B12 deficiency.
• Serum methylmalonic acid (MMA) and
  homocysteine levels are increased. Not
  offered in NHLS

Question 30

How to treat B12 deficiency?

Answer 30

• Body stores should be replaced after six
  1000μg intramuscular injections given
  every 3-7 days.
• Maintenance therapy involves 1000μg IMI
  given every three months

Question 30

TREATMENT FOR MEGALOBLASTIC ANAEMIA

Answer 30

• In most cases replacement therapy with the appropriate vitamin is given based on the laboratory test results.
• In patients with severe symptoms:
  – Blood for both folate and Vitamin B12 levels should be taken.
  – Bone marrow aspirate and trephine biopsy may be necessary.
  – Replacement therapy with both B12 and folate should then be started. If large doses of folate are given in the presence of Vitamin B12 deficiency they may aggravate the neuropathy.
  – Patients must be monitored for hypokalaemia (increased potassium levels).
• Avoid blood transfusion if possible.
  – If absolutely essential packed cells should be given very slowly.
  Patients must be closely monitored for pulmonary oedema and
  circulatory overload.
  – Platelets may be necessary for patients with spontaneous
  bleeding due to a severe thrombocytopenia.

Question 31

How to treat folate deficiency?

Answer 31

• Given orally.
• Dose 5-15mg daily. Adequate folate is absorbed at
  these high doses even in patients with malabsorption.
• Therapy should continue for 4 months.
• Prior to giving large doses of folate it is important to
  exclude Vitamin B12 deficiency.
• Long term therapy is indicated in patients in whom the
  cause for the deficiency cannot be corrected such as
  chronic haemolysis due to thalassaemia or sickle cell
  anaemia.
• Patients receiving long term folate therapy should
  have their Vitamin B12 levels checked annually.

Question 32

MEGALOBLASTOSIS PROPHYLACTIC THERAPY

Answer 32

• Folic acid is given during pregnancy (5mg daily)
  and it is recommended that woman of child
  bearing age have an increased intake of folate in
  an attempt to avoid NTD’s.
• Mandatory food fortification with folate was implemented in South Africa in October 2003.
• Folic acid is given to patients on chronic dialysis, with severe chronic haemolytic anaemias and to premature infants.
• Vitamin B12 is given for life to patients who have
  had a total gastrectomy or ileal resection.