W2 LECT 2: Megaloblastic Anaemia Flashcards
What is macrocytosis?
Is an increase in the size of the erythrocytes, where the red
cells are abnormally large.
Reflects the red cell size on the Full blood count (FBC)
Mean Cell Volume diagnosed on the peripheral blood smear
What is the normal shape of macrocytes IN MEGALOCYTOSIS?
Oval
What is the cause of macrocytic anemias?
They result from abnormal
erythropoiesis, and are broadly subdivided into megaloblastic and non-megaloblastic macrocytic anemias.
What are the causes of megaloblastic anaemia?
- Vitamin B12/Cobalamin deficiency
- Folic acid deficiency
- Abnormalities of B12 or folate
metabolism (eg: antifolate drugs,
nitrous oxide, TC deficiency).
Features of megaloblastic anemias
- 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.
Describe the biochemical basis of megaloblastosis
- 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
What are the sources of folate?
liver, yeast, nuts, spinach
and other leafy greens
What is folate daily requirement?
~100μg.
* Folate is easily destroyed by cooking.
How long can folate last on the body stores upon it uptake?
4 months; therefore
folate deficiency can develop rapidly
Discuss the absorption of folate from food in the stomach to the plasma
- 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.
What are the causes of folate deficiency?
- 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).
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
Dihydrofolate reductase
Drugs that intefere with utilisation or absorbtion of folate into the plasma by inhibiting dihydrofolate reductase
- 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
What is the human source of Vitamin B12?
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
What are the daily requirements of B12?
Daily requirements are
about 1-5μg.
How long can B12 last on the body stores upon it uptake?
Body stores are sufficient
for 3-4 years.
Discuss the trasportation of B12/cobalamin from the stomach into the plasma
- 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.
Why is most B12 bound to haptocorrin?
for storage in the liver
CLINICAL FINDINGS COMMON TO ALL MEGALOBLASTIC ANAEMIAS
- 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.
What are the causes of severe B12 deficiency?
- 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.
Name the dividing, DNA synthesizing cells, affected by Vitamin B12 and/or folate deficiency
- 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.
Associated with the development of neural tube defect on foetus;
and may also cause mental changes such as depression
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%.
NEUROLOGICAL ABNORMALITIES
DUE TO VITAMIN B12 DEFICIENCY
- 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)
Discuss the laboratory investigations in the diagnosis of megaloblastic anaemia
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
- PANCYTOPENIA: anaemia, neutropenia and thrombocytopenia
- RPI – Low for the degree of anaemia.
Biochemical evidence of ineffective erythropoiesis includes:
- Unconjugated hyperbilirubinaemia.
- Decreased haptoglobin levels.
- Raised urine urobilinogen.
- Raised LDH.
These findings are the same as those noted in certain haemolytic anaemias
FEATURES OF NORMAL MCV IN
MEGALOBLASTOSIS;WITH CO-EXISTING IRON DEFICIENCY
- 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
HOW TO DIAGNOSE FOLATE
DEFICIENCY?
- 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!
HOW TO DIAGNOSE VITAMIN B12
DEFICIENCY?
- 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
How to treat B12 deficiency?
- Body stores should be replaced after six
1000μg intramuscular injections given
every 3-7 days. - Maintenance therapy involves 1000μg IMI
given every three months
TREATMENT FOR MEGALOBLASTIC ANAEMIA
- 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.
How to treat folate deficiency?
- 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.
MEGALOBLASTOSIS PROPHYLACTIC THERAPY
- 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.
