Microlytic-Macrolytic Anaemias Flashcards
Explain the term ‘anaemia’
Definition
Anaemia is a reduction in the amount of haemoglobin in a given volume of blood below what would be expected in comparison with a healthy subject of the same age and gender
- By definition, the Hb is reduced
- The RBC and Hct are usually also reduced
(NOTE: you could have a low haemoglobin concentration if you have an increase in plasma volume
But anaemia is usually due to a reduction of the absolute amount of haemoglobin in the blood stream
In a healthy person, anaemia due to an increase in plasma volume cannot persist because the excess fluid in the circulation is excreted)
Describe the mechanisms of action of anaemia and classify anaemia on the basis of the mechanism
Mechanisms of Anaemia
- Reduced production of red blood cells/haemoglobin in the bone marrow
- Loss of blood from the body (haemorrhage)
- Reduced survival of red blood cells in the circulation (haemolytic)
- Pooling of red blood cells in a very large spleen
Mechanisms and causes of anaemia
- The mechanism of the anaemia might be reduced synthesis of haemoglobin in the bone marrow
- The cause of this could be either a condition causing reduced synthesis of haem or one causing reduced synthesis of globin
(•Sometimes neither the mechanism of the anaemia or the specific cause is immediately apparent
•Classification on the basis of cell size can help to suggest specific causes)
Classify anemia on the basis of cell size (macrocytic, normocytic, microcytic)
Classification of anaemia on the basis of cell size
Microcytic -‐ usually HYPOchromic
Normocytic -‐ usually NORMOchromic
Macrocytic -‐ usually NORMOchromic
Microcytic Anaemia (hypochromic: smaller cells but also reduced concentration in cells)
Common causes of Microcytic Anaemia
1. Defect in HAEM synthesis
- Iron deficiency
- Anaemia of chronic disease (type of anaemia in people with chronic infection, TB, rheumatoid arthritis – iron trapped in macrophages and is not mobilised
2. Defect in GLOBIN synthesis (THALASSAEMIA)
- Defect in ALPHA chain synthesis (alpha thalassaemia)
- Defect in BETA chain synthesis (BETA thalassemia)
Macrocytic Anaemia
Macrocytic anaemia usually results from abnormal haemopoiesis so that the red cell precursors continue to synthesise haemoglobin and other cellular proteins but they fail to divide normally
As a result, the red cells end up being much LARGER than normal
Delay in maturation of the nucleus while the cytoplasm continues to mature and the cell continues to grow – Megaloblastic
Macrocytic or Megaloblastic?
Macrocytic -‐ MCV is increased
- One cause of this is megaloblastic erythropoiesis: This refers specifically to a delay in the maturation of the nucleus while the cytoplasm continues to mature and the cell continues to grow
- A megaloblast is an abnormal bone marrow erythroblast
- In the megaloblasts, the nuclear development is not matching the cytoplasmic development
- Megaloblasts are larger than normal and shows nucleo-‐cytoplasmic dissociation
- It is possible to suspect megaloblastic anaemia from the peripheral blood features but to be sure requires bone marrow examination
- An alternative mechanism of macrocytosis is premature release of cells from the bone marrow
- Young red cells (reticulocytes) are about 20% LARGER than mature red cells so if there is an increase in the proportion of young red cells (reticulocytosis) in the circulation, the MCV will be increased
Normocytic Normochromic Anaemia -‐ Mechanisms
- MCV and MCH are normal
- Recent blood loss
- Failure of production of red blood cells
- Pooling of red blood cells in the spleen
Describe the common causes of microcytic, normocytic and macrocytic anaemia
Common Causes of Macrocytic Anaemia
- Megaloblastic anaemia as a result of a lack of vitamin B12orfolic acid
- Use of drugs interfering with DNA synthesis e.g. chemotherapy or azathioprine
- Other causes: Liver disease and ethanol toxicity
REMEMBER: in megaloblasts there is an asynchrony between the nucleus and cytoplasm
- Recent major blood loss with adequate iron stores (reticulocytes increased)
- Haemolytic anaemia (reticulocytes increased)
Normocytic Normochromic Anaemia -‐ Causes
- Any blood loss – barices, trauma
- Peptic ulcer
- Oesophageal varices
-
Failure of production of red blood cells
- Early stages of iron deficiency or anaemia of chronic disease
- Renal failure
- Bone marrow failure or suppression
- Bone marrow infiltration
- Hypersplenism e.g. portal cirrhosis
List the major causes of hemolysis and state important causes of hemolytic anemia
Haemolytic Anaemia
- Haemolytic anaemia is anaemia resulting from shortened survival of red blood cells in the circulation
CAUSES:
- Haemolysis can result from an intrinsic abnormality of the red cells
- Haemolysis can result from an extrinsic factor acting on normal red cells
Normal red cell survival = 120 days
It can also be classified as inherited or acquired:
- Inherited hemolytic anemia -‐ can result from abnormalities of the cell membrane, the haemoglobin or the enzymes in the red blood cell
- Acquired haemolytic anaemia -‐ usually results from extrinsic factors such as micro-‐organisms, chemicals or drugs that damage the red cell
NOTE: extrinsic factors can interact with red cells that have an intrinsic abnormality
Haemolytic anaemia can also be classified as intravascular or extravascular
- Intravascular haemolysis occurs if there is very acute damage to the red cell
- Extravascular haemolysis occurs when defective red cells are removed by the SPLEEN
Often hemolysis is partly intravascular and partly extravascular
Usual causes:
- abnormal cell membrane
- abnormal hemoglobin
- defect in the glycolytic pathway e.g. pyruvate kinase deficiency
- defect in the pentose phosphate shuttle
I
Glucose-6-phosphate dehydrogenase and pyruvate Kinase Deficiency deficiencies
- G6PD is an enzyme involved in the pentose phosphate pathway
- Pentose phosphate pathway is a metabolic pathway parallel to glycolysis that generates NADPH and pentoses
- The G6PD pathway is the only source of reduced glutathione in red blood cells
- The oxygen carrying role of red blood cells puts them at substantial risk of damage from oxidising free radicals except for the protective effects of G6PD
- Oxidants may be generated in the blood stream e.g. during infection, or may be exogenous
- So people with G6PD deficiency are at risk of haemolytic anaemia in states of oxidative stress
- Extrinsic oxidants may be foodstuffs (e.g. broad beans), chemicals (e.g.naphthalene) or drugs (e.g. dapsone, primaquine)
NOTE: if you’re treating patients with malaria, some anti-‐malarials will interact in patients that have G6PD deficiency
- The gene for G6PD is on the X chromosome so affected individuals are usually hemizygous males (but occasionally homozygous females)
- G6PD deficiency usually causes intermittent, severe intravascular haemolysis as a result of infection or exposure to an exogenous oxidant
- These episodes of intravascular haemolysis are associated with the appearance of considerable numbers of irregularly contracted cells
- Haemoglobin is denatured and forms round inclusions known as Heinz bodies, which can be detected by a specific test
- You can get more than one Heinz body in one cell, unlike Howell-‐Jolly bodies
- Heinz bodies are removed by the spleen, leaving a defect in the cell
- Acute haemolysis sometimes requires blood transfusion so prevention is important
Pyruvate Kinase Deficiency
- Pyruvate kinase is involved in the last stage of glycolysis
- A deficiency of pyruvate kinase will result in a red blood cell with decreased energy
- Because red blood cells cannot synthesise ATP, cellular death occurs
- It could be caused by an autoimmune problem where red blood cell antigens are recognised as being foreign and, subsequently, removed
Describe how a haemolytic anaemia is recognised and describe typical important examples
Haemolytic Anaemia -‐ when to suspect it
- Otherwise unexplained anaemia that is normochromic and usually either normocytic or macrocytic
- Evidence of morphologically abnormal red cells
- Increase in bilirubilin production (unconjugated specifically) + gall stones because of that + jaundice
- Evidence of increased red cell breakdown
- Evidence of increased bone marrow activity – high reticulocyte count)
All these patients would be suspected of having haemolytic anaemia for different reasons
hereditary eliptocytosis -‐ this leads to haemolysis
irregularly contracted cells
fragments -‐ if you see fragments, this suggests that the red cells are being broken down within the circulation (in the small capillaries) -‐ rather than the whole red cell being taken up and broken down in the spleen, they are being smashed up within the circulation
This is called microangiopathic haemolytic anaemia (microangiopathic -‐ occurs in small blood vessels)
Hereditary Spherocytosis
This is haemolytic anaemia or chronic compensated haemolysis resulting from an inherited intrinsic defect of the red cell membrane
After entering the circulation the cells lose membrane in the spleen and thus become spherocytic
These cells become LARGER and ROUNDER
They will have an increased MCHC
Red cells become less flexible and are removed prematurely in the spleen
This is extravascular haemolysis
The bone marrow responds to haemolysis by an increased output of red cells leading to polychromasia and reticulocytosis
Polychromasia -‐ the presence of certain BLUE red blood cells seen in blood films stained with Romanowsky stains, as well as the normal pink cells. The cells that appear blue are juvenile red blood cells.
Spherocytes, because of their shape, can’t manoeuvre themselves through these gaps so you get early removal of these cells in the spleen
The only effective treatment for hereditary spherocytosis is SPLENECTOMY -‐ but this has its own risks and so is only done in extreme situations
Cause:
The immunoglobulin bound to the red cell membrane is recognised by the splenic macrophages, which remove parts of the cell membrane, leading to spherocytosis
This is very sudden and dramatic -‐ can occur out of the blue
So spherocytosis can be caused by:
- Hereditary Spherocytosis
- Autoimmune Haemolytic Anaemia
Diagnosis is by:
- Finding spherocytes and an increased reticulocyte count
- Detecting immunoglobulin on the red cell surface
- Detecting antibodies to red cell antigens or other antibodies in the plasma
Treatment is by:
- Use of corticosteroids and other immunosuppressive agents
- Splenectomy for severe cases
Polycythaemia: recall the classification of polycythaemia and explain the mechanisms underlying polycythaemia
Definition: Polycythaemia literally means ‘many cells’ but it refers specifically to too many red cells in the circulation
- The Hb, RBC and Hct/PCV are all increased compared with normal subjects of the same age and gender could result from a DECREASE in plasma volume -‐ PSEUDOPOLYCYTHAEMIA or APPARENT POLYCYTHAEMIA
- When the abnormalities result from an increase in the number of circulating red blood cells is TRUE POLYCYTHAEMIA
Causes
Pseudo- Reduced plasma volume
True- Increase in total volume of red cells in the circulation
- Blood doping or overtransfusion
- Appropriately increased erythropoietin (as a result of hypoxia)
- Inappropriate erythropoietin synthesis: renal or other tumors inappropriately secrete erythropoietin
- Inappropriate erythropoietin use: cyclists
- Independent of erythropoietin: polycythemia vera: is an intrinsic bone marrow disorder (is classified as a myeloproliferative neoplasm)
Evaluation
- Start with a clinical history and physical examination (splenomegaly, abdominal mass or cyanosis could be relevant)
- Next compare with an appropriate normal range
- Note: the Hb, RBC and Hct are higher in the neonate than at other times of life, lower in children than in adults and lower in women than in men
Pseudo or True?
- A high Hb, RBC and PCV/Hct can result from a decrease in plasma volume, referred to as ‘pseudopolycythaemia’ or ‘apparent polycythaemia’
- When the abnormalities result from an increase in the number of circulating red cells there is a true polycythaemia
Treatment
- If there is no physiological need for a high haemoglobin, or if hyperviscosity is extreme, blood can be removed to thin the blood
- If there is intrinsic bone marrow disease, drugs can be used to reduce bone marrow production of red cells
Discuss autoimmune haemolytic anaemia
Autoimmune haemolytic anaemia
- Autoimmune haemolytic anaemia results from production of autoantibodies directed at red cell antigens
- The immunoglobulin bound to the red cell membrane is recognized by splenic macrophages, which remove parts of the red cell membrane, leading to spherocytosis Complement components can also be bound to the immunoglobulin molecule, and they are also recognised by receptors on splenic macrophages
- Anaemia and spherocytosis in autoimmune haemolytic anaemia
- The spherocytes are less flexible than normal red cells
- The combination of cell rigidity and recognition of antibody and complement on the red cell surface by splenic macrophages leads to removal of cells from the circulation by the spleen
Diagnosis is by
- Finding spherocytes and an increased reticulocyte count
- Detecting immunoglobulin ± complement on the red cell surface
- Detecting antibodies to red cell antigens or other autoantibodies in the plasma
Treatment:
- Use of corticosteroids and other immunosuppressive agents
- Splenectomy for severe cases
Microangiopathic haemolytic anaemia
Treatment may be by
- Removing the cause, e.g. treating severe hypertension or stopping a causative drug
- Plasma exchange when it is caused by an antibody in the plasma that is leading indirectly to fibrin deposition
Explain the structure and function of the hemoglobin molecule
General Notes on Haemoglobin
- Found exclusively to RBC’s (because it is a potent oxidative agent so if it is free – toxic)
- Normal concentration in adults:120-165g/L
- Approximately 90 mg/kg produced and destroyed in the body every day
- Each gram of Hb contains 3.4mg Fe
- It is a composite molecule consisting of FOUR haem groups and FOUR globin chains
- Each haem group is bound to a Fe2+
- Each hemoglobin molecule can bind FOUR oxygen molecules
Globins
Types of globin proteins: Alpha/ Beta/ Gamma/ Delta
- Primary structure
- α 141 AA
- Non- α 146 AA
- Secondary structure
- 75% α and b chains-helical arrangement
- Tertiary structure
- Approximate sphere
- Hydrophilic surface (charged polar side chains), hydrophobic core
- Haem pocket
Explain the genetic control of Hb synthesis and explain how genetic defects lead to thalassemias
