Haemoglobinopathies Flashcards
What is sickle cell?
Chronic condition with sickling of red blood cells caused by inheritance of HbS.
What does HbA refer to?
Normal Hb with two alpha and two beta chains.
What is the aetiology sickle cell anaemia? What happens to the resultant haemoglobin?
- Autosomal recessive
- Single missense mutation at codon 6 of the B-globin chain gene. The resultant B-chain is almost identical to normal beta-globin but the AMINO ACID Valine replaces Glutamic acid at position 6 in the chain. HbA is therefore replaced by HbS.
- Glu is polar and soluble; valine is non-polar and insoluble.
- This mutation is normally benign; however, under low oxygen concentration i.e. when HbS becomes DeoxyHb-S, the Deoxy-HbS becomes INSOLUBLE. This occurs because Deoxy-HbS polymerises to form fibres called TACTOIDS within the RBC.
What are the types of sickle cell?
- ANAEMIA: Refers to the homozygous genotype (SS).
- TRAIT: produces 50% normal and 50% abnormal Hb (HbA and HbS). The two alleles are codominant which leads to normal blood count and life expectancy, usually asymptomatic but needs caution in high altitude and exertion or under anaesthetics.
- DISEASE: includes sickle cell anaemia and compound heterogenous states
What examples are there of heterogenous sickle cell disease? (x2)
SC patients (sickle cell mutation coinherited with another mutation seen in Africa) and SB thalassaemia (where sickle cell mutation is coinherited with B thalassaemia. Here, NO HbA is produced because B-globin chains are lacking. As such, although the condition is heterogenous, and the S and B alleles are co-dominant, only HbS is produced).
What are the effects of HbS in red blood cells? (x3)
- DISTORTION – polymerisation and subsequent presence of long-chain polymers of HbS distorts shape of RBC into sickle shape. This makes cells fragile (at risk of HAEMOLYSIS) and rigid. Initially, this polymerisation is reversible if oxyHbS is formed, but becomes subsequently irreversible.
- DEHYDRATION of RBCs.
- Increased ADHERENCE to vascular endothelium. This increases transit time of cells through body in large vessels which gives more time for Hb to deoxygenate, become DeoxyHbS, and Hb to polymerise.
How does RBC lifespan differ in sickle cell and healthy patients?
20 days vs 120 days.
What is the pathophysiology of sickle cell? (x5)
- SHORTENED RED CELL LIFESPAN leading to haemolysis. This can lead to anaemia and pigment gallstones.
- REDUCED ERYTHROPOIETIC DRIVE: HbS has low affinity for oxygen. This leads to low oxygen saturation (hypoxia). However, low affinity means that oxygen release at bone marrow is sufficient enough to prevent hypoxia-induced increase in erythropoietic drive.
- BLOCKAGE TO MICROVASCULAR CIRCULATION: because distorted and adherent, sickle cells are vaso-occlusive.
- PULMONARY HYPERTENSION: haemolysis results in cell-free Hb in the plasma which limits NO bioavailability and can cause vasoconstriction in the lungs.
- STROKE: occurs from thrombotic occlusion of LARGE cerebral arteries. The mechanism is UNKNOWN.
What is the epidemiology of sickle cell disease: Where? Age? Disease concurrence?
Mediterranean, Caribbean and most of Africa. Rarely presents before 6 months (due to foetal Hb). Common in areas with high malaria rates.
How does sickle cell protect against malaria?
Molecular alteration is a deletion which protects against malaria.
What are the early symptoms of sickle cell disorders? (x3)
- Dactylitis (inflammation of digit) from vaso-occlusion affected bones
- Splenic sequestration happens early because spleen clears sickle cells, so more affected by the vaso-occlusive effects of sickle cells.
- INFECTION with S. pneumoniae.
!!! What are the signs and symptoms of sickle cell disease? (x5)
- VASO-OCCLUSIVE CRISIS
- SPLENIC SEQUESTRATION CRISIS: (red cell pooling) painful splenomegaly and hypovolaemic SHOCK
- ACUTE CHEST SYNDROME: chest pain, fever, pulmonary infiltrate and hypoxia.
- APLASTIC CRISIS: acute worsening of baseline anaemia resulting in pallor, tachycardia and fatigue.
- HAEMOLYTIC CRISIS: acute drops in Hb resulting in anaemia, jaundice and reticulocytosis
What are the signs and symptoms of vaso-occlusive crisis? (x7)
- Skin ulceration
- Bone/joint pain (dactylitis and avascular necrosis, commonly femoral head)
- Auto-splenectomy (splenic infarct and ATROPHY leading to splenomegaly and increased risk of infections with encapsulated organisms (pneumococcus, H. influenza, meningococcus, Salmonella))
- Stroke
- Visual loss with cotton-wool spots on examination from ischaemic retina (pathology is called proliferative retinopathy)
- Haematuria (renal necrosis), renal failure
- Priapism (erect penis not under stimulation)
!!! What is the aetiology of crises in sickle cell? (x5)
Infection, acidosis, hypoxia, dehydration, cold.
What is the pathophysiology of splenic sequestration crisis?
Sickle cells cause vessel narrowing, reduced splenic function, and spleen becomes engorged with blood.
What is the pathophysiology of acute chest syndrome? (x3 points)
It comes as a result of occlusion of the pulmonary vessels leading to hypoxia and inflammation. Sickle cell is also associated with pulmonary vasoconstriction. Hypoxia leads to further sickling which makes condition worse.
What is the aetiology of aplastic crisis in sickle cell?
Parvovirus B19: Parvovirus affects developing cells in bone marrow and arrests their maturation and production for a few days. This is harmless in healthy patients as they have normal RBCs which last 120 days. However, in sickle cell patients where RBCs last 20 days, it can lead to rapid onset anaemia – APLASTIC CRISIS.
What are the investigations for sickle cell disorders? (x6 +2) Definitive diagnosis and clinical application?
- FBC: anaemia, reticulocytosis in haemolytic crisis, reticulocytopenia in aplastic crisis
- Deranged U&Es in renal compromise
- BLOOD FILM (see photo): sickle cells, anisocytosis (unequal size), features of hyposplenism (target cells, Howell-Jolly bodies (nuclear remnants in RBC))
- SICKLE CELL SOLUBILITY TEST: add reducing agent (dithionate) to oxyHb to convert to deoxyHb. If HbS is present, you will get turbid (cloudy) solution indicating insolubility. However, does not differentially diagnose between sickle cell trait and homozygous state (anaemia).
- HAEMOGLOBIN ELECTROPHERESIS and CHROMATOGRAPHY: most commonly used for diagnosis; separates proteins based on charge. Shows HbS, absence of HbA and raised levels of HbF
- DNA ASSAYS: definitive diagnosis but not always used
- HIP X-RAY: common site for avascular necrosis
- MRI/CT HEAD: if there are neurological complications
What types of electrophoresis can be used to diagnose sickle cell? (x2)
Isoelectric focussing and cellulose acetate electrophoresis
How is sickle cell anaemia managed acutely? (x3)
Treat triggers; oxygen, warmth, IV fluids. Analgesics for pain crises. Antibiotics for infection
How is sickle cell anaemia managed non-acutely? (x4 (x3 indications))
- Infection prophylaxis: penicillin V, regular vaccinations e.g., against pneumococcus
- Folic acid: in haemolysis or pregnancy (pregnancy can be a trigger)
- Hydroxyurea: increased HbF levels and decreases frequency and duration of sickle cell crises
- Red cell transfusion: for anaemia, stroke (exchange transfusion to keep volume constant (you don’t want to transfuse on top of normal blood volume as it would increase BP), and acute chest syndrome (exchange transfusion for the same reason).
How is sickle cell anaemia surgically managed? (x2)
Haemopoietic stem cell transplant (bone marrow transplant) and joint replacement for avascular necrosis.
What are the complications of sickle cell disorders? (+2)
- Crises (discussed in signs and symptoms)
- Septic shock from infection
- Erectile dysfunction from priapism (prolonged erection) lasting more than 4 hours
What is thalassaemia?
Group of genetic disorders characterised by reduced globin chain synthesis
What are the types of thalassemia? (x2 and x3)
- Based on globin chain affected: alpha and beta
- Based on severity: minor (or thalassaemia ‘trait’), intermedia and major (or transfusion dependent)
What is beta thalassemia?
Reduced or absent production of beta globin chains.
What is the aetiology of beta thalassemia? Each type?
Autosomal recessive, affecting CHROMOSOME 11. Some mutations result in NO B-globin production (B^O), while others result in DECREASED B-globin production (B^+).
What characterises each beta thalassaemia type? (x4)
- Based on PHENOTYPE rather than genotype, but major tends to be B^OB^O and trait tends to be BB^O/BB^+:
- MAJOR: complete absence of HbA. Severe anaemia and incompatible with life without transfusions
- INTERMEDIA: microcytic anaemia, reduced alpha-chain synthesis or raised gamma-chain synthesis. Has a broad phenotype
- TRAIT: asymptomatic with mild microcytic anaemia and raised RCC
- SILENT CARRIER.
When does beta thalassaemia major clinically present?
First 4-6 months of life when beta globins start to take over gamma globins, and HbF –> HbA.
What is the pathophysiology of beta thalassaemia?
Deficient beta globin production leads to microcytic and hypochromic anaemia. There is clumping of excess alpha chains in erythroid precursors and maturing RBCs leading to membrane damage and haemolysis. As such, there is anaemia and compensatory erythroid hyperplasia. Erythroid hyperplasia may manifest as bony changes in the skull and extramedullary haematopoiesis in the liver and spleen resulting in organomegaly.
What is alpha thalassaemia?
Reduced or absent production of beta globin chains.
What is the aetiology of alpha thalassemia? Four presentations?
- Autosomal recessive. Affecting CHROMOSOME 16 which contains four alleles (HBA1 and HBA2 genes).
- ONE MISSING GENE: silent carrier, asymptomatic.
- TWO MISSING GENES: known as alpha thalassaemia trait with minor anaemia and symptoms.
- THREE MISSING GENES: known as haemoglobin H (HbH) disease leading to mild-moderate anaemia and symptoms. HbH results in excess beta chains (and excess gamma chains in newborns). The excess B chains form unstable tetramers called HbH of FOUR beta chains, and excess gamma chains form Hb Barts of four GAMMA chains –> abnormal oxygen dissociation curves and haemolysis.
- FOUR MISSING GENES: known as hydrops fetalis – foetal death usually occurs after birth.
What is the pathophysiology of alpha thalassaemia?
(THE SAME AS BETA) Deficient alpha globin production leads to microcytic and hypochromic anaemia. There is clumping of excess beta chains in erythroid precursors and maturing RBCs leading to membrane damage and haemolysis. As such, there is anaemia and compensatory erythroid hyperplasia. Erythroid hyperplasia may manifest as bony changes in the skull and extramedullary haematopoiesis in the liver and spleen resulting in organomegaly. (EXTRA BIT IN ALPHA) In addition, there is formation of Hb Bart and HbH which have high oxygen affinity and are non-functional oxygen carriers.
What is the epidemiology of thalassemia: Where? In relation to malaria?
Mediterranean and Arabian Peninsula. Thalassemia is seen in the same areas where malaria is prevalent, because thalassemia protects against malaria – a bit of evolution there.
What are the signs and symptoms of thalassaemia major? (x6) SHIF+2
- SEVERE ANAEMIA presenting after four months
- HEPATOSPELENOMEGALY: from extramedullary haematopoiesis (reflecting a continuation of haematopoiesis in foetal liver and spleen)
- IRON OVERLOAD leading to CARDIAC FAILURE, endocrinopathies and LIVER FAILURE.
- FRONTAL BOSSING and THALASSAEMIC (or chipmunk) FACIES: bone marrow cavity expansion in forehead and cheeks respectively to compensate for anaemia and haemolysis
- JAUNDICE and GALL STONES – from bilirubin excess, from chronic haemolysis because alpha chains without beta globin chains are unstable.
- Delay in growth and puberty.
! What is the cause of iron overload in thalassaemia patients? (x2)
- From frequent blood transfusions in transfusion-dependent thalassaemia.
- Compensatory increased iron uptake in the GI tract in transfusion-independent patients.
What are the signs and symptoms of beta thalassaemia trait and intermedia in relation major?
Trait, intermedia and major all lie on a SPECTRUM, so each subtype will all of major’s symptoms to an extent
What are the signs and symptoms of alpha thalassaemia?
The same as beta.
What are the investigations for thalassaemia? (x6) Definitive diagnosis?
- FBC: low Hb, low MCV, low MCH, high RBC count
- Iron studies: high ferritin and serum iron reflecting iron overload
- BLOOD FILM: hypochromic, microcytic anaemia, target cells, nucleated RBCs and high reticulocyte count, but no anisocytosis
- Hb ELECTROPHORESIS and CHROMATOGRAPHY: absent or reduced HbA. Beta thalassaemia easy to diagnose as associated with compensatory increases in levels of HbF and HbA2 (two alpha and two delta chains). Alpha harder to diagnose unless three missing genes as you would see HbH
- DNA STUDIES: definitive diagnosis for alpha and used if clinician unsure about beta.
- BONE MARROW: hypercellular and erythroid (precursor) hyperplasia
Why are thalassaemia patients at risk of infection? (x2)
From splenomegaly, especially in splenectomy because spleen has immune function against some bacteria; increased blood iron concentration seen in thalassaemic patients also provides breeding-ground for some microorganisms.
