Heritable Bleeding Disorders

Question 1

Describe the formation of a haemostatic plug

Answer 1

1. Normal platelets in flowing blood
2. Damaged endothelium leads to platelets adhering to the wall (bind to collagen in sub endothelium) and becoming activated
3. Activated platelets aggregate to form a thrombus (platelet plug)
4. Receptors are expressed and fibrinogen binds to platelets
5. Granules are released, stimulating further platelet activation
6. Coagulation cascade follows – clotting factors are soluble proteins and contribute to fibrin formation.

Question 2

Which enzyme converts fibrinogen to fibrin?

Answer 2

thrombin (serine protease)

Question 3

How is thrombin formed?

Answer 3

Prothrombin (coagulation factor II) is cleaved to form thrombin

Question 4

What is Von Willebrand factor?

Answer 4

Von Willebrand factor helps platelets aggregate and adhere to (collagen in) walls of blood vessels.

Question 5

What is the most common heritable bleeding disorder?

Answer 5

Von Willebrand disease

Question 6

What is Von Willebrand disease?

Answer 6

low or dysfunctional Von Willebrand factor –> bleeding

Question 7

What acts as receptors on platelet surfaces?

Answer 7

Membrane glycoproteins

Question 8

The membrane glycoproteins of platelets act as receptors that mediate 2 important functions. What are these?

Answer 8

1. adhesion to the subendothelial matrix
2. platelet aggregation

Question 9

What are the 2 important membrane glycoproteins found on platelets? What is each one a receptor for?

Answer 9

1. 2b3a –> receptor for fibrinogen
2. 1b9 –> receptor for vWf

Question 10

Inside the platelets are granules. What are the 3 different types of granules found?

Answer 10

1. alpha granules
2. dense granules
3. lysosomal granules

Question 11

What do alpha granules contain?

Answer 11

contain adhesion proteins and calcium (role in coagulation cascade)

Question 12

What do dense granules contain?

Answer 12

contain ATP and ADP which are released when platelets become activated (ADP forms positive feedback loops – activates more platelets)

Question 13

Metabolic processes are going on inside cytoplasm of platelets when they become activated. What 2 major metabolic processes are going on?

Answer 13

1. Formation of prostaglandins
2. Formation of thromboxane

Question 14

How are prostaglandins formed in the cytoplasm of platelets?

Answer 14

• Mobilisation of fatty acids from platelet membrane
• Conversion of the fatty acid arachidonic acid to intermediate prostaglandins

Question 15

How is thromboxane formed in the cytoplasm of platelets?

Answer 15

• Synthesised by activated platelets from arachidonic acid (via the enzyme cyclooxygenase)
• Released from phospholipid membrane upon platelet activation
• Secreted into surrounding and stimulates activation of new platelets as well as platelet aggregation

Question 16

What enzyme converts arachidonic acid to thromboxane?

Answer 16

cyclooxygenase

Question 17

how does aspirin act as an anti-thrombotic agent?

Answer 17

Aspirin inhibits cyclooxygenase (COX) –> prevents production of thromboxane A₂

Question 18

Name 4 anti-platelet drugs

Answer 18

1. Aspirin
2. Clopidogrek
3. Dipyridamole
4. IIb and IIa anatagonists

Question 19

How does Clopidogrel act as an anti-platelet drug?

Answer 19

• blocks ADP receptor (P2Y12 inhibitor)
  
  • P2Y12 is a chemoreceptor for ADP

Question 20

How does Dipyridamole act as an anti-platelet drug?

Answer 20

Stimulated prostacyclin to inhibit binding site for collagen, thrombin, thromboxane A₂

Question 21

How do IIb and IIa antagonist act as an anti-platelet drugs?

Answer 21

blocks GPIIb/IIIa

Question 22

Overview of coagulation cascade

Answer 22

Question 23

Describe the steps of the extrinsic pathway

Answer 23

1. Coagulation is triggered by tissue factor
2. Factor VII binds to tissue factor and becomes activated –> VIIa
3. VIIa can directly activated X –> Xa

Question 24

What is tissue factor? Where is it found?

Answer 24

a cellular protein found on surface of cells – damaged endothelium, inflamed blood vessels, WBCs, brain cells

Question 25

Describe the steps of the intrinsic pathway

Answer 25

Doesn’t rely on molecules external from the blood but instead becomes activated when blood is exposed to a foreign surface: 1. Factor 12 is activated when exposed to a foreign surface --\> XIIa 2. XIIa then activates XI --\> XIa 3. XIa then activates IX --\> IXa 4. IXa (with cofactor VIII) activates X --\> Xa

Question 26

Describe the steps of the common pathway of the coagulation cascade

Answer 26

1. Xa (with cofactor Va) will convert prothrombin --\> thrombin 2. Thrombin converts fibrinogen to fibrin (crosslinked by Factor 13)

Question 27

What is the 'prothrombin time'?

Answer 27

* Used to measure extrinsic pathway: * Adding a tissue factor substitute (thromboplastin) to plasma in a test tube * Time how long it takes blood to clot

Question 28

What is the Activated Partial Thromboplastin Time (APTT) used to measure?

Answer 28

Intrinsic pathway

Question 29

What is the thrombin clotting time?

Answer 29

* Used to measure final step * Thrombin is added to plasma * Time how long it takes to clot

Question 30

What is the thrombin clotting time dependent on?

Answer 30

* Very dependent on level of plasma fibrinogen * Dependent on there being nothing in blood inhibiting formation of fibrin from fibrinogen * E.g. heparin

Question 31

What is often the first line of investigation of someone with abnormal bleeding?

Answer 31

*Prothrombin Time (PT)*

Question 32

What does a coagulation screen comprise?

Answer 32

1. Prothrombin time 2. Activated Partial Thromboplastin Time (APTT) 3. Thrombin Clotting Time

Question 33

Why are the top parts of the intrinsic pathway is less physiologically important?

Answer 33

* Extrinsic pathway is thought to start it off. * Intrinsic pathway acts as amplification system instead of initiator. * Thrombin has positive feedback effect Factor 11 can therefore be activated through extrinsic pathway activating thrombin and then the thrombin activating Factor 11 through a feedback system.

Question 34

Does a patient with Factor VII deficiency have a bleeding disorder? What will the APTT be like?

Answer 34

No - this factor is less important But will have a long APTT (instrinsic pathway time)

Question 35

Will a patient with a factor XI deficiency have a bleeding disorder?

Answer 35

Yes but only mild

Question 36

Protein C and Protein S also play a role in the coagulation cascade. What is their function?

Answer 36

Both have a role in **anticoagulation**: * Protein C plays an important role in regulating **_anticoagulation_** by **inhibiting Factor V** * Protein S has a role in the **_anticoagulation_** pathway, where it functions as a **cofactor** to **Protein** **C** in the inactivation of Factors Va and VIIIa

Question 37

Antithrombin also has a role in the coagulation cascade. What is its function?

Answer 37

Anticoagulant effects; It inactivates several enzymes of the coagulation cascade, in particular thrombin and factor Xa

Question 38

The fibrinolytic system also has a role in the coagulation cascade. What is its function?

Answer 38

Anticoagulant: The fibrinolytic system comprises an inactive proenzyme, **plasminogen**, which can be converted to the active enzyme, **plasmin**, which in turn **degrades fibrin into soluble fibrin** **degradation** products.

Question 39

When conducting an assessment of a suspected bleeding disorder, clinical history is key. What questions should you ask?

Answer 39

* Date of onset * Previous history (longer is congenital, shorter if acquired) * Clinical pattern (more severe presents in childhood) * Response to challenges: surgery, dental extraction * Young children: bleeding from umbilical stump, vaccinations, circumcision * Requirement for medical/surgical intervention to ascertain extent of bleeding * Systemic illness, **drug** **history**, **family** **history** – helps to identify if congenital

Question 40

When conducting a clinical examination during an assessment of a suspected bleeding disorder, waht should you look for?

Answer 40

* Pattern of any bruising * Signs of underlying disease: Joints, muscles, skin

Question 41

Exampls of blood tests done during an assessment of a suspected bleeding disorder

Answer 41

* FBC and blood film * Coagulation screen * Clauss fibrinogen (D-dimer if suspicion of acquired disorder) * Mixing studies: if clotting tests are longer than expected, they can be performed with the patient’s plasma and normal plasma. Test will correct significantly if there is a lack of factors. It will not correct significantly if the patient’s blood contains an inhibitor of clotting. * Von Willebrand profile * Coagulation factor assays * Inhibitor assays (antiphospholipid type) * Platelet function tests * If all normal and suspicion remains: factor 13 and alpha2antiplasmin assays.

Question 42

During mixing studies, if clotting tests are longer than expected, they can be performed with the patient’s plasma and normal plasma. * If the clotting test corrects significantly, what does this indicate? * If the clotting test does not correct significantly, what does this indicate?

Answer 42

* Lack of factors in patients blood * Patients blood contains an inhibitor of clotting

Question 43

If a patient has a normal coagulation screen (APTT, PT, TCT), what are other potential causes of a bleeding disorder?

Answer 43

* Thrombocytopenia (low platelets) * Disorder of platelet function * vWD * Factor 13 deficiency * Mild coagulation factor deficiency * Vascular disorder * Disorder of fibrinolysis (rare) * Platelet/vessel wall defects (1ary haemostasis) --\> give rise to prolonged bleeding time

Question 44

How can a factor 13 deficiency lead to bleeding? How does this bleeding present?

Answer 44

* Factor 13 covalently crosslinks the fibrin polymer and stabilises it * Patients with this deficiency typically present with late bleeding (a few days after sometimes)

Question 45

How can platelet/vessel wall defects give rise to bleeding disorders?

Answer 45

* Reduced number of platelets (thrombocytopenia) * Abnormal platelet function (could be iatrogenic, caused by aspirin) * Abnormal vessel wall * E.g. Ehlers-Danlos syndromes; a group of rare inherited conditions that affect connective tissue --\> bleeding due to defects in collagen * Abnormal interaction between platelets and vessel wall * E.g. vWD – reduction in platelet adhering molecules to the wall.

Question 46

What is Ehlers-Danlos syndromes?

Answer 46

Ehlers-Danlos syndromes; a group of rare inherited conditions that affect connective tissue --\> bleeding due to defects in collagen

Question 47

Difference in presentation between vascular/platelet defects and coagulation defects

Answer 47

Question 48

What is this? What would it indicate?

Answer 48

*Petechiae* * Little red spots often around hair follicles * Don’t blanch with pressure * Not palpable (flush against skin) * Think about; thrombocytopenia, meningitis, vascular/platelet defects

Question 49

There are 3 types of **Von Willebrand Disease**. How does each type differ?

Answer 49

* Type 1: * **Reduced** production of **normal** vWf, tend to have normal/slightly reduced Factor VIII * Common, **mild**; especially in blood group O * Variable penetrance in a family with all the same genetic defect. * Type 2: * Patient is producing enough vWf but often is **structurally abnormal** so is dysfunctional (low molecular weight polymers) * Slightly more severe than type 1 * Type 3: * vWf is **absent altogether** and Factor VIII tends to be greatly reduced too * **Severe** and **spontaneous;** * **​**bleeding into joints and vessels as well as mucosal surfaces

Question 50

Why can vWd often present with reduced Factor VIII levels?

Answer 50

Vwf is a carrier protein for factor VIII (carries it around circulation)

Question 51

Inheritance pattern of vWf (remember different types)?

Answer 51

* Mainly autosomal dominant inheritance. * Type 3 is autosomal recessive.

Question 52

Which blood group is often associated with low vWf levels?

Answer 52

Blood group O (this is not vWd)

Question 53

Symptoms of vWd?

Answer 53

* Postoperative/post-partum bleeding * Mucocutaneous bleeding * Menorrhagia

Question 54

vWf has 2 functions. What are these?

Answer 54

1. binds to **protein 1b9 on the platelet surface** and helps it bind to the vessel wall (**collagen**) 2. forms a complex with factor 8 (**carrier** protein) to activate factor 10

Question 55

What factors are deficient in vWd?

Answer 55

In vWD, there is less 8a, F10a and fibrin and so bleeding is more likely

Question 56

Treatment options for vWd:

Answer 56

* **Anti**-**fibrinolytics** --\> tranexamic acid * DDAVP * **Factor concentrates** containing vWF (pool plasma derived) * Vaccination again hepatitis * Combined OCP for menorrhagia (or Mirena coil)

Question 57

What is tranexamic acid? What is it useful in treating?

Answer 57

Anti-fibrinolytic - good for bleeding of the mouth, periods and nosebleeds.

Question 58

What is DDAVP? What is it used to treat?

Answer 58

* DDAVP is an analogue of vasopressin * Analogue of **vasopressin** * Side effect of promoting secretion of vWf from where it is stored inside endothelial cells inside lining of blood vessels --\> short term solution

Question 59

What is Factor I?

Answer 59

Fibrinogen

Question 60

What is Factor II?

Answer 60

Prothrombin

Question 61

Haemophilia A is a defect in what?

Answer 61

Factor VIII

Question 62

Haemophilia B is a defect in what? What is this also known as?

Answer 62

Factor IX - Christmas disease

Question 63

Summary of **Heritable Coagulation Factor Deficiencies**

Answer 63

Question 64

Inheritance of haemophilias?

Answer 64

X-linked recessive

Question 65

How does haemophilia affect men vs women? Why?

Answer 65

* Typically expressed in males and carried by females. * In females, the normal X chromosome will compensate for the faulty X chromosome so tend to be carriers only

Question 66

Is haemophilia A or B more common?

Answer 66

**Haemophilia A**: 1 in 5000 males Haemophilia B: 1 in 30,000 males

Question 67

Which part of the coagulation cascade is affected in haemophilia A/B?

Answer 67

8 and 9 are **intrinsic** **pathway** factors and contribute to the **common** **clotting** cascade.

Question 68

Factor VIII or IX levels determine if haemophilia is mild, moderate or severe. What are the Factor VIII or IX levels for each of these?

Answer 68

* Normal: Factor VIII or IX level between 50-150% * Mild: Factor VIII or IX level between 6-50% * Moderate: Factor VIII or IX level between 1-5% * Severe: Factor VIII or IX level less than 1%

Question 69

How does mild haemophilia present?

Answer 69

* Bleeding only with moderate trauma or surgery. * Diagnosis often due to family history or later in childhood

Question 70

How does moderate haemophilia present?

Answer 70

Variable, occasional spontaneous bleeds, often proceeding trauma

Question 71

How does severe haemophilia present?

Answer 71

Frequent bleeds into muscle and joints

Question 72

Types of bleeding - reflect the severity of haemophilia

Answer 72

* Spontaneous/post traumatic * Can be diagnosed after surgery, trauma, dental procedure etc * Joint bleeding = haemarthrosis, sets of synovitis and causes abnormal, damaged joints * Muscle haemorrhage – hallmark of severe haemophilia, leading to contractures, compartment syndrome, shortening, MSK disability * Soft tissue – bleeds into tongue, pharynx, primary teeth eruption, can threaten the airways (life threatening). * Life threatening bleeding – intracranial bleeding in severe/moderate

Question 73

Treatment of haemophilia;

Answer 73

* Replacement of missing clotting protein (daily): on demand or prophylaxis. * Using factor concentrates; recombinant are products of choice, no longer blood derived * DDAVP (only for mild/moderate haemophilia A) * Antifibrinolytic agents – for mucosal bleeding, GI bleeding * Vaccination against hepatitis A and B * Supportive measures: icing, immobilisation, rest

Question 74

What are the supportive measures for haemophilia?

Answer 74

icing, immobilisation, rest

Question 75

Which haemophilia does DDAVP treat?

Answer 75

A

Question 76

One complication of treatment of haemophilia is the potential development of inhibitors. What are these?

Answer 76

Antibodies formed against Factor VIII or IX that stop the factor concentrates from working due to immune response: * More common in haemophilia A (25%) * Genetic predisposition

Question 77

Presentation of development of inhibitors during haemophilia treatment?

Answer 77

* Poor recovery, bleeding difficult to manage, muscle/joint damage. * Poor clinical response to treatment, factor ineffective or less effective * Occurring early in course of condition, around 10 exposure days

Question 78

Emicizumab is a treatment used for Haemophilia. Mechanism?

Answer 78

Used for **haemophilia** **A** only, with and without inhibitors * Is a “**monoclonal** **antibody**” * Monoclonal antibodies are proteins that are designed to bind to a specific target in the body * They can be used to treat diseases or conditions * Emicizumab: * **Mimics the action of the missing FVIII** in the clotting pathway * The cue for its action is FIXa, which appears after an injury * Emicizumab binds to FIXa and FX, forming a link between these two clotting factors * FIXa can then activate FX, allowing the pathway to continue and the blood to clot

Question 79

Factor VIII Prophylaxis can also be used in haemophilia A treatment. How does this work?

Answer 79

* FVIII concentrate replaces missing or low levels of FVIII * Prophylaxis is the regular administration of FVIII concentrate, to try and prevent bleeds * FVIII rises quickly after infusion and then decreases over a number of hours