Thrombocytopenia, VWD and platelet disorders Flashcards
1
Q
Outline the steps in thrombopoiesis
A
- Megakaryocytes are the bone marrow precursors
- programmed via transcription factors to express platelet cell surface proteins and form platelet-specific organelles
- Final stages of maturation - long pseudopodia develop
- Forms a protoplatelet process
- organelles move along microtubules to the end of the proplatelet
- Proplatelets branch and formed bead like constrictions
- The proplatelets fragment and for smooth disc like platelets
- The platelets are released from the marrow into the vascular space as quiescient non-adhesive platelets.
- Platelets have a life span of 6-10 days in the blood stream
- ~ 100 billion are release each day!
2
Q
Briefly describe the process of primary haemostasis
A
- Vessel wall injury leads to exposure of subendothelial collagen and VWD
- Platelets interact with collagen and VWD triggering activation
- Platelet granules are released via cell signalling pathways
- adenine nucleotides, calcium, serotonin, fibrinogen, VWF, fibronectin and P-selectin
- Granules recruit additional platelets
- Fibrin mesh helps to strengthen platelet plug
3
Q
Discuss how activated platelets up-regulate and amplify clot formation, consolidation and retraction
A
- Platelets are activated via exposure to subendothelial collagen and VWF
- Platelet granules are released - recruiting more platelets and liberating fibrinogen
- coated platelets act as a scaffolding for tenase and prothrombinase (FXa and Va) which binds to the phospholipid surface
- thrombin and fibrin are produced locally to enhance clot maturation
- Platelet cytoskeletal elements linked to fibrin receptors contract resulting in retraction of the growing clot
4
Q
Clopidogrel: Mechanism of action
A
- Platelet aggregation inhibitor
- Effects via an active, highly unstable, unidentified compound
- Compound selectively binds to platelet surface low-affinity ADP receptors - irreversibly alters ADP receptor
- Inhibits activation of the platelet-glycoprotein Ib/IIIa complex necessary for platelet-fibrinogen binding
- Also inhibits granule release - decreasing serotonin, calcium, fibrinogen, ADP, thrombospondin, all necessary for up-regulation of clot formation.
5
Q
Aspirin: mechanism of action
- in thromboprophylaxis
- Analgesia
- Anti-inflammatory
- Anti-pyretic
A
- Inhibits COX-1 (prostaglandin synthetase)
- Reduces synthesis of prostaglandin and thromboxanes
- Platelets cannot synthesize new COX-1, so the effect on platelets is irreversible
- Doses of 1-2 mg/kg/day inhibit platelet function and aggregation (current recommendation is ~ 2 mg/kg/day
- Prostaglandin E2 responsible for pyrogen effec
6
Q
Heparin: Mechanism of action and use
A
7
Q
Discuss the interaction between platelets and VWF
A
- VWF and subendothelial collagen are exposed with vascular damage.
- VWF binds with collagen and undergoes a conformational change. The strength of the bond is dependent on VWF multimer size
- The largest multimeric form of VWF is stored within endothelial cells.
- Platelets adhere to VWF once exposed. This adherance causes platelet activation.
- Platelets adhere to VWF and slowly roll and become activated by interactions with collagen
- Activated platelets release alpha-granules
- These granules store ~ 20% of the total blood VWF (humans).
- calcium released in the granules catalyses binding of GPIIb and IIIa to form the fibrinogen receptor
- VWF then binds to GPIIb/IIIa receptors (fibrinogen receptor) on activated platelets allowing further adhesion and aggregation.
8
Q
List causes of primary haemostatic disorders.
A
- Severe thrombocytopenia
- Von Willebrand disease
- Thrombopathia
- Vasculopathy
9
Q
Discuss potential clinical signs cause by primary haemostatic disorders
A
- Surface mucosal bleeding is the hallmark with petechiae most often seen
- Epistaxis, haematuria and melena can be severe enough to warrant blood transfusion
- Bleeding into the brain and lungs is uncommon but can be life-threatening
- Cavity bleeding, haemarthrosis and haematoma are rarely reported
- Excessive bleeding after trauma / surgery may be first sign
10
Q
Describe the pathophysiological mechanisms that lead to immune mediated thrombocytopenia
A
- ITP is mediated by the humoral immune system.
- There may be a reduction in Treg activity leading to development of auto-antibodies
- Antibodies bind to various platelet surface antigens
- Abs directed against the fibrinogen receptor have been identified
- A monoclonal antibody directed against GPIIb causes dose depended thjrombocytopenia
- Antibody binding leads to opsinisation and subsequent removal by the reticuloendothelial system
11
Q
List known causes of secondary ITP
A
- Drug reaction
- sulphonamides and cephalosporins most common
- Infectious agents
- Babesia
- Anaplasma phagocytophilium
- Ehrlichia canis
- Leptospira
- Leishmania infantum
- Neoplasia
- Thrombocytopenia can also be caused by tumour bleeding, platelet consumption, DIC, splenic sequestration, decreased production (with myelophthisis)
- histiocytic sarcoma, lymphoma, haemangiosarcoma
- Inflammatory disease
- Chronic hepatitis
- Pancreatitis
- SIRS
12
Q
Describe the diagnostic pathway for a dog with suspected severe thrombocytopenia
A
- Clinical exam - surface bleeding (petechiae, ecchymoses, epistaxis), pallor, splenomegaly, fever
- CBC and blood smear evaluation
- Care to assess for clumped platelets
- Always a presumptive diagnosis
- Consider imaging
- Consider infectious disease screening
- In clinical practice - document SEVERE thrombcytopenia, exclude underlying disease, assess response to immunosuppression
- detection of platelet bound antibody - Platelet surface associated IGg - flow cytometry assay. Sensitive but not specific for idiopathic IMT
13
Q
List potential treatment options for IMT
A
- Corticosteroids
- Vincristine
- hIVIG
- Mycophenolate
- Azathioprine / cyclosporine
- Transfusion - pRBC or whole blood
- fresh PRP, platelet concentrate or cryopreserved platelets not readily available in clinical practice
- lyophilised platelets???
14
Q
Discuss the treatment approach for IMT
A
- Ensure any offending medications are stopped
- Assess for and treat infectious diseases (especially when endemic)
- Blood or PRBC tansfusion as needed
- Immunosuppression for idiopathic and if anti-microbials are not working
- Vincristine been shown to reduce duration of severe thrmobocytopenia
- hIVIG has also shown reduced time to platelet recovery compared to prednisolone alone (3.5 d vs 7.5 d)
- Vincristine vs hIVIG showed not difference in platelet recovery in a further study.
- Concurrent immunomodulatory drugs
- no prospective studies comparing efficacy
- azathioprine, cyclosporine and mycophenolate
- Mycophenolate - platelet recovery in 2-6 days in 5 dogs as a single agent
15
Q
Discuss the implications of concurrent IMHA and IMT in dogs
A
- In one study of 21 dog with concurrent IMHA and IMT (< 50k/uL), 16 died or were euthanised within 30 days of hospital admission.
- Another study, 12 dogs (Orcutt 2010) with IMHA and thrombocytopenia, but platelet count < 15 k/uL had an in-hospital mortality rate of 25% - similar to the individual diseases.
- Looking closer, 2 further dogs died or were lost to follow up at 7 days and 2 more at 30 days and 30 day survival was 58% (but 2 were then lost to follow up at 30 days…)
- Definition of IMT component was more stringent in the second paper and was more likely to exclude dogs with DIC
- The clinical inplications and treatment options are the same as for each disease.
- Immunosuppressive treatment and supportive care is similar.
