Haemostatic disorders Flashcards
Describe the process of primary haemostasis
- Injury to vessel
- Platelets adhere to collagen
- Von Willbrand’s factor released from endothelium
- Platelets change shape following adhesion with secretion of substances from granules which potentiate platelet aggregation and contraction of the platelet plug
Describe the process of secondary haemostasis
Stabilisation of the platelet pug by deposition of fibrin
Describe the process of tertiary haemostasis
- Formation of plasmin from plasminogen
- Fibrinolysis to break down fibrin clot
- Predominantly by action of plasmin
Describe hyperfibrinolysis
- Breakdown of clots too quickly leading to bleeding
- Sight and Greyhounds predisposed
- Can bleed badly from minor wounds or surgical sites
What are the 2 pathways of clotting activation?
Intrinsic, extrinsic
Briefly outline the intrinsic pathway of clotting activation
- Damaged surface
- Activation of cascade
- Production of FXa
- FXa converts prothrombin to thombin, which converts fibrinogen to fibrin
- Clot forms from cross linked fibrin
Briefly outline the extrinsic pathway of clotting activation
- Trauma/tissue factor activates cascade
- Production of FXa from FVII
- FXa activates prothrombin to thrombin, which converts fibrinogen to fibrin
- Clot forms from cross linked fibrin
Describe the clinical signs of disorders of primary haemostasis
- Petechiae (<3mm), echymosis (>1cm)/purpura (3-10mm)
- Bleeding from MM
- often more than one site of bleeding
- Haematomas rare-
Describe what is meant by diascopy
- Using glass slide to determine if a wheal is from a bite, reaction or if a haemorrhage
Press slide against skin - if haemorrhage will not blanche
What sign may be seen at the teeth in Von Willebrand’s disease?
Bleeding from gingiva as teeth come through
Describe the clinical signs of disorders of secondary haemostasis
- Petechiae/echymoses rare
- Deep or cavity bleeds, can bleed from mucous membranes
- Sometimes single sites of bleeding
- Haematomas common
Describe the clinical signs of Von Willebrands disease
- Typical primary haemostatic defect: mucosal haemorrhage, cutaneous bruising, prolonged bleeding from surgical and traumatic wounds
- More profound bleeding incl. epistaxis, haematuria, GI haemorrhage, prolonged oestral bleeding and gingival bleeding at tooth eruption
- Classically seen around first events e.g. first season, first tooth
What are potential causes of disorders of primary haemostasis?
- Thrombocytopaenia
- Thrombocytopathia
- von Willebrand factor deficiency
Describe the typical signalment for disorders of primary haemostasis
- Young animals more likely to have inherited vs acquired
- Certain breeds predisposed to inherited disorders of primary haemostsis
- Certain breeds and female dogs prone to immune mediated thrombocytopaenia
- Acquired immune mediated thrombocytopaenia is the most common cause for haemostatic disorder
What may cause thrombocytopaenia?
- Defective platelet production
- Accelerated platelet removal
- Platelet sequestration or loss
- Thrombocytopaenia in cattle: BVD, bracken, alloantibodies
Outline potential causes for defective production of platelets
- Bone marrow neoplasia
- Drug/chemical/toxin induced bone marrow suppression
- Bone marrow infections (esp. viral and rickettsial)
- Less severe
List the potential causes of accelerated platelet removal
- Immune mediated destruction
- Consumption in microangiopathic conditions (DIC)
What is the most common acquired cause of primary haemostatic defects in the dog?
Immune mediated thrombocyte destruction
Explain how immune mediated destruction of platelets leads to thrombocytopaenia
- Platelets destroyed faster than they are produced
- Can be primary (idiopathic) or secondary (e.g. drug induced, infection, neoplasia related)
Describe the signalment for idiopathic immune mediated destruction of platelets
- Young to middle aged female dogs
- Cockers, miniature/toy poodles, Old English sheepdogs predisposed
List the potential causes of platelet sequestration or loss
- Splenomegaly/vascular pooling
- Acute ongoing haemorrhage (rare)
List the potential causes of thrombocytopaenia in cattle
- Bovine neonatal pnacytopaenia (bleeding calf syndrome)
- Bracken poisoning
- Bovine viral diarrheoa virus
Describe the pathogenesis of bovine neonatal pancytopaenia
- Caused by alloantibodies absorbed form colostum of particular cows
- Commercial BVD vaccine is likely source of alloantigens eliciting BNP associated antibodies
Describe the pathogenesis of bone marrow toxicity
- Leads to decreased platelets = bleeding = non-regenerative anaemia
- Decreased erythrocyte production leads to non-regenerative anaemia
- Decreased leukocytes leads to secondary bacterial infection
List the potential causes of thrombocytopathia
- Inherited thrombopathias
- Drug induced defects of platelet function
- Platelet dysplasia
Describe the common presentation of inherited thrombopathias
- Age important part of diagnosis
- First suspicion at vacc or neuter or dental eruption - stimulus required for bleeding
- Secondary haemostatic disorders when still with litter mates e.g mild trauma = huge bleeds
Outline von Willebrands factor deficiency
- Most common of the inherited bleeding disorders
- Wide range of dog breeds
- Very rare in cats
- 3 classification sbased on severity and abnormality of the vWF protein
- Variable bleeding tendency
List the breeds prediposed to von Willbrand factor deficiency
- Doberman pinschers
- GSD, German shorthaired pointer
- Corgi
- Golden retrievers
- Shetland Sheepdog
- Standard poodle
Describe the plasma vWF and clinical severity of type I von Willebrand Factor deficiency
- Abnormally low concentration of structurally norma vWF
- Milder/variable
Describe the plasma vWF and clinical severity of type II von Willebrand factor defiency
- Structurally abnormal vWF
- Severe
Describe the plasma vWF and clinical severity of type III von Willebrand factor deficiency
- Essentially no plasma vWF, diagnosed by ELISA
- Severe
LIst tests that are used to investigate disorders of primary haemostasis
- Platelet count
- Buccal mucosal bleeding time
- Von Willebrand factor antigen
- Platelet function assays
What methods can be used to perform a platelet count?
- Estimated ocunt from blood smear
- Automated cell count
- Haemocytometer
Describe the use of a platelet count in the investigation of disorders of primary haemostasis
- Do first to rule out thrombocytopaenia
- Perform as soon as possible to avoid clumping once sampled
- Scan for evidence of platelet clumps before examining the monolayer of the film under oil immersion
- assess morphology
- Care with breeds: e.g. cavvies have lower numbers of larger paltelets
Which sample tube may be best for performing a platelet count?
Citrated
How many platelets per high power field would indicate no risk of bleeding?
5-6 platelets per high power filed (each platelet/hpf = 20x10^9/L)
Describe the morphology of platelets that would be suggestive of increased platelet production
Large or shift platelets
If the platelet concentration is normal, but the animal is bleeding, what does this suggest?
Thrombocytopathia
Evaluate the use of buccal mucosal bleeding time to investigate disorders primary of haemostasis
- If platelet count low on smear, do not perform BMB as it will be prolonged and can be dangerous
- Can be difficult
- Usually performed sedated, but some sedatives can interfere with platelet function
- Very subjective
Describe the method for a buccal mucosal bleeding time
- Use simplate devide
- Makes 2 parallel cuts into mucosa
- Assess and time the clot formation
- Remove blood coming out without disturbing any potential clots that may be forming
- Increased time to stop bleeding indicates defective primary haemostasis
What is a normal buccal mucosal bleeding time in dogs and cats?
- Dogs: 1.7-3.3 mins (up to 4.2 if anaesthetised)
- Cat: up to 3.3. minutes
Outline the use of platelet function assays in the investigation of disorders of primary haemostasis
- Thromboelastography/ROTEM
- Provides dynamic global functional assessment of primary, secondary and tertiary haemostasis fibrinolysis
- Investigates interaction of coagulation factors, their inhibitors, anticoagulant drugs, blood cells, specifically platelets
Where should samples for the investigation of disorders of primary haemostasis be taken from and why?
- Peripheral veins e.g. cephalic, saphenous
- Can be pressure bandaged to prevent animal bleeding out until secondary haemostasis takes place
Outline the diagnosis of thrombocytopathia
- Normal PLT but prolonged BMBT
- Normal vWF levels
- Diagnosis of exclusion
- PLT function tests (not first opinion practice)
Give examples of platelet function tests
- Aggregometry
- Adhesion assays
- Flow cytometry
What methods are used in the diagnosis of von Willebrands disease?
- Platelet count
- Buccal mucosal bleeding time
- ELISA for vWF antigens
Outline the buccal mucosal bleeding time in the diagnosis of von Willebrands disease
- Useful screening test for vWD
- Normal: 2-4 min
- Mild to moderate (type I): 5-10 min
- Severe vWD: >12 min
Outline the use of ELISA for vWF antigens in the diagnosis of von Willebrands disease
- Diagnosis confirmed by demosntration of low vWF antigen concentrations
- ELISA shows vWF:Ag ratio
- Compare with species specific plasma pool
- <50% = affected, >70% = normal
Evaluate the use of ELISA in the diagnosis of von Willebrands disease
- Does not always accurately predict risk of haemorrhage
- May need repeat if clinical signs are present but not seen on ELISA
- Can wax and wane depending on how much vWF released from endothelium at time of testing
What condition may be mistaken as vWD and why?
Haemophilia A due to low factor VIII
Describe the genetic testing available for von Willebrands disease
- 5 mutations responsible for most vWD in dogs
- Autosomal
- Is not a diagnostic test
- 3 possible results: clear, carrier or affected status
- Not predictive of clinical bleeding (type I)
Outline the treatment of immune mediated thrombocytopaenia
- Suppress immune system
- Avoid subcut injections
- Do not use IM injections
- Minimise invasive procedures
- Gentle handling
- Poor surgical candidates
- Will often get intracranial bleeds which are fatal but can do very little about
OUtline the management of thrombocytopathia
- No specific therapy
- Platelet transfusions not possible
- Withdraw any durgs
- Treat symptomatically e.g. blood transfusion if marked anaemia
Discuss the use of platelet rich plasma transfusion in the treatment of acute short term bleeding
Platelets only last 24 hours, but good if animal needs surgery as will be able to clot for this period of time until secondary haemostasis can take over
Outline the treatment of vWD
- Plasma: stabilisation and cessation of active haemorrhage
- Cryoprecipitate
- Red cells if oxygen carrying capacity is compromised, whole blood may be required
- Administer demospressin 1ug/kg BW SC to patient
Describe how the use of plasma in the treatment of vWD can be optimised
- If donor dog, administer DDAVPs before taking blood to elevate vWF in the donor
- Supplements vWF of patient even more
Describe the clinical signs of disorders of secondary haemostasis
- Deep or cavity bleeds
- Petechiae/echymoses rare
- Haematomas comon
- radiographs may show areas of increased opacity (identify if are solid soft tissue masses or fluid)
- Melena and epistaxis common
How can you differentiate between a solid soft tissue mass or fluid in the thorax on a radiograph?
Solid soft tissue masses generally lead to displacement of the trachea, not the case for fluid
What are the potential aetiologies of disorders of secondary haemostasis?
- Congenital (haemophilias)
- Acquired
What factor is affected in:
1) haemophilia C?
2) haemophilia B?
3) haemophilia A?
1) Factor XI
2) factor IX
3: Factor VIII
Describe the history for disorders of secondary haemostasis
Signs present from young age e.g. bruises, lame (haemarthroses), subcut haematoma from playing
Discuss the importance of factor XII in haemophilia
- Clinically irrelevant, not required for activation of intrinsic pathway but can be seen in cats
- Occasionally seen in cats, asymptomatically carried, in vitro artefact
What gender is susceptible to haemophilia A and B?
Males
Describe haemophilia A
- Factor VIII deficiency
- Sex linked inherited in males
- Suffer from spontaneous life threatening bleeding episodes
Outline the pathogenesis of haemophilia A
Factor VIII is essential cofactor in the intrinsic pathway and deficiencies result in prolingation of the APTT
What is the APTT?
Activated Partial Thromboplastin Time
How is haemophilia A diagnosed?
Confirmation by a specific FVIII assay
Describe haemophilia B
- Sex-linked inherited condition
- Clinically identical to F VIII deficiency
- Active at same point in coagulation cascade
How is haemophilia B diagnosed?
Specific F. IX assay
What should samples being tested for haemophilia be tested for? Why?
Haemophilia A and B as these are clinically identical
Describe the clinical signs of vitamin K antagonism/coumarin toxicity
- Clinical signs may appear within 1-3 days (variable onset of signs)
- Haemorrhage may occur in large number of sites externally and into body cavities
- Can occur without major clinical signs, esp. if bleeds into cavity rather than epistaxis or coughing up blood
Explain the pathogenesis of coumarin toxicity
- Inhibit vit K epoxide reductase with normally reactivates vit K in the liver
- Vit K needed to activate factors II, VII, IX, X
- VII has the shortest half life so is quickest to become abnormal (but also quickest to normalise)
- Depletion of intrinsic and extrinsic pathway factors
Describe the OSPT and APPT in coumarin toxicity
- OSPT prolonged initially
- Then later APTT increases
What is the OSPT?
One stage prothrombin time
Outline the treatment of coumarin toxicity
- Induce emesis, gastric lavage, enema, activated charcoal (aim to remove 80% f ingested rat bait to decrease clinical risk dramatically)
- Vitamin K supplementation initially subcut
- Several weeks to months of treatment may be required depending on type of coumarin
- OSPT promptly returns to normal with appropriate therapy, but if withdrawn too soon will become prolonged again
Why shoudl vit K be given subcut?
IV associated with systemic anaphylaxis and shock
Explain why hepatic disease can lead to disorders of secondary haemostasis
- Many clotting factors and inhibitors of coagulation are synthesised in the liver
- Vit K dependent factors activated in the liver
Describe the signs that may indicate disorders of secondary haemostasis are secondary to hepatic disease?
- Elevated OSPT and APTT
- Increased plasma concentrations of liver enzymes showing liver damage
- Increased bile acid concentrations showing hepatic dysfunction
- Jaundice
- rarely spontaneously bleed
What is the main risk regarding disorders of secondary haemostasis secondary to hepatic disease?
Bleeding during liver biopsy so liver disease diagnosis is difficult
Outline the treatment of hepatic disease leading to disorders of secondary haemostasis
- If coagulation parameters prolonged, provide vit K parenterally
- Some animals may require plasma therapy
List appropriate tests for the investigation of disorders of secondary haemostasis
- Whole blood clotting time
- Activated clotting time
- One stage prothrombin time
- Activated partial thromboplastin time
- Specific factor assays
Describe the use of whole blood clotting time in the investigation of disorders of secondary haemostasis
- Crude measure of intrinsic and common pathways
- Will be prolonged in severe thrombocytopaenia
Describe the interpretation of whole blood clotting time in the investigation of disorders of secondary haemostasis
- Prolonged when single factor depleted to <5-10% of normal
- Normal values: dogs 3-13 mins, cats 8 mins
- Collect in a warmed glass tube
What does one stage prothrombin time assess?
Extrinsic and common pathways, becomes prolonged due to marked deficiency of a single factor <30% of its normal concentration)
Describe the interpretation of one stage prothrombin time (PT/OSPT)
- Reference intervals depend on machine used
- May have normal values on APTT
- If signs suggestive but results say normal, run again
What does activated partial thromboplastin time assess (APTT)?
Intrinsic and common pathways
Describe the interpretation of the activated partial thromboplastin time
- Prolonged due to marked deficiency of a single factor (<30% of its normal concentration but can be significantly lower)
- Reference intervals depend on machine used
- Haemophilia: 18019% of factor VIII but may give normal APTT occasionally, care regarding clinical signs and results
Outline the use of specific factor assays for the investigation of disorders of secondary haemostasis
- Level of individual clotting factors can be measured
- Most commonly indicated in the investigation of inherited coagulation disorders
Give an important consideration when taking samples for specific factor assays for the investigation of disorders of secondary haemostasis
- Ensure are not using up clotting factors by slow sampling, or old blood in needle/hub from previous sampling attempts
- Clotting will use up factors
List the fibrinogen degradation products that are produced in tertiary haemostasis
Fragments X, Y, D, E
List the fibrin degradation products that are produced in tertiary haemostasis
X-oligomers, D-dimer
List the tests used to asses fibrinolysis
- Fibrin(ogen) degradation products (FDPs)
- D-dimer (not used alone as diagnostic tool)
Outline the process of tertiary haemostasis
- Formation of plasmin from plasminogen
- Carries out fibrinolysis to break down fibrin clot
How can tertiary haemostasis be assessed?
- Test for fibrinolysis
- Test for inhibitors (antithrombin)
Outline the testing of inhibitors in the investigation of tertiary haemostasis
- Antithrombin III in particular is measured
- % of species specific plasma pool
What antithrombin value would be expected in a healthy animal?
> 80%
What antithrombin value indicates that an animal is at risk of thrombosis?
<50%
What is the consequence of decreased level of antithrombin III and very marked increased D-dimer (FDPs)
Thrombosis - clot forms due to reduced resistance to clot formation
Give examples of conditions that may lead to thrombosis
- Feline thromboembolic disease
- Protein losing nephropathy
- Hyperadrenocorticism
- Hypertrophic cardiomyopathy in cats
Explain how hyperadrenocorticism may lead to thrombosis
- Increase in factors V, VIII, IX, X, fibrinogen and plasminogen
- Decrease in antithombin from hypertension induced urinary loss
- Obesity and hypercholestreolaemia may also contribute to thrombus formation
