haemostatic disorders

• The haemostatic system is a complex balance of processes that involves clot formation and clot lysis. It is triggered when blood vessels are injured and involves platelets, fibroblasts, coagulation factors (and inhibitors) and extracellular matrix proteins.  Haemostasis always needs a “surface” on which to occur, for example on platelets or on the fibrin in a clot. It is not a process that happens freely in liquid blood. The cells and matrix involved are required for enzymatic processes that allow the clot to form. 
• haemostasis can be divided into 3 processes, all of which are occurring, to some degree, simultaneously:

  • Primary hemostasis: Formation of the platelet plug.
  • Secondary haemostasis: Formation of the fibrin clot via coagulation factors. Stabilisation of the platelet plug is achieved by the deposition of fibrin, the end product of the coagulation cascade.  
  • Clot lysis – breakdown of the clot to restore blood flow.
• When a vessel is injured, platelets adhere to collagen. Von Willebrand factor (vWF) is also important in platelet adhesion. Platelets change shape following adhesion with secretion of substances from granules which potentiate platelet aggregation and contraction of the platelet plug
• If primary haemostasis goes wrong - look at platelets and von Willebrand
• If secondary haemostasis goes wrong - look at coagulation factors and toxins
• If the balance of the haemostatic system is lost, haemorrhage or thrombosis may be seen. Most animals with haemostatic disorders present with clinical signs of excessive haemorrhage which may be seen externally eg epistaxis, or internally eg cavity bleeding. Thrombosis is much more difficult to detect clinically or by laboratory tests, especially if it is occurring on a microscopic level, but if the inhibitory processes do not occur then clots may form elsewhere and disseminated intravascular coagulation (DIC) could occur
• Screening tests are...

  • Hemogram, including platelet count
  • Biochemistry: To assess for underlying diseases
  • Screening coagulation assays: The most commonly used screening assays are the prothrombin time (PT) and activated partial thromboplastin time (APTT), with the activated coagulation time (ACT) being available as a less sensitive and more subjective point-of-care test
  • Buccal mucosal bleeding time (BMBT), vonWillebran Factor testing may be indicated.
• Standard coagualtion panel: PT and APTT Some laboratories also include fibrinogen measurement and Total clotting time (TCT) in the standard panel
• Disseminated intravascular coagulation (DIC) panel: Usually includes PT, APTT, fibrinogen, FDP/D-dimer and antithrombin (AT)
• primary haemostasis disorders present with...

  • petechiae / ecchymoses
  • bleeding from mucosal membranes e.g., gingiva or melena
  • there is often more than one site of bleeding
  • prolonged bleeding at injury sites
• secondary haemostasis disorders present with...

  • deep or cavity bleeds (also possible to bleed from mucosal membranes)
  • hematomas
  • there is often a singular site of bleeding
  • petechiae and eccymoses is rare
• Hyphema is the collection of blood in the anterior chamber of the eye.

  • The most common cause of hyphema is blunt trauma, though spontaneous hyphemas can occur in the setting of sickle cell disease or other increased bleeding states.
  • Hyphemas are graded based on the degree of blood obscuring the cornea.
• Primary haemostasis = formation of a platelet plug
• The formation of a platelet plug is the first thing that the body will attempt to do if there is vascular injury. This process requires: 

  • Cells: Platelets, endothelial cells (source of von Willebrand factor and inhibitors)
  • Proteins: vWf and others help platelets stick together and to the subendothelial matrix on a damaged vessel. 
  • Facilitators: Platelet agonists such as thrombin, which is produced by secondary haemostasis
  • Collagen is also a platelet activator.
  • Physiologic inhibitors: Nitric oxide, prostacyclin (prostaglandin E12) produced by endothelial cells.
• Normally, the intact endothelium is a physical barrier separating circulating platelets from thrombogenic substances (such as extracellular matrix proteins) in the extravascular space. When the endothelium is injured, the procoagulant subendothelial matrix (consisting of proteins such as collagen, laminin, and fibronectin) is exposed and immediately initiates primary hemostasis.
• primary hemostasis consists of these events:

  • Platelet capture: (occurs in vessels with high blood flow (shear rates) and is mediated by long strands of vWf, which capture and slow down circulating platelets)
  • Platelet adhesion: Platelets firmly adhere to the exposed subendothelial matrix
  • Platelet activation: Once platelets adhere, they then become activated. They recruit and activate additional platelets to the injured site.
  • Platelet plug formation: Fibrinogen forms bridges between activated platelets to form the platelet plug.
• thrombin generated by the coagulation cascade is an extremely powerful platelet activator.
• Primary haemostatic disorders are caused by failure of platelet plug formation due to quantitative or qualitative platelet disorders, or due to von Willebrand factor deficiency.
• Diascopy is used to determine whether erythema in a lesion is due to blood within superficial vessels (inflammatory or vascular lesions) or is due to haemorrhage (petechiae or purpura). A microscope slide is pressed against a lesion (diascopy) to see whether it blanches. A positive diascopy result occurs when the applied pressure results in blanching of the skin, as seen in cases of erythema secondary to simple vascular vasodilation. A negative diascopy result occurs when the applied pressure does not result in skin blanching.
• what are the three causes of a primary hemostasis disorder?

  low platelet numbers (thrombocytopenia), platelet dysfunction (thrombocytopathia) or a vWF deficiency
• what are the four tests of primary hemostasis?

  platelet count, buccal mucosal bleeding time, vWF antigen and platelet function assays
• An increased time to stop bleeding after a Buccal mucosal bleeding time test indicates defective primary haemostasis (vWF, platelets, vascular defects)

  • Dog: 1.7—3.3 minutes (up to 5.5 if anaesthetised)
  • Anaesthetised cat: less than 3.5 minutes
• von Willebrand factor antiqen (VWF:Aq) test measures the amount or concentration ofVWF in a blood sample. The Comparative Coagulation Laboratory reports each dog's result as %VWFAg compared to a 100% standard
• Thromboelastography (TEM, Platelet function assay) investigates the interaction of coagulation factors, their inhibitors, anticoagulant drugs, blood cells, specifically platelets, during clotting and subsequent fibrinolysis
• Causes of thrombocytopaenia include:

  • Defective platelet production
  • bone marrow neoplasia e.g leukaemia
  • Drug/chemical/toxin-induced bone marrow suppression
  • bone marrow infections (especially viral and rickettsial)
  • Accelerated platelet removal
  • Immune-mediated thrombocytopaenia (IMTP) (most common)
  • Consumption in microangiopathic conditions (DIC)
  • Platelet sequestration or loss
  • Splenomegaly / vascular pooling
  • Acute ongoing haemorrhage
• IMTP is the most common acquired haemostatic defect in the dog. In IMTP, platelets are destroyed in the circulation and tissues faster than they can be made in the bone marrow. IMTP can be categorised as:

  • Primary - idiopathic
  • Secondary -. drug-induced, or secondary to infection or neoplasia related

  Young to middle-aged, female dogs are over-represented, especially Cocker spaniels, miniature / toy poodles and old English sheepdogs which are all specifically predisposed.
• Thrombocytopathia refers to any kind of abnormal platelet function. It can be congenital or acquired and will result in similar signs to thrombocytopaenia, except platelet count will be normal.

  • Inherited thrombopathias
  • Drug-induced defects of platelet function
  • Various drugs, particularly NSAlDs
  • Platelet dysplasia
  • Myeloproliferative disease and other forms of neoplasia
• thrombocytopathia is often a diagnosis of exclusion

  • Normal PLT count but prolonged BMBT
  • Normal levels of vWF
  • PLT function tests: aggregometry, adhesion assays, flow
  • cytometry (expression of surface molecules)
• There is no specific therapy for thrombocytopathia but you should withdraw any drugs e.g. NSAlDs and treat symptomatically e.g. blood transfusion if marked anaemia
• Desmopressin  acts by causing release of vWF from endothelial cells
• what is the treatment for type 1 vWD?

  Desmopressin
• vWD is autosomal and there are five mutations responsible. Genetic testing is available and all genotypes are recessive: clear, carrier or affected status. This is also not predictive of clinical bleeding (type 1).
• Clinical Signs of vWD are typical of a primary haemostatic defect: mucosal haemorrhage; cutaneous bruising; prolonged bleeding from surgical and traumatic wounds. Occasionally more profound bleeding occurs and you may see epistaxis, haematuria, gastrointestinal haemorrhage, prolonged oestral bleeding and gingival bleeding at tooth eruption reported.
• in vWD the platelet count will be normal. Buccal mucosal bleeding time is a useful screening test for vWD

  • normal: 2 – 4 min
  • mild to moderate (type 1) 5–10 min
  • severe forms of vWD > 12 min

  The diagnosis is confirmed by the demonstration of low vWF antigen concentrations. BUT the measurement does not always accurately predict the risk of haemorrhage.
• Secondary haemostasis = formation of a fibrin clot
• Vitamin K is an essential cofactor in the functioning of factors II, VII, IX and X,

  • fibrin formation is defective when vitamin K is deficient. 

  Vitamin K is a fat-soluble enzyme so deficiency can also occur in severe hepatic/cholestatic disorders. Clinical signs mimic that seen with coagulation factor deficiencies, including spontaneous haemorrhage into body cavities, subcutaneous haematomas and prolonged bleeding from wounds
• Warfarin and warfarin-based anticoagulant rodenticides (rat bait) cause a relative vitamin K deficiency because they inhibit vitamin K epoxide reductase, which is necessary for recycling of vitamin K to its active form in the vitamin K-epoxide cycle.
• Traditionally, coagulation has been divided into three distinct pathways: extrinsic, intrinsic and common.
• Secondary haemostasis is the formation of a fibrin clot under the influence of clotting factors, specifically thrombin. This process is required because the platelet plug needs to be stabilised.  Secondary haemostasis constituents:

  • Cells: Fibroblasts, platelets, endothelial cells, leukocytes
  • Enzymatic Coagulation factors: Factors XI, X, IX, VII, and II. 
  • Non-enzymatic coagulation factors – cofactors: Tissue factor (TF), Factors V and VIII.
  • Fibrinogen 
  • Calcium
• Blue pellets = rat poison or slug pellets

  • Slug pellets cause neurological signs quickly
  • Rat bait has a delay period (5 - 7 days) before bleeding occurs