Article #2 CE Diagnosing Bleeding Disorders Jeffery W. Smith, DVM Thomas K. Day, DVM, MS, DACVA, DACVECC Andrew Mackin, BVMS, MVS, DVSc, DACVIM ABSTRACT: The hemostatic system is a very complex, coordinated, and balanced interaction among endothelial cells, platelets, circulating clotting factors, fibrinolytic agents, and inhibitors of hemostasis. The purposes of the hemostatic system are to maintain vascular integrity to prevent excessive blood loss during health and injury as well as maintain adequate blood flow through the vessels to provide oxygen to tissue. Mild to fatal hemorrhage can result from defects in the hemostatic system.This article discusses normal hemostasis (i.e., primary hemostasis, secondary hemostasis, fibrinolysis, amplification and inhibitory steps), testing to evaluate the hemostatic system, and interpretation of results, with the goal of helping practi- tioners feel more comfortable evaluating patients with suspected bleeding disorders. rom the patient with epistaxis due to low Negative charges on endothelial cell surface platelets to the patient with hemothorax membranes repel platelets, and endothelial cells Fdue to anticoagulant rodenticide poison- secrete substances that dilate vessels and inhibit ing, disorders of the hemostatic system can platelet function, including prostacyclin (PGI2), manifest in numerous ways (Figure 1). Clini- adenosine diphosphatase, and nitric oxide.1–4 cians need to be able to recognize signs of a Endothelial cells separate circulating blood from bleeding disorder during the physical examina- thrombogenic subendothelial components such tion and to adequately evaluate the hemostatic as von Willebrand factor (vWf ), collagen, tissue system to make a rapid and accurate diagnosis. factor (TF), and fibroblasts. vWf is also secreted Hemostatic disorders can be classified as pri- into circulation and must undergo a conforma- mary or secondary. Primary hemostatic disor- tional change before it can participate in platelet ders involve a qualitative and/or quantitative adhesion. Once the vascular endothelium is defect in platelets or vessels, whereas secondary damaged, however, the antithrombotic capabili- disorders involve qualitative and/or quantitative ties of endothelial cells decrease and subendothe- defects in clotting factors. Primary and second- lial components are exposed, initiating a complex ary hemostatic disorders can occur simultane- and well-regulated hemostatic process.1,4 ously. To understand what can go wrong with hemostasis, clinicians must understand how the Primary Hemostasis normal hemostatic system works. Primary hemostasis is the initial response to endothelial damage associated with either nor- Send comments/questions via email NORMAL mal endothelial turnover or tissue damage and [email protected] HEMOSTASIS results in the formation of a platelet plug via or fax 800-556-3288. Endothelial Damage interactions between vascular endothelium and Visit CompendiumVet.com for Healthy intact endothelial platelets (Figure 2). When vascular endothe- full-text articles, CE testing, and CE cells lining the vascular system lium is damaged, local vasoconstriction is initi- test answers. are naturally antithrombotic. ated and maintained by substances secreted COMPENDIUM 828 November 2005 Diagnosing Bleeding Disorders CE 829 Figure 1. Hemostatic defects. Hyphema associated with defective primary hemostasis Cervical hematoma and bruising associated with jugular (i.e., severe immune-mediated thrombocytopenia) in a venipuncture in a feist terrier with defective secondary shih tzu. Hyphema can also be caused by secondary hemostatic hemostasis (i.e., anticoagulant rodenticide toxicosis). defects. Postvenipuncture bruising can also be seen in patients with primary hemostatic defects, although hematoma formation is uncommon. from nearby activated platelets.2,5 Vasoconstriction Secondary Hemostasis decreases blood flow through the damaged endothe- Secondary hemostasis is the process of formation of a lium. Endothelial damage also exposes subendothelial stable fibrin clot over the already-formed platelet plug procoagulant components to the circulation. Platelets (Figure 3). Secondary hemostasis involves the sequen- then adhere to subendothelial collagen via specific tial activation of multiple coagulation factors—a membrane receptors on the platelet—a process that ini- process that ultimately results in the formation of tiates platelet activation. Platelets also bind to vWf—a thrombin at the site of vessel damage, the central event process that promotes further platelet adherence and of secondary hemostasis.5–7 The traditional concept of activation at the site of vessel injury. Activated platelets the secondary hemostatic system has been of two path- The hemostatic system involves complex interactions among primary hemostasis, secondary hemostasis, fibrinolysis, and amplification and inhibitory steps. change shape to increase surface area as well as promote ways, the intrinsic and extrinsic, both activating a com- adherence and aggregation of other platelets via the mon pathway and leading to the formation of thrombin release of the contents of platelet-dense and alpha gran- and, ultimately, cross-linked fibrin (Figure 3). The ulti- ules.6–8 Fibrinogen-mediated platelet-to-platelet adhe- mate formation of thrombin and cross-linked fibrin is sion (aggregation) follows exposure of fibrinogen still the main endpoint of coagulation, but the distinc- receptors on the surface of activated platelets. The tion of separate extrinsic and intrinsic pathways leading resultant platelet plug is composed of platelets adhered to that endpoint is changing. The TF (extrinsic) path- to the exposed vascular subendothelium and aggregated way is now thought to be the main initiator of coagula- to each other.1 The platelet plug provides only a tempo- tion, with intrinsic factors serving to sustain the rary seal for damaged vessels and is not sufficient to sus- process.1,9 The division into intrinsic and extrinsic path- tain long-term hemostasis. ways does, however, aid in interpreting coagulation November 2005 COMPENDIUM 830 CE Diagnosing Bleeding Disorders Figure 2. Outline of the events in primary hemostasis. (Reproduced with essary for proper hepatocyte formation of permission from Day M, Mackin A, Littlewood J [eds]: Manual of Canine and Feline functional factors FII (prothrombin), Haematology and Transfusion Medicine. Gloucester, British Small Animal Veterinary FVII, FIX, and FX.6 Association, 2000.) Secondary hemostasis is initiated by Fibrinogen Platelet (inactive) vWf Fibrinogen Platelet (inactive) vWf vascular endothelial damage—the same event that initiates primary hemostasis. Fibrinogen Platelet (inactive) vWf Fibrinogen Platelet (inactive) vWf Endothelial Subendothelial TF is exposed to circulat- Endcellothelial ing blood and combines with a small cell Endothelial amount of circulating factor FVIIa, form- cellEndothelial ing the TF-VIIa complex.1,6,10 The TF- vWf Basement membranecell vWf Subendothelial collagen fibrils FVIIa complex is the driving force for Subendothelial collagen fibrils Basement membrane further activation of coagulation factors vWf Subendothelial collagen fibrils Basement membrane IntactvWf endothelium and components of primary hemostasis. Basement membrane and is the classically taught “extrinsic Subendothelial collagen fibrils pathway.” The TF-FVIIa complex directly activates factor X. Activated FXa and FVa A B A B (FV is activated to FVa by thrombin) combine with ionized calcium on the sur- A B A B face of activated platelets (prothrombinase complex) to initiate the conversion of pro- thrombin to thrombin. The platelet sur- face provides the necessary phospholipids Initial response of platelets to vascular damage and exposure to subendothelial for coagulation to proceed.10 Thrombin collagen fibers and vWf. Activated platelets change shape, release the contents of then converts fibrinogen to soluble fibrin their granules (A), and form fibrinogen-mediated platelet-to-platelet bridges (B). monomers, which are cross-linked into an insoluble mesh via the action of factor FXIIIa—a process that is also activated by thrombin. The process of the prothrombi- nase complex converting prothrombin to thrombin, with ultimate formation of cross-linked fibrin, is known as the com- mon pathway.1,7 Recruitment of platelets to the growing platelet plug in response to agonists released The components of the classic intrinsic from the platelet granules and generation of thrombin in secondary hemostasis. pathway are contact factors XIIa, prekallikrein, bradykinin, and high molec- ular weight kininogen. The contact factors are not a relevant source of thrombin gen- eration in vivo but do serve to activate FXI and cause coagulation in vitro; therefore, they are part of laboratory coagulation testing. In live animals, instead of being activated by these contact factors, FXI is Stabilization of the platelet plug by the fibrin mesh formed in secondary hemostasis. activated by thrombin generated by the TF-VIIa complex. The classic extrinsic pathway (via the TF-FVIIa complex) is tests. Clotting factors, the key components of second- therefore the main initiator of coagulation, and the clas- ary hemostasis, are produced primarily by hepatocytes sic intrinsic pathway serves as a sustainer of coagulation. and are released into the circulation by the liver in inac- Factor FXIa activates factor FIX, which combines with tive forms (i.e., FV, FX) that must be activated (i.e., ionized calcium