Update. Diagnostic. 1. Physiology. 2. Bleeding disorders

Transcription

1 Diagnostic Update May 08 Notes on Disorders of Haemostasis Part 1 of 2 Part 2 will cover hypercoagulation and thrombosis and management of the common bleeding disorders. 1. Physiology Injury occurs to the vascular system on a daily basis; however, blood loss is minimised by activation of complex processes: Reflex vasoconstriction Formation of a platelet aggregate (Primary haemostasis) The Coagulation Cascade (Secondary haemostasis) These mechanisms are activated if blood comes into contact with sub-endothelial structures (collagen fibres, basal membrane) or tissue (release of tissue thromboplastin, phospholipids). Once the development of the fibrin clot is completed, there is retraction of the fibrin threads and further stabilisation of the platelet thrombus. inactive platelet fibrinogen activated platelet intima sub-endothelium von Willebrand factor collagen 1.1 Primary haemostasis (see Figure 1) In the event of a vascular injury, reflex vasoconstriction occurs and platelets are deposited on the exposed collagen fibres. Platelet adhesion is mediated via the von Willebrand factor (vwf), which is synthesised and secreted by endothelial cells. The platelet aggregate causes an initial, unstable closure of the lesion. 1.2 Secondary haemostasis During secondary haemostasis, fibrin accumulates at the site of the lesion and it is cross-linked and bound to the vascular wall. Platelets and red-blood cells are trapped within the mesh and a stable thrombus is produced. The formation of fibrin from fibrinogen is the result of the coagulation cascade a sequential activation and amplification scheme. The coagulation cascade is made up of the extrinsic and intrinsic systems as well as the common pathway (see Figure 2, page 2). Separating the coagulation cascade into these components is only useful in order to interpret in vitro coagulation tests. It is now apparent that the initiation and maintenance of the coagulation cascade in vivo is far more complex than suggested by this representation. Activation of the extrinsic system occurs through tissue thromboplastin, which is released in the event of tissue injury. Activation of the intrinsic system occurs through exposure of sub-endothelial collagen. primary thrombus secondary thrombus Figure 1: Primary and secondary haemostasis 1.3 Coagulation inhibitors Once coagulation has been initiated at a particular site, the process must be prevented from becoming widespread. Various mechanisms exist for inhibiting excessive coagulation. The most important substance is antithrombin III (AT-III). As the name indicates, AT-III neutralises thrombin and thrombin is crucial for the formation of a fibrin clot. Thus, AT-III acts to limit coagulation. AT-III requires heparin for activity. 1.4 Fibrinolysis As healing occurs degradation of the thrombus occurs, with fibrin being split by the fibrinolytic enzyme plasmin. Plasmin is produced from plasminogen under the effect of various activators. Finally, repaired tissue replaces the thrombus. 2. Bleeding disorders In general practice, dogs are more likely to present with haemostatic disorders than cats. Bleeding disorders may be hereditary or acquired, and can be classified into the following categories:

2 2.1 Disorders of primary haemostasis Thrombocytopenia (quantitative platelet disorders) Thrombopathia (qualitative platelet disorders) Von Willebrand s disease Vascular disease (rare) e.g. Ehlers-Danlos syndrome Thrombocytopenia is the most common disorder of haemostasis in dogs and cats and there are many causes for this (see Table 1). Platelet dysfunctions (thrombopathias) may be acquired or hereditary and the clinical significance is highly variable. As platelet function tests are difficult to perform, they are probably under-diagnosed. Many drugs can impair platelet function and well-known examples include most NSAIDs and acepromazine. Von Willebrand s disease is the most common hereditary haemostatic defect in domestic animals and occurs in three forms. Clinical severity varies from sub-clinical to a markedly increased tendency to bleed. In some breeds, the incidence of von Willebrand s disease is relatively high. For example, nearly three quarters of Doberman pinschers are either carriers or affected. The same figure in German Shepherd dogs is close to one third. Table 1: Causes of thrombocytopenia 1. Diminished production (often in combination with leucopenia and anaemia) Tumours in the bone marrow Bone marrow hypoplasia, bone marrow aplasia (idiopathic) Myelofibrosis Toxins, drugs (oestrogens, sulphonamides, chloramphenicol, cytostatic agents) FeLV, FIV Radiation 2. Increased consumption Profound, acute haemorrhage Disseminated intravascular coagulation (DIC) Microangiopathy (haemangiosarcoma) Hypothermia Some infections 3. Increased destruction (lysis) Immune-mediated (primary or secondary) Drugs (e.g. phenylbutazone) Some infections (e.g. FeLV, FIV) To reach a diagnosis, the von Willebrand factor concentration in the patient s blood should be determined. With some breeds, the location of the genetic defect is known and can be diagnosed by means of PCR analysis. 2.2 Disorders of secondary haemostasis Disorders of secondary haemostasis may also be hereditary or acquired. Relatively common hereditary disorders include haemophilia A (factor VIII ) and haemo- intrinsic system extrinsic system XII XI IX VII appt VIII X PT V II (prothrombin) I (fibrinogen) clot common pathway Figure 2: The Coagulation Cascade

3 philia B (factor IX ). Acquired secondary haemostatic disorders can result from liver disease, vitamin K and some neoplasias. The liver has various coagulation functions including production of both coagulation factors and inhibitors. It also influences the catabolism of activated factor inhibitors and is the site of vitamin K-related activation of factors II, VII, IX and X. Although many cats and dogs with severe hepatic dysfunction have abnormal coagulation tests, spontaneous bleeding rarely occurs. However, these patients are prone to excessive bleeding after surgery and liver biopsies, and with gastrointestinal ulceration. Vitamin K is a fat-soluble vitamin, stored in the liver, which is absorbed from food in the small intestine and synthesised in the ileum and colon by bacteria. The main cause of vitamin K in dogs and cats is coumarin intoxication, but it can also be caused by severe inflammatory intestinal changes, exocrine pancreatic insufficiency (EPI), a complete bile duct obstruction or chronic antibiotic therapy. Neoplasia can cause bleeding disorders as a result of various mechanisms. With dogs, lymphosarcoma, leukaemia and haemangiosarcoma are associated with coagulopathies. 2.3 Combined disorders of primary and secondary haemostasis In the case of disseminated intravascular coagulation (DIC), components of both primary and secondary haemostasis are impaired. DIC is always a secondary disease and represents the terminal stage in many disease processes. It is caused by excessive activation of coagulation. The primary disease causes the production of numerous thromboses, either regionally or throughout the patient. This consumes coagulation factors and platelets, leading to bleeding. Other consequences of DIC include hypoxia, organ failure and death. Initially, the patient may be found to have reduced clotting times; however, later these become prolonged. An increased serum fibrin degradation products (FDP) provides indirect evidence of increased intravascular coagulation. The concentration of FDPs is increased as a reflection of breakdown of the thromboses formed due to the excessive tendency to clot. 3. Diagnosis of disorders of haemostasis 3.1 Indications for investigating the haemostatic system Occurrence of spontaneous bleeding at several sites in the body (petechiae, ecchymoses, haematomas, haemothorax, haemoperitoneum, haemarthrosis) Traumatically induced bleeding which is heavier than would be expected from the severity of the trauma Intake of rodenticides or other toxins Diseases associated with disorders of haemostasis (e.g. hepatopathy) Suspected DIC e.g. in connection with a neoplasia, immune-mediated haemolytic anaemia, infections, shock, gastric torsion, pancreatitis, trauma Preoperative screening for at risk patients Monitoring patients receiving anticoagulant and fibrinolytic therapies 3.2 Signals, history and clinical symptoms A thorough history can give indications as to the aetiology: Animal s age (hereditary disorders tend usually become apparent young animals) Breed (see Table 2) Family history of bleeding problems (supporting a hereditary disorder) Coagulation problems already known in the past (second ary dentition, operations) Vaccinations in recent weeks (possibly triggering an immune-induced thrombocytopenia), administration of certain drugs Stay abroad/tick infestation (suspicion of ehrlichiosis, anaplasmosis, babesiosis, leishmaniosis etc.) A past medical history including hepatic disease and/or chronic pancreatitis Possible intake of rodenticide To clarify the cause of a haemostatic disorder, it may be helpful to consider the nature of the bleeding. For example, with a disorder of primary haemostasis, petechiae, ecchymoses and purpura occur relatively commonly, with haematomas being rare. Typically, the haemorrhage is superficial (gums, GI tract, sclera and retina) and bleeding occurs immediately after venepuncture. A defect in secondary haemostasis is more commonly associated with large haematomas and with bleeding in to the musculature, joints and body cavities. There is a delay in the onset of bleeding after venepuncture.

4 Epistaxis can be a symptom of disorders of both primary and secondary haemostasis. (see below), when an abnormality is first discovered the clinician is advised to repeat the testing. Coagulation factor Factor I Factor II Factor IIV Factor VIII Factor IX Factor X Hereditary diseases Hypo/dys fibrinogenaemia Prothrombin Factor VII Haemophilia A Haemophilia B Factor X Known breed associations (not exhaustive) St. Bernard, Borzoi Boxer Beagle, Malamute Numerous dog breeds, mongrels, cats Numerous dog breeds, British Shorthair cats Cocker Spaniel PT and aptt PT ã, PT ã, PT ã PT ã, 3.4 Evaluation of haemostasis (see Table 3) Disorders of primary haemostasis Platelet enumeration/estimation A platelet count and examination of a blood smear should be performed in all patients with a suspected bleeding disorder. Particular care should be taken to examine the blood smear for large platelets (macroplatelets usually indicative of a regenerative response), platelet clumping (this may result in artefactually low platelet counts) and schistocytes (suggestive of DIC). Unless concomitant bleeding disorders exist, spontaneous bleeding is unlikely unless the platelet count is less than 40 x10 9 /l. Factor XI Factor XII (Hageman factor) Factor XI Factor XII Generally, this is considered to be clinically insignificant. Table 2: Hereditary coagulopathies Springer Spaniel, Great Pyrenees Numerous dog breeds, cats Where an automated count is not available, or verification is required, approximately platelets per high-power field (hpf) is considered normal. Spontaneous bleeding is likely when the count is fewer than 3 to 4 per hpf. 3.3 Blood sampling Before sampling blood from a patient with a suspected bleeding disorder it is prudent to consider how easy it will be to apply sufficient pressure to ensure the bleeding stops for this reason many clinicians choose to avoid using the jugular vein. In certain breeds, such as Greyhounds and Cavalier King Charles Spaniels, it is expected that the patient will have a relatively low concentration of platelets, but an apparently increased mean platelet volume. If this is the case, assessing the plateletcrit (PCt) may be more clinically useful than the platelet concentration. Correct specimen preparation is essential to ensure accurate results. Blood sampling should be as close to atraumatic as possible and the vein should not be occluded ( raised ) for too long. This minimises release of tissue thromboplastin (which can initiate coagulation) and platelet activation. Ideally, the first drops of blood obtained should be discarded (or used for other purposes). Buccal Mucousal Bleeding Time (BMBT) A screening test for disorders of primary haemostasis. The patient is placed into lateral recumbency; cats and some dogs will require sedation and the drugs used for this are likely to affect the bleeding time. Certain NSAIDs may also increase the BMBT. Take care to ensure: rapid and clean venepuncture the sodium citrate tube is filled exactly to the mark after replacing the cap, without delay, the sample tube is gently inverted several times to ensure thorough mixing that the sample has not clotted prior to testing or submission to the reference lab Despite the best efforts of the person responsible for sampling, a significant proportion of samples will be compromised. Irrespective of the methodology utilised for the screening coagulation tests, such as PT, appt and ACT The top lip is folded back and gently held in place with a gauze bandage. This should result in moderate mucosal engorgement. A Simplates device (Organon Teknika; available from IDEXX Customer Services) or similar is placed on the surface of a smooth area of the exposed mucous membrane and activated. This produces one or two standardised wound(s) of 1mm in depth. The time taken until bleeding stops is timed, meanwhile the flowing blood is soaked up with filter paper every 10 seconds, taking care not to touch the cut itself. The reference range is 1 to 5 minutes. The initial haemostasis reflects platelet function and num-

5 bers. A re-bleed phenomenon may be observed if there is a defect in secondary haemostasis. An extended BMBT indicates the possibility of thrombocytopenia, a platelet dysfunction or a distinct reduction in the von Willebrand factor. If a patient with a prolonged BMBT has a normal platelet concentration and no obvious cause for failure of platelet function, then von Willebrand disease should be suspected. The vwf should be assayed for confirmation. Thrombopathias only rarely occur as congenital diseases. Causes of an acquired thrombopathia include DIC, paraproteinaemia (lymphatic leukosis, multiple myeloma), cholestases, portovenous shunts, uraemia and certain drugs. quickly replace. Repeat every 10 seconds. The stopwatch is stopped at the first soft clot formation. The ACT will be prolonged with deficits within the intrinsic and/or common pathways; however, it is an insensitive test when compared to the appt. Furthermore, severe thrombocytopenia (<10 x 10 9 /l) may cause mild prolongation. Fibrinogen concentration IDEXX Reference Laboratories Fibrinogen can be lowered through reduced production (e.g. severe hepatopathy) or as a result of elevated consumption (e.g. in case of a DIC). It may be increased during inflammation. Fibrin degradation products (FDP) See section Disorders of secondary haemostasis The term coagulopathy refers to excessive bleeding resulting from (or abnormal function) of one of more of the clotting factors. Secondary haemostasis Prothrombin time (PT) Coag Dx or IDEXX Reference Laboratories If all the factors of the extrinsic and common pathways are in normal concentration, the PT will be normal. If the PT is prolonged, the deficient factor(s) may be part of either the extrinsic or common pathways or both. Determination of coagulation factors Consult with the IDEXX Clinical Pathologists for advice If, as a result of secondary haemostatic screening tests, there is the suspicion of a hereditary of an individual factor (e.g. with a selectively extended aptt), determination of the activity of individual factors is advisable. The proportion by percentage of a specific factor is compared with the concentration of the factor in the case of healthy animals, which is set as 100%. The factors must be reduced to less than 30% of the normal concentration in order to extend aptt or PT. Activated partial thromboplastin time (aptt) Coag Dx or IDEXX Reference Laboratories If all the factors of the intrinsic and common pathways are in normal concentration, the appt will be normal. If the aptt is prolonged, the deficient factor(s) may be part of either the intrinsic or common pathways or both. These two screening tests (PT and aptt) thus allow the entire coagulation cascade to be evaluated. Activated clotting time (ACT) The ACT is a simple, somewhat crude, screening test for the common and intrinsic pathways that can be performed in the practice environment without the need for specialised equipment. Draw 2ml of blood into a prewarmed (37 C) commercial tube containing diatomaceous (Fuller s) earth. A stopwatch should be started as blood enters the tube. Invert the tube five times and place in a 37 C water-bath. After one minute remove the tube, examine for the presence of a clot and vwf is a multimeric glycoprotein which is formed in endothelial cells and megakaryocytes and is circulated in the blood as a complex with factor VIII. vwf-antigen determination can be used in order to make a diagnosis. If the patient s vwf concentration is > 70 %, this is consider normal. An amount between % is in the grey area. If the amount is < 50 %, it is described as reduced and if it is < 30 % there is a bleeding tendency. With some breeds, the location of the genetic defect is known so that PCR can be used to prove whether the animals are carriers of the disease. For certain breeds, this may be important information prior to breeding. IDEXX Coag Dx Coagulation-Analyser

6 Diagnostic Update Table 3: Interpreting the screening test (N = normal) BMBT Platelet count PT aptt Fibrinogen Thrombocytopenia ã å N N N Thrombopathia ã N N N / ã N vwf ã N N N / ã N Defect in intrinsic system (e.g. haemophilia A or B) N N N ã N Defect in extrinsic system (e.g. factor VII ) N N ã N N Vitamin K (e.g. coumarin intoxication) N N ã* ã* N Hepatopathy N / (ã) N / (å) N / ã ã N / å DIC ã å ã ã N / å Assay Sample Requirements Turnaround Want to know more? Coagulation Profile Includes platelet count, morphological assessment, PT, aptt, fibrinogen. Test Simplates for buccal mucosal bleeding time available. 1 ml EDTA & air dried smear, 2 ml citrate plasma. After mixing the blood in the citrate tube, please separate citrated plasma into the plain tube supplied Same day Factor VIII* 1 ml citrate blood 7-10 days FDP 1 ml citrate blood 1-2 days Fibrinogen 1 ml EDTA Same day PT (Prothrombin Time) 1 ml citrate blood Same day appt (Activated Partial Thromboplastin Time) 1 ml citrate blood Same day PT + appt 2 ml citrate blood Same day Simplate device Contact laboratory Next day Von Willebrand s Factor* 2 ml citrate blood 7-10 days Then join us for an IDEXX Webinar (a free-of-charge, interactive, on-line seminar) Tuesday 24 June 2008, 19:00 Coagulation Sergio Serrano, Vet24 Wednesday June , 17:00 and Thursday June , 20:00 The Management of Common Bleeding Disorders Dr Urs Giger, University of Pennsylvania To register, please webinarsuk@idexx.com *Referral test please note discount do not apply to referral test IDEXX Reference Laboratory Southwater 4 Oakhurst, Business Park, Southwater, Horsham, West Sussex RH13 9RT Tel: , Fax: labhelp@idexx.com IDEXX Reference Laboratory Wetherby Grange House, Sandbeck Way Wetherby, West Yorkshire LS22 7DN Tel: , Fax: labhelp@idexx.com UK