Drs Du Buisson, Bruinette & Kramer

Transcription

1 INVESTIGATION OF THROMBOPHILIA Thrombophilia is a tendency to thrombosis. It is important to remember that the aetiology of thrombosis is multifactorial, and usually thrombi are the result of a combination of both acquired and inherited factors interacting. In the past decade there has been a markedly improved understanding of the common inherited and acquired coagulopathies that increase a patients risk of venous thromboembolism (VTE). Physicians normally only apply the term thrombophilia when patients have thrombosis with one or more of the following features : early age; frequent recurrences; a strong family history; or an unusual site or severity. The term inherited thrombophilia acknowledges a genetic predisposition toward thrombosis. On investigation of patients with VTE, it is possible to identify an inherited predisposing factor in approximately 50% of cases. The more significant and common inherited risk factors that can be readily identified in the laboratory include activated protein C resistance (APCR), prothrombin gene G20210A variant, protein C and S deficiencies, antithrombin deficiency, and hyperhomocysteinaemia. Less common or less certainly associated factors (eg dysfibrinogenaemia, raised factor VIII levels, plasminogen activator inhibitor) are acknowledged, and it is likely that further genetic risk factors remain to be described. Acquired risk factors include anti-phospholipid antibodies, the oral contraceptive pill, hormone replacement therapy, pregnancy, prolonged periods of stasis (eg bed rest, air travel, plaster cast), acquired high levels of homocysteine (eg vitamin deficiency) or factor VIII, surgery, obesity, malignancy, myeloproliferative disorders (eg polycythaemia vera, essential thrombocytosis), nephrotic syndrome, haemolysis (eg PNH) and liver disease. Individuals with more than one inherited risk factor, or those with a genetic risk factor combined with an acquired factor are particularly prone to thrombosis. These individuals need to be recognized for appropriate clinical management. Inherited thrombophilia and risk of obstetric complications It has long been recognized that the anti-phospholipid syndome is associated with venous and arterial thromboembolic events, recurrent foetal loss and low platelet counts (see lab update August 1999). While hormonal, uterine, immune system and chromosomal abnormalities are widely accepted as causes of repeated miscarriage, recent studies implicate inherited thrombophilia in a significant number of cases. Identifiable thrombophilias are associated with approximately two thirds of deep vein thromboses (DVT) in women on oral contraception, and up to three quarters of DVTs in pregnancy and the puerperium 1. Obstetric complications such as severe pre-eclampsia, foetal loss, foetal growth retardation, and abruptio placentae are associated with inadequate placental perfusion. In one study 38% of 110 women with obstetric complications were reported to have inherited t hrombophilia 2. Page 1 of 5

2 Pathogenesis of thrombophilia Intravascular haemostasis is maintained through a fine balancing act between thrombogenic and anticoagulant forces to keep the blood thin enough to circulate easily, but thick enough to clot when necessary. Any factor that tips this balance results in a tendency to bleed, or a tendency to clot. Clot formation is held in check principally by three mechanisms: inhibition of thrombin, inactivation of factor V, and the breakdown of fibrin. Thrombophilias arise from a defect in any of these mechanisms. 1. Deficiency of natural coagulation inhibitors (protein C, protein S, antithrombin) Functional or quantitative deficiencies of the natural anticoagulant systems (antithrombin, protein C, protein S) lead to thrombophilic states. These are rare deficiencies present in less than 1% of the general population and in less than 10% of unselected patients with VTE. The most powerful natural anticoagulant is antithrombin. Antithrombin (AT) inactivates thrombin (this reaction catalysed by heparin), as well as activated factors XII, XI, IX, X and kallikrein. Thrombin binds to thrombomodulin on the endothelial surface and activates protein C (PC). Activated protein C, in the presence of its cofactor protein S (PS) and phospholipid, then inactivates factors Va and VIIIa, thus inhibiting the conversion of prothrombin to thrombin by factor X. Homozygous AT deficiency is usually incompatible with life, and homozygous PC or PS may develop purpura fulminans at or soon after birth. Among carriers (heterozygotes) of such deficiencies the risk of VTE is 5 to 8-fold higher than the general population, with an annual incidence of 1% to 2 %. Approximately half of the thrombotic events occur in association with circumstantial risk factors and the first event commonly occurs before age Activated protein C resistance (APCR) / Factor V Leiden In 1993 Dahlbach and coworkers described a resistance to the anticoagulant effect of protein C in certain patients. This is usually (>90%) due to a genetically inherited variant of factor V with a point mutation (Arg506Gln). This factor V Leiden has normal procoagulant function but is resistant to cleavage and inactivation by activated protein C. Activated protein C resistance is the most common inherited thrombophilia in the European population (prevalence 5%), but is thought to be rare in the Black population. The risk of VTE is 2-7 fold higher among heterozygotes and fold higher for homozygotes3. Pregnancy and oral contraceptives increase the risk, as do minor events (prolonged travel, minor illness or surgery). Page 2 of 5

3 3. Prothrombin G20210A A relatively recently described mutation of the prothrombin gene occurs in approximately 3% of healthy European individuals and 7% of patients with VTE. It appears to be rare in individuals of Asian and African descent. The mutation leads to an increase in prothrombin levels and a significant increase in thromboembolic risk (3-11 fold in heterozygotes). 4. Homocysteinaemia Homocysteine is a sulfydryl amino acid derived from metabolic conversion of methionine. It may be raised in the blood due to genetic defects in the enzymes involved with methionine metabolism (eg MTHFR, CBS), or as a result of deficiencies of vitamins B12, B6 or folate. Raised homocysteinemia is a risk factor for arterial and venous occlusion. The proposed mechanisms of pathology of raised homocysteine levels mainly involve damage to the endothelial cell resulting in platelet or coagulation pathway activation. The combination of mild homocysteinaemia with factor V Leiden or prothrombin G20210A has been reported to produce a 20 to 50-fold increase in the risk of VTE4. There is still considerable debate regarding the best way to screen for homocysteinaemia and the therapeutic benefit of lowering levels. At present we screen a fasting blood sample, but methionine loading tests and 24 hour urinary homocysteine levels are available if required. It is heartening to know that raised blood levels of homocysteine usually respond to simple and safe vitamin supplementation. 5. Antiphospholipid antibodies These antibodies directed against phospholipids are identifiable acquired causes of thrombosis. Their pathogenesis is unclear but may involve epithelial damage, platelet activation or inhibition of natural anticoagulants. They may occur in patients secondary to autoimmune disorders, infections, malignancies or drugs (especially phenothiazines); or without underlying pathology (primary antiphospholipid syndrome). Two types are identified in the laboratory (requires multiple screening tests) lupus anticoagulants (LA associated with a prolonged PTT) and anticardiolipin antibodies (ACA). Page 3 of 5

4 Which patients should be investigated This remains a highly controversial question and stringent criteria for screening are best avoided. In general, laboratory testing should be carried out when the results of the investigations are likely to influence therapeutic decisions and further management. In the field of thrombophilia, laboratory results rarely (eg antithrombin deficiency may require higher doses of heparin) influence the management of acute events, as the management of thrombosis is usually not dependent on its cause. However the results of tests may influence the decision on the prevention of re-thrombosis (secondary prophylaxis), and help the physician decide on how long and how intensively to treat patients. Patients often want to know the reason for their illness and the future risks. Testing may also be beneficial for family members who may be offered primary prophylaxis when they are exposed to risk situations. Because the inherited thrombophilias are autosomal dominant traits, screening of first degree relatives will give a diagnostic yield of 50%. Screening also identifies those individuals with combined defects that may be at very high risk. Sometimes acquired risks (eg moderate hyperhomocysteinaemia) can then be addressed. For effectiveness to be maximized, laboratory screening should be comprehensive and include all the above mentioned markers. Consider screening the following patients for thrombophilia :- Venous thrombosis before 45 years of age Spontaneous thrombosis and thrombosis after trivial provocation (even up to 70 yrs) Recurrent venous thromboses Venous thrombosis at an unusual site (eg mesenteric vein) Recurrent foetal loss Strong family history Coumarin induced skin necrosis or neonatal purpura fulminans Mass screening is not recommended prior to oral contraceptive (OC) or hormone replacement therapy. If there is a personal history of thrombosis OC therapy should be avoided, but screening should be performed when there is a family history of VTE. When is it appropriate to test? Acute thrombotic events, with or without therapy, may influence laboratory investigations (except DNA tests) and make interpretation of results difficult. If possible, tests on plasma should be performed at least 6 months after the acute event. In addition, oral anticoagulants affect the measurement of PC, PS, APCR and LA, while heparin affects the measurement of AT and LA. It is preferable to defer laboratory testing until 2 weeks after the discontinuation of oral anticoagulants. Page 4 of 5

5 Occasionally it will be deemed too dangerous to take a patient off anticoagulation to perform thrombophilia testing. In such a case the problem should be discussed with a haematologist. The patient may need to be switched to heparin treatment for a period (to measure PC, PS, APCR), or a provisional diagnosis could be attempted by comparing natural anticoagulant levels to those of factor VII (eg PC:VIIc ratio) 5. Conclusion Venous thromboembolism occurs in approximately 1 per thousand individuals per year in Western countries 3. It is associated with mortality (mainly pulmonary emboli), and considerable morbidity. The aetiology of venous thromboembolism is usually multi-factorial, commonly the result of a combination of inherited and acquired risk factors. Recently a great deal has been learned about these risk factors. In carefully selected patients, thrombophilia screening in the laboratory helps patients understand their disease, and enables doctors to identify high risk patients and optimally manage their primary and secondary prophylaxis. References 1. Katz Vern L. Detecting thrombophilia in ob/gyn patients. Contempory OB/GYN Archive 2002;10: Kupferminc MJ, Eldor A, Steinman N, Many A, Bar-am A, Jaffa A, et al. Increased frequency of genetic thrombophilia in women with complications of pregnancy. N Engl J Med 1999; 340: De Stefano V, Rossi E, et al. Screening for inherited thrombophilia: indications and therapeutic implications. Haematologica 2002;87: De Stefano V, Zappacosta B, at al. Prevalence of mild homocysteinaemia and association with thrombophilic genotypes (factor V Leiden and prothrombin G20210A) in Italian patients with venous thromboembolic disease. Br J Haematol 1999; 106: Jones DW, Mackie IJ, Winter M, et al. Detection of protein C deficiency during oral anticoagulant therapy use of the protein C : factor VII ratio. Blood Coagulation and Fibrinolysis 2: Drs Mike King, Carol Moore and Tania Ihlenfeldt. Page 5 of 5