Relationship between Venous and Arterial Thrombosis: A Review of the Literature from a Causal Perspective Downloaded by: Universitaetsbibliothek - Med. Universitaet Wien. Copyrighted material. Willem M. Lijfering, M.D., Ph.D.,1 Linda E. Flinterman, M.Sc.,1 Jan P. Vandenbroucke, M.D., Ph.D.,1 Frits R. Rosendaal, M.D., Ph.D.,1,2 and Suzanne C. Cannegieter, M.D., Ph.D.1 ABSTRACT Venous thrombosis and arterial thrombosis are traditionally regarded as two different diseases with respect to pathophysiology, epidemiology, and treatment strategies. Research findings of the past few years suggest that this categorical distinction may be too strict. However, whether the described relationship between venous and arterial thrombosis is real or a result of other factors such as confounding, chance, or bias is still unclear. In this review, we discuss the current literature while using causal diagrams to better understand possible causal relations between cardiovascular risk factors, atherosclerosis, arterial thrombosis, and venous thrombosis. Furthermore, we propose study designs to investigate the causal link between venous and arterial thrombosis. In addition, we comment on the effect of statin use on the occurrence of both arterial and venous thrombosis. The possible clinical implications of these findings are discussed. KEYWORDS: Venous thrombosis, arterial thrombosis, atherosclerosis, vascular diseases, epidemiology V enous thrombosis and arterial thrombosis are traditionally regarded as two different diseases with respect to pathophysiology, epidemiology, and treatment strategies.1 According to textbooks, arterial thrombi tend to occur at places where plaques are formed and where shear stress is high, which results in platelet-rich ‘‘white thrombi.’’ In contrast, in venous thrombotic disease, thrombi tend to occur at sites where the vein wall is undamaged, but where blood flow and shear stress are low, resulting in red cell-rich ‘‘red thrombi.’’ Therefore, antiplatelet therapy is considered the favorite choice in preventing arterial throm- bosis, while anticoagulant therapy (vitamin K antagonists, heparinoids, or direct factor X inhibitors) is the recommended therapy in venous thrombosis. If venous thrombosis and arterial thrombosis were completely separate entities, then cardiovascular drugs should be specific, that is, antiplatelet agents and statins should only affect arterial disease; classical arterial risk factors should not increase the risk of venous thrombosis; and conversely, risk factors for venous thrombosis, such as elevated levels of coagulation factor VIII, or impaired fibrinolysis should not increase the risk of arterial thrombosis. 1 M.A.I.M.S., F.F.Sc. (RCPA), Massimo Franchini, M.D., and Giuseppe Lippi, M.D. Semin Thromb Hemost 2011;37:885–896. Copyright # 2011 by Thieme Medical Publishers, Inc., 333 Seventh Avenue, New York, NY 10001, USA. Tel: +1(212) 584-4662. DOI: http://dx.doi.org/10.1055/s-0031-1297367. ISSN 0094-6176. Department of Clinical Epidemiology; 2Department of Thrombosis and Hemostasis, Leiden University Medical Center, The Netherlands. Address for correspondence and reprint requests: Willem M. Lijfering, M.D., Ph.D., Department of Clinical Epidemiology, Leiden University Medical Centre, P.O. Box 9600, 2300 RC Leiden, The Netherlands (e-mail: w.m.lijfering@lumc.nl). Coagulopathies and Thrombosis: Usual and Unusual Causes and Associations. Part V; Guest Editors, Emmanuel J. Favaloro, Ph.D., 885 SEMINARS IN THROMBOSIS AND HEMOSTASIS/VOLUME 37, NUMBER 8 2011 Table 1 Risk Estimates for Venous Thrombosis and Arterial Thrombosis Venous Thrombosis Relative Risk* Venous thrombosis Arterial thrombosis NA 1.0–4.5 Arterial Thrombosis Discussed in This Article Reference Relative Risk* Reference NA 1.2–3 NA 5,6 97 Yes 4 NA Yes Yes Atherosclerosis 0.8–2.3 8–10 1.5–2.5 Surgery, trauma, immobilization 5–50 71 Unknown to us Pregnancy and puerperium 3–5 72 3–4 98 No Oral contraceptives 4–7 73,74 2–4 99,100 Yes Overweight and obesity 2–3 28,38 1.5–2.0 101 Yes Hypertension 0.7–1.3 37,38 1.5–2.0 101 Yes Diabetes mellitus Male sex 0.7–2.0 0.8–1.3 37,39 2–3 1.5–2.0 101 Yes No Smoking 1.2–1.4 38,49 2.5–3.5 47,103 Yes Dyslipidemia 0.8–1.5 38,46 2–3 104 Yes Increasing age 1-1 35 1-1 105 Yes Malignancy 7–20 75 Unknown to us Lupus anticoagulant 3–10 76 5–50 106 No Rheumatoid arthritis 2–2.5 77 1.5–2.5 107 No Systemic lupus erythematosus Inflammatory bowel disease 3–8 3–4 78 3–8 1.0–2.0 108 No No Hyperthyroid disease 1.5–3 80 1.0–1.7 110 No HIV infection 3–10 81 2–5 81 No Nephrotic syndrome 3–10 82 3–10 82 No Chronic kidney disease 1.3–1.7 83,84 1.2–1.7 111 No Microalbuminuria 1.5–2.5 36 1.5–2.0 112 No Air travel 1.5–3 85 Unknown to us Transient infectious disease Antithrombin deficiency 1.0–3.0 15–20 86 3–5 1.0–1.5 113 No Yes 15–20 87 1.0–4.5 55 Yes Protein S type I deficiency 15–20 87 1.3–7 55 Yes Factor V Leiden 5–7 88 1.1–1.3 54 Yes Prothrombin G20210A 2–3 89 1.1–1.5 54 Yes Non-O blood group 1.5–1.8 90 1.2–1.8 114 No Low free protein S levels 5–10 91 1.5–2.5 51 No High factor VIII Hyperhomocysteinemia 3–5 1.5–2.5 30,92 1.5–3 1.5–2.5 30,51,115 No No Hypofibrinolysis 1.5–2.5 94,119 1.5–2.5 118,119 No High platelet reactivity Unknown to us 2–3 120 No High CRP 1.3–2.0 1.3–1.7 121 Yes Protein C deficiency 35,38 79 87 93 95,96 Yes 102 No 109 No 55 116,117 *As compared with the normal population. Nevertheless, recent research has shown 40 to 50% risk reductions for venous thrombosis occurrence in patients taking statin, and also a 1.5- to 3-fold increased venous thrombosis risk in individuals who were exposed to traditional arterial risk factors (Table 1).2,3 Furthermore, it appears from the literature that patients with arterial thrombosis have a 1.2- to more than 4-fold increased risk of developing subsequent venous thrombosis,4 that venous thrombosis is a predictor of subsequent arterial thrombosis risk,5–7 and that patients with atherosclerosis have a twofold increased risk of venous thrombosis,8 although the latter finding has been refuted by others.9,10 A drawback of these studies is that it is not immediately obvious how these associations can be explained from a causal perspective. A BRIEF NOTE ON CAUSALITY Causal factors are those antecedents that are the producer of an effect, result, or consequence, which can graphically be summarized as ‘‘cause ! effect.’’ In medicine, causal factors are often called risk factors, and it is believed that if a risk factor can be taken away, it will no longer produce disease.11 Causes can Downloaded by: Universitaetsbibliothek - Med. Universitaet Wien. Copyrighted material. 886 Figure 1 Directed acyclic graphs that illustrate four hypotheses of how venous and arterial thromboses may be associated. AS, atherosclerosis; AT, arterial thrombosis; RF, risk factors; VT, venous thrombosis. lead to an effect either directly or indirectly. Noncausal relationships are associations that are due to other factors, such as chance, bias, or confounding. Chance is the unknown and unpredictable element in happenings that seems to have no assignable cause, which is most often a result of small numbered studies. Bias is a misrepresentation of data in a study that arises from the way that subjects or data are collected. Confounding is a mixing or blurring of effects that arises when a confounding variable is associated with the exposure and affects the outcome, yet is not an intermediate link in the chain of causation between exposure and outcome.11 For a better understanding of possible causal relations for the associations between cardiovascular risk factors, atherosclerosis, arterial thrombosis, and venous thrombosis, we propose four different hypotheses, illustrated by directed acyclic graphs (DAGs) (Fig. 1),11 to be discussed in this review. In short, they consist of either a direct or indirect causal effect of one disease on the other, or occurrence of both diseases due to the presence of common confounding variables. ARTERIAL THROMBOSIS AND SUBSEQUENT RISK OF VENOUS THROMBOSIS Patients hospitalized with acute myocardial infarction are at high risk of subsequent venous thrombosis. In a meta-analysis published in 1996, reviewing placebocontrolled studies of antithrombotic drugs, it was shown that 4% of patients with myocardial infarction (MI) had symptomatic pulmonary embolism within 2 weeks after hospitalization.12 Patients with acute ischemic stroke are also at a high risk of venous thrombosis. In one study, the incidence of symptomatic deep vein thrombosis or pulmonary embolism was 3 and 5%, respectively, after 21 days of hospitalization.13 One direct causal explanation is the temporary inflammatory or procoagulant response due to sudden tissue damage or necrosis after the arterial thrombotic event onset. This tissue damage may consequently result in temporary or long-term increased venous thrombosis risk. However, all these patients were, obviously, subjected to hospitalization which is generally accompanied by periods of immobilization that offers an alternative (indirect causal) explanation of venous thrombosis due to stasis. Therefore, one cannot conclude from these studies that the arterial event itself has been the causative factor and thus a risk factor for venous thrombosis. A way to distinguish between hospitalization or to the event itself as an explanation is to use a study design in which a longer period between arterial and venous event is studied. This was done in a recent large population based case–control study.4 Cases (n ¼ 5824) were individuals with a venous thrombosis, and they were compared with age- and sex-matched controls (n ¼ 58,240). An arterial thrombotic event was associated with a large increase in venous thrombotic risk in the subsequent 3 months: relative risk 4.2 (95% confidence interval [CI], 2.3–7.6) after MI, and 4.4 (95% CI, 2.9–6.7) after stroke. However, this increased risk did not extend beyond that of accompanying hospitalization in patients with MI. Stroke patients, however, seemed to have a long-term 1.2-fold increased risk of venous thrombosis. As ischemic stroke may lead to longterm impaired mobility, this may explain the extended excess risk of venous thrombosis. As alternative explanations for a direct effect of arterial thrombotic disease on venous thrombosis, as postulated in Fig. 1A-1, we propose the following mechanisms: Fig. 1A-2 summarizes the explanations given above: the arterial event leads to a certain situation (immobilization, necrosis) which in its turn increases the risk of venous thrombosis. Second, there is a possibility 887 Downloaded by: Universitaetsbibliothek - Med. Universitaet Wien. Copyrighted material. RELATIONSHIP BETWEEN VENOUS AND ARTERIAL THROMBOSIS/LIJFERING ET AL SEMINARS IN THROMBOSIS AND HEMOSTASIS/VOLUME 37, NUMBER 8 that the presence of atherosclerosis is a common cause of both arterial and venous thromboses (Fig. 1B-2) (see below). Third, risk factors that are associated with both venous and arterial thromboses, such as obesity, could explain the association (Fig. 1D-1) (see below). Adjustment for immobilization, common risk factors, or atherosclerosis should help distinguish between a true direct effect and any of the other three explanations: in the first case the association will remain, while it will disappear when one of the other mechanisms is present. ATHEROSCLEROSIS AND THE RISK OF VENOUS THROMBOSIS In 2003, Prandoni et al reported that atherosclerosis (measured by carotid intima media thickness [IMT]) was twice as prevalent in patients with unprovoked venous thrombosis as in age- and sex-matched controls.8 As in this study IMT was performed after venous thrombosis occurred, its temporal relation and causality were not entirely clear.14 Currently, there is little evidence available, apart from the study by Prandoni et al,8 that venous thrombosis and atherosclerosis are causally associated. However, there is some biological evidence that may give credence to the causality of this association, as hemostatic and inflammatory markers are elevated in individuals who have atherosclerosis.15–17 Two options are theoretically possible in the association—(1) that the presence of atherosclerosis directly increases the risk of venous thrombosis (Fig. 1B-1) or (2) that venous thrombosis leads to an acceleration of the atherosclerotic process (Fig. 1C-2, 3). For the latter, it could be hypothesized that an inflammatory process, activated by venous thrombosis, leads to acceleration of growth of atherosclerotic plaques in individuals with previous venous thrombosis. Adjustment for hemostatic or inflammatory risk markers (e.g., C-reactive protein or factor VIII) should then lead to an attenuation of effect. The former explanation, (Fig. 1B-1), that the presence of atherosclerosis may (e.g., through elevation in hemostatic markers) lead to venous thrombosis, has as far as we know not been studied yet. Alternatively, the relation can be explained by the presence of risk factors for both atherosclerosis and venous thrombosis (Figs. 1B-2 and D-1). This was the case in two large prospective population-based studies where the relation disappeared after adjustment for various potential confounders.9,10 For example, in the Atherosclerosis Risk in Communities (ARIC) study (n > 13,000) a graded positive effect (dose response relation) was found between IMT thickness and venous thrombosis risk; relative risks of venous thrombosis across quartiles of baseline IMT were 1.0, 1.2, 1.6, and 1.5.10 However, this association disappeared after adjustment for various cardiovascular risk factors. A presentation of such a finding is depicted in Fig. 1D-1. 2011 VENOUS THROMBOSIS AND SUBSEQUENT RISK OF ARTERIAL THROMBOSIS Recent studies have shown that the incidence rate of arterial thrombotic events after a first venous thrombosis may be as high as 2 to 5% per year.6,7 In a retrospective age- and sex-matched case–control study from Sørensen et al, containing more than 40,000 patients with venous thrombosis and 160,000 controls, a two- to three-fold increased risk of MI or ischemic stroke after a first episode of venous thrombosis was shown, most predominantly in the first year following the initial event.5 According to a meta-analysis of published studies on this issue, detection bias (by which we mean that patients with venous thrombosis are closely monitored and therefore sooner diagnosed with ‘‘soft’’ arterial thrombotic endpoints such as angina pectoris or transient ischemic attacks) cannot explain the association. The meta-analysis included only ischemic stroke and MI as clinical endpoint, leading to a 1.9-fold (95% CI, 1.2–2.9) increased risk of arterial thrombosis after venous thrombosis.18 Several other (unbiased) explanations for the relationship between venous thrombosis and subsequent arterial thrombosis exist. Because atherosclerosis enhances inflammation,19 and coagulation,20 it can be speculated that arterial thrombosis and venous thrombosis are associated diseases through atherosclerosis (Fig. 1B-2). Studies on this issue, as far as we know, have not been performed yet. The link between venous and arterial thrombosis may also be causal (Fig. 1C-1–3). One causal explanation is that the inflammatory process instigated by the thrombus,21 leads to arterial thrombosis. This hypothesis may also clarify why the risk of arterial thrombosis is highest in the first few months after the venous event takes place, as inflammation is most pronounced shortly after venous thrombosis is diagnosed, and diminishes slowly afterward.22 A second causal explanation is that treatment with vitamin K antagonists (at present one of the mainstays of venous thrombotic disease treatment) may lead to increased arterial calcification,23,24 thereby increasing arterial thrombotic risk (Fig. 1C-3). Nevertheless, as the results of these investigations are based on animal studies,23 or are small numbered,24 this explanation should be interpreted with caution. Other causal explanations are that other unknown or not well-studied mechanisms (for example, paradoxical emboli in patients with deep vein thrombosis),25 explain the link between venous and arterial thrombosis. If we assume that the association is not causal (Figs. 1B-2 and D-1), then adjustment for risk factors (e.g., smoking and obesity) would lead to a diminishment of the relative risk. In the one study that reported on this issue (n ¼ 302) the authors could adjust the findings for classical arterial risk factors (such as Downloaded by: Universitaetsbibliothek - Med. Universitaet Wien. Copyrighted material. 888 hypertension, overweight/obesity, or smoking) but found no attenuation of the effect.26 venous thrombosis before adjustment for age, which disappeared after adjustment for age.36–38 COMMON RISK FACTORS FOR VENOUS AND ARTERIAL THROMBOTIC DISEASE (CONFOUNDING) All relationships between atherosclerosis, arterial and venous thromboses described above could be explained by the presence of common risk factors (Figs. 1B-2 and D-1). Below we will discuss some well-known risk factors for arterial disease and their relation with venous thrombotic disease. Moreover, we will discuss the effect of statin use on the occurrence of both arterial thrombosis and venous thrombosis. Diabetes Mellitus Diabetes mellitus, like hypertension, is an age-related disease, but in contrast to hypertension, studies on diabetes have shown an age-adjusted 1.5- to 2-fold increased risk of venous thrombosis.37,39,40 However, after controlling for hospitalization, major surgery or medical illness, and nursing home confinement, diabetes was no longer associated with venous thrombosis in a large population-based case–control study (n ¼ 4037).39 A similar result was found in the Copenhagen City Heart Study.38 Therefore, diabetes mellitus does not seem to be causally related to venous thrombotic disease. Overweight/Obesity Of all classical arterial cardiovascular risk factors (including overweight/obesity, hypertension, diabetes mellitus, metabolic syndrome, smoking), obesity has most consistently been reported as a risk factor for venous thrombosis.3,27,28 It is, however, not completely understood why obesity predisposes to venous thrombosis. People with overweight or obesity tend to be more immobile, which may lead to clot formation through stasis. It is also possible that these individuals acquire a prothrombotic state. Indeed, there are studies available that have shown a correlation between increase in body mass index and factor VIII,29 which is a risk factor for venous thrombosis and arterial thrombosis.30 Adipose tissue may contribute to enhanced coagulation by direct tissue factor production, but hypercoagulability could also be due to direct effects of adipose tissue on the hepatic synthesis of coagulation factors.31,32 Another explanation is that estrogen levels are higher in obese males and females due to an increased conversion from androgen to estrogen in adipose tissue.33 As estrogens and progestagens increase factor VIII levels,34 this may also be a possible pathway. A third explanation may be that obesity is considered a chronic low-grade inflammatory state that may result in increase of clotting factors leading to venous thrombosis.32 Hypertension A recent meta-analysis reported that hypertension was associated with a 1.5-fold increased risk (95% CI, 1.2– 1.9) of venous thrombosis compared with normotensive individuals. However, in this analysis it was not possible to adjust for age,3 which would have been necessary, because hypertension is strongly associated with increasing age, as is the occurrence of venous thrombosis.35 Indeed, original studies on venous thrombotic risk in hypertensive individuals showed increased risks for Metabolic Syndrome The metabolic syndrome has also been associated with venous thrombosis.41,42 As the metabolic syndrome is a cluster of abdominal obesity, hypertension, high triglycerides, reduced levels of high-density lipoprotein (HDL) cholesterol, and elevated fasting plasma glucose, an interesting question is whether this increased risk is a result of one or more of the individual components or of the total interplay between these cardiovascular risk factors. Two recent studies showed that the clustering of components of the metabolic syndrome was predictive of venous thrombosis only in the presence of abdominal obesity.43,44 Particularly, total/HDL/low-density lipoprotein (LDL) cholesterol levels and triglycerides levels do not increase venous thrombotic risk.36,38,45,46 Therefore, the metabolic syndrome nor its other components (apart from obesity) seem to be important in venous thrombosis etiology. Smoking Smoking, a well-known risk factor for arterial thrombosis,47 did not increase the risk of venous thrombosis in some studies.36,46 This could have been a result of chance, as these studies were small numbered, and because biological evidence of smoking (which results in systemic activation of coagulation and inflammation),48 on venous thrombotic risk suggests otherwise. In the Multiple Environmental and Genetic Assessment (MEGA) study of risk factors for venous thrombosis, a large population-based, case–control study, 3989 patients with venous thrombosis were compared for smoking habit with 4900 controls.49 Relative risk of venous thrombosis was 1.4 (95% CI, 1.3–1.6) in current smokers and 1.2 (95% CI, 1.1–1.4) in ex-smokers compared with those who had never smoked. Those who smoked heaviest or longest had the highest relative risk of 4.3 (95% CI, 3.0–7.1). So, this study showed a 889 Downloaded by: Universitaetsbibliothek - Med. Universitaet Wien. Copyrighted material. RELATIONSHIP BETWEEN VENOUS AND ARTERIAL THROMBOSIS/LIJFERING ET AL SEMINARS IN THROMBOSIS AND HEMOSTASIS/VOLUME 37, NUMBER 8 clear dose-dependent and reversible association of smoking habit on risk of venous thrombosis.49 This result has recently been confirmed in two large followup studies.38,50 Thrombophilia Although elevated levels of factors VIII, IX, XI, and fibrinogen have been associated with 1.5- to 3-fold increased risks of arterial thrombosis,51–53 and the inherited thrombophilias with 1.2- to 2.5-fold increased risks,54,55 the effects are up to 10 times weaker than in venous thrombosis (Table 1). This difference in relative risk is probably explained by the role of atherosclerosis in arterial thrombosis. It is plausible that there must be a local thrombotic focus (plaque rupture) for occlusion to occur because of exposure of tissue factor to circulating blood and platelets.56 This may be so strong in promoting local clot formation that it matters only minimally whether the coagulation system is competent or overcompetent, as in factor V Leiden or prothrombin G20210A carriers. While further studies are required, one study suggested that the lower risk estimates found for the genetic (in this case natural anticoagulant deficient) thrombophilias on arterial thrombotic risk could also be due to the influence of competing classical arterial risk factors.55 These individuals had an overall risk of arterial thrombosis that was 2.3-fold increased compared with individuals without a deficiency. However, this risk was 5.5-fold increased for subjects aged <55 years and only 1.3-fold increased for age 55 years. Apparently, classical arterial risk factors were more important in predicting arterial thrombotic risk after the age of 55 years than natural anticoagulant deficiencies in that study. Results from studies on incompetent clotting, however, have yielded consistent results arguing for a role for coagulation in arterial thrombotic disease. Patients with hemophilia have a strongly reduced risk of MI (80% reduction).57,58 Female carriers of hemophilia, who have an intermediate reduction of factors VIII or IX, have an intermediate 25% reduced risk of MI.59 Furthermore, treatment with oral anticoagulants, reducing factors II, VII, IX, and X has a substantial effect on risk of recurrent MI.60 Statins Statins, a class of medications first introduced in the 1980s,61 reduce blood levels of LDL and boost levels of HDL. That statins are able to reduce mortality and morbidity in patients with arterial thrombosis is ascribed to antidyslipidemic and anti-inflammatory properties.62 The results of recent observational and experimental studies that showed 40 to 50% risk reductions for first venous thrombosis occurrence when 2011 using a statin,2,63,64 are somewhat surprising, particularly because the mechanism behind this effect is unclear.65 Dyslipidemia may be the most plausible culprit to be considered. However, as dyslipidemia is not related to an increased risk of venous thrombosis,38,46,64 or to hypercoagulability,45 and because nonstatin lipidlowering drugs (i.e., fibrates) do not reduce venous thrombosis risk,2,63 it is unlikely that statins decrease venous thrombosis risk by lipid-lowering activities. Based on reports showing that statins can reduce thrombin generation,66,67 one would suspect that their use protects, to some extent, against venous thrombosis by downsizing coagulation activation. However, these findings should be interpreted with caution as not all studies consistently reported a favorable outcome of statin treatment on coagulation factor levels. For example, in a small randomized study of Dangas et al (n ¼ 93), an increase in fibrinogen level was observed when individuals were exposed to pravastatin compared with placebo.68 For arterial thrombosis, statins may, next to their ability to influence lipid levels, have an anti-inflammatory component. However, the impact of such a component on arterial risk is hard to assess as the reduction of inflammatory response in statin users is accompanied by similar reductions of dyslipidemia.69 Because dyslipidemia is not associated with increased venous thrombosis risk,38,84 it is possible that reduction of inflammation is the driving force behind the reduced venous thrombosis risk that has been observed in statin users. It should be noted that the latter statement is hypothetical and it is presently not known whether these results can be explained by a reduction in inflammation or coagulation activation in individuals who use a statin. THE LINK BETWEEN VENOUS AND ARTERIAL THROMBOSIS EXPLAINED? Which of the four putative pathways in Fig. 1 may be considered most likely to explain the link between venous and arterial thrombosis is in our opinion presently an unanswerable question as the necessary study material on this issue is absent. Therefore, we will not try to answer this question but propose study designs that are needed to sufficiently study these hypotheses. First of all, a population-based study is essential as selection of subjects could give rise to bias. Second, the best design would be a follow-up study for two reasons: (1) we unequivocally need to know the temporal relation between venous and arterial thrombosis and (2) in a case– control study several potential confounders or factors in the causal pathway (laboratory or radiological) will have been measured after the thrombosis, and the event may have changed these variables (post-hoc phenomenon). Third, such a follow-up study should not end its followup after the first arterial or venous thrombotic event is Downloaded by: Universitaetsbibliothek - Med. Universitaet Wien. Copyrighted material. 890 RELATIONSHIP BETWEEN VENOUS AND ARTERIAL THROMBOSIS/LIJFERING ET AL 891 documented because then the temporal relationship between the two diseases cannot be obtained. Next, the studies must have recorded data of variables that have been associated with both venous and arterial thrombosis, and most preferably data on atherosclerosis at different time points (as for example was done in the ARIC and Cardiovascular Health Study). This is important for answering the study question whether the relationship between venous and arterial thrombosis is causal (Fig. 1A, C) or mediated through atherosclerosis or through common risk factors (Fig. 1B, D). Although the list of potential confounders is long (as shown in Table 1), one should keep in mind that many of these confounding variables occur rarely in the general population (such as human immunodeficiency virus [HIV] infection or nephrotic syndrome) and are therefore unlikely to fully elucidate the (in this case confounded) relationship between venous and arterial thrombosis. For other more common exposures, such as factor V Leiden, the relative risk increases for arterial thrombosis are too low to fully explain the relationship (20% of patients with venous thrombosis have factor V Leiden, but the relative risk for arterial thrombosis in factor V Leiden carriers is less than 1.2).54 Therefore, probably the main contributors to a supposed confounded relationship of venous to arterial thrombosis, and vice versa, could be variables such as age, sex, oral contraceptive use, smoking, and overweight/obesity. Because some of these variables may change over time, it is also important to have data available that record these differences over time. Otherwise, it will be difficult, or even impossible, to make a distinction between Fig. 1B, 1 (in Fig. 1B-2 the risk factor, e.g., atherosclerosis confounds the association between venous and arterial thrombosis, while in Fig. 1C-3 the risk factor, atherosclerosis, is a result of venous thrombosis that finally causes arterial thrombosis). Although it is possible to record data on several variables in a population cohort study at different time moments, this will be time-consuming and expensive, given the large number of individuals that need to be tested and retested. However, retesting during followup will be necessary to distinguish between several causal pathways, but can be restricted to a subgroup. We will elucidate this with a hypothetical example on atherosclerosis and venous thrombotic disease, graphically illustrated in Fig. 2A–C. If we assume that atherosclerosis is not independently associated with future venous thrombotic risk, then progression of atherosclerosis should be similar for individuals who will or will not develop venous thrombotic disease. Venous thrombosis cases will develop during followup period. When they are compared with an appropriate control group in whom IMT measurements were performed at time of enrollment (similar to the cases) and at the end of follow-up (similar to cases), then one can calculate whether atherosclerosis progressed more in cases after they had venous thrombosis than in controls or not. Such a finding is graphically illustrated in Fig. 2A. In another scenario, it could be that atherosclerosis is an independent risk factor for venous thrombosis. One could still analyze if venous thrombosis itself accelerated atherosclerosis even more after the event occurred using the same nested case–control design. If atherosclerosis progression at the second measurement is more than at the first measurement, venous thrombosis may have lead to this increased atherosclerosis progression. Such a finding is illustrated in Fig. 2B. When one conducts a study in which at baseline everyone has the same extent of atherosclerosis progression (but cases with venous thrombosis develop atherosclerosis faster than others during follow-up, independent of venous thrombosis occurrence [Fig. 2C]), it will appear to the researcher that venous thrombosis accelerated atherosclerosis progression, while in fact it was not dependent on venous thrombosis disease, but on other underlying (unknown) factors. To distinguish these scenarios, one need at Downloaded by: Universitaetsbibliothek - Med. Universitaet Wien. Copyrighted material. Figure 2 Hypothetical example of a nested case–control study to determine if venous thrombosis leads to atherosclerosis progression. Solid line indicates a person who develops venous thrombosis at follow-up, dashed line indicates a control subject without venous thrombosis who was enrolled and followed similar as the individual who developed venous thrombosis. Of note, this example assumes that atherosclerosis is somehow associated with venous thrombosis which may not be true. *Atherosclerosis progression can be measured, for example, by carotid intima media thickness. VT, venous thrombosis. SEMINARS IN THROMBOSIS AND HEMOSTASIS/VOLUME 37, NUMBER 8 least three atherosclerosis measurements as well as information on which individual in the cohort develops venous thrombosis at a certain time point (for example, by direct access of the researcher to chart reviews in electronic medical records). The first measurement should take place at time of enrollment in all individuals. When someone develops a venous thrombosis, this participant’s IMT should be remeasured as soon as possible, together with that of a (matched) control person(s). A third measurement should be taken at the end of follow-up period in both the case and control(s). Although theoretically possible, one will need a scrupulous system that regularly screens individuals, on possible venous thrombosis occurrence, which may practically be unachievable. CLINICAL RELEVANCE OF THE SUPPOSED ASSOCIATION BETWEEN VENOUS AND ARTERIAL THROMBOSIS In terms of clinical policy, it is pivotal to meticulously investigate the proposed hypotheses for the association between venous and arterial thrombosis. If, on the one hand, the association between venous and arterial thrombosis is causal, then all patients with venous thrombotic disease should not only be treated with anticoagulants, but also with arterial thrombosis prevention strategies to avoid subsequent arterial thrombosis. If, on the other hand, the association between venous and arterial thrombosis is not caused by venous thrombotic disease, but if arterial thrombotic risk is increased via common etiological risk factors such as smoking or obesity, then targeting venous thrombosis patients with arterial thrombosis prevention strategies is not rational. Still, individuals with cardiovascular risk factors have an increased venous thrombotic (and arterial thrombotic) risk in this scenario. Therefore, they should receive arterial thrombotic risk prevention strategies from a clinician who is specialized in both venous and arterial thrombotic disease, and has access to laboratories that are specialized in both diseases. This is probably the most beneficial scenario for the individual patient.70 CONCLUSIONS Venous thrombosis and arterial thrombosis seem to share many pathogenic mechanisms. Inflammation, elevated levels of coagulation factors, and inherited thrombophilia are characteristics of both disorders. 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