Pathogenesis of varicose veins - lessons from biomechanics.

L. Pfisterer et al.: Biomechanics of varicosis DOI 10.1024/0301-1526/a000335

Vasa 2014; 43: 88 – 99 © 2014 Hans Huber Publishers, Hogrefe AG, Bern

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Pathogenesis of varicose veins – lessons from biomechanics Larissa Pfisterer, Gerd König, Markus Hecker, and Thomas Korff Department of Physiology and Pathophysiology, University of Heidelberg, Germany

Summary

Zusammenfassung

The development of varicose veins or chronic venous insufficiency is preceded by and associated with the pathophysiological remodelling of the venous wall. Recent work suggests that an increase in venous filling pressure is sufficient to promote varicose remodelling of veins by augmenting wall stress and activating venous endothelial and smooth muscle cells. In line with this, known risk factors such as prolonged standing or an obesity-induced increase in venous filling pressure may contribute to varicosis. This review focuses on biomechanically mediated mechanisms such as an increase in wall stress caused by venous hypertension or alterations in blood flow, which may be involved in the onset of varicose vein development. Finally, possible therapeutic options to counteract or delay the progress of this venous disease are discussed.

Pathogenese der Varizenbildung – Lehren aus der Biomechanik Eine maladaptive Reorganisation der venösen Gefäßwand geht der Entstehung von varikösen Venen bzw. der Etablierung einer chronischen venösen Insuffizienz voraus, ist aber auch mit deren weiterer Progression assoziiert. Jüngste Arbeiten legen nahe, dass ein Anstieg des Füllungsdrucks durch die resultierende Erhöhung der Wandspannung und die Aktivierung venöser Endothel- und glatter Gefäßmuskelzellen ausreicht, um in Venen variköse Veränderungen zu initiieren. In Übereinstimmung mit dieser Hypothese scheinen bekannte Risikofaktoren wie langes Stehen oder eine durch Übergewicht verursachte Erhöhung des hydrostatischen Drucks die Bildung variköser Venen zu begünstigen. Der vorliegende Übersichtsartikel befasst sich mit verschiedenen biomechanisch induzierten Mechanismen wie etwa eine durch venösen Hochdruck oder Veränderungen im Blutfluss hervorgerufene Erhöhung der Wandspannung, die in der Frühphase der Varizenbildung eine Rolle spielen könnten. Abschließend werden mögliche therapeutische Ansatzpunkte diskutiert, die die Progression dieser venösen Erkrankung verhindern bzw. verzögern könnten.

Key words: Varicose veins, shear stress, circumferential wall tension, endothelial cells, vascular smooth muscle cells, remodelling

Introduction Veins are exposed to various biomechanical forces which are determined by the flowing blood, the intra- and extraluminal pressure, and the longitudinal tensile load originating from different blood volumes stored in compartments of larger veins, which are separated by venous valves. Depending on their magnitude, these physical determinants may either stabilize the architecture of the venous vessel wall or stimulate (mal) adaptive remodelling processes. In the latter case, a chronic rise in biomechanical load may evoke pathophysiological responses of the venous wall promoting its weakening, which may subsequently lead to the development of venous insufficiency and/ or varicose veins, predominantly in the lower extremities. Chronic venous insufficiency (CVI) is charac-

terized by a more or less defined set of symptoms in the leg including skin hyperpigmentation, edema, and ulceration [1, 2]. Although development of venous insufficiency may be associated with varicose veins, these venous diseases may simply be coincidental but not necessarily causally related. While it appears to be well accepted that valve reflux is the predominant determinant of both CVI and varicose veins, there is an ongoing discussion as to whether valve dysfunction initiates these venous diseases or is a secondary event to venous remodelling [3 – 5]. Clinically, varicose veins appear swollen and bulged with a twisted or tortuous corkscrew-like morphology. At the histological level they show enlarged diameters, increased endothelial and smooth muscle cell activities and an altered composition of the extracellular matrix [6]. Char-

acteristically, the regular architecture of the collagen-elastin network in the vessel wall is lost, the collagen type I to type III ratio varies significantly [7], and the activity of matrix metalloproteinases (MMPs) such as MMP– 2 and MMP– 9 is elevated in varicose veins [7 – 9]. Despite detailed knowledge about the clinical appearance or prevalence of this disease, not much is known about the mechanisms ultimately triggering its onset. In fact, there is an ongoing debate whether alterations in biomechanical forces are sufficient to cause pathophysiological venous remodelling and whether such hemodynamic abnormalities are cause or consequence of chronic venous insufficiency. For instance, weakening of the structural integrity of the venous wall may be a consequence of and the cause for a dysfunction of the venous valves at the same time [10].


L. Pfisterer et al.: Biomechanics of varicosis

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One general functional consequence of varicose vein formation and/or venous insufficiency is a decrease in venous return and an increase in the filling pressure of the associated venous network – a phenomenon that is referred to as ’venous hypertension‘. Likewise, most individual risk factors for this disease such as prolonged standing or tight undergarment [11, 12] contribute to an increase in venous filling pressure, which along with valvular dysfunction and venous dilatation raises circumferential wall stress in the affected veins. This review will discuss the impact of this biomechanical force on venous remodelling, possible analogies to arterial remodelling processes, the implications of risk factors and surgical interventions, and potential therapeutic consequences.

Biomechanical forces in the venous system Essentially adaptations of the vessel wall, such as an enlarged lumen diameter and remodelling of the extracellular matrix (ECM), may be evoked by changes in hemodynamic forces such as shear stress and/or (circumferential) wall stress, hence biomechanical stretch. Their chronic alteration often promotes cardiovascular pathologies such as hypertension [13, 14], atherosclerosis [15] and venous valve dysfunction [16]. Endothelial cells (ECs) are directly affected by changes in both hemodynamic forces, whereas only biomechanical stretch stimulates vascular smooth muscle cells (SMCs) in the media. Under physiological conditions, both forces stabilize the function of arteries and veins and maintain a normotensive blood pressure. In this situation, laminar shear stress (LSS)-mediated expression and activity of endothelial nitric oxide synthase (eNOS) stimulate

the production of nitric oxide (NO). NO – as a freely diffusing and membrane-penetrating signalling molecule – transmits the rise in LSS to the SMCs, inhibiting their proliferation and triggering their production of cyclic guanosine 3’,5’-monophosphate (cGMP), which subsequently promotes relaxation of these cells and, as a consequence, vasodilatation [17, 18]. Furthermore, NO bears several anti-inflammatory effects, as it may react with reactive oxygen species (ROS) such as superoxide to form peroxynitrite, thereby inactivating these well-characterized proinflammatory mediators. In ECs, LSS inhibits pro-inflammatory CD40/ CD154 signalling by up-regulating TRAF– 3, attenuates attachment of macrophages and activation of the transcription factor NF-κB, and ultimately the expression of corresponding pro-inflammatory gene products [19]. Consequently, a decrease in or lack of LSS promote pro-inflammatory responses and pathophysiological remodelling of the vessel wall, which predominantly occurs in areas exposed to disturbed blood flow. This transition from laminar to turbulent flow depends on the inner radius of the vessel, the average flow velocity and both blood density and viscosity. Flow – or precisely shear stress (τ) – itself is defined by the relation of blood viscosity (η), the length (l) of the blood vessel and its radius (r), whereby: . Conditions supporting changes in these parameters are usually given at curvatures and branches of large arteries, which are referred to as arteriosclerosisprone sites because arteriosclerotic plaques primarily develop here. However, considering the anatomy of larger veins, such conditions may also occur in them, e. g. following alternating contractions of the skeletal muscle (flexor vs. tensor), which tremendously boost venous blood flow velocity temporarily, but repeatedly.

Generally, the overall level of laminar shear stress ECs are exposed to is much lower in veins as compared to arteries, which may in fact limit the venous capability to release NO [20] and to counteract pathophysiological remodelling processes. While the level of LSS is strictly regulated by changes in the inner radius of the blood vessel (inverse correlation, namely in small blood vessels) and hence blood flow, the level of biomechanical stretch, hence wall stress, on the other hand is orchestrated by blood pressure and volume, and in veins most notably by hydrostatic or filling pressure. This exposure of the venous SMCs to increased biomechanical stretch elicits the release of Ca2+ from intracellular stores, which in turn stimulates signalling pathways [21] contributing to the adjustment of myogenic tone, and thus contractile capacity of the SMCs [22]. Consequently, Ca2+-mediated phasic constriction constitutes a mechanism for acutely adapting the vessel diameter to alterations in blood pressure and/ or volume [23, 24]. The rise in biomechanical stretch of the vessel wall following an increase in wall stress (σt) depends on the relation between the transmural pressure difference (Ptm) and the inner radius (ri) divided by the wall thickness (w) whereby (Law of Laplace). This equation represents the circumferential tensile stress applied to the wall, which is higher during vasodilatation (due to the increase in ri and decrease in w) than during constriction. However, dilation-related stretch is partially counteracted by the tensile strength of collagen and elastic fibres within the ECM, while active constriction of the medial SMCs increases wall thickness and thus reduces wall stress. Besides spontaneous responses (contraction or relaxation of SMCs) to temporary changes in blood flow or pressure, a chronic rise in transmu-



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Figure 1: Schematic view of the two main hemodynamic forces acting on the wall of a blood vessel, including veins and venules. Laminar shear stress exerted by the flowing blood (red arrows) on the inner lining of the vessel wall (i. e. endothelial cells) is indicated by the blue arrow. Circumferential wall tension or stretch (green arrows) evoked by a change in the radially acting intraluminal pressure, i. e. filling or hydrostatic pressure, acts on both the intimal cell layer (i. e. endothelial cells) and, more profoundly, the medial cell layer (i. e. vascular smooth muscle cells). If such a difference in hydrostatic pressure occurs in a blood vessels fixed at both ends, the blood vessel may start to twist or shrink as indicated by the arrow pointing along the longitudinal axis of the blood vessel.

ral pressure (e. g. during arterial or venous hypertension) elicits (mal) adaptive vascular remodelling foremost to normalize wall stress, which in arteries primarily occurs through thickening of the vessel wall due to hypertrophy and/or hyperplasia of the medial SMCs (eutrophic remodelling), thereby counterbalancing the pressure-dependent increase in the inner radius – and thus keeping the quotient σt roughly the same. In the comparatively thin-walled and SMC-poor veins, the corresponding structural adaptations of the vessel wall preferentially lead to the typical

corkscrew-like morphology of remodelling (varicose) veins that rather points to a (longitudinal) growth between fixed ends [25]. Recently, the possibility that a chronic increase in venous pressure may be sufficient to drive venous remodelling was investigated by using a novel mouse model. To prove this concept, an individual vein in the mouse auricle was occluded to elevate the filling (hydrostatic) pressure in the connected venous network located proximally to the ligature. In accordance with the Law of Laplace, wall stress in these blood vessels increased and

led to the development of enlarged and tortuous veins [25]. As indicated by further histological analyses, endothelial and smooth muscle cells in these veins proliferated and increased the expression of MMP– 2 – observations which were also made in human samples of varicose veins. In fact, prolonged exposure of isolated mouse veins to elevated pressure levels was sufficient to induce the observed changes. Interestingly, activation of the transcription factor activator protein 1 (AP– 1) appeared to be a prerequisite for venous remodelling, proliferation and MMP– 2 expression in this context, as blockade of its activity essentially abolished all these processes. The relevance of wall stress for venous remodelling is further underlined by the fact that AP– 1 is activated by ROS, namely hydrogen peroxide (H2O2), whose intracellular concentration is controlled by biomechanical stretch. Stretch-stimulated ECs or SMCs increase the expression and activity of certain NADPH oxidases and thereby production of ROS, which in low to intermediate amounts trigger signalling pathways sensitive to oxidative modification of pivotal effector proteins [16, 26]. This is quite in analogy to processes elicited by arterial hypertension, where ROS are a major component in the initiation of SMC dedifferentiation [27], partially by the increase in H2O2 formation [28], and additionally support a pro-migratory and growthpromoting phenotype [29, 30]. Exposure to elevated biomechanical stretch also stimulates pro-inflammatory responses, for instance by diminishing suppression of endothelial cell CD40 interaction with its ligand CD154 on activated platelets or T-cells, which promotes adhesion of monocytes to the endothelial surface and their final transmigration into and differentiation within the vessel wall [31]. It is unknown, though, whether development of varicose veins is asso-



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ciated with or even preceded by an increase in inflammation of the venous wall. Although AP– 1 controls the expression of many pro-inflammatory adhesion molecules (e. g. VCAM– 1) and cytokines (e. g. MCP–1), accumulation of macrophages has been observed in both healthy and varicose veins [32], and pro-inflammatory adhesion molecule expression appears to be more pronounced at later stages of varicose vein development [33]. Likewise, recent publications indicate that inflammation may not necessarily be linked with the development of varicose veins [34, 35]. However, in animal models acute and high intravenous pressure levels appear to promote pro-inflammatory responses [36 – 38], suggesting that they may be a consequence rather than the cause of advanced varicose vein development. An increase in the level and duration of stretch also evokes adaptation of the smooth muscle cell phenotype enabling a reorganization of the ECM, e. g. by altering the expression and activity of matrix metalloproteinases. Consequently, an increase in wall stress stimulates expression of MMP– 2 and elevates gelatinase activity both in the media of stretched mouse veins and in human venous SMCs [25, 39]. Likewise, MMP– 9 activity is increased in varicose veins of human patients [40] and in rat veins upon exposure to an increased transmural pressure level [41]. This may explain for the fact that the abundance of collagen types I and III is altered in varicose veins as compared to healthy veins [42, 43]. In fact, the amount of rigidity-mediating collagen type I is increased in the varicose vessel wall while the distensible collagen type III fibre network is degraded. Such a reorganisation of the ECM is a hallmark of the remodelling processes in varicose veins and associated with an increase in rigidity enabling it to withstand a chronic increase in wall stress.

Figure 2: Venous remodelling in murine auricles following ligation. After ligation of a main draining vein (day 0) collateral small veins and venules are exposed to altered hemodynamic stimuli, most notably an increase in wall tension. Accordingly, compensatory adaptive processes in the venous wall are initiated. The most obvious sign of this remodelling process is the typical corkscrew-like morphology (arrow) due to hyperplastic cell proliferation (day 2). The mouse model clearly shows that venules or small veins rapidly enlarge following downstream occlusion of a larger draining vein. The two images are not perfectly superimposable, since the mouse ear was positioned slightly differently when the second picture was taken.

Lessons from arteriogenesis – ‘varicose’ remodelling of arterioles and small arteries Not much is known about the onset and progression of varicose vein development and the role of biomechanical stressors. Therefore, an interesting approach to get an idea about mechanisms contributing to venous remodelling is the comparison with arterial remodelling processes triggered by elevated wall stress, such as arteriogenesis, which is also referred to as collateral artery formation. Arterioles undergoing arteriogenesis are characterized by an enlarged vessel lumen and a corkscrew-like appearance – morphological characteristics that are also observed in varicose veins. In humans, formation of collateral arteries through arteriogenesis is usu-

ally promoted by a slowly progressing and/or chronic occlusion of a main conduit artery [44]. For instance, atherosclerosis may lead to the formation of plaques that narrow or partially occlude the lumen of the femoral artery. As a consequence, peripheral artery disease develops. Due to occlusion of the main feeding artery the longitudinal pressure difference in the accompanying collateral arterioles bypassing the site of occlusion increases. In analogy to Poiseuille’s equation blood flow and consequently shear stress is increased in the collateral arterioles, which dilate in response to the resulting release of NO [45 – 47]. As can be deduced from the Law of Laplace, any increase in diameter is also accompanied by an increase in wall stress, so this biomechanical force acts in concert with shear stress to initiate remodelling of the collateral arterioles.



arterial and venous vessel wall differ, current data strongly suggests an important role of wall stress as trigger and moulding force during arterial and venous remodelling processes. Although wall stress predominantly controls the release of MCP– 1 via activation of AP– 1 and the subsequent recruitment of macrophages during arteriogenesis, it may also be responsible for the initial changes in matrix metalloproteinase expression that alter the structural properties of the media during varicose vein development. Another hallmark of both remodelling processes pointing to an involvement of wall stress is the proliferation of medial SMCs. This cellular response may ultimately lead to the thickening of the vessel wall counteracting the increase in this biomechanical force. The fact that varicose veins almost always develop in the lower extremities [56] also underlines the relevance of hydrostatic (filling) pressure and hence wall stress for this disease. When standing upright, hydrostatic pressure and thus transmural pressure and the level of wall stress to which the veins are exposed increases in these areas of the venous network. Collectively, arteriogenesis as well as varicose vein development may be viewed as an adaptive structural (ECM) and cellular (ECs, SMCs) remodelling process of the vessel wall to counteract an increase in wall stress.

such as nutrition, lifestyle, hormonal constitution (e. g. pregnancy) or obesity, have not been defined in detail so far, but there are several (clinical) studies assessing their contribution to the severity of varicosis and/or CVI. However, both venous disorders may be differently affected by individual risk factor as can be deduced from many corresponding epidemiologic studies [1, 2, 57, 58]. The prevalence of both varices and CVI increases with age [57], which is discussed to be mediated by an increased pressure on superficial veins as a consequence of a gradual weakening of calf muscles in combination with a deterioration of the vessel wall [1]. However, female gender appears to be robustly associated with a greater risk for developing varicose veins but there is some controversy as to whether females more likely develop CVI [1]. One of the most relevant risk factors promoting venous disease appears to be obesity or an increased body mass, as it is associated with a higher incidence of varicose veins [1, 59] as well as CVI [60]. In addition to its negative impact on the function of endothelial and smooth muscle cells, excess weight often leads to an obstruction of venous return, which results in an increase in venous filling pressure and thus wall stress. In a study analysing the impact of postmenopausal age and hormonal status on varicosis, patients with an elevated BMI (≥ 28) or an increased hip circumference showed a higher tendency to develop varicose veins [61]. Furthermore, a higher incidence of varicose veins in obese patients (BMI > 30) was observed. Nonetheless, the limitation of this study is the additional association with the postmenopausal hormonal status and the advanced age. But a range of studies focusing on certain ethnicities postulate that an increased BMI alone is a major risk factor for the initiation of varicose veins [61, 62] and venous remodelling [59, 63,

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Arteriogenic remodelling is defined as the radial and longitudinal growth of pre-existing small arteries or arterioles to bypass an occlusion [48]. Vessel enlargement is predominantly based on medial but also intimal hyperplasia [49], while lengthening of a vessel between fixed ends results in the macroscopically visible corkscrew-like appearance of remodelling collateral arterioles, eventually attaining the size of a small conduit artery. This is in analogy to the morphology of varicose veins, whose development is also at least in part triggered by an increase in wall stress. In fact, comparing the remodelling of veins and arterioles upon occlusion of larger conduit blood vessels in the mouse auricle revealed that both partially depend on similar mechanisms and result in a similar reorganization of the vessel wall [25, 50]. As in varicose veins, growth of collateral arterioles is accompanied by an increase in MMP– 2 and 9 expression or activity and, as a consequence, a shift between degradation and synthesis of the ECM [51]. Moreover, one major regulator of these gene products is activator protein 1 (AP– 1), which itself is controlled by a stress-responsive cis element designated TRE [52]. AP– 1 is also a potent determinant of the expression of molecules modulating pro-inflammatory responses such as MCP– 1, a cytokine that is critical for the onset of arteriogenesis [53] and controls SMC proliferation [54, 55]. Consequently, blunting the activity of AP– 1 through administration of AP– 1-specific decoy oligodeoxynucleotides in the mouse auricle model inhibited the progression of both venous and arteriolar remodelling [25, 50]. Likewise, anti-MCP– 1 treatment of human and murine vein grafts or MCP– 1 receptor blockade augmented vein graft thickening [54]. Despite the fact that the dynamics of venous and arterial remodelling are different and the architecture of the

Increasing the biomechanical load – risk factors or CVI and varicose vein development Considering risk factors contributing to the development of varicose veins and/or CVI it stands out that they oftentimes affect venous pressure and thus wall stress. In fact, there are multiple factors which influence the onset and progression of these diseases. The impact of the individual factors,



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64]. So one could assume that the mild association between varicosis and obesity seen in some studies is also due to the increased bodyweight of elderly women rather than a direct consequence of older age [65]. Consequently, an increase in BMI irrespective of the cause may support the initiation and progression of varicosis. The Tampere study interviewed 3,284 randomly chosen men and 3,590 women 40 to 60 years of age [66]. The questionnaire covered family status, sex, age, profession and weight. With these parameters it is possible to correlate the incidence of varicosis with specific life style factors. From the data one can conclude that varicose veins correlate with female sex, obesity, extensive standing type of work, parity (proportional to the number of given births) and a family history of varicosis. The latter relation points to a possible genetic predisposition to develop varicose veins. Interestingly, most risk factors, such as BMI, parity and prolonged standing, lead to a rise in hydrostatic pressure and may therefore augment venous remodelling. This process can also be triggered or augmented by a pathology called venous valve dysfunction, which can either be hereditary or develop as a consequence of the aforementioned risk factors [67]. As has been well described for aortic valves and arterial hypertension [68, 69], an increase in venous pressure may also affect the integrity and compliance of venous valves and consequently impair the return of blood to the heart [10]. Moreover, reflux of venous blood due to insufficient valves greatly enhances venous hypertension, which subsequently results in a rise in capillary pressure. Thus another complication often observed in patients suffering from varicosis and/or CVI is edema formation, especially in the lower extremities.

Besides obesity or venous valve dysfunction, prolonged standing or a sedentary lifestyle accelerate the pathophysiological reorganization of the venous wall by limiting the reflux of venous blood and supporting its accumulation especially in the lower extremities. An increase in blood volume in distinct segments of larger veins separated by valves elevates the load to which they are exposed and increases hydrostatic pressure. In healthy veins such a scenario is usually prevented by an intact muscle pump and/or regular physical exercise supporting venous return [70]. But low physical activity often comes along with an increased BMI or even obesity, which consequently is associated with a higher incidence of varicosis. Prolonged standing, also by raising venous pressure, is a relevant risk factor for the development of varicosis [71]. In contrast to the aforementioned risk factors, the reason for the association of varicosis, but not necessarily CVI, with female sex [1,56] are not clear yet. While pregnancy is assumed to greatly contribute to varicose vein development due to the fetal growth- and weight-dependent increase in intraabdominal pressure [72], there is also some evidence that the biomechanical properties of blood vessels differ between men and women [73]. For instance, the contractile capacity of the coronary arteries in male rats is higher than in female rats. Accordingly, the distensibility of these blood vessels was higher and the elastic modulus lower in males as compared to females. Female blood vessels therefore seem to respond differently to biomechanical stimuli and may thus be more prone to an increase in venous pressure and/or remodelling. In a nutshell, the multifactorial disease varicosis is caused and influenced by a plethora of environmental and physiological effectors. They

either modulate venous compliance (valve dysfunction, genetic predisposition) or the level of wall stress (valve dysfunction, prolonged standing, less amount of physical activity, high BMI) and the response of vascular cells to these stressors (hormonal status, age, health, and lifestyle).

Hemodynamic consequences of surgical varicose vein removal Although some 250 million people worldwide may suffer from varicose veins, no causative therapy is available so far. For this reason, depending on the severity and progression of the disease, insufficient varicose veins have to be surgically removed, namely to prevent thrombosis and/ or phlebitis. Despite the need for surgical extraction (stripping) of insufficient veins, for individual veins this intervention can functionally be viewed as an obstruction of a partially insufficient pathway for returning blood to the heart, which is comparable to the experimental occlusion of veins in the mouse auricle model. On the one hand, vein stripping is usually accompanied with a partial decrease in venous pressure in veins affected by the reflux-mediated volume load. On the other hand, however, such an intervention will subsequently result in an increase in venous pressure and wall stress in other parts of the venous network that originally were connected to the insufficient vein. Accordingly, a situation is created which entails the slow, progressive remodelling of other veins and the development of varicose recurrences. Considering the putative impact of risk factors on venous wall stress in this context, the observed obesity-dependent increase in the risk of developing varicose recurrences upon crossectomy seems plausible [74].



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Interestingly, clinical studies comparing the long-term outcome of the classical stripping surgery with the non-surgical hemodynamic CHIVA technique (Cure Hémodynamique de I’Insuffisance Veineuse en Ambulatoire) after five and ten years indicate that the latter technique is associated with a lower rate of varicose recurrences [75, 76]. This observation again underlines the relevance of walls stress for venous remodelling processes as the CHIVA technique limits venous reflux, hence filling pressure, by consequently bypassing insufficient veins, which allows for an optimized venous return. As can be deduced from the physical laws discussed above, this surgical method lowers transmural pressure in veins connected to the bypassed vein, which in turn lowers their wall stress. Another interesting observation in the context of surgical interventions is the formation of neovascular (varicose) recurrences within six months up to two or three years after crossectomy [74, 77 – 79]. Although angiogenesis is often discussed as the main mechanism through which these neovessels form, it appears unlikely that their de novo formation is solely dependent on this process, as it usually does not last for years. However, angiogenesis may connect collateral veins to an alternative venous network which undergoes a steady and progressive remodelling process. In fact, the tortuous morphology of neovascular recurrences [77] is quite comparable to remodelling collateral arterioles which develop over years through arteriogenesis, which is evoked by nearly concomitant changes in both shear and wall stress, as discussed above [47, 50, 80]. A similar process could be initiated when small collateral veins are chronically exposed to a higher level of shear and wall stress upon stripping of a larger vein. These biome-

Figure 3: Signalling pathways and cellular responses affected by hemodynamic forces in endothelial (light grey) and smooth muscle (orange) cells. Black arrows indicate changes in protein activity and/or expression under pathophysiological conditions, + and – mark activation or inhibition at haemostasis. The NO-dependent signalling pathway as the major vascular control element is indicated by a bold arrow.

chanical stimuli may ultimately promote the enlargement and increased conductivity of the collateral veins, which may finally form neovascular varicose recurrences with the typical corkscrew-like appearance. Angiogenesis may significantly support this process, as it is capable to postsurgically connect smaller veins to the stump of the surgically removed vein. As soon as a functional connection has been established, blood flow and pressure would successively increase in the collateral network. Furthermore, angiogenic stimulation of endothelial cells is closely linked to their stimulation by an increase in wall stress, as the wall stress-me-

diated activation of AP– 1 supports the expression of vascular endothelial growth factor (VEGF) [81, 82], a crucial mediator of angiogenesis. Therefore, an increased expression of VEGF in the wall of primal or neovascular recurrences may also be a consequence of wall stress-induced venous remodelling [79].

Therapeutic approaches ‘targeting’ biomechanical forces Considering the discussed mechanisms underlying venous remodelling processes, novel aspects of well-



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established therapeutic methods may evolve. For instance, the usage of compression stockings for the prevention and concomitant treatment of varicose veins increases the extravascular tissue pressure by some mmHg [83, 84] and thus decreases the transmural pressure difference – a parameter directly controlling wall stress as can be deduced from the Law of Laplace. The intravascular, hence hydrostatic pressure does not change much or may even be diminished due to the displacement of some blood within the vein. As the transmural pressure gradient is determined by the difference between intra- and extravascular pressure, a decrease in the former and an increase in the latter translate into an overall decrease in the transmural pressure difference. This will reduce circumferential wall stress within the venous wall and thus should hamper the progression of varicosis. Under these conditions, blood flow in the vein will rise [83] due to the tissue compression-mediated reduction in internal diameter. An increase in flow, however, is accompanied by an increase in shear stress stimulating the endothelial cells to activate and express eNOS and to produce elevated levels of NO [85]. During varicose vein development, the release of NO would counteract the pro-inflammatory activation and proliferation of endothelial and smooth muscle cells, which is promoted by an increase in wall stress. Moreover, NO would neutralize ROS such as superoxide anions, whose production is elevated by a wall stress-dependent stimulation of NADPH oxidase expression and activity. In addition, prevention of superoxide dismutation to H2O2 would simultaneously limit the activity of AP– 1, as has been shown to be the case in arterial hypertension [86]. In fact, enhanced ROS production has recently been verified in varicose veins [87], suggesting that the

aforementioned mechanisms may play a so far underestimated role in the pathogenesis of the disease. Especially an elevation of wall stress could account for many of the phenotypic changes occurring in the vessel wall of varicose veins and would provide a plausible explanation for the impact of several risk factors on varicosis. As wall stress itself does not appear to be a suitable target for sustained therapeutic strategies, the molecular mechanisms driven by this biomechanical force, however, such as ROS production and subsequent activation of AP– 1, seem to be promising targets for a pharmacological intervention. To this end, corresponding studies implicate that treatment of patients suffering from chronic venous insufficiency with Pycnogenol – a mixture of flavonoids that exert anti-oxidative effects – improves the symptoms associated with varicose veins [88]. Although it has not been shown whether such a treatment would limit the progression or stage of varicose vein development, the results underline the option for a pharmacological intervention. As we learned from the mouse auricle model, the inhibition of AP– 1 may also effectively limit pathophysiological venous remodelling. Against this background unpublished results from our laboratory indicate that HMG-CoA reductase inhibitors (statins) may also inhibit varicose vein remodelling. Although statins are usually utilized to lower LDLcholesterol levels in the blood, they also effectively inhibit the activity of AP– 1, both in endothelial and smooth muscle cells [89]. Although such an approach may affect the progression of varicosis or prevent formation of varicose recurrences, it is unlikely that already manifest varicose veins will be cured by pharmacological intervention alone and be retransformed into healthy veins. In this regard, preventing a chronic

increase in venous pressure by minimizing the risk factors of the disease may more likely maintain a healthy and functional venous network.

Acknowledgments The authors’ work is supported by grants from the German Research Foundation (DFG), the German Centre for Cardiovascular Research (DZHK), the European Commission and the German Cardiac Society (DGK).

Conflicts of interest There are no conflicts of interest existing.

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Correspondence address Prof. Dr. Markus Hecker, Ph.D., D.Sc. Department of Physiology and Pathophysiology University of Heidelberg Im Neuenheimer Feld 326 69120 Heidelberg Germany [email protected]

Submitted: 07.10.2013 Accepted after revision: 03.12.2013

Morphological alterations of non-varicose and varicose veins. (A morphological contribution to the discussion on pathogenesis of varicose veins).

Varicose veins.

Varicose veins.

Varicose veins.

Varicose veins.

Varicose veins.

Letter: Treatment of varicose veins.

The management of varicose veins.

Connective tissue in varicose veins.

Operative Treatment of Varicose Veins.

Compression sclerotherapy of varicose veins.

Varicose veins in tropical Africa.

Varicose veins in developing countries.

Diverticular disease and varicose veins.

The surgical cure of primary varicose veins.

Acquired varicose veins of the penis.

Letter: Support for varicose veins.

Letter: Support for varicose veins.

Results of stripping operation for varicose veins.

Editorial: Tailored treatment for varicose veins.

From the Veins to the Heart: A Rare Cause of Varicose Veins.

The morphology of recurrent varicose veins.

Letter: Varicose veins in developing countries.

Isoenzymes of lactate dehydrogenase in varicose veins.