In recent years, the field of venous thromboembolism has undergone numerous innovations, starting from the recent discoveries on the role of biomarkers, passing through the role of metabolomics in expanding our knowledge on pathogenic mechanisms, which have opened up new therapeutic targets. A variety of studies have contributed to characterizing the metabolic phenotype that occurs in venous thromboembolism, identifying numerous pathways that are altered in this setting. Among these pathways are the metabolism of carnitine, tryptophan, purine, and fatty acids. Furthermore, new evidence has emerged with the recent COVID-19 pandemic. Hypercoagulability phenomena induced by this viral infection appear to be related to altered von Willebrand factor activity, alteration of the renin–angiotensin–aldosterone system, and dysregulation of both innate and adaptive immunity. This is the first literature review that brings together the most recent evidence regarding biomarkers, metabolomics, and COVID-19 in the field of venous thromboembolism, while also mentioning current therapeutic protocols.
The last few years have seen the maturation of numerous discoveries in the field of venous thromboembolism. In particular, more and more evidence has been reported regarding biomarkers, the role of metabolomics, and that of COVID-19, not to mention the advent of direct oral anticoagulants, which have rapidly established themselves among the treatment options. Deep vein thrombosis (DVT) and pulmonary embolism (PE) represent the major diseases of venous thromboembolism (VTE). DVT usually involves the deep veins of the lower or upper limbs but can occur in other sites. Occlusion of the deep veins in a limb by a thrombus damages drainage of blood, thereby leading to pain and swelling distal to the obstruction. Pulmonary embolism refers to a block of a pulmonary artery by a thrombus that has traveled from elsewhere in the body, through the bloodstream, and to the lungs. DVT in the legs—or less commonly, the arms—is by far the leading source of pulmonary embolism. The incidence of thromboembolism increases with increasing age. Women are usually affected at a younger age. Approximately two-thirds of VTE cases are associated with deep vein thrombosis and 80% are proximal. Distal (below the knee) DVTs are more transient episodes, whereas proximal DVTs are related to chronic conditions. Deep venous thrombosis is frequently secondary to heritable and acquired risk factors. Heritable risk factors include abnormalities associated with hypercoagulability of the blood, the most common of which are factor V Leiden and the prothrombin G20210A gene mutations. Acquired risk factors include advanced age, history of previous VTE, obesity, and active cancer, all of which limit mobility and may be associated with hypercoagulability. Superimposed on this background risk, VTE often occurs in the presence of triggering factors, which increase the risk above the critical threshold. In 25–50% of first episodes of DVT, no trigger is identified.
Triggering factors such as surgery, trauma, and pregnancy or estrogen therapy lead to endothelial cell activation, stasis, and hypercoagulability, which are the components of the Virchow triad. In 25–50% of first episodes of DVT, no predisposing factor is identified. The main complications involve the extension of thrombosis and the recurrence of PE and DVT. Long-term complications include post-thrombotic syndrome (PTS), which is characterized by chronic venous symptoms and/or signs secondary to DVT. This is the most frequent chronic complication of DVT and occurs in 30–50% of patients within 2 years of proximal DVT. A previous ipsilateral DVT, proximal location (ileo-femoral > popliteal), and stenosis of residual veins are the most significant risk factors for PTS. According to the recent guidelines, we can use the Villalta score for the diagnosis and treatment of PTS (post-thrombotic syndrome). In the diagnosis of DVT, clinical signs and symptoms remain the bedrock of diagnostic strategy, even if unspecific and variable. This is the first literature review that brings together the most recent evidence regarding biomarkers, metabolomics, and COVID-19 in the field of venous thromboembolism, while also mentioning current therapeutic protocols.
In addition to possible metabolic alterations, it is necessary to mention that infectious processes can also be triggers of thromboembolic episodes. In this regard, it is essential to note SARS-CoV2 infection. The Coronavirus disease of 2019 (COVID-19) is caused by the SARS-CoV2 coronavirus. Thromboembolic complications have been reported in COVID-19 patients from different groups. Organs such as the lungs, spleen, lower limbs, and brain are affected by the hypercoagulability phenomena induced by viral infection. In severe cases of the disease, these complications are associated with a high risk of mortality. SARS-CoV2 uses its spike protein (S protein) to bind to the human angiotensin-converting enzyme 2 (ACE2) receptor. ACE2 is not only expressed at the level of hair cells in the nasopharynx but is also found in blood vessels, the heart, the brain, and the kidney; it is a molecule that also regulates the activity of the renin–angiotensin–aldosterone system (RAAS). As a result of infection with COVID-19, downregulation of ACE2 occurs; consequently, the action of the RAAS system is altered, with altered blood flow and increased hypercoagulability phenomena. To this aspect, it should be added that the immune/inflammatory condition alone is a risk factor associated with increased blood clotting. With angiotensin II impairment, as a result of ACE2 alteration, the oxidative stress process via the NADPH pathway is enhanced. This is followed by progressive endothelial dysfunction and overexpression of LOX-1, COX-2, and VEGF in the endothelium. Endothelial dysfunction is also associated with endothelial expression of many prothrombotic molecules and receptors including P-selectins and angiopoietin 2 and endothelin 1, which are specific activators of thrombotic phenomena. From the study of patients who died from COVID-19, compared with H1N1 influenza, it was found that angiogenesis and alveolar capillary microthrombi in COVID-19 were up to nine times more prevalent than in flu. This phenomenon appears to be related to altered von Willebrand factor (vWF) activity and dysregulation of both innate and adaptive immunity. Since this is an infection that is a daily cause for scientific research, knowing that thrombotic processes are triggered on the one hand by dysregulation of RAAS and on the other by an excessive innate immune response to SARS-CoV2 may offer new opportunities for the development of innovative therapies for the treatment of COVID-19-induced coagulopathy.
Considering that VTE can often present with few symptoms, it would be useful to know biomarkers that enable early identification of patients at high or low risk of primary and recurrent VTE. Various established and novel biomarkers associated with VTE have been investigated with regard to their potential for predicting primary or recurrent VTE, for facilitating the diagnosis of VTE, and for optimizing the clinical management of VTE. Actually, these biomarkers can be divided into two categories from the pathobiology of DVT or thrombotic disease. One is coagulation markers, such as D-dimer, Thrombin, etc. while the other is inflammatory markers, including P-selectin, inflammatory cytokines, and microparticles.
D-Dimer is a cross-linked fibrin degradation product that forms right after throm-bin-generated fibrin clots are broken down by plasmin and indicates a general stimulation of blood coagulation and fibrinolysis. Testing for D-Dimer was investigated as a tool for the diagnosis of VTE and has been incorporated into diagnostic algorithms in the management of patients with suspected VTE since D-Dimer levels rise during a critical incident of VTE. D-Dimer is the best-recognized biomarker for the first assessment of suspected VTE; a negative result of D-Dimer may confidently rule out both DVT and PE with a high sensitivity of up to 95% and a negative predictive value of almost 100%. D-Dimer testing must be incorporated into thorough sequential diagnostic methodologies that involve clinical probability assessment and imaging tools because of its poor specificity for proving VTE. D-Dimer was examined as a risk factor for the occurrence of a future first incident of VTE and was related with a three-fold higher risk in a population-based cohort analysis. Additionally, in prospective cohort studies, D-Dimer levels are a well-researched biomarker for the estimation of the risk of VTE recurrence following the cessation of oral anticoagulant therapy. In subjects with prior unprovoked VTE, Palareti et al. measured D-Dimer levels 1 month after discontinuing oral anticoagulation and found that normal levels (500 ng/mL) had a high negative predictive value for VTE recurrence. In a different investigation, Eichinger et al. demonstrated that elevated D-Dimer levels were linked to an even greater risk of recurrent VTE, particularly in individuals with congenital thrombophilia, such as a factor V Leiden or prothrombin variant. On the basis of the above evidence, the authors Eichinger et al. concluded that the de-termination of the duration of oral anticoagulation for secondary VTE prevention may be influenced by the measurement of D-Dimer, which has become a pillar in the diagnostic work-up of patients with suspected VTE and is essential in the identification of hyper-coagulable conditions.
Thrombin is crucial for the acceleration of the coagulation cascade because it activates platelets, Factor V, and FVIII and because it is an essential part of a positive feedback loop that causes the production of a significant amount of additional thrombin, the conversion of fibrinogen to fibrin, and ultimately the formation of clots. Some studies in the past have demonstrated that TG is one of the risk factors for VTE and can be used as a predictive marker to assess thrombosis. Many authors across the years, such as Lutsey et al. and Vilieg et al., have tried to show the lack of an association between increased TG level and the recurrence of VTE; however, some investigators thought TG parameters alone were inappropriate for the exclusion of DVT or to predict the risk of recurrence of VTE.
P-selectin, which is stored in the granule membrane of resting platelets (a-granules) and endothelial cells (Weibel–Palade bodies), is a member of the selectin family of cell adhesion molecules together with E-selectin and L-selectin. The primary ligand for P-selectin in vivo is P-selectin glycoprotein ligand 1 (PSGL-1), which is expressed in the majority of leukocytes and is also present in trace levels on platelets. Transmembrane P-selectin is redistributed onto the cell surface after cell activation and partially discharged into the bloodstream in its soluble form (sP-selectin). It facilitates the interaction of leukocytes that express PSGL-1 with activated platelets and endothelial cells. In humans, elevated levels of soluble P-sel (sP-sel) are typical in DVT and VTE. The interaction between P-selectin and PSGL-1 is crucial for thrombus development. P-selectin was shown to have an impact on fibrin deposition in the thrombus by Palabrica et al. They discovered that inhibiting P-selectin interactions selectively prevented fibrin from being deposited on a thrombogenic graft in a baby as well as leukocyte adherence to platelets.
Korne l Miszti-Blasius et al. discovered that PSGL-1-null mice had milder thrombocytopenia, less fibrin deposition, and a smaller number of thrombosed blood vessels after giving collagen with epinephrine to wild-type and PSGL-1 knockout mice. As a result, it is conceivable that a lack of PSGL-1 might prevent leukocyte–platelet interactions and lessen the likelihood of thrombus development. Using sP-sel as a biomarker may improve the positive predictive value (as determined by a positive duplex ultrasound). According to a study evaluating the use of sP-sel in combination with a Wells risk prediction score for diagnosing VTE, this combination may be able to rule in the diagnosis of DVT with a sensitivity of 91% (low sP-sel and low Wells score to rule out the diagnosis) and a specificity of 98% (high sP-sel and high Wells score to rule in the diagnosis).
In conclusion, a significant antithrombotic impact was seen when PSGL-1 and P-selectin interacted. As a result, focusing on P-selectin or its ligand PSGL-1 may offer a viable treatment strategy for clinical circumstances. Numerous studies have revealed that the P-selectin–PSGL-1 interaction induces a procoagulant state by causing the formation of leukocyte-derived microparticles and mediating the transfer of tissue factor (TF) to platelets. This is in addition to its roles in mediating the binding of platelets and endothelial cells with leukocytes and enhancing fibrin deposition.
An increasing number of studies point to a role for inflammatory markers in VTE, including CRP and interleukin (IL)-1b, 6, 8, and 10. An initiator of the extrinsic route of coagulation, TF, may be affected by inflammatory cytokines, potentially setting off thrombotic illness. Recent laboratory investigations have shown that elevated CRP levels significantly impact the development of VTE.
