Altered fibrin clot properties in patients with retinal vein occlusion

Growing evidence indicates that formation of a fibrin clot composed of compact fiber networks, resistant to fibrinolysis, is associated with thrombotic events. Reduced fibrin clot permeability and impaired fibrinolysis have been reported in patients with a history of myocardial infarction [1], ischemic stroke [2] and venous thromboembolism (VTE) [3]. It is estimated that 16 million people worldwide suffer from retinal vein occlusion (RVO) [4], involving branch retinal vein occlusion (BRVO) and central retinal vein occlusion (CRVO). The pathogenesis of RVO is not fully understood [5]. Proven risk factors for RVO are age, arterial hypertension and glaucoma. Data on links between prothrombotic state and RVO are inconsistent [5]. A role of known thrombophilic factors in RVO appears minor [5]. To our knowledge, there has been no published report on fibrin clot characteristics in RVO. Given the association between atherosclerosis risk factors and prothrombotic fibrin clot phenotype, we hypothesize that abnormal properties of a fibrin clot can be observed in patients who experienced RVO, and therefore contribute to the occurrence of this thrombotic complication. Of the 120 screened white adult patients in whom first-ever RVO was diagnosed during the preceding 6 to 36 months, 59 individuals were included in this case–control study from March 2009 to May 2010 in Krakow. The diagnosis of RVO was based on typical findings in the fundus of the eye documented by digital photography, fluorescein angiography and optical coherence tomography. Inclusion criteria were age between 18 and 75 years, and absence of chronic and acute disorders known to affect clot properties. Exclusion criteria were: age above 75 years (n = 26); C-reactive protein (CRP) > 10 mg L (n = 2); cancer (n = 2); hepatic or renal dysfunction (n = 0); anticoagulant therapy (n = 1); acute myocardial infarction or stroke within the preceding 6 months (n = 4); previous VTE (n = 4); and diabetes (n = 22). One hundred and eighteen subjects matched for age, sex, body mass index (BMI), ischemic cardiovascular events and smoking from the general population of the south of Poland, described previously [6], served as disease controls. The same exclusion criteria were used for controls. Individuals with abnormal findings during ophthalmologic evaluation were excluded from the controls. The University Bioethical Committee approved the study and patients provided written, informed consent. Fasting blood samples were drawn from the antecubital vein with the minimal stasis in the morning. Lipid profile, glucose, platelet count and creatinine were measured using routine laboratory techniques. Total homocysteine (tHcy) was assessed using high pressure liquid chromatography as described [7]. Immunoenzymatic assays were used to determine tissue plasminogen activator (tPA) and plasminogen activator inhibitor-1 (PAI-1) antigens. CRP and fibrinogen were measured by nephelometry (Dade Behring, Marburg, Germany). Thrombophilia screening was performed as described [6]. Clot variables were determined in citrated plasma at least in duplicate by technicians blinded to the origin of the samples (intra-assay and inter-assay coefficients of variation, 5%–8%), as described in detail [3]. 1 Fibrin clot permeation using a pressure-driven system, with calculation of a permeation coefficient (Ks), which indicates the pore size. LowerKs values indicate reducedpermeability. 2 The lag phase of the turbidity curve, which reflects the time required for initial protofibril formation and maximum absorbance at 405 nm at the plateau phase (DAbsmax), indicating the number of protofibrils per fiber. 3 Clot lysis time, defined as the time required for a 50% decrease in clot turbidity at 405 nm (t50%), induced by 1 lg mL recombinant tissue plasminogen activator (rtPA; Boerhinger Ingelheim, Ingelheim, Germany). 4 Maximum rate of increase in D-dimer levels (American Diagnostica, Greenwich, CT, USA) and maximum Ddimer concentrations measured every 30 min in a buffer containing 0.2 lmol L rtPA (Boerhinger Ingelheim) percolating through fibrin clots prepared as in the permeation evaluation. Data are given as mean ± SD or median (interquartile range), or as otherwise stated. Inter-group comparisons were analyzed by Fischer exact, chi-squared, t-Student and Mann– Whitney tests as appropriate. Correlations were assessed by the tau-B Kendall test. To determine whether any fibrin clot variables are independent correlates for RVO, a forward multivariate logistic regression for matched pairs data using Correspondence: Anetta Undas, Institute of Cardiology, Jagiellonian University School of Medicine, 80 Pradnicka Street, 31-202 Krakow, Poland Tel. : +48 12 6143004; fax: +48 12 4233900. E-mail: mmundas@cyf-kr.edu. pl