Platelet function and thrombin generation in ischemic stroke : clinical correlates and prognostic importance

Sammanfattning: Platelet function is central to atherothombotic diseases and antiplatelet agents prevent non-cardioembolic ischemic stroke (IS). Individual differences in platelet reactivity may limit the response to antiplatelet treatment, leading to so-called high on-treatment platelet reactivity (HPR). HPR is associated with increased risk of cardiovascular events. Platelet activation leads to the release of platelet microvesicles (PMV), and circulating PMV are considered a measure of in vivo platelet function. Activated platelets and PMV enhance thrombin generation and thrombin in turn is a strong platelet agonist. The overall aim of this thesis was to study the importance of platelet function, circulating PMV, thrombin generation and associations with clinical characteristics and prognosis in patients with IS or transient ischemic attack (TIA). The prevalence of HPR to clopidogrel was assessed by whole blood aggregation in a cross-sectional study of 72 patients treated with clopidogrel one month after IS or TIA. Associations between HPR and clinical variables were determined, focusing on glucose metabolism, IS subtype and arterial vessel changes (study 1 and 2). Circulating PMV, expressing activation markers P-selectin or tissue factor (TF), and thrombin generation variables were measured in the acute and convalescent phase of IS/TIA in a cohort study of 211 patients (study 3 and 4). Associations with prognosis were evaluated after a 5 to 7 year follow-up. The primary outcome was a composite of recurrent IS, myocardial infarction and ischemic cardiovascular death; recurrent IS was a secondary outcome. HPR to clopidogrel was found in 16/72 IS/TIA patients (22%) and was associated impaired glucose tolerance/diabetes, insulin resistance, hypertension and radiological white matter changes (WMC), indicating cerebral small vessel disease (CSVD). There were no associations between HPR and carotid atherosclerosis, other manifestations of large vessel atherosclerosis or IS stroke subtype. PMV populations expressing P-selectin or TF were substantially elevated in the acute phase of IS/TIA, and remained elevated in the convalescent phase. Only PMV expressing TF and lacking phosphatidylserine (PS) were associated with poor outcome, with a hazard ratio (HR) of 1.86 [1.04-3.31] p=0.036 after adjustment for cardiovascular risk factors. Unexpectedly, high levels of several PMV populations appeared inversely associated with poor outcome. Despite being elevated in patients, endogenous thrombin generation potential (ETP) and peak thrombin measured in the acute phase were associated with reduced risks of primary outcome and recurrent IS after adjustment for cardiovascular risk factors, HR 0.40-0.53, p < 0.05 for all. MV-induced thrombin generation potential and MV peak thrombin were associated with increased risk of recurrent IS in univariate analysis. F1+2 was lower in patients than healthy controls, but not associated with outcome. There were no correlations between thrombin generation variables and PMV populations. In summary, HPR to clopidogrel in patients with IS or TIA was common and the risk of HPR increased in the pre-diabetic phase. HPR was associated with CSVD but not with large artery atherosclerosis. Circulating levels of PMV populations after an episode of IS/TIA had different associations with outcome; PMV expressing TF but lacking PS merit further investigation as they were associated poor prognosis. High levels of ETP and peak thrombin in the acute phase were associated with reduced risk of recurrence, while MV-induced thrombin generation was associated with increased risk of recurrent IS.