Cerebral microbleeds and thrombolysis

Introduction Thrombolytic therapy remains the only treatment for acute ischemic stroke proven to improve clinical outcome. However, the development of intracerebral hemorrhage (ICH) following treatment remains an important limitation of this therapy and is symptomatic in 2.4-10% of patients depending on the definition used [1-3]. Of note, hemorrhage occurs in 20-30% of patients in a location remote from the acute ischemic field, suggesting an underlying hemorrhage-prone vasculopathy in this subgroup of patients [1,4]. Identifying patients at higher risk for hemorrhagic transformation following thrombolysis is an important goal, and this information, in turn, could increase the overall safety profile of thrombolytic therapy. Since the 1990s, when MRI increasingly came into use as an imaging modality for acute stroke, a growing body of data has shown that cerebral microbleeds (CMBs) are, in fact, an important marker for a bleeding-prone vasculopathy. Yet, only a few studies have systematically evaluated the risk of ICH following thrombolysis in patients with CMBs. While the results of studies available to date have begun to elucidate the risk, a number of important questions remain. Does the presence of cerebral CMBs increase the risk of hemorrhage with thrombolytic therapies? If there is an increased risk, does this outweigh the benefits? Can patients at the highest risk be identified: is there a threshold number, a pattern of location (e.g. lobar) or other patient characteristics that change the risk-benefit ratio? In the first report addressing the role of CMBs in thrombolysis-induced symptomatic ICH, Kidwell et al. described a series of 41 patients screened with a baseline MRI followed by intra-arterial thrombolysis for large vessel acute stroke[5].