Recurrent Stroke and Aspirin Resistance
Among high-risk patients, aspirin has been shown to reduce the odds of suffering a vascular event (stroke or myocardial infarction) or dying by about 25%. Our stroke service at the NNI, however, encounters a not uncommon clinical problem – that of patients who are already on aspirin because of coronary heart disease or a prior cerebral ischemic event, and then suffering a first or recurrent TIA or stroke. This so-called "aspirin failure" occurs in about 1 out of every 8 patients despite being on aspirin.
Not a single clinical trial has investigated the use of anti-platelets in this particular situation. Possible strategies include simply continuing aspirin, adding dipyridamole, adding or switching over to clopidogrel or ticlopidine, or switching to anticoagulation with warfarin. Until further evidence become available, it is prudent for physicians to make sure that the index stroke is atherothrombotic in origin and not due to conditions requiring anticoagulation (e.g. cardioembolic) or other treatments (e.g. severe carotid disease, vasculitis), that the dose of aspirin is sufficient (50 to 325 mg/day), that the patient is compliant with his medications, and that modifiable risk factors such as hypertension, diabetes, dyslipidemia, smoking, excessive alcohol intake, and sedentary lifestyle are corrected.
One topic that is gaining attention lately is the concept of "aspirin resistance." It appears that certain individuals may not show the expected platelet inhibitory response to aspirin as determined by laboratory tests like platelet aggregometry and urinary thromboxane metabolite, and they may be at higher risk for a recurrent event than "aspirin responders." The tests, however, have not been standardised and the clinical relevance of such condition is not yet clearly established. Here at the NNI, we are hoping to validate the laboratory definition of "aspirin resistance" and determine the frequency and accompanying risk for this condition, with the goal of eventually recommending the optimal treatment for individuals with "aspirin resistance."
References
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Eikelboom JW, Hankey GJ. Aspirin resistance: A new independent predictor of vascular events? J Am College Cardiol 2003;41:966-968.
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Gum PA, Kottke-Marchant K, Welsh PA, White J, Topol EJ. A prospective, blinded determination of the natural history of aspirin resistance among stable patients with cardiovascular disease. J Am College Cardiol 2003;41:961-965.
Hankey GJ, Eikelboom JW. Aspirin resistance. May be a cause of recurrent ischaemic vascular events in patients taking aspirin. BMJ 2004; 328:477-479.
Helgason CM, Tortorice KL, Winkler SR, Penney DW, Schuler JJ, McClelland TJ, Brace LD. Aspirin response and failure in cerebral infarction. Stroke 1993;24:345-350.