Patients with acute ischemic stroke who test positive for aspirin resistance had both larger stroke volume and increased severity, compared with patients without resistance, in an observational study of 311 patients at Korean centers.
Given that previous studies have shown that the use of aspirin is associated with lower stroke severity and decreased infarction growth, the current study’s findings may help to define the effect of aspirin resistance (AR) on stroke severity, since previous studies had provided inconclusive results, Dr. Mi Sun Oh and colleagues at Hallym University Sacred Heart Hospital, Anyang, South Korea, wrote in their abstract. The findings were released Feb. 23 in advance of the annual meeting in April of the American Academy of Neurology.
The investigators enrolled patients with acute ischemic stroke confirmed by diffusion-weighted imaging (DWI) who had received at least 7 days of aspirin therapy before initial stroke symptoms and had been checked for AR within 24 hours of hospital admission. Patients with high prestroke disability scores (modified Rankin Scale score > 2) were excluded, as were those who were taking another antiplatelet or anticoagulant medication concurrently with aspirin on hospital admission.
The abstract did not report detailed patient characteristics or information about type or dose of aspirin; the full results of the study will be presented at the meeting in Washington.
Enrollees were deemed aspirin resistant if a rapid assay detected greater than 550 Aspirin Reaction Units. DWI-observed stroke volume was assessed via a semiautomated threshold technique, and investigators employed the National Institutes of Health Stroke Scale (NIHSS) score to measure initial stroke severity.
Seventy-eight of the 311 patients (25.1%) had AR. Dr. Oh and colleagues reported that median stroke volume was higher for these patients, compared with the aspirin-sensitive group (2.8 cc vs. 1.6 cc), as was least-square mean on multivariate analysis (1.6 cc [95% CI, 1.1-2.1] vs. 1.1 cc [95% CI, 0.7-1.4], P = .036). Median NIHSS scores were also higher for the AR group (4 vs. 3), indicating greater stroke severity, a result that was confirmed by multivariate analysis.
Aspirin resistance is a complicated and heterogeneous concept, and not a well defined entity, according to vascular neurologist Dr. Philip Gorelick, head of the Hauenstein Neuroscience Center at St. Mary’s Health Care in Grand Rapids, Mich. Dr. Gorelick is an honorary member of the Korean Stroke Society but was not involved in the present study. In an interview, he expanded on the diverse mechanisms that can impede the stroke prevention effect of antiplatelet agents such as aspirin (Stroke Res. Treat. 2013;Article ID 727842 [doi:10.1155/2013/727842]).
In contrast to the traditional notion of “resistance” as an inherent or acquired defense or chemical blockage of a drug, whether by a microbe or the host, aspirin resistance may be either a laboratory-defined lack of inhibition of thromboxane A2, or a clinically-defined entity. In either case, a host of factors may contribute, Dr. Gorelick said. Poor adherence to an aspirin therapy regimen may be a primary contributor to AR. Further, enterically coated aspirin may not be as well absorbed in the gut, leading to lower effective aspirin dosing. A host of other factors, including concurrent medication administration, comorbidities impacting platelet turnover, and genetic polymorphisms may also contribute to aspirin failure.
Although patient characteristics were not reported in this study, Dr. Gorelick did issue a general note of caution: “Another major issue in these types of studies,” he noted, is to determine if “patients are similar in terms of background factors. Patients on aspirin therapy may be more likely to have more severe preexisting vascular disease,” predisposing them to more severe stroke.
The Korea Healthcare Technology R&D Project, Ministry of Health and Family Welfare, and the Republic of Korea supported the study. The authors had no disclosures.