Aspirin resistance signals increased stroke severity

Stroke severity and infarct volume are significantly increased among patients who become resistant to aspirin, show study findings published in Neurology.

"Therefore laboratory tests for aspirin resistance should be considered before modifying an antiplatelet regimen in patients who experience new or recurrent ischemic stroke while taking aspirin", recommend Byung-Chul Lee (Hallym University College of Medicine, South Korea) and co-researchers.

They studied 310 patients who were admitted within 48 hours of acute ischemic stroke onset and had been taking aspirin 100 mg/day for at least 7 days before.

Aspirin resistance was seen in 86 (27.7%) patients, based on high residual platelet reactivity (HRPR) of at least 550 on the VerifyNow (Accumetrics, San Diego, California, USA) assay.

Stroke severity was significantly worse for these patients than for those without aspirin resistance, with a median National Institutes of Health Stroke Scale (NIHSS) score of 6 versus 3, supporting findings from previous studies but in a larger sample size.

The researchers also used the diffusion-weighted imaging (DWI) to clinically assess neurological deficits in the patients, finding that those with aspirin resistance had significantly larger infarct volumes, at a median 5.4 cm³ versus 1.7 cm³.

NIHSS scores and infarct volume remained significantly higher in patients with aspirin resistance after taking into account age, gender and other variables such as hypertension, antihypertensive drug use, stroke type and delay in symptom onset to hospital arrival, with median between group differences of 2.1 points and 2.3 cm³, respectively.

While the study design ruled out conclusions regarding the cause and effect relationship between HRPR and stroke severity and infarct volume, the researchers found that the negative effects of aspirin resistance were more apparent in patients with NIHSS scores and DWI infarct volumes in the upper quantiles (50th, 75th and 90th) relative to lower ones (10th, 25th).

Therefore, "HRPR had a greater negative association with stroke severity in patients with greater thrombosis", they explain.

There was also evidence of a relationship between the preventive effects of aspirin and the underlying stroke mechanism. Aspirin resistance was significantly associated with higher NIHSS scores and larger infarct volumes in patients with large-artery atherosclerosis and those with stroke of other or undetermined aetiology, but not in those with cardioembolic stroke or small vessel occlusion.

This suggests that aspirin resistance has a greater influence on stroke severity in patients with atherosclerotic versus nonatherosclerotic stroke, says the team.

By Lucy Piper

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