Ischemic stroke in patients previously anticoagulated for non-valvular atrial fibrillation: Why does it happen?

Oral anticoagulants have proved to be highly effective in preventing atrial fibrillation (AF)-related strokes. The occurrence of stroke despite oral anticoagulation is unexpected and little is known about the mechanisms responsible. The aim of this study was to assess possible mechanisms for stroke occurrence, such as poor treatment adherence and inappropriate dosage, in patients chronically anticoagulated for AF.

We performed a prospective observational study of 60 consecutive patients with non-valvular AF, chronically medicated with an oral anticoagulant and admitted due to ischemic stroke. Treatment adherence was assessed through the Brief Medication Questionnaire (BMQ) and the Medição da Adesão ao Tratamento (MAT) scales. Patient characteristics, stroke etiology, and appropriacy of anticoagulant dosage were also assessed.

Patients' mean age was 78.6±8.0 years, and 51.7% were male. Overall, the proportion of patients with good adherence to anticoagulants was 63.3%. Adherent patients were more frequently illiterate (26.3% vs. 4.5%, p=0.012). The proportion of patients under vitamin K antagonists (VKAs) with good treatment adherence was significantly higher than that of patients under novel oral anticoagulants (NOACs) (83.3% vs. 54.8%, respectively, for BMQ, p=0.035). However, 91.7% of patients under VKAs presented an admission INR <2. Subtherapeutic prescriptions were found in 43% of patients under NOACs.

In the majority of patients, stroke occurrence despite chronic anticoagulation appears to be explained by subtherapeutic dosage, poor treatment adherence or non-cardioembolic etiology, and not by inefficacy of the anticoagulants.