A new suspected cause of cryptic strokes is “silent atrial fibrillation.” Pacemakers and other implanted devices allow continuous recording of cardiac rhythm for months or years. They have discovered that short periods of atrial fibrillation lasting minutes or hours are frequent and usually are asymptomatic. A meta-analysis of 50 studies involving more than 10,000 patients with a recent stroke found that 7.7% had new atrial fibrillation on their admitting electrocardiogram. In 3 weeks during and after hospitalization, another 16.9% were diagnosed. A total of 23.7% of these stroke patients had silent atrial fibrillation; that is, atrial fibrillation diagnosed after hospital admission. Silent atrial fibrillation is also frequent in patients with pacemakers who do not have a recent stroke. In a pooled analysis of 3 studies involving more than 10,000 patients monitored for 24 months, 43% had at least 1 day with atrial fibrillation lasting more than 5 minutes. Ten percent had atrial fibrillation lasting at least 12 hours. Despite the frequency of silent atrial fibrillation in these patients with multiple risk factors for stroke, the annual incidence of stroke was only 0.23%. When silent atrial fibrillation is detected in patients with recent cryptogenic stroke, anticoagulation is indicated. In patients without stroke, silent atrial fibrillation should lead to further monitoring for clinical atrial fibrillation rather than immediate anticoagulation, as some have advocated.
More than 25% of ischemic strokes per year in the US are classified as cryptogenic: that is, after extensive evaluation, no cause is determined.1
Some2, 3 have suggested that cryptogenic strokes may be due to paradoxical embolism of venous thrombi across a patent foramen ovale. This led to surgical closure of patent foramen ovales in some patients, and then patent foramen ovale closure by a transvenous device in thousands of patients.4 However, 3 randomized clinical trials failed to find a benefit of patent foramen ovale closure.5 Paradoxical embolism is a rare cause of ischemic strokes.5
Cardioembolism is the commonest cause of ischemic stroke,6 and atrial fibrillation is the commonest cause of cardioembolism.7 The percentage of ischemic strokes due to cardioembolism increases with age; in one series it accounted for <5% of strokes in patients younger than 45 years of age and 53% in patients older than 70 years. This parallels the increasing prevalence of atrial fibrillation with increasing age.8
The risk of stroke in patients with atrial fibrillation can be determined by a score based on the presence or absence of congestive heart failure, hypertension, diabetes, history of stroke or transient ischemic attack (TIA), vascular disease, age >75 years, sex (CHA2DS2-VASc).9
Multiple randomized clinical trials have shown that anticoagulation with warfarin or one of the newer oral anticoagulants reduces the rate of stroke in patients with atrial fibrillation by at least two-thirds and reduces death by 25%.10
Anticoagulation is effective in patients with paroxysmal atrial fibrillation as well as patients with chronic atrial fibrillation. Hohnloser et al11 randomized 6706 patients with atrial fibrillation to treatment with oral anticoagulants or aspirin plus clopidogrel to prevent strokes. Twelve hundred two of these patients had paroxysmal atrial fibrillation defined as sinus rhythm at randomization, but atrial fibrillation by electrocardiogram (ECG) on 2 separate occasions, at least 2 weeks apart in the 6 months prior to randomization. The other 5495 patients had chronic atrial fibrillation. The incidence of stroke during follow-up in those with paroxysmal atrial fibrillation (2.0%/y) was not significantly different from those with chronic atrial fibrillation (2.2%/y).
Hart et al12 compared the rate of stroke in patients treated with aspirin in 1126 patients with permanent atrial fibrillation with 460 patients with intermittent atrial fibrillation on admission, but sinus rhythm documented within the prior 6 months. The rates of stroke during 2-year follow-up was nearly the same: 3.2% vs 3.3%.12
Detection of Silent Atrial Fibrillation
The ability of modern dual-chamber pacemakers to function as permanently implanted cardiac monitors has led to the observation that transient atrial fibrillation lasting minutes or hours is frequent in patients with pacemakers. The vast majority of these episodes are asymptomatic and are termed “silent atrial fibrillation.”13
These observations lead to many important questions. Are patients with episodes of silent atrial fibrillation at increased risk of stroke? Is silent atrial fibrillation a major cause of cryptic stroke? How long must these episodes last to increase the incidence of left atrial thrombi? Would oral anticoagulants prevent strokes in these patients? Which patients with silent atrial fibrillation should be anticoagulated?
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-James E. Dalen, MD, MPH, ScD (hon), Joseph S. Alpert, MD
This article originally appeared in the March 2017 issue of The American Journal of Medicine.
