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Cryptogenic stroke survivors who went on to get an insertable cardiac monitor (ICM) had a similar risk of recurrent stroke whether or not any atrial fibrillation (Afib, AF) ever turned up on monitoring, an observational study from Japan showed.

Ischemic stroke recurred in a similar 4.0% of Japanese patients without Afib detected and 5.8% of those who did have Afib detected during nearly 2 years of monitoring after their index stroke (OR 1.47, 95% CI 0.54-3.95), reported study investigators led by Kenichi Todo, MD, PhD, of Osaka University Graduate School of Medicine in Japan.

The stroke risk likewise was not higher immediately after Afib detection or within 90 days in a landmark analysis. An Afib burden at or above the median of 0.1% (a low threshold) also did not appear to affect stroke recurrence rates, according to the CRYPTON-ICM registry published in the .

Complicating interpretation, though, was that the study was done in Japan, where guidelines call anticoagulation reasonable for cryptogenic stroke regardless of Afib detection. Thus, about one-third of the comparator no-Afib group was on anticoagulation, which might have impacted comparison between groups.

A diagnosis of cryptogenic stroke or embolic stroke of undetermined source is given to people whose cause of stroke is unknown despite a thorough diagnostic workup. In these patients, covert paroxysmal Afib is often suspected and may be confirmed with close monitoring for subsequent treatment.

"However, the superiority of long-term monitoring with external cardiac monitor or ICM over conventional monitoring for the prevention of stroke recurrence has not been demonstrated," Todo and colleagues noted. "One of the explanations for this may be the lower cardiovascular burden and lower risk of stroke recurrence in patients with AF detected after stroke compared with patients with known AF before stroke."

In an , Michela Rosso, MD, and Brett Cucchiara, MD, both of the University of Pennsylvania in Philadelphia, stressed the questions surrounding the proliferation of mobile cardiac outpatient telemetry devices and implantable loop recorders used to hunt low-burden intermittent Afib.

"Does low-burden AF detected on prolonged monitoring really have the same risk of stroke as permanent AF? Is it really the cause of patients' strokes, or just an incidental finding? Is the stroke prevention effect of anticoagulation, and the balance of benefit compared with bleeding risk, similar in such patients? These critical questions were leapfrogged in the thrill of the hunt for AF," the duo wrote.

The similarity of recurrent stroke risk between cryptogenic stroke with and without Afib in the study was "possibly because of a relatively small burden of AF and appropriate initiation of anticoagulation therapy after AF detection with >90% anticoagulation rate," Todo's group wrote.

In the U.S., antithrombotic therapy -- typically an antiplatelet -- is for nearly all cryptogenic stroke patients who do not have contraindications. For embolic stroke of undetermined source (about half of cryptogenic stroke), direct oral anticoagulants are not recommended.

Another implication of the study findings, Todo and colleagues suggested is in support of anticoagulation in these patients: "We confirmed that it is reasonable to switch from antiplatelet therapy to anticoagulation therapy after detection of AF by ICM instead of empirically initiating anticoagulation therapy before AF detection in cryptogenic stroke."

The investigators reported that almost 32% of the study population had already started anticoagulation therapy by the time of ICM implantation. Then, anticoagulation therapy was initiated in 92% of the Afib group.

Todo's team reported ischemic stroke recurrence in 5.5% of those with Afib receiving anticoagulation therapy and in 10% of the few Afib patients not receiving anticoagulation therapy -- a difference that did not reach statistical significance.

However, the sample size was inherently too small to show an association between Afib and stroke recurrence, the researchers recognized.

The retrospective observational study was based on 370 patients with cryptogenic stroke who underwent ICM implantation at eight stroke centers in Japan from 2016 to 2020. Median age was 71 years, and 65% of participants were men.

Patients waited a median 22 days from index stroke to ICM implantation. Monitoring continued for an average of 637 days. During that time, 121 had Afib detected and showed an "extremely low" 0.1% median Afib burden.

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