Atrial Fibrillation Before and After Patent Foramen Ovale Closure Study

Inclusion Criteria: 1. History of embolic events such as cryptogenic stroke or systemic embolism (single event or multiple events). A stroke is considered to be cryptogenic if no possible cause can be determined despite extensive workup according to the standard protocol of the participating center (TOAST classification 5b) (29). Before inclusion in the study, the following tests are required as standard tests to establish the diagnosis of cryptogenic stroke: * MRI or computed tomography (CT) of the neurocranium (documenting ischemic embolic stroke) * 12-lead ECG (exclusion of AF) * Continuous ECG monitoring for at least 7 days (inpatient telemetry or Holter-ECG as an in- or outpatient * Ultrasonography, CT or MRI angiography of head and neck to rule out arterial disease as a cause of stroke (see below), or other potential causes of stroke 2. Cardiac monitoring is planned to be performed with the BIOMONITOR III(m) device 3. Presence of right-to-left shunt through a PFO as assessed by means of transesophageal echocardiography (TEE) with agitated saline while the patient is at rest or while a Valsalva maneuver is being performed. 4. Occlusion of PFO is planned to be performed with the AMPLATZERTM PFO OCCLUDER device. 5. Patient is willing to sign patient consent form. 6. Age ≥18 years. Exclusion Criteria: 1. Known etiology of the embolic event (based on neuro-/cardiac/vascular imaging), such as: * Evidence of large-artery atherosclerosis (ultrasonography, CT or MRI angiography, or digital subtraction angiography) with stenosis of 50% or more in the artery feeding the acute ischemic territory. * Small vessel disease, defined by radiographic appearance consistent with ischemic infarction in the territory of a perforating arteriole, with ≤20 mm in diameter on axial sections and not involving the cortex, located in the white matter, internal or external capsule, deep brain nuclei, thalamus, or brainstem. * Evidence of a high-risk cardiac or aortic arch source of embolism (left ventricular or left atrial thrombus or "smoke," emboligenic valvular lesion or tumor, aortic arch plaque \>3 mm thick or with mobile components or any other high-risk lesion) * Stroke of other determined cause such as presence of non-atherosclerotic vasculopathies (i.e. dissection, fibromuscular dysplasia), hypercoagulable states (must be tested in patients \<55 years old) and hematologic disorders 2. Atrial septal defect or ventricular septal defect. 3. Coronary or valvular disease requiring surgical intervention. 4. Documented history of AF or atrial flutter. 5. Permanent indication for therapeutic oral anticoagulation at enrollment. 6. Already included in another clinical trial that will affect the objectives of this study. 7. Life expectancy \<1 year. 8. Pregnancy. 9. Patient underwent or is scheduled for implantation of a pacemaker, implantable cardioverter defibrillator or cardiac resynchronization therapy device. 10. Unable or unwilling to follow the required procedures of the Clinical Investigation Plan.