• 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

• Cardiac monitoring is planned to be performed with the BIOMONITOR III(m) device

• 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.

• Occlusion of PFO is planned to be performed with the AMPLATZERTM PFO OCCLUDER device.

• Patient is willing to sign patient consent form.

• Age ≥18 years.