Apixaban for Treatment of Embolic Stroke of Undetermined Source

Inclusion criteria Must be ≥ 18 years at the time of signing the informed consent. * ESUS must be defined according to following criteria: * Stroke detected by CT or MRI that is not lacunar * Absence of extracranial or intracranial atherosclerosis causing ≥50% luminal stenosis in arteries supplying the area of ischaemia * No major-risk cardioembolic source of embolism * No other specific cause of stroke identified * \* At least one of the following non-major but suggestive risk factors for cardiac embolism: * LA size \>45mm (parasternal axis) * spontaneous echo contrast in LAA * LAA flow velocity \<=0.2m/s * atrial high rate episodes * CHA2DS2-Vasc score \>=4 * persistent foramen ovale * Understand and voluntarily sign an informed consent document * Women of childbearing potential (WOCBP) must be using an adequate method of contraception. Exclusion Criteria: * History of hypersensitivity to the investigational medicinal product * Participation in other clinical trials or observation period of competing trials. * Arteria cerebri media stroke affecting \> 30% of c o r r e s p o n d i n g territory * Diagnosis of haemorrhage or other pathology, * Clear indication for anticoagulation * Inability to control following risk factors for Hemorrhagic Transformation of fresh cerebral Infarction (HTI) during index hospital stay: presence of HTI at the time of anticoagulation, blood pressure \>140 mmHg systolic, abnormal blood glucose Clear indication for dual antiplatelet therapy * Clear stroke-/non-stroke-indication for concomitant long-term therapy with antiplatelets (e.g. acetylsalicylic acid (ASA), Clopidogrel, or Prasugrel) or with non-steroidal anti-inflammatory drugs (NSAID). * Concomitant systemic therapy with strong inhibitors of cytochrome P450 3A4 (CYP3A4) and P-glycoprotein (P-gp), i.e. azole antimycotics and human immunodeficiency virus (HIV)-protease inhibitors. * Contraindication to investigational medications * Planned or likely therapy with fibrinolytic agents within 48 hours of first study medication * History of intracranial, intraocular, spinal, retroperitoneal or atraumatic intra-articular bleeding * Gastrointestinal bleed or major surgery within 3 months * Planned or likely revascularization (any angioplasty or vascular surgery) within the next 3 months * TIA or minor stroke induced by angiography or surgery * Severe non-cardiovascular comorbidity with life expectancy \< 3 months * Severe renal failure, defined as Glomerular Filtration Rate (GFR) \<15ml/min * Severe hepatic insufficiency (Child-Pugh score B to C), * Active liver disease, * Contraindications against performance of MRI (pacemaker/ICD), previous implantation non-MRI capable protheses * Patients considered unreliable by the investigator, or having a life expectancy less than the expected duration of the trial