A PFO serves as a right-to- left conduit for paradoxical emboli originating in the veins, whereas ASA may be a nidus for thrombus formation. PFO and ASA have been associated with stroke in many, but not In the Patent Foramen Ovale in Cryptogenic Stroke Study (PICSS), a PFO was detected by transesopha- geal echocardiography more often in patients with crypto- genic stroke than in those with known causes of stroke (39.2% Another study also found that the prevalence of PFO was greater among patients with cryp- togenic stroke than among those with known causes of stroke, including patients <55 years of age (OR, 4.70; 95% CI, 1.89 A meta-analysis of case-control studies of patients who have had an ischemic stroke found that among patients 55 years of age there are significant associations with PFO (OR, 3.10; 95% CI, 2.29 4.21), ASA (OR, 6.14; 95% CI, 2.47 15.22), and PFO plus ASA (OR, 15.59; 95% In patients >55 years of age, the associa- tion with PFO was not significant (OR, 1.27; 95% CI, 0.80 2.01), although it was for ASA (OR, 3.43; 95% CI, 1.89 6.22) and for PFO plus ASA (OR, 5.09; 95% CI, 1.25 20.74). 