Patent foramen ovale (PFO) is a frequent finding on echocardiography and occurs in over 25% of the population (Wechsler 2008). Therefore, in general the finding of a patent foramen ovale in a patient with a stroke certainly by itself does not establish this as etiologically responsible for the stroke.
In young adults with cryptogenic stroke, the likelihood that a patent foramen ovale is responsible for a stroke is much greater than in the general adult population. In young adults (generally less than 55 years of age) up to 40% of acute ischemic strokes are cryptogenic (Ghosh et al 2007). A patent foramen ovale is present in more than half of these patients, with an increased incidence of atrial septal aneurysm (ASA), suggesting that paradoxical embolization may be responsible for the clinical events in a sizeable fraction of these patients. An atrial septal aneurysm is associated with the presence of a large patent foramen ovale (Homma et al 2004).
Studies examining the association between patent foramen ovale and stroke have produced somewhat variable results, but in general the risk of initial and recurrent stroke in individuals with patent foramen ovale is relatively low (Khairy et al 2003; Meissner 2005; Wöhrle 2006; Messé and Kasner et al 2008; Windecker and Meier 2008; Almekhlafi et al 2009). According to the American Academy of Neurology (AAN) practice parameter, "PFO is not associated with increased risk of subsequent stroke or death among medically treated patients with cryptogenic stroke. However, both PFO and ASA possibly increase the risk of subsequent stroke (but not death) in medically treated patients younger than 55 years" (Messé et al 2004). Similarly, in a meta-analysis of data by Devuyst and Bogousslavsky, the authors concluded, "among patients who have had a cryptogenic stroke under treatment (with either warfarin or aspirin), the main data from the French PFO/ASA [study] and PICSS (Patent Foramen Ovale in Cryptogenic Stroke Study), analyzed separately and in combination, indicate that PFO alone does not announce a significantly increased risk of recurrent stroke or death. But a small increase or decrease in risk cannot be excluded by this meta-analysis" (Devuyst and Bogousslavsky 2005). A recent meta-analysis of medically treated patients with prior cryptogenic stroke by Almekhlafi and colleagues concluded that "available evidence does not support an increased relative risk of recurrent ischemic events in those with vs without a patent foramen ovale" (Almekhlafi et al 2009).