Patent foramen ovale: Seeing through the mist

In 1877 Connheim was the first to describe paradoxical embolism through a patent foramen ovale (PFO), a phenomenon that has since been documented in numerous autopsy and echocardiographic studies.Besides migration of thrombi through the septal defect, other mechanisms have been proposed that implicate PFO in cardioembolic phenomena, including increased vulnerability to atrial arrhythmias and local thrombus formation. 1arious studies have demonstrated an association between PFO and cryptogenic stroke, but others have questioned the causal nature of this relationship. 2---4 The figures show the importance of this link: around 25% of the general population have PFO, and around 40% of ischemic strokes are cryptogenic (of undetermined cause); PFO is a plausible mechanism that might explain many of them.But with such a common finding as PFO, how can its guilt or innocence be established, particularly regarding ischemic stroke?Distinguishing association from causality is always a challenge, but in this context is crucial.