Is stroke more common in PFO or ASD?

Is stroke more common in PFO or ASD?

Abstract: Stroke is more likely because of the potential for  R > L shunt in strain phase in PFO. In ASD the shunt is left to right and hence paradoxical embolism is less likely.

Paradoxical embolism can cause stroke in the presence of an interatrial communication. Emboli originating in the systemic  veins usually travel to the pulmonary circulation causing pulmonary embolism. But in the presence of an interatrial communication, emboli originating from the systemic veins, usually from the deep veins of lower limb and pelvic veins, can traverse the interatrial septum and reach the left side of the heart. From the left side of the heart the emboli are carried by the arterial circulation and can get lodged in the cerebral arteries causing ischemic stroke.

Atrial septal defect is an unrestricted communication between the two atria so that in the postnatal life, blood flows from the left atrium to right atrium because of the higher compliance of the right ventricle compared to the left ventricle. Patent foramen ovale on the other hand is a valvular opening with hardly any flow in the postnatal life. The valvular nature of the opening prevents flow due to the higher left atrial pressure which keeps the opening closed.

But in certain phases of Valsalva like strain, the valvular patent foramen ovale opens up, allowing shunt from the right atrium to the left atrium. This phasic shunt can allow a critically timed embolus to travel from the systemic venous side to the systemic arterial side, potentially leading to ischemic stroke. Hence theoretically, stroke due to paradoxical embolism is more likely to occur with a patent foramen ovale than an atrial septal defect.