, ▁sleep ▁ap nea , ▁mig raine ▁with ▁a ura , ▁and ▁decom pression ▁sick ness . ▁No ▁cause ▁is ▁established ▁for ▁a ▁for amen ▁ov ale ▁to ▁remain ▁open ▁instead ▁of ▁closing ▁naturally , ▁but ▁her ed ity ▁and ▁gen et ics ▁may ▁play ▁a ▁role . ▁P FO ▁is ▁not ▁treated ▁in ▁the ▁absence ▁of ▁other ▁sympt oms . ▁ ▁The ▁mechanism ▁by ▁which ▁a ▁P FO ▁may ▁play ▁a ▁role ▁in ▁stroke ▁is ▁called ▁par ado x ical ▁em bol ism . ▁ ▁In ▁the ▁case ▁of ▁P FO , ▁a ▁blood ▁cl ot ▁from ▁the ▁ven ous ▁circul atory ▁system ▁is ▁able ▁to ▁pass ▁from ▁the ▁right ▁at rium ▁directly ▁into ▁the ▁left ▁at rium ▁via ▁the ▁P FO , ▁rather ▁than ▁being ▁filtered ▁by ▁the ▁l ungs , ▁and ▁there up on ▁into ▁system ic ▁circul ation ▁toward ▁the ▁brain . ▁ ▁P FO ▁is ▁common ▁in ▁patients ▁with ▁an ▁at rial ▁sept al ▁an eur ys m ▁( AS A ), ▁a ▁much ▁r ar er ▁condition , ▁which ▁is ▁also ▁linked ▁to ▁crypt ogen ic ▁( i . e ., ▁of ▁unknown ▁cause ) ▁stroke . ▁ ▁P FO ▁is ▁more ▁common ▁in ▁people ▁with ▁crypt ogen ic ▁stroke ▁than ▁in ▁those ▁with ▁a ▁stroke ▁of ▁known ▁cause . ▁While ▁P FO ▁is ▁present ▁in ▁ 2 5 % ▁in ▁the ▁general ▁population , ▁the ▁probability ▁of ▁someone ▁having ▁a ▁P FO ▁increases ▁to ▁about ▁ 4 0 ▁to ▁ 5 0 % ▁in ▁those ▁who