Before birth, the foramen ovale is a vital foetal opening between the right and left atria, allowing oxygen-rich placental blood to bypass the inactive lungs. After the first breath, pressure in the left atrium increases, closing the flap naturally. In around 25% of adults, the flap does not seal fully — this persistent communication is called a Patent Foramen Ovale (PFO) which is an important cause of cryptogenic stroke. All standard tests including an echocardiogram can be normal- it can be only be diagnosed using a bubble contrast study and once diagnosed, treatment with clam shell closure is high effective.

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