Patent foramen ovale (PFO) occurs if the foramen ovale, a hole between the heart's atria made in normal foetal development, does not close after birth as it should.
PFO is common, affecting about 1 in 4 people.
PFO can increase risk of ischaemic stroke, because thrombi (blood clots) can pass from the right to the left side of the circulation and more easily reach the brain - normally small thrombi are filtered out by the lungs.