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A paradoxical embolism is a blood clot that does not travel with normal blood flow. Normally, blood flows from the right side of the heart through the pulmonary arteries and lungs before it returns to the left side of the heart.

This type of embolism often causes a stroke because the clot moves directly from the right side of the heart to the left through a hole (defect) in the septum, which separates the upper right and left heart chambers. This hole is called a patent foramen ovale. The clot is pumped out of the heart and toward the brain, where it can cause a stroke.