Wyburn-Mason syndrome is an extremely rare nonhereditary disorder that is present at birth (congenital). Affected infants have arteriovenous malformations (AVMs), which are developmental abnormalities affecting the blood vessels, specifically the arteries, veins and capillaries. Arteries typically carry oxygen-rich blood from the heart to body cells, while veins transport oxygen-deficient blood to the heart and lungs for the exchange of oxygen and carbon dioxide. The network of very tiny blood vessels (capillaries) that normally connects arteries and veins may be absent and the arteries and veins may be directly linked together. Larger AVMs may consist of a tangled mass of abnormal or malformed blood vessels. AVMs associated with Wyburn-Mason syndrome are usually found in the eyes and midbrain. The exact cause of Wyburn-Mason syndrome is unknown.

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