What causes an enlarged aorta? is it fatal?

Few things: High blood pressure, a congenital condition like Marfan syndrome, or structural heart disease like having a bicuspid valve or aortic regurgitation. It can be fatal if the aorta gets beyond a certain size. There is debate and each patient is different, but most surgeons would elect to replace an aorta that is bigger than about 50-52 mm in diamter.