Aneurysms in the brain occur when there is a weakened area in the wall of a blood vessel. They may occur as a congenital (present from before birth) defect or may develop later in life.
A saccular aneurysm (berry aneurysm) can vary in size from a few millimeters to over a centimeter. Giant berry aneurysms can reach well over 2 cm. The aneurysm resembles a sack of blood attached to one side of the blood vessel by a narrow neck. These are more common in adults. Other types of cerebral aneurysm may involve widening (dilatation) of the entire circumference of the blood vessel in an area, or may appear as a ballooning out of part of a blood vessel. These types of aneurysms can occur in any blood vessel that supplies the brain. Trauma and infection, which can injure the blood vessel wall, can cause such aneurysms. It is estimated that 5% of the population has some type of aneurysm in the brain. However, the incidence of ruptured aneurysm is approximately 10 out of 100,000 people per year. Multiple berry aneurysms are not unusual. About 10% of patients who have one aneurysm will have at least one more. Risk factors include a family history of cerebral aneurysms, and some medical problems, including polycystic kidney disease and coarctation of the aorta.
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