Cerebral arteries have a thinner media and adventitia than arteries elsewhere in the body and no external elastic lamina. The berry aneurysm is a small, saclike structure at the branchpoints of arteries with a wall which does not contain a media or internal elastic lamina and therefore can easily rupture during periods of hypertension or stress. Most commonly ruptures when the patient is 40-60 yrs. May also rupture intracerebrally.
Vascular malformations are much less common.
Anterior communicating artery and middle cerebral artery at the trifurcation are the most comman places for one to arise. Subarachnoid hemorrhage is blood in the leptomeninges usually at the base of the brain and most often due to rupture of a berry aneurysm, vascular malformation, or trauma. After three days macrophages can be seen phagocytosing degenerating blood cells. If the patient is in good enough condition an aneurysm may be clipped with a metal clip at its neck or a vascular malformation may be occluded by intrarterial plastic material. Robbins Pathologic Basis of Disease.