Most common cause of subarachnoid hemorrhage outside of trauma is a ruptured berry aneurysm. 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 in his 40's or 50's. May also rupture intracerebrally. Aneurysms are thought to be due to congenital absence of the media and internal elastic lamina with ballooning as the blood pulsates through the vessels over the years.
Vascular malformations are much less common.
The Anterior communicating artery and middle cerebral artery at the trifurcation are the most common sites of origin. Subarachnoid hemorrhage is blood in the leptomeninges, usually at the base of the brain, and most often due to rupture of a berry aneurysm or vascular malformation, or trauma. After three days, macrophages clean up 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.