Subarachnoid hemorrhage is bleeding in the area between the brain and the thin tissues that cover the brain. This area is called the subarachnoid space.

Subarachnoid hemorrhage can be caused by:

• Bleeding from an arteriovenous malformation (AVM)
• Bleeding disorder
• Bleeding from a cerebral aneurysm
• Head injury
• Unknown cause (idiopathic)
• Use of blood thinners

Injury-related subarachnoid hemorrhage is often seen in the elderly who have fallen and hit their head. Among the young, the most common injury leading to subarachnoid hemorrhage is motor vehicle crashes.

Subarachnoid hemorrhage due to rupture of a cerebral aneurysm occurs in approximately 10-15 out of 10,000 people. Subarachnoid hemorrhage due to rupture of a cerebral aneurysm is most common in persons age 20 to 60. It is slightly more common in women than men.

Risks include:

• Aneurysms in other blood vessels
• Fibromuscular dysplasia (FMD) and other connective tissue disorders associated with aneurysm or weakened blood vessels
• High blood pressure
• History of polycystic kidney disease
• Smoking

A strong family history of aneurysms may also increase your risk.