Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Subarachnoid Hemorrhage

What Is It?

A subarachnoid hemorrhage is bleeding from a damaged artery at the surface of the brain. This bleeding often causes a sudden, severe headache. It is a medical emergency. Subarachnoid hemorrhage is a type of stroke. It can cause permanent brain damage.

Blood from a subarachnoid hemorrhage pulses into the space between the brain and the skull. It mixes with the cerebrospinal fluid that cushions the brain and spinal cord. As blood flows into the cerebral spinal fluid, it increases the pressure that surrounds the brain. The increased pressure can interfere with brain function.

In the days that immediately follow the bleeding, chemical irritation from clotted blood around the brain can cause brain arteries to go into spasm. An artery spasm can cause additional new brain damage.

Most often, a subarachnoid hemorrhage happens because a bulge in the wall of an artery ruptures. The sac-like bulge is called a saccular aneurysm. A subarachnoid hemorrhage also can occur because blood leaks from an abnormal tangle of blood vessels called an arteriovenous malformation (AVM).

Ruptured Aneurysm
Several large arteries form a circle at the base of your brain. When a subarachnoid hemorrhage is caused by a ruptured saccular aneurysm, the aneurysm usually is located where a blood vessel branches from one of these large arteries. About 20% of patients who have had a subarachnoid hemorrhage have multiple aneurysms.

Although it is not possible to predict whether an aneurysm will rupture, an aneurysm is more likely to rupture when it has a diameter of 7 millimeters or more. In most cases, a person who has a brain aneurysm never has a symptom related to it.

Leaking AVM
An AVM is a tangled, abnormal mesh of blood vessels that connects an artery and a vein in the brain. AVMs form accidentally prior to birth. An AVM can form almost anywhere in the brain or spinal cord, but they usually are near the back of the brain.

AVMs can appear in several generations of the same family and are more common in men. Bleeding from an AVM most often occurs between the ages of 10 and 30. If an AVM is too deep to cause bleeding onto the outer surface of the brain (a subarachnoid hemorrhage), it can cause bleeding inside the brain itself (intracerebral hemorrhage).