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Brain Aneurysms-The Basics

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Updated February 17, 2009

What is a brain aneurysm?

Aneurysms are weak areas on the wall of a blood vessel, usually an artery -- the type of blood vessel that carries oxygen-rich blood to tissues. Over time, these weak areas on the walls of arteries balloon out causing the blood vessel wall to become even weaker as the aneurysm grows. Occasionally aneurysms can rupture and cause a hemorrhagic stroke, the type of stroke that is caused by bleeding inside the brain. Approximately 8% of all strokes are caused by ruptured aneurysms.

Why do people develop aneurysms?

Little is known about why aneurysms develop in the brains of some people and not in the brains of others. In many cases, aneurysms are inherited through the genes, but high blood pressure and cigarette smoking appear to predispose people to develop aneurysms, too. About 5% of the population in the United States has at least one aneurysm in the brain, but up to 80% of them will never be affected by bleeding in the brain.

Brain aneurysms usually occur at sites where arteries divide into branches. The following arteries in the brain are most likely to have aneurysms:

  • The anterior communicating artery (30%)
  • The posterior communicating artery (25%)
  • The middle cerebral artery (20%)

What are the risk factors for aneurysm bleeding and rupture?

When an aneurysm ruptures, it causes profuse bleeding in the brain leading to a hemorrhagic stroke. In general, aneurysms bleed during situations when blood pressure is high. This can happen even to people who do not suffer from chronic high blood pressure. Episodes of high blood pressure that come and go can develop for many reasons, including:

Aneurysms are also more likely to bleed after they reach a size of more than 10 millimeters, or about a third of an inch.

If I have an aneurysm, will I feel any symptoms?

Very often small aneurysms do not cause any symptoms unless they bleed. Sometimes, however, the growing aneurysm might push against blood vessels or other structures around it as it grows and cause headaches, double vision, or pain around the eyes when you look to the sides. If the aneurysm bleeds, people often feel a "thunderclap headache" they might call the “worst of their lives,” as well as neck pain and stiffness. They might also develop typical stroke symptoms. In some 10% of people with a ruptured aneurysm, bleeding inside the brain is so profuse that they die before ever reaching a hospital.

What happens after an aneurysm ruptures and bleeds?

The prognosis after an aneurysm bleed is variable, depending on the size of the bleed. In general, up to 50% of people with bleeding in the brain die from the complications of the bleeding itself. There is also a very high risk of bleeding again around the time of the first bleed. Up to 4% of people can bleed again within the first 24 hours after the initial episode of bleeding. By the end of the second week after the bleed, they have a 15 to 20% chance of bleeding again.

Source: Jonathan L. Brisman, Joon K. Song, and David W. Newell New England Journal of Medicine 2006, 355:928-939

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