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Definition and Causes of Aneurysms
What is an Aneurysm?
An aneurysm is a blood-filled dilation of a blood vessel caused by a weakening of the vessel’s wall. Aneurysms typically develop because of the general wear and tear of blood pressure on the arteries’ walls, called hemodynamic forces. A weak spot in the artery wall bulges out, giving way to the pressure of the blood flow, forming a sac. For this reason, areas of artery or vessel wall which are submitted to higher blood pressures are more susceptible to aneurysm development, such as the branching points of arteries, called bifurcations. In the same way that a balloon stretches, the aneurysm sac balloons out from the vessel wall usually slowly, progressively becoming weaker.
An aneurysm
You are unlikely to be born with a cerebral aneurysm; they usually develop as you get older in particular past the age of 40. They are more prevalent in women. In the brain, causation has been linked to:
- Traumatic injury such as a blow to the head (less than 1% of all cases)
- An infection (termed a mycotic aneurysm, (2% of all known cases)
- A hereditary predisposition (They can run in families; this accounts for 20% of all aneurysms)
- Cigarette smoking and hypertension or high blood pressure
- Use of drugs such as cocaine and amphetamines
- Certain blood disorders: fibromuscular dysplasia, cerebral arteritis, arterial dissection…
3-D Volumetric reconstruction of an anterior
communicating artery aneurysm. - Click image for a larger view.
To view a video clip of the image, select one of the formats below.
MPEG (MP4 format -
141 KB)
QuickTime (1.33 MB)
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AVI (2.86 MB)
For more details about the causes and to find out if you are in the ‘at-risk’ population, for information about prevention, detection and treatment options, go to these sections:
One of the biggest dangers with an aneurysm is when some of the blood flowing through the artery diverts into the aneurysm sac, causing an eddy-like effect. The diverted blood hits the opposing wall of the aneurysm sac at a high pressure. Eventually this pressure can cause the aneurysm sac to rupture. Rupture then leads to bleeding within the head (hemorrhaging) which causes brain tissue compression and damage often resulting in a severe headache, and other symptoms such as, stiff neck, weakness, confusion, or even loss of consciousness. Aneurysm ruptures result in bleeding within the subarachnoid space, that is, the space between the skull and the brain; this is called a subarachnoid hemorrhage (SAH).
X-ray reconstruction of top of head/brain
- Click image for a larger view.
To view a video clip of the image, select one of the formats below.
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125 KB)
QuickTime (691 KB)
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AVI (1.43 MB)
Approximately 2 million Americans have silent unruptured aneurysms. Most of these people do not know they have an aneurysm and have no symptoms (estimates vary between 1.5-6% of the U.S. population). The overall annual rupture rate is approximately 12/100,000, which means that each year 240 of the 2 million with unruptured aneurysms will experience a rupture. However, the risk of rupture of a particular aneurysm can be much higher and varies from 1/1000 to over 100/1000 depending on the characteristics of the aneurysm. There are 30,000 new aneurysm ruptures or "SAH" cases reported in the US annually and about half of these will lead to death. Of those surviving, only one third will recover without disabilities.
What increases your risk for an aneurysm rupture (SAH)?
- Medical family history
- Hypertension (high blood pressure)
- Tobacco use
- Female (3:2 female to male ratio for aneurismal rupture)
- Between the ages of 35 and 60
Multiple Aneurysms
If you have one aneurysm, there is a 15 to 20 percent chance that you have at least one or more additional aneurysms. It is unknown whether the presence of multiple aneurysms affects their rupture rates. Further research is needed in this area.
