Aneurysm
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An aneurysm (or aneurism) (from Greek ανευρυσμα, a dilatation) is a localized dilation or ballooning of a blood vessel. Aneurysms most commonly occur in the arteries at the base of the brain (the circle of Willis) and in the aorta (the main artery coming out of the heart) - this is an aortic aneurysm.
The layer of the artery that is in direct contact with the flow of blood is the tunica intima, commonly called the intima. This layer is made up of mainly endothelial cells. Just deep to this layer is the tunica media, known as the media. This "middle layer" is made up of smooth muscle cells and elastic tissue. The outermost layer (furthest from the flow of blood) is known as the tunica adventitia or the adventitia. This layer is composed of connective tissue.
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Types
Aneurysms are also described according to their shape: Saccular or fusiform. Aneurysms can be broken down into two groups: true aneurysms and false aneurysms. A true aneurysm involves an outpouching of all three layers of a blood vessel: the intima, the media, and the adventitia. True aneurysms can be due to congenital malformations, infections, or hypertension. A false aneurysm, also known as a pseudoaneurysm, involves an outpouching of only the intima. Pseudoaneurysms can be due to trauma involving the intima of the blood vessel, and are a known complication of percutaneous arterial procedures.
Locations
Aneurysms can occur anywhere where there is an artery. Most non-intracranial aneurysms (95%) arise distal to the origin of the renal arteries at the infrarenal abdominal aorta, a condition mostly caused by atherosclerosis. The thoracic aorta can also be involved. One common form of thoracic aortic aneurysm involves widening of the proximal aorta and the aortic root, which leads to aortic insufficiency.
- While most aneurysms occur in an isolated form, the occurrence of berry aneurysms of the anterior communicating artery of the circle of Willis is associated with autosomal dominant polycystic kidney disease (ADPKD).
- The third stage of syphilis also manifests as aneurysm of the aorta, which is due to loss of the vasa vasorum in the tunica adventitia.
Formation
The occurrence and expansion of an aneurysm in a given segment of the arterial tree involves local hemodynamic factors and factors intrinsic to the arterial segment itself.
The human aorta is a relatively low-resistance circuit for circulating blood. The lower extremities have higher arterial resistance, and the repeated trauma of a reflected arterial wave on the distal aorta may injure a weakened aortic wall and contribute to aneurysmal degeneration. Systemic hypertension compounds the injury, accelerates the expansion of known aneurysms, and may contribute to their formation.
Hemodynamically, the coupling of aneurysmal dilation and increased wall stress is defined by the Laplace law. Specifically, the Laplace law states that the (arterial) wall tension is proportional to the pressure times the radius of the arterial conduit (T = P X R). As diameter increases, wall tension increases, which contributes to increasing diameter. As tension increases, risk of rupture increases. Increased pressure (systemic hypertension) and increased aneurysm size aggravate wall tension and therefore increase the risk of rupture.
Aneurysm formation is probably the result of multiple factors affecting that arterial segment and its local environment.
Treatment of brain aneurysms
Previously, a relatively complex surgical intervention (i.e. invasive surgery) was required in order to eliminate the potential risk of a brain aneurysm. In this operation, called clipping, a trepanation was performed, and afterwards a titanium clip was affixed around the aneurysm's neck. This operation, carrying a very high risk of infection or death of brain tissue, was associated with a low patient survival rate.
A new method associated with a higher survival rate is now used, currently more common in Europe than in the United States. In this type of non-invasive intervention, a catheter carrying a very thin platinum coil (named GDC for Guglielmi Detachable Coil) is inserted through the vein of the patient's groin. Later, using an angiogram as guidance, the catheter is moved through the vein up into the affected area of the brain. The aneurysm sac is then fitted with the GDC.
It usually takes less than three days for the aneurysm to become occluded, or blocked with embolized blood, thereby preventing potential or further bleedings. After this period, however, there is still a risk of re-bleeding.
Not every type of aneurysm can be treated with the described method. For example, certain wide-necked and inaccessible aneurysms currently still require surgical intervention, even though new methods (that use a type of stent) are already being studied and tested.
Related topics
External links
- Merck Manual (http://www.merck.com/mrkshared/mmanual/home.jsp):Aneurysms (http://www.merck.com/mrkshared/mmanual/section16/chapter211/211a.jsp)de:Aneurysma
es:Aneurisma fr:Anévrisme nl:Aneurysma pt:Aneurismo sv:Aneurysm