An aneurysm or aneurism (from Ancient Greek: ἀνεύρυσμα - aneurusma "dilation", from ἀνευρύνειν - aneurunein "to dilate"), is a localized, blood-filled dilation (balloon-like bulge) of a blood vessel[1] caused by disease or weakening of the vessel wall.
Aneurysms most commonly occur in arteries at the base of the brain (the circle of Willis) and in the aorta (the main artery coming out of the heart, a so-called aortic aneurysm). As the size of an aneurysm increases, there is an increased risk of rupture, which can result in severe hemorrhage, other complications or even death.
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.
Aneurysm formation is probably the result of multiple factors affecting that arterial segment and its local environment.
Hemodynamically, the coupling of aneurysmal dilation and increased wall stress is approximated by the law of Laplace. Specifically, the Laplace law applied to a cylinder states that the (arterial) wall tension is equal to the pressure times the radius of the arterial conduit (T = P x R). As diameter increases, wall tension increases, which contributes to more increase in diameter and risk of rupture. Increased pressure (systemic hypertension) and increased aneurysm size aggravate wall tension and therefore increase the risk of rupture. In addition, the vessel wall is supplied by the blood within its lumen in humans (? although aorta has Vasa vasorum). Therefore[citation needed]in a developing aneurysm, the most ischemic portion of the aneurysm is at the farthest end, resulting in weakening of the vessel wall there and aiding further expansion of the aneurysm. Thus eventually all aneurysms will, if left to complete their evolution, rupture without intervention.
Treatment
Historically, the treatment of arterial aneurysms has been surgical intervention, or watchful waiting in combination with control of blood pressure. Recently, endovascular or minimally invasive techniques have been developed for many types of aneurysms.
[edit]Treatment of cranial aneurysms
Currently there are two treatment options for brain aneurysms: surgical clipping or endovascular coiling.
Surgical clipping was introduced by Walter Dandy of the Johns Hopkins Hospital in 1937. It consists of performing a craniotomy, exposing the aneurysm, and closing the base of the aneurysm with a clip. The surgical technique has been modified and improved over the years. Surgical clipping remains the best method to permanently eliminate aneurysms.
Endovascular coiling was introduced by Guido Guglielmi at UCLA in 1991. It consists of passing a catheter into the femoral artery in the groin, through the aorta, into the brain arteries, and finally into the aneurysm itself. Once the catheter is in the aneurysm, platinum coils are pushed into the aneurysm and released. These coils initiate a clotting or thrombotic reaction within the aneurysm that, if successful, will eliminate the aneurysm. In the case of broad-based aneurysms, a stent is passed first into the parent artery to serve as a scaffold for the coils ("stent-assisted coiling").
At this point it appears that the risks associated with surgical clipping and endovascular coiling, in terms of stroke or death from the procedure, are the same. The major problem associated with endovascular coiling, however, is the high recurrence rate and subsequent bleeding of the aneurysms. For instance, the most recent study by Jacques Moret and colleagues from Paris, France, (a group with one of the largest experiences in endovascular coiling) indicates that 28.6% of aneurysms recurred within one year of coiling, and that the recurrence rate increased with time. (Piotin M et al., Radiology 243(2):500-508, May 2007) These results are similar to those previously reported by other endovascular groups. For instance Jean Raymond and colleagues from Montreal, Canada, (another group with a large experience in endovascular coiling) reported that 33.6% of aneurysms recurred within one year of coiling. (Raymond J et al., Stroke34(6):1398-1403, June 2003) The long-term coiling results of one of the two prospective, randomized studies comparing surgical clipping versus endovascular coiling, namely the International Subarachnoid Aneurysm Trial (ISAT) are turning out to be similarly worrisome. In ISAT, the need for late retreatment of aneurysms was 6.9 times more likely for endovascular coiling as compared to surgical clipping. (Campi A et al., Stroke 38(5):1538-1544, May 2007)
Therefore it appears that although endovascular coiling is associated with a shorter recovery period as compared to surgical clipping, it is also associated with a significantly higher recurrence and bleeding rate after treatment. Patients who undergo endovascular coiling need to have annual studies (such as MRI/MRA, CTA, or angiography) indefinitely to detect early recurrences. If a recurrence is identified, the aneurysm needs to be retreated with either surgery or further coiling. The risks associated with surgical clipping of previously-coiled aneurysms are very high. Ultimately, the decision to treat with surgical clipping versus endovascular coiling should be made by a cerebrovascular team with extensive experience in both modalities. At present it appears that only older patients with aneurysms that are difficult to reach surgically are more likely to benefit from endovascular coiling. These generalizations, however, are difficult to apply to every case, which is reflected in the wide variability internationally in the use of surgical clipping versus endovascular coiling.
[edit]Treatment of aortic and peripheral aneurysms
Aneurysms are treated by either endovascular techniques (angioplasty with stent) or open surgery techniques. Open techniques include exclusion and excision. Exclusion of an aneurysm means tightly tying suture thread around the artery both proximally and distally to the aneurysm, to cut off blood flow through the aneurysm. If the aneurysm is infected or mycotic, it may then be excised (cut out and removed from the body). If uninfected, the aneurysm is often left in place. After exclusion or excision, a bypass graft can be placed, to ensure blood supply to the affected area. For some aneurysm repairs in the abdomen, where there is adequate collateral blood supply, bypass grafts are not needed.
For aneurysms in the aorta, arms, legs, or head, the weakened section of the vessel may be replaced by a bypass graft that is sutured at the vascular stumps. Instead of sewing, the graft tube ends, made rigid and expandable by nitinol wireframe, can be inserted into the vascular stumps and permanently fixed there by external ligature.[1] [15] New devices were recently developed to substitute the external ligature by expandable ring allowing use in acute ascending aorta dissection, providing airtight, easy and quick anastomosis extended to the arch concavity [16][2]Less invasive endovascular techniques allow covered metallic stent grafts to be inserted through the arteries of the leg and deployed across the aneurysm.
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