An aneurysm is defined as a permanent restrictive dilation of the arterial wall exceeding 50% of the normal vessel diameter. Therefore, a precise definition of abdominal aortic aneurysm (AAA) would require calculation of the ratio of normal to dilated abdominal aorta in the same individual, with correction for influencing factors such as age, sex, race and body surface area. Usually, an abdominal aortic aneurysm can be diagnosed with an abdominal aortic diameter of more than 3 cm.
1. Morbidity
The occurrence of abdominal aortic aneurysm is related to many factors, such as age, gender, race, family history, and smoking. The incidence of abdominal aortic aneurysms is higher in patients of advanced age, males, Caucasians, positive family history and long-term smokers, and can reach 5.9% in males over 80 years of age.
2. Etiology
The biological mechanisms of aneurysm occurrence are complex, and genetic susceptibility, atherosclerosis and various proteases have been shown to be directly related to its occurrence. All etiologies ultimately manifest as degenerative changes in the middle layer of the aorta, followed by expansion under blood flow pressure to form aneurysms.
Two specific types of abdominal aortic aneurysms.
(1) Inflammatory abdominal aortic aneurysms. Inflammatory abdominal aortic aneurysm is a special type of aneurysm with an exceptionally thick, shiny white, hard aneurysm wall that is highly susceptible to fibrotic adhesions to intra-abdominal organs (e.g., ureter, duodenum). The distribution of macrophages and cytokines within the aneurysm wall is abnormally increased in inflammatory aneurysms compared to normal AAA. The incidence of inflammatory abdominal aortic aneurysms accounts for about 5% of all abdominal aortic aneurysms. There is no significant difference between inflammatory abdominal aortic aneurysms and common abdominal aortic aneurysms in terms of risk factors, treatment options and prognosis. In terms of clinical presentation, inflammatory abdominal aortic aneurysms are more likely to present with symptoms such as back or abdominal pain, and are usually associated with increased blood sedimentation. Chronic abdominal pain, weight loss, and increased hematocrit are the triad signs for the diagnosis of inflammatory abdominal aortic aneurysm.
(ii) Infectious abdominal aortic aneurysm. Infectious abdominal aortic aneurysm is a rare disease. In recent years, its incidence has been decreasing with the continuous development of antibiotics. Primary infections of the aortic wall resulting in aneurysms are rare, and most infected abdominal aortic aneurysms are caused by secondary infections. Staphylococcus and Salmonella are the most common causative agents of infectious abdominal aortic aneurysms, while Mycobacterium tuberculosis and syphilis can also cause aortic aneurysms to occur.
3. Natural course and complications of abdominal aortic aneurysm
The natural course of abdominal aortic aneurysms is the gradual enlargement of the aneurysm and the formation of an appendage thrombus due to the continuous turbulent flow of blood within the aneurysmal cavity. Therefore, the most common complications of abdominal aortic aneurysm are: aneurysm rupture, distal organ embolism and adjacent organ compression.
When the diameter of abdominal aortic aneurysm is less than 4 cm, the annual growth rate is around 1 mm to 4 mm; when the diameter of the aneurysm is 4 cm to 5 cm, the annual growth rate is around 4 mm to 5 mm; when the diameter of the aneurysm is more than 5 cm, the annual growth rate is greater than 5 mm, and the eventual rupture rate of the aneurysm reaches 20%; if the diameter of the aneurysm is greater than 6 cm, the annual growth rate of the aneurysm is 7 mm to 8 mm, and the eventual rupture rate of the aneurysm If the diameter of the aneurysm is greater than 6 cm and the annual growth rate of the aneurysm is 7 mm to 8 mm, the final rupture rate increases to 40%. The risk of ruptured abdominal aortic aneurysms is extremely high, with a mortality rate of 90%. Therefore, it is now generally accepted that surgery is required for abdominal aortic aneurysms larger than 5 cm in diameter. In women, due to the thin diameter of the abdominal aorta, surgery should be considered if the aneurysm is greater than 4.5 cm in diameter. In addition, if the diameter of the abdominal aortic aneurysm grows too rapidly and is greater than the average value mentioned above, early surgery should also be considered. Factors associated with abdominal aortic aneurysm rupture, in addition to aneurysm diameter, include hypertension, chronic obstructive pulmonary disease, long-term smoking, female and positive family history, all of which increase the risk of abdominal aortic aneurysm rupture.
When abdominal aortic aneurysms are large, they can compress the duodenum and cause symptoms of upper gastrointestinal obstruction such as difficulty in eating, and in severe cases, they can invade the duodenum to form a duodenal fistula and cause gastrointestinal hemorrhage, which is one of the most fatal complications of abdominal aortic aneurysms. In addition, abdominal aortic aneurysm can also compress inferior vena cava or renal vein, and even occur abdominal aorta-inferior vena cava or abdominal aorta-renal vein fistula, leading to acute heart failure and death.
4.Diagnosis
(1) Symptomatic abdominal aortic aneurysm
Pain is the most common complaint of abdominal aortic aneurysm. The pain is usually located in the middle abdomen or low back and is usually dull in nature and can last for hours or even days. This pain is characterized by the fact that it does not usually change with position or movement, which is different from the common low back pain of the elderly and needs to be differentiated. When the pain suddenly increases, it often indicates an impending rupture of the abdominal aortic aneurysm. Blood is often confined to the posterior peritoneum after aneurysm rupture, so blood pressure does not drop too rapidly and bruising of the abdominal wall can occur bilaterally, spreading further to the perineum. The aneurysm may also rupture into the abdominal cavity, which is associated with abdominal muscle tension and hypotension due to massive blood loss; rupture into the duodenum may result in upper gastrointestinal hemorrhage and death due to rapid onset of hypovolemic shock.
(2) Asymptomatic abdominal aortic aneurysm
Most abdominal aortic aneurysms are asymptomatic and are discovered inadvertently as a pulsating abdominal mass or during physical examination. Since abdominal aortic aneurysms and peripheral arterial occlusive disease share the same high-risk factors, regular aortic and peripheral arterial examinations should be performed in this high-risk group to achieve early detection and diagnosis and to reduce the rate of abdominal aortic aneurysm rupture and mortality.
(3) Imaging examinations
①4.3.1 Color Doppler ultrasound
Ultrasound is characterized as noninvasive, inexpensive, radiation-free, and has reliable data. Color Doppler ultrasound has been widely used for screening, preoperative evaluation and postoperative follow-up of abdominal aortic aneurysms, and its sensitivity can reach more than 90%. However, its shortcomings are that it is operator-dependent, and different cut lines of the probe will yield different data, which affects the objectivity of the result measurement; for abdominal aortic aneurysms and iliac artery aneurysms in deeper locations, its diagnostic accuracy is also reduced due to intestinal gas interference.
②Abdominal X-ray plain film
A significant proportion of abdominal aortic aneurysms are detected when abdominal X-ray is performed, and the images show arcuate calcifications in the dilated aortic region; they can also show large soft tissue shadows in the abdomen, making the contours of the psoas major muscle poorly displayed, all of which suggest the presence of abdominal aortic aneurysms.
③Enhanced CT of the abdomen
Enhanced CT examination of the abdomen can accurately measure various data of abdominal aortic aneurysm and has basically replaced transcatheter angiography. CTA has gradually become the gold standard for preoperative examination and postoperative follow-up of abdominal aortic aneurysms. The preoperative CT evaluation of abdominal aortic aneurysm includes: the maximum diameter of the aneurysm; the relationship between the aneurysm and the renal artery; the length, diameter, and angulation and calcification of the normal aorta under the renal artery (i.e., the neck of the aneurysm); the diameter and tortuosity of the iliac artery; and also requires careful analysis of any vascular variants, such as the collateral renal artery, the bilateral inferior vena cava, or the left renal vein behind the aorta. All these data can be clearly understood by a high-quality CT angiography.
5.Conservative treatment
(1) Close monitoring
For abdominal aortic aneurysms found after screening, color Doppler ultrasound is recommended every 2 to 3 years if the aneurysm is less than 4 cm in diameter; if the aneurysm is larger than 4 cm but less than 5 cm in diameter, close monitoring is needed, and color Doppler ultrasound or CT angiography is recommended at least once a year. Once the tumor is found to be more than 5cm in diameter, or if the tumor grows too fast during the monitoring period, early surgery is required.
(2) Drug treatment
After the diagnosis of abdominal aortic aneurysm is confirmed, during the observation period, one should strictly quit smoking and pay attention to controlling blood pressure and heart rate. It has been found that oral beta-blockers can reduce the rate of expansion of atherosclerosis-induced abdominal aortic aneurysms, effectively reducing the rupture rate and mortality due to perioperative adverse cardiac events, which is the only drug proven to be effective in the conservative treatment of abdominal aortic aneurysms. The rationale may be that by slowing down the heart rate, the intra-aortic pressure is reduced, thereby reducing the impact of blood flow on the aortic wall and slowing down the rate of aneurysm expansion.
6.Open surgery for abdominal aortic aneurysm
The earliest abdominal aortic aneurysm resection and artificial vessel grafting originated in the 1960s. After more than 40 years of development, it has evolved and matured, and has become one of the classic surgeries. Although, in recent years, EVAR has developed rapidly, causing a great impact on the dominance of open surgery. However, for patients with low-risk factor abdominal aortic aneurysm who are in good general condition and can tolerate surgery, open surgery is still the standard of care because of its definite immediate and long-term results.
7. Endoluminal abdominal aortic aneurysm repair (EVAR)
It can be performed with regional block anesthesia or local anesthesia and is particularly indicated for patients with combined severe cardiopulmonary insufficiency and other high-risk factors. The stent grafts currently used in EVAR are made by suturing and fixing the artificial blood vessel inside a metal stent to prevent distortion and ectasia of the artificial blood vessel and to maintain stability. To accommodate aortic bifurcation structures and to increase the stability of the stented vessel, most current stent graft products are designed in a patterned fashion, with the main body and one iliac branch placed through one femoral artery and the other iliac branch placed through the contralateral femoral artery, positioned in docking. An important prerequisite for the implementation of this procedure is the presence of a normal aorta of sufficient length beneath the renal artery that can serve as a proximal anchorage zone for the stent to prevent ectopic stent grafts distally and to prevent the occurrence of postoperative endoleaks.
The main complications after EVAR are endoleak, stent graft ectasia, torsion, graft occlusion, and infection. The larger the preoperative AAA aneurysm diameter, the higher the incidence of postoperative endoleaks, stent ectasia and other complications.
(1) Problems of endoluminal repair of abdominal aortic aneurysm
With the continuous improvement of interventional devices and techniques, EVAR for abdominal aortic aneurysm has become increasingly mature, but there are still some problems with this procedure that need further development and improvement.
①Vascular anatomical limitations.
Compared with traditional open surgery, EVAR procedure requires higher vascular anatomical conditions. First, a normal aorta of at least 1.5 cm in length under the renal artery is required as the proximal anchorage area, i.e., the neck of the aneurysm must be at least 1.5 cm long; at the same time, the diameter of the neck of the aneurysm is required to be less than or equal to 28 mm, while it must not be severely angulated. It is also required that the external iliac artery and the femoral artery have sufficient diameter to ensure the passage of the delivery vehicle carrying the stent graft. Because of the thin external iliac artery in women, the proportion of women who forgo endoluminal treatment due to poor delivery routes is considerably higher than that of men.
② Endoleaks.
Endoleaks refer to persistent blood flow into the closed aneurysm cavity after EVAR of abdominal aortic aneurysms and can be classified into the following four types. type I endoleaks refer to blood flow into the aneurysm cavity due to failure of proximal or distal anchorage zone closure, which generally results in high pressure in the aneurysm cavity and can easily lead to aneurysm rupture. Once detected, it needs to be corrected by adding a laminated stent proximally or distally. Type II endoleaks refer to the return of blood into the tumor cavity through branch arteries (e.g., lumbar artery, inferior mesenteric artery, etc.) and occur in about 40% of cases. Type III endoleaks refer to leakage at the interface due to breakage or distortion of the stent vessel and require immediate intervention or surgical correction once they occur. type IV endoleaks refer to blood entering the lumen due to high permeability of the stent vessel and usually occur within 30 days after the placement of the overlapping stent. In conclusion, it is due to the presence of inexact factors such as endoleaks that patients after EVAR for abdominal aortic aneurysms need to be followed up regularly. The follow-up interval is usually 3, 6, and 12 months postoperatively and annually thereafter. If the imaging data reveals progressive aneurysm enlargement, further investigations are needed to clarify the cause.
(iii) Stent graft occlusion.
There is a high incidence of stent graft occlusion after EVAR for early abdominal aortic aneurysms. An important reason for occlusion is the twisting of the graft into an angle. It was later found that using a metal stent as an external support could reduce the twisting of the vascular graft, thus greatly reducing the incidence of graft thrombosis occlusion.
(iv) Aneurysm neck expansion.
After EVAR for abdominal aortic aneurysms, the aorta in the proximal anchorage area will dilate further over time, which can lead to ectasia of the stent graft distally. Currently, the stent body diameter is generally chosen to exceed the proximal aneurysm neck diameter by 10%-20% when EVAR is performed to accommodate future aortic dilation, but even so, late ectasia of the stent graft cannot be completely prevented.