An aneurysm is a full-thickness dilation of a segment of a blood vessel so that it’s 50 percent greater than its normal diameter, and it’s usually caused by a weakness in the blood vessel wall.
True aneurysms involve all three layers of the arterial wall - the tunica intima, which has endothelial cells; the tunica media, which has smooth muscle; and the tunica adventitia, which has connective tissue as well as vasa vasorum which are the vessels nourishing the aortic wall itself.
There are two major types of aneurysms: fusiform aneurysms, which are uniform in shape with symmetrical dilatation and involves the entire circumference of the aortic wall; and saccular aneurysms, which are localized outpouchings of only a portion of the aortic wall.
Aortic aneurysms are classified by their location along the aorta. Abdominal aortic aneurysms or AAAs involve the segment of the aorta within the abdominal cavity.
Thoracic aortic aneurysms or TAAs are found within the chest; and these are further classified as ascending or descending aneurysms.
Finally, there’s thoracoabdominal aortic aneurysms, which involve both the thoracic and abdominal aorta.
AAAs are the most common form of aneurysm, usually involving the infrarenal segment of the aorta, inferior to the renal arteries and superior to the iliac bifurcation. An AAA is defined as dilation with a transverse diameter exceeding 3 centimeters.
Most individuals with abdominal aortic aneurysm, or AAA, have no symptoms and are only detected as an incidental finding on imaging studies performed to evaluate an unrelated condition.
Some cases, especially in thinner individuals, may be discovered on a routine physical examination as a pulsating abdominal mass that can be felt on palpation and heard as a bruit on auscultation.
For earlier detection of asymptomatic AAAs, it’s generally recommended to perform one-time screening with ultrasound in males that are 65 to 75 years old who have ever smoked, as well as in males 60 years or older who are the sibling or offspring of a person with AAA.
One-time screening with ultrasound can also be done in individuals with risk factors, which includes hypertension, increasing age, male sex, smoking, atherosclerosis, having a family history for aneurysmal diseases, having aneurysms in other arteries, and rarely genetic syndromes - such as Turner syndrome or connective tissue diseases like Marfan’s syndrome or Ehlers-Danlos syndrome.
Symptoms of an AAA, include abdominal pain that can radiate. For proximal aneurysms, there’s upper abdominal or back pain, and for distal aneurysms, there’s lower abdominal or pelvic pain.
The pain is typically unrelated to movement or body position. Unruptured aneurysms typically present with an indolent onset of pain that is vague and nonspecific, while ruptured aneurysms cause sudden-onset severe pain.
AAAs that cause symptoms may be about to rupture or may have already ruptured - and so all of them are considered medical emergencies.
The most important risk factors for rupture include large diameter, rapid expansion, and current smoking.
Now, some cases of abdominal aortic aneurysm rupture present with the classical triad of severe acute pain, a pulsatile abdominal mass, and hypotension.
Often, only two of the features are present, and sometimes none of them are. Individuals may show signs of hemodynamic compensation with tachycardia, peripheral cyanosis, and mottling.
They may also complain of vague symptoms, such as pain in the back, flank, pelvis or hip, nausea and vomiting, or syncope, leading to frequent misdiagnosis.
If the AAA ruptures into the peritoneal cavity, then there’s typically sudden cardiovascular collapse and death prior to hospital arrival.
On the other hand, if it ruptures into the retroperitoneal space, then the individual may stabilize temporarily, but they may rapidly deteriorate from massive hemorrhaging, with a high risk of evolving into shock and death.
A rare complication that can occur from rupture into the retroperitoneum is the creation of an aortocaval fistula with the inferior vena cava, leading to venous congestion in retroperitoneal organs like the bladder.
In addition, aneurysms cause turbulent blood flow and that can lead to complications like vessel wall damage, areas of stagnant blood flow, and the formation of thrombi.
Thrombi can also embolize to various sites causing symptoms like claudication, painful blue digits due to recurrent focal ischemia - known as blue toe syndrome - or a painful pulseless cool extremity due to acute limb ischemia.
Rarely, AAA can lead to acute aortic thrombosis leading to bilateral lower extremity ischemia, or rarely, spinal cord ischemia.
Finally, large aneurysms can cause local compression of nearby structures, like the gastrointestinal tract or the spine. A definitive diagnosis of AAA requires seeing a focal aortic dilation on imaging.
Small aneurysms have a diameter smaller than 4 centimeters; medium aneurysms have a diameter between 4 and 5.5 centimeters; and large aneurysms have a diameter larger than 5.5 centimeters.
Based on prior imaging, a rapidly expanding aneurysm is defined as one that grows more than 0.5 centimeters in 6 months or 1 centimeter per year.
For symptomatic individuals, a diagnosis of AAA is guided by their hemodynamic status. Hemodynamically stable individuals with symptoms of an AAA should get an urgent abdominal CT scan with contrast, which helps confirm diagnosis and allows for treatment planning.
CT scan findings consistent with a ruptured AAA include a retroperitoneal hematoma, indistinct aortic walls, retroperitoneal fat stranding, loss of the fat plane between the aorta and surrounding tissue, and extravasation of IV contrast outside the aorta into the retroperitoneum, into a vein, or even into the bowel. Individuals with a ruptured aneurysm should be sent for emergency repair.
On the other hand, findings that may be associated with potentially unstable aneurysms or impending rupture include a crescent sign of layering hematoma within the aorta, aortic blebs that bulge from the surface of the aorta, "draping" of the aorta over a vertebral body, irregularity of the aortic wall, breaks in the calcification of the aortic wall, and localized areas of higher attenuation within a mural thrombus.
If these findings are present, the individual should be sent for urgent repair. While if these findings aren’t present, the individual has a stable aneurysms without impending rupture, so they should be assessed to determine whether their symptoms are related to the aneurysm or to another cause; if no other cause is apparent, the individual can be sent for urgent repair.
Now, in hemodynamically unstable individuals not previously known to have AAA but with any suspicion or symptoms a bedside ultrasound should be done to confirm the presence of an aneurysm.
If the presence of an AAA is confirmed, they should be taken right away for emergency repair. On the other hand, hemodynamically unstable patients with known AAA who present with classic symptoms of rupture should be taken directly to emergency repair.
The treatment options for asymptomatic AAA include conservative management, surveillance with a plan for eventual repair, and immediate repair.
Conservative treatment involves management of modifiable risk factors like smoking cessation, antihypertensive therapy to control blood pressure, statin therapy to lower cholesterol levels, and antiplatelet therapy for cardiovascular disease.
Asymptomatic AAA smaller than 5.5 centimeters in diameter should be followed up with annual ultrasound with periodic clinical evaluation and surveillance of aneurysm diameter.
Repair is recommended if an asymptomatic aneurysm is or becomes bigger than 5.5 centimeters in males or 5 centimeters in females, or if it’s rapidly expanding, which is defined as an increase in maximal aortic diameter of 5 millimeters or more over a six month period of time or more than 1 centimeter over one year.
For a symptomatic unruptured AAA of any size, urgent AAA repair should be performed, because the presence of symptoms suggests that the aneurysm is rapidly changing and might rupture.
A ruptured AAA can be fatal and needs emergency repair. For individuals with ruptured AAA, the systolic blood pressure should be maintained between 80 and 100 mm Hg prior to repair, which is known as permissive hypotension, to minimize further tearing of the aorta and reduce blood loss.
There are two modes of repair: open aneurysm repair, and endovascular aneurysm repair or EVAR.
Open aneurysm repair involves replacement of the diseased aortic segment with a prosthetic graft through a midline abdominal or retroperitoneal incision depending on the location of the aneurysm.
The dilated portion of the aorta is opened for insertion of a synthetic graft. Once the graft is sewn into the proximal and distal portions of the aneurysm, the aneurysmal sac is closed around the graft to avoid adhesions or fistulization, especially with the duodenum.
Complications associated with open repair include bleeding during or after surgery, thrombosis, infection of the graft, irregular heart rhythms and myocardial infarction, injury to the bowel, bowel ischemia, kidney damage, and spinal cord injury.
The other repair option is EVAR, which involves the placement of an expandable stent graft through the iliac or femoral arteries to line the aorta.
Sealing of the device against the aortic wall is achieved proximal and distal to the aneurysm, excluding it from the systemic circulation.
To perform EVAR though, the individual must meet some specific requisites. Firstly, there has to be a stable proximal and distal aneurysm necks, both long enough to ensure that the stent graft seals - 1.5 centimeters of neck are generally needed.
In addition, there must be no thrombus or calcification of the aneurysm, no occlusion of the iliac or femoral arteries for access, and the individual can’t be affected by connective tissue disorders, since that means they have weaker arterial walls.
Complications associated with EVAR include damage to surrounding blood vessels, organs, or other structures, as well as groin wound infection, or an endoleak - which is a continual leaking of blood out of the graft and into the aneurysm sac with potential rupture.
Compared to open AAA repair, EVAR has shown significantly improved short-term morbidity and mortality, but no significant differences in long-term outcomes.
So in general, EVAR is the preferred method for repair of ruptured AAA, and it is recommended to intervene within 90 minutes of arrival for the best outcomes.
It is also an option for individuals that are poor candidates for open repair, such as those with advanced age or comorbidities.
Now, EVAR does appear to be associated with the need for more secondary procedures, and for that reason, open surgical repair may be preferred for younger individuals with low or average perioperative risk.
Moving on to thoracic aortic aneurysms or TAAs, they’re defined as a permanent localized dilation of the thoracic aorta having at least a 50 percent increase in diameter compared with the expected normal diameter for that aortic segment.
They’re divided into the ascending thoracic or the descending thoracic, depending on whether they’re proximal or distal to the origin of the left subclavian artery, respectively.
Ascending TAAs are most common, and the main risk factors are cystic medial necrosis which often occurs with aging, or connective tissue disorders like Marfan and Ehlers-Danlos; while for descending TAAs, the main risk factor is atherosclerosis, and some might even be associated with tertiary syphilis.
Thoracic aneurysms are often asymptomatic and discovered through chest radiography done for other reasons.
