Thoracic Aortic Aneurysm

Most TAA patients are asymptomatic and the majority of aortic aneurysms is incidentally detected on radiographic images taken for any other reason [7].

Space-occupying aneurysms compress adjacent structures, mainly nerves or vessels, and trigger these symptoms. For instance, patients may claim persistent hoarseness if the aneurysm is affecting the recurrent laryngeal nerve. Compression of coronary arteries may result in myocardial infarction. Patients may present with dyspnea and cough if airways are constricted; compression of the esophagus may manifest as dysphagia.

Tears in the aortic wall may lead to tenderness and pain. In this context, some patients may claim chest or back pain.

Thoracic aortic dilation, especially if affecting the ascending part of the artery, is often associated with aortic regurgitation and the latter may cause symptoms of heart failure. Palpitations, reduced tolerance to exercise and dyspnea particularly under exercise may indicate this disorder.

Also, endothelial lesions due to aneurysm formation increase the risk of thrombus development. While acute thrombosis of an aortic aneurysm is a very rare event [8], thromboembolism due to displacement of thrombus particles is more frequently observed. Thus, a TAA patient may present with any form of thromboembolism rather than with thoracic symptoms.

While TAA ruptures may lead to death within very short periods of time, hemorrhages into smaller, enclosed spaces may not cause lethal bleedings. In this context, aneurysms may rupture into the right ventricle or into the mediastinum and cause right heart insufficiency or, similarly to the above described unruptured aneurysms, compression of anatomical structures located within the mediastinum. Hemoptysis or hematemesis may be observed if bleeding into trachea or esophagus occurs. Ruptures into the pericardial space will cause cardiac tamponade and possibly cardiogenic shock.

As has been mentioned above, most TAA are incidentally diagnosed in radiographic images taken for any other reasons. If radiographs prompt suspicion of an aneurysm, computed tomography angiography should be carried out to confirm the tentative diagnosis and to assess localization and extent of this vascular pathology. Leakage, if present, may also be detected. Magnetic resonance angiography may serve as an alternative in stable patients [9]. If an aneurysm is located in the ascending part of the aorta, transesophageal echocardiography may be as informative as the aforementioned methods but it bears less risks.

If the finding is suspicious for infectious aortitis and mycotic aneurysm, blood cultures should be established.

Of note, radiography is little sensitive for TAA, so this technique should not be utilized to clarify the origin of chest pain, dyspnea or other symptoms possibly triggered by an aneurysm.

Watchful waiting and risk factor reduction are the mainstays of therapy in patients presenting with small TAA. Hypertension is the most common risk factor and blood pressure should be strictly controlled to prevent formation of new aneurysms and to delay growth of existing ones. Beta-sympatholytics are most frequently administered to this end. Such therapy may be complemented with vasodilators like sodium nitroprusside. Smokers should strongly be advised to stop consumption of tobacco.

TAA above 5 cm in diameter should be considered for surgical intervention. Patients may either be submitted to endovascular repair, sometimes also referred to as TEVAR, short for thoracic endovascular aortic aneurysm repair, or open surgery. Several studies argue for preference of TEVAR over open repair, but others provide evidence against that recommendation [10] [11].

Most TAA do not grow more than 5 mm per year, but this can only be considered a rule of thumb. The larger the aneurysm, the faster it grows. Large TAA are generally associated with a higher risk of rupture than small aneurysms and any vascular pathology further increases the probability of life-threatening hemorrhages. Rupture usually occurs in TAA larger than 6 cm in diameter. If not adequately treated, survival rates for large TAA are low. One out of three patients may die within a year, more than three out of four within five years.

Patients with connective tissue disorders have a higher risk of TAA rupture even though the specific aneurysm may not yet have reached the above mentioned size.

The vast majority of TAA is associated with atherosclerotic plaques. Therefore, any condition augmenting the risk of atherosclerosis - particularly hypertension, elevated serum levels of low-density lipoproteins and total cholesterol, decreased concentrations of high-density lipoproteins, diabetes mellitus and tobacco consumption - also increases the individual risk of TAA [3].

While atherosclerosis is typically a disease of the elderly that may be associated with sporadic TAA, familial accumulation of this disease may be seen due to genetic diseases. These are sometimes also referred to as heritable TAA. In this context, pathological alterations of the aortic tunica media may be caused by several connective tissue disorders. TAA and aortic dissections, i.e., tears of the aortic wall that predispose for TAA, are frequently detected in patients suffering from Marfan syndrome, Ehlers Danlos syndrome and Loeys Dietz syndrome [4]. These patients often present aortic valve insufficiency and aortic regurgitation, too.

In rare cases, inflammation of the aortic wall may decrease its flexibility and resistance. Infectious agents may reach the aorta during bacteremia or viremia, or directly from foci of infection in surrounding tissues. For instance, mycobacteria and Treponema pallidum may cause aortitis and subsequent TAA.

While traumas may cause aortic lesions and fatal hemorrhages, these events are not considered in this article because they don't result from pathological aortic dilation.

The annual incidence of TAA has been estimated to be up to 9 per 100,000 women and up to 16 per 100,000 men [5]. Elsewhere, the male-to-female ratio is specified even higher: It has been reported that men are affected four times more often than women.

TAA are usually diagnosed in patients aged 60 years and older. However, this does not apply to heritable forms of TAA. Patients suffering from Marfan syndrome or Ehlers Danlos syndrome may develop aneurysms much earlier.

Hypertension seems to be the most important risk factor. More than half of all TAA patients present with permanently increased arterial blood pressure.

Because all risk factors for TAA, those related to atherosclerosis as well as genetic disorders, correspond to systemic diseases, the presence of TAA increases the individual risk for other aneurysms and vice versa [6].

Both hemodynamic factors and intrinsic properties of the arterial wall contribute to TAA.

Atherosclerotic plaques mainly consist of extracellular lipids, debris and lipid-loaded macrophages that release pro-inflammatory mediators and matrix metalloproteinases. These enzymes degrade structure proteins of the aortic wall, particularly elastin and collagen fibers, and thus induce loss of flexibility and resistance. Space-occupying atherosclerotic lesions also cause turbulences that may expose weakened wall sections to even higher physical stress.

Patients suffering from Marfan syndrome present a defective gene for fibrillin-1, a major component of microfibrils of elastin. This genetic disorder is associated with a reduced aortic wall content of elastin and in pathological alterations of the properties of this fiber. The aorta of Marfan syndrome patients is abnormally flexible and therefore dilated, but constant dilation also induces remodeling processes that result in a diminished resistance and propensity to TAA [1]. Similar alterations account for increased susceptibility to TAA in other connective tissue disorders.

Hypertension aggravates the aforementioned pathological conditions with regards to the likelihood of aneurysms. It also contributes to aneurysm growth and unfortunately, wall tension further augments with increasing vessel diameters. Thus, aortic dilation promotes even more severe dilation. This relation is described in Laplace's law.

Aneurysm growth increases the probability of a rupture, which may lead to fatal hemorrhages.

No direct preventive measures can be recommended to avoid TAA. However, the main risk factor for TAA is atherosclerosis and lifestyle decisions as well as dietary adjustments may significantly contribute to lowering blood pressure, serum cholesterol concentrations and blood glucose levels. Regular exercise and a healthy, balanced diet are the mainstays of atherosclerosis prophylaxis. Also, patients should be advised against smoking.

Similar to other arteries, the aortic wall is composed of three layers: tunicas intima, media and adventitia. The aorta is exposed to high systolic blood pressures and also compensates for the rhythmic action of the heart. It dilates immediately after the systole, but contracts rather slowly. This feature is often described with the term Windkessel effect. To a certain degree, this results in a smoother pulse. Thus, the wall of this main artery has to be extremely flexible and resistant and this applies particularly to the proximal parts of the artery.

Under physiological conditions, the strength of the aortic media and its elevated content of elastic fibers allow it to meet these high demands. However, any pathological alteration will immediately lead to dilation of the vessel, a so-called aortic aneurysm. If such an aneurysm extends regularly and symmetrically over a certain part of the artery, it is described as a fusiform aneurysm. In contrast, focal weakness of the aortic wall will result in a sac-like protrusion. Consequently, this type of aneurysm is denoted saccular aneurysm. Of note, a saccular aneurysm is not to be confused with a pseudoaneurysm. The latter term refers to a hematoma located outside but adjacent to the artery. From the outside, it may appear similar to a saccular aneurysm, but it may not (any longer) be connected to the arterial lumen.

The aorta originates at the left cardiac ventricle. It ascends from here, describes an arch and subsequently descends. The descending aorta passes through the diaphragm and its branches supply distinct abdominal and pelvic organs. All parts of the aorta proximal to aortic hiatus in the diaphragm may be referred to as thoracic aorta. If an aortic aneurysm is detected within the thoracic aorta, the respective patient is diagnosed with thoracic aortic aneurysm (TAA). Approximately one out of four aortic aneurysms affect the thoracic part of this main artery [1].

TAA are life-threatening vascular lesions. They may rupture and give rise to fatal hemorrhages. Some TAA may not ever rupture, but compromise adjacent tissues due to them being space-occupying processes. Other vessels and nerves may be compressed and their function may be impaired. Indeed, these are the most frequent symptoms of TAA, although this condition is mostly an incidental finding on radiographs taken for any other reason. Of course, TAA may also be detected with more sophisticated imaging techniques such as magnetic resonance imaging or computed tomography scans. These are also applied to precisely assess the extent of aortic wall damage and to make a decision regarding surgical intervention. Survival rates are significantly higher in patients that undergo planned surgery for TAA than for those submitted to emergency procedures [2]. In very mild cases, watchful waiting may be less risky than surgery.

The aorta is the largest artery of the body. It originates from the left cardiac ventricle, ascends, describes an arch, descends and passes through the diaphragm into the abdominal space. That part of the aorta between heart and diaphragm is referred to as thoracic aorta.

Being located that close to the heart, the aorta is exposed to high blood pressure with every heartbeat. It thus has to be very flexible and resistant. If the arterial wall weakens for any reason, the aorta may dilate and form what is called an aneurysm. If an aneurysm is detected within the thoracic aorta, the patient is diagnosed with thoracic aorta aneurysm (TAA).

Causes

The main cause for pathological alterations of the aortic wall and subsequent formation of TAA is atherosclerosis. This condition, in turn, is much more common in smokers and individuals suffering from hypertension, elevated serum levels of low-density lipoproteins and total cholesterol or diabetes mellitus.

Certain connective tissue disorders, namely Marfan syndrome, Ehlers Danlos syndrome and Loeys Dietz syndrome, are associated with high risks of TAA.

Rarely, infectious diseases account for aortic aneurysms.

Symptoms

Most TAA patients do not present any symptom; the aneurysm is usually detected incidentally on radiographic images taken for any other reason. Some patients may suffer from difficulties to breathe or swallow, from hoarseness or chest and back pain.

TAA may lead to myocardial infarction, stroke and other serious conditions that may dominate the clinical picture.

Diagnosis

Imaging techniques such as computed tomography angiography and transesophageal echocardiography are usually applied to assess localization and extent of the TAA. These information are of utmost importance to take decisions regarding individual treatment.

Treatment

Risk factor reduction by means of lifestyle decisions, dietary adjustments and medication are indicated for patients suffering from small TAA. In this context, patients should stop smoking, exercise regularly and keep a healthy, balanced diet. This will contribute to lower blood pressure, serum cholesterol and glucose levels. Beta-blockers are often prescribed to aid in reduction of hypertension.

Large TAA pose a considerable risk for rupture and fatal hemorrhages and require a surgical intervention.

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