Thoracic aortic aneurysm overview

# Overview
A thoracic aortic aneurysm (TAA) is defined as an expansion or dilation of the aorta greater than 4.5 cm.  Thoracic aortic aneurysms (TAAs) encompass all aneurysms the aorta between the aortic root and the crura of the diaphragm. The aortic root and the ascending thoracic aorta are the most common sites of TAAs (60%), followed by the descending thoracic aorta (40%). Involvement of the aortic arch and/or extension of a TAA into the abdominal aorta (AA) (so called thoracoabdominal aneurysms) are relatively uncommon subsets (each encountered in 10% of cases or less).

# Pathophysiology
An aneurysm occurs when a part or entire circumference of the vessel is pathologically dilated. A true aneurysm involves all three layers of the vessel, whereas pseudoaneurysm is characterized by disruption of the intima and media, and the dilated segment of the aorta is lined by adventitia alone.

# Causes
Two of the most common causes of thoracic aorta aneurysm are marfan's syndrome and atherosclerosis.  In some centers, a prior history of dissection with aneurysmal dilation is the most common cause of surgical repair.

# Differentiation of Thoracic Aortic Aneurysm from Other Disorders
Thoracic aortic aneurysms are often asymptomatic and as a result can be easily missed.  If chest pain is present, it is critical to differentiate a thoracic aortic aneurysm with dissection from other potential life-threatening diseases.

# Epidemiology and Demographics
Thoracic aortic aneurysms are relatively uncommon (6 to 10 new ones per 100,000 person years) and are less prevalent than abdominal aortic aneurysm.  Thoracic aortic aneurysms are generally diagnosed after the sixth and seventh decades of life.  Although men are affected 2 to 4 times as often as women, women are at higher risk of rupture by a factor 6.8.

# Risk Factors
Hypertension and cigarette smoking are the most important risk factors, though the importance of genetic factors in the development of thoracic aortic aneurysm (such as familial thoracic aortic aneurysm; familial aortic dissection) has been increasingly recognized.  Approximately 10% of patients may have other family members who have aortic aneurysms. It is also important to note that individuals with a history of aneurysms in other parts of the body have a higher chance of developing a thoracic aortic aneurysm.

# Natural History, Complications and Prognosis
Thoracic aneurysms enlarge at a more rapid rate than abdominal aneurysms (0.42 vs. 0.28 cm/yr), with aneurysms of the aortic arch growing at ~ 0.56 cm/yr. Aneurysms that are 5-6 cm in diameter have a faster rate of growth and a greater tendency to rupture than smaller ones.  Aortic dissection and aortic rupture are the two most feared complications of an aortic aneurysm.  The five-year survival of patients with a thoracic aortic aneurysm is 56%.  The following are associated with a poorer prognosis: the presence of diastolic dysfunction; trauma as a cause of the aneurysm;  the presence of coronary artery disease and the presence of carotid artery disease.

# Screening
Screening for TAA is not recommended in the general population.  However, certain population substrates, such as those with history of marfan's syndrome, turner's syndrome, ehlers-danlos type IV syndrome, familial thoracic aortic disease syndromes, bicuspid aortic valve, takayasu arteritis, giant cell arteritis, loeys-dietz syndrome or a confirmed genetic mutation known to predispose to aortic aneurysms and aortic dissections (TGFBR1, TGFBR2, FBN1, ACTA2, or MYH11) should have imaging study to screen for TAAs.  First-degree relatives of people with thoracic aortic aneurysm or dissection should have aortic imaging to identify asymptomatic disease.  Patients with high risk syndromes such as marfan's syndrome and loeys-dietz syndrome should undergo complete aortic imaging at initial diagnosis and 6 months thereafter to establish if enlargement is occurring.

# Natural History, Complications and Prognosis
Thoracic aortic aneurysms typically expand slowly  at a rate of approximately 0.1-0.2 cm/year.  Aortic dissection and aortic rupture are the two most feared complications of an aortic aneurysm.  The five-year survival of patients with a thoracic aortic aneurysm is 56%.  The following are associated with a poorer prognosis: the presence of diastolic dysfunction; trauma as a cause of the aneurysm;  the presence of coronary artery disease  and the presence of carotid artery disease.

# Diagnosis

## History and Symptoms
Most thoracic aortic aneurysms are asymptomatic and diagnosed incidentally on imaging studies.  The development of symptoms can indicate that the thoracic aortic aneurysm is expanding.  When symptoms are present, they are generally due to compression of adjacent structures.  These compressive symptoms include dysphagia (compression of the esophagus), dyspnea and chronic cough (compression of the airway), or hoarseness (compression of the recurrent laryngeal nerve).

## Physical Examination
A large thoracic aortic aneurysm may be associated with findings on physical examination secondary to aortic insufficiency (wide pulse pressure, diastolic murmur, heart failure) and compression of adjacent structures such as the recurrent laryngeal nerve (hoarseness), superior vena cava obstruction (selling of the face and neck), and the airway (stridor and wheezing).

## General Approach to Imaging in the Patient with a  Thoracic Aortic Aneurysm
The diagnosis of a thoracic aortic aneurysm is often made as an incidental finding on either chest x-ray (widening of the mediastinum), on transthoracic echocardiography (dilated aortic root) or on trans esophageal echocardiography (enlarged ascending aorta or aortic arch).  The definitive diagnosis is made using CT angiography but MRA is also an excellent test. Once diagnosed, serial CTA (or MRA) are recommended every 6-12 months.  The frequency of surveillance imaging should be increased if the aneurysm is larger at baseline, if there is a recent history of accelerating expansion, if the patient is pregnant,  and if the patient has marfan syndrome, loeys-dietz syndrome or a confirmed genetic mutation known to predispose to aortic aneurysms and aortic dissections (TGFBR1, TGFBR2, FBN1, ACTA2, or MYH11).  Indeed in these high risk syndromes, the patient should undergo complete aortic imaging at the time of the initial diagnosis and then every 6 months to determine if enlargement is occurring.  Bicuspid aortic valve, coarctation of the aorta, and/or dilatation of the ascending thoracic aorta should be excluded in a patient with turner's syndrome.  Follow-up imaging should be obtained every 5 to 10 years in the patient with turner's syndrome if the initial imaging is normal, and yearly if any abnormalities are present.  Patients with bicuspid aortic valve disease and first-degree relatives of patients with bicuspid aortic valve disease should undergo imaging to rule out the presence of the thoracic aortic aneurysm.

## Chest X-Ray
Thoracic aortic aneurysm is associated with a abnormality on chest x-ray in 80 to 90% of patients. Smaller aneurysms (saccular aneurysms) may not be apparent on a chest x ray.

## MRI
The definitive diagnosis of thoracic aortic aneurysm can be made using magnetic resonance angiography (MRA). Once the diagnosis is made, serial MRAs (or CT) are recommended every 6-12 months.

## CT
Computed tomography is the most effective method for measuring and monitoring the maximum outer diameter of a thoracic aortic aneurysm over long periods of time.

## Echocardiography or Ultrasound
The diagnosis of a thoracic aortic aneurysm is often made as an incidental finding on transthoracic echocardiography (dilated aortic root) or on trans esophageal echocardiography (enlarged ascending aorta or aortic arch).  The definitive diagnosis is made using CT angiography but MRA is also an excellent test. Once diagonosed, serial CTA (or MRA) are recommended every 6-12 months.

# Treatment

## Medical Therapy
Medical therapy for patients with a thoracic aortic aneurysm includes aggressive blood pressure control, smoking cessation, and aggressive lipid management.

## Surgery
Indications for surgical repair of a thoracic aortic aneurysm include rupture; symptoms such as pain consistent with impending rupture; aortic regurgitation; growth  0.5 - 1 cm/year; bicuspid aortic valve; asymptomatic patients with degenerative thoracic aneurysm, chronic aortic dissection, intramural hematoma, penetrating atherosclerotic ulcer, mycotic aneurysm, or pseudoaneurysm, who are otherwise suitable candidates once a TAA reaches a pre-specified size (>5 cm in the ascending aorta, >6 cm in the descending segment) referral for surgical or endovascular repair sholuld be initiated. Most patients undergo repair once they reach >5.5 cm diameter in the ascending aorta and > 6.5 cm in the descending thoracic aorta, respectively.  Patients with marfan syndrome or other genetically mediated disorders (vascular ehlers-danlos syndrome, turner syndrome, bicuspid aortic valve, or familial thoracic aortic aneurysm and dissection) should undergo elective operation at smaller diameters (4.0 to 5.0 cm depending on the condition. To avoid acute dissection or rupture, adult patients with loeys-dietz syndrome should undergo surgery for an aortic diameter of >4.4 to 4.6 cm.  If a marfan syndrome patient is contemplating pregnancy, they should undergo aortic root replacement if the diameter is greater than 4 cm, and patients undergoing aortic valve repair or replacement and who have an ascending aorta or aortic root of greater than 4.5 cm should be considered for concomitant repair of the aortic root or replacement of the ascending aorta.  A woven dacron tube graft is most commonly used in the repair of thoracic aortic aneurysms.