Thoracic aortic aneurysms develop when the wall of the aorta weakens and balloons outward in the chest. The causes range from inherited genetic conditions to lifestyle factors like smoking and high blood pressure, and the specific cause often depends on where in the thoracic aorta the aneurysm forms. An ascending aorta measuring 4.0 cm or more is considered dilated, and the risk of a dangerous tear (dissection) rises significantly once it reaches 4.5 cm.
Genetic Conditions and Family History
Genetics play a larger role in thoracic aortic aneurysms than most people realize. Several inherited syndromes directly weaken the aortic wall by disrupting the connective tissue that holds it together. Marfan syndrome, caused by a mutation in the FBN1 gene, was the first to be identified in 1991 and remains one of the most well-known. People with Marfan syndrome produce a defective version of the protein fibrillin-1, which is essential for the structural integrity of the aortic wall.
Loeys-Dietz syndrome is another major genetic cause, and it comes in multiple forms (types 1 through 5), each involving a different gene in a signaling pathway that regulates how the body builds and repairs connective tissue. Vascular Ehlers-Danlos syndrome, caused by a mutation in the COL3A1 gene, weakens collagen throughout the body and makes the aorta especially fragile. Together, mutations in the genes behind Marfan syndrome and the various types of Loeys-Dietz syndrome account for roughly 10% of familial cases where no outward signs of a syndrome are present.
Even without a named syndrome, thoracic aortic aneurysms often run in families. Researchers identified the familial nature of these aneurysms in the late 1990s, and specific gene variants can now help predict how quickly the aorta will enlarge and when intervention might be needed. If a first-degree relative (a parent or sibling) has been diagnosed, your own risk is meaningfully elevated.
Bicuspid Aortic Valve
A bicuspid aortic valve, where the valve has two flaps instead of the usual three, is the most common congenital heart defect. It’s also a significant cause of thoracic aortic aneurysms. Between 50% and 70% of people born with a bicuspid valve experience some form of complication, whether valve dysfunction or aortic aneurysm, either in childhood or later in life.
There are two reasons this happens. First, the misshapen valve changes how blood flows through the ascending aorta, creating turbulent jets that put uneven stress on the aortic wall. Second, and perhaps more importantly in younger patients, people with bicuspid valves often have an underlying weakness in the aortic wall itself, a condition sometimes called aortopathy. This means the aorta can enlarge even when the valve is functioning well and blood flow patterns aren’t particularly abnormal. The root of the aorta (closest to the heart) appears especially susceptible to this genetic component, while the ascending aorta further along may be affected by both the inherited weakness and the altered blood flow.
High Blood Pressure and Wall Degeneration
Chronic high blood pressure is one of the most common contributors to thoracic aortic aneurysm. The mechanism is straightforward in concept: years of elevated pressure strain the aortic wall, and over time, the wall’s ability to withstand that strain breaks down. On a cellular level, this involves several overlapping processes. The smooth muscle cells that give the aorta its strength begin to malfunction. The structural matrix of proteins surrounding those cells degrades. Oxidative stress accumulates. Normal cellular signaling pathways go awry.
The result is a condition that pathologists call cystic medial necrosis, where the middle layer of the aortic wall develops deteriorated, cyst-like areas filled with damaged tissue. This same type of degeneration occurs in genetic conditions like Marfan and Ehlers-Danlos syndromes, but it also happens as a consequence of aging and uncontrolled blood pressure in people with no genetic predisposition at all. Over years or decades, the weakened areas gradually stretch outward under the constant pressure of each heartbeat.
Atherosclerosis and the Descending Aorta
Atherosclerosis, the buildup of fatty plaque inside artery walls, plays a different role depending on where the aneurysm is located. In the ascending aorta (the section nearest the heart), atherosclerosis is a rare cause. But in the descending thoracic aorta (the section that runs along the spine), it’s the most commonly associated factor.
The risk factors that drive atherosclerosis are the same ones that increase your chances of a descending thoracic aneurysm: age over 55, male sex, high cholesterol, high blood pressure, smoking, and diabetes. Plaque deposits damage and inflame the aortic wall, weakening it from the inside. This is one reason why thoracic aortic aneurysms are sometimes discovered incidentally during imaging for heart disease or other vascular problems.
How Smoking Damages the Aortic Wall
Smoking contributes to thoracic aortic aneurysms through a specific and well-documented mechanism. Nicotine triggers the aortic wall to overproduce two types of enzymes (MMP-2 and MMP-9) whose job is to break down structural proteins, particularly elastin. Elastin is what gives the aorta its ability to stretch and recoil with each heartbeat. In animal studies, chronic nicotine exposure caused these enzymes to ramp up dramatically in the aortic wall within 40 days, leading to visible fragmentation of elastin fibers. The resulting damage increased aortic stiffness irreversibly, even after nicotine exposure stopped. A stiffer, less elastic aorta is more vulnerable to further stretching and aneurysm formation.
Inflammatory Diseases
Chronic inflammation of the aortic wall, called aortitis, can come from autoimmune conditions that attack blood vessels. Giant cell arteritis is the most significant example. In a population-based study from Olmsted County, Minnesota, patients with giant cell arteritis were 17.3 times more likely to develop a thoracic aortic aneurysm than people of the same age and sex in the general population. Of 96 patients tracked, 11 developed thoracic aortic aneurysms, with most appearing a median of 5.8 years after the initial diagnosis. Six of those 11 patients ultimately died from acute aortic dissection.
Takayasu arteritis, another inflammatory condition affecting large vessels, can also lead to thoracic aneurysms. Historically, untreated syphilis was a well-known infectious cause of thoracic aortitis and aneurysm, though this has become rare with modern antibiotics.
Chest Trauma
A sudden, forceful blow to the chest, most commonly from a high-speed car accident, can injure the thoracic aorta through rapid deceleration. The aorta is partially fixed in place at certain points, and when the body stops suddenly, the forces of shearing, stretching, and compression between the chest wall and spine can tear through the inner layers of the aortic wall. This injury is graded on a scale from a minor intimal tear (grade I) to a full pseudoaneurysm (grade III) or complete rupture (grade IV). A pseudoaneurysm is not a true aneurysm in the classic sense. It’s a contained leak where blood pushes through the damaged inner layers but is held in by the outermost layer. The progression from initial tear to pseudoaneurysm or rupture happens over a variable timeline, which is why these injuries require close monitoring even when they initially appear minor.
Age and Sex
Thoracic aortic aneurysms are significantly more common in men, who make up roughly 74% of diagnosed cases. Men also tend to be diagnosed younger, with a mean age around 62, while women are typically diagnosed closer to 68. The aorta naturally stiffens and weakens with age as elastin fibers degrade and the wall loses its resilience, making older adults more vulnerable regardless of other risk factors. This age-related degeneration compounds the effects of high blood pressure, smoking, or any genetic predisposition that may already be present.