Key Features of Thoracic Aortic Aneursym (TAA)
and Abdominal Aortic Aneurysm (AAA)
TAA is often recognized as a distinct disease from AAA and is associated with heterogenous etiologies including:
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5% connective tissue systemic disease; Marfan syndrome, Loeys-Deitz syndrome, and Ehlers-Danlos syndrome.
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21% have a family history but do not have syndromic disease.
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~50% of Bicuspid Aortic Valve (BAV) patients develop TAA and have an 8‐fold higher risk of aortic dissection than the general population.
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Trauma and aortic arteritis.
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Idiopathic, but likely genetic predisposition.
Among the connective tissue genetic diseases, Marfan syndrome, has a relatively favorable prognosis with a median survival of 70 years. Loeys–Dietz has a more aggressive vascular course with a median survival of only 37 years. Ehlers–Danlos syndrome (EDS) is a group of rare genetic disorders which has 13 subtypes. Although some subtypes are associated with only mild symptoms, life-threatening complications such as aortic dissection or rupture, may cause fatalities. Bicuspid aortic valve or BAV is the most common congenital cardiac defect in adults, affecting 0.5% to 2% of people, and is responsible for more deaths and complications than all other congenital cardiac defects combined. BAV is highly heritable with a prevalence of 9% among first-degree relatives of patients with BAV. Up to 50% of patients with BAV develop TAA.
Risk factors associated with TAA are hypertension, atherosclerosis, family history, smoking, high body mass index, male gender, and age. Thoracic aneurysms are often asymptomatic and are diagnosed incidentally. In symptomatic patients, the most common presentations are pain in the chest, neck, or back, hoarse voice, distended veins, and dyspnea or cough among others. Thoracic aortic aneurysms often have a risk of rupture or dissection, and acute aortic syndrome with sudden pain in the chest or back, neck, or abdomen.
Thoracic aortic dissection is a tear in the aorta wall's intimal layer that allows blood to flow between the tunica intima and media. It is classified as acute (when detected within 14 days) or chronic (when diagnosed after 14 days). They are more common in men and people with connective tissue problems, and they peak between the ages of 50 and 70. The pooled incidence of TAA is estimated to be 5.3 per 100,000 individuals/year. Thoracic aortic dissection (TAD) is estimated to occur at a rate of ~3.3 cases per 100,000 persons per year and is associated with a high mortality. Because of the catastrophic risks associated with TAAs, asymptomatic patients with aneurysms of the ascending aorta or arch measuring 5.5 cm or greater generally should be considered for surgical repair. For patients who may present with TAA or for complications related to TAA, such as aortic dissection, medical management is important. Aortic wall stress is determined by the velocity of ventricular contraction (dP/dt), the heart rate, and blood pressure. Maintaining adequate perfusion while addressing these parameters may be critical, in particular in patients with dissection to minimize the risk for propagation. Recommendations for patients with chronic thoracic aortic disease include BP goals less than 140/90 (without diabetes) or 130/80 (with diabetes or chronic kidney disease). Beta blockers or ACE inhibitors or ARBS are recommended. For patients with acute dissection, guidelines suggest a heart rate target less than 60 bpm and a systolic blood pressure between 100- and 120-mm Hg. Initial treatment with beta blockers such as short-acting esmolol infusion or IV propanolol, metoprolol, or labetalol may be good options. In patients who cannot tolerate beta blockers, nondihydropyridine calcium channel blockers such as verapamil or diltiazem are recommended as an acceptable alternative.
Unfortunately, there is no medication that arrests the expansion of TAA. Nor is there a medication that acts in a direct way to reduce the risk of rupture or dissection. An effective medication to treat aortic disease is desperately needed for those with strong predisposition to TAA/TAAD such as connective tissue disease, identified genetic risk, or BAV.
A common pathology of the aorta is the development of an aneurysm. A true aneurysm is dilation of all three layers of the aortic wall more than 50% of the expected diameter. AAA is characterized by aortic enlargement in a portion of the lower aorta (see figure left). Stated in units of measure, this turns out to be a diameter >3.0 cm, since the diameter >3.0 cm, since the diameter of a healthy aorta in the same region is ~2 cm. Well-defined risk factors for AAA development are advanced age, male sex, being White, a positive
family history, smoking, the presence of other large vessel aneurysms, and atherosclerosis. Most AAAs are asymptomatic until they rupture. Ruptured AAA is the 14th leading cause of mortality in the United States each year. Between 59% and 83% of patients who experience AAA rupture die before they reach a hospital or have surgery.
AAA affects about 1.5% of men older than 60 years and about 1% of women older than 64 years. Women with AAA are known to have worse outcomes than men. The reasons for this difference are not well understood. AAA is often detected as an incidental finding when imaging is performed for other purposes. In 2005, the U.S. Preventive Services Task Force (USPSTF) recommended one-time ultrasonography to screen for AAA in men 65 to 75 years of age who have smoked. This recommendation was updated in 2014. About 1MM people in the US have been diagnosed with AAA based on imaging studies.
The risk and rate of AAA growth increases with aneurysm size. Larger aneurysms tend to grow faster and have a higher risk of rupture. No medication has been demonstrated to arrest the inexorable expansion of an aortic aneurysm. The current goal of therapy for patients diagnosed with AAA is to maintain general cardiovascular health and to control blood pressure to try and avert a catastrophic rupture. Medical management of AAA predominantly involves administering antihypertensive, statin, and antiplatelet therapy. Although the risk of rupture for most small aneurysms is low, ruptures of diagnosed AAA occur. For men, the current treatment guidelines indicate that when an AAA reaches ~5.5 cm the risk of a potentially fatal rupture outweighs the risk of morbidity and mortality of an intervention to repair it. For women, an aortic diameter exceeding ~5 cm is typically deemed to present an unacceptably high risk of rupture. Two procedures are used to repair large AAA, open repair and endovascular aneurysmal repair or EVAR. EVAR is the most common surgical repair for AAA. EVAR involves a small incision on the femoral vessels to gain vascular access to the aorta. A stent graft is deployed in the aneurysmal sac and extends into the iliac vessel. The purpose of the endograft is to decrease the pressure on the aortic wall and prevent aneurysmal sac enlargement by excluding the aneurysm from circulatory pressures. An aortic stent graft must be reassessed at one month, six months, and then yearly to detect graft migration, evaluate for endoleaks, and monitor for expansion of the aortic aneurysmal sac. Outcomes after repair procedures reinforce the unmet medical need for a therapy that arrests AAA expansion and ideally obviates the need for any type of repair. A systematic review and meta-analysis of 73 studies including 299,784 patients, found that EVAR was associated with higher long-term, all-cause mortality, reintervention, and secondary rupture rates compared with open repair. Late rupture remains a devastating complication, with an estimated 30-day mortality of 32%, according to a 2015 meta-analysis. Rupture of an abdominal aortic aneurysm after EVAR has been documented in 5.4% of patients. The need for reintervention to maintain exclusion of the aneurysm from the circulation and to guard against late rupture has been reported at early, midterm, and long-term follow-up and does not plateau with time; therefore, lifelong follow-up is necessary.
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