Thoracic Aortic Aneurysm

Background

Thoracic aortic aneurysm represents aneurysmal dilatation of ascending, arch, or descending thoracic aorta. Aneurysm is defined as a localized or diffuse dilatation of more than 50% normal diameter of the aorta. It occurs with the highest frequency of all of the diseases of the thoracic aorta that require surgical treatment. Atherosclerosis or connective tissue disorders may be contributing underlying disorders that facilitate aortic dilatation. Frequently associated factors include advanced age, hypertension, smoking, atherosclerosis, and aortic dissection.Thoracic aneurysms are classified by the portion of aorta involved: the ascending thoracic aorta, the arch, or the descending thoracic aorta. This anatomic distinction is important because the etiology, natural history,and treatment of thoracic aneurysms vary for each of these segments.Aneurysms of the descending aorta are most common, followed by aneurysms of the ascending aorta, whereas arch aneurysms occur less often. In addition, descending aortic thoracic aneurysms may extend distally to involve the abdominal aorta and create a thoracoabdominal aortic aneurysm.Sometimes, the entire aorta may be ectatic, with localized aneurysms seen at sites in both the thoracic and abdominal aorta. Interestingly, thoracic aortic aneurysms are less common than aneurysms of the abdominal aorta.

Pathophysiology

Cystic medial necrosis Aneurysms of the ascending thoracic aorta most often result from the process of cystic medial degeneration (or cystic medial necrosis).Histologically, cystic medial degeneration has the appearance of smooth muscle cell necrosis and elastic fiber degeneration, with the presence in the media of cystic spaces filled with mucoid material. Although these changes occur most frequently in the ascending aorta, in some cases the entire aorta may be similarly affected.The histologic changes lead to weakening of the aortic wall, which in turn results in the formation of a fusiform aneurysm. Such aneurysms often involve the aortic root and may consequently result in aortic regurgitation. The term annuloaortic ectasia is often used to describe this condition.Cystic medial degeneration is found in virtually all cases of Marfan syndrome and may be associated with other connective tissue disorders as well, such as Ehlers-Danlos syndrome. Marfan syndrome is an autosomal dominant heritable disorder of connective tissue that has been discovered to be due to mutations in one of the genes for fibrillin, a structural protein that helps direct and orient elastin in the developing aorta.These mutations result in a decrease in the amount of elastin in the aortic wall, together with a loss of the normally highly organized structure of elastin. As a consequence, a marfanoid aorta exhibits markedly abnormal elastic properties and increased systemic pulse wave velocities from an early age, and over time, the aorta exhibits progressively increasing degrees of stiffness and dilatation.In patients without Marfan syndrome, however, it is not possible to recognize the histologic diagnosis of cystic medial degeneration prospectively (eg, without surgery or necropsy). This fact has significantly limited our understanding of medial degeneration and its natural history, and it remains unclear to what extent this syndrome may represent an independent disease process versus a manifestation of another disease state.It has long been suspected that some patients who have annuloaortic ectasia and proven cystic medial degeneration without the classic phenotypic manifestations of Marfan syndrome may, in fact, have a variation, or forme fruste, of Marfan syndrome, although this theory remains unproved. On the contrary, many patients with ascending thoracic aortic aneurysms appear to have nothing more than idiopathic cystic medial degeneration.

Atherosclerosis

Atherosclerotic aneurysms infrequently occur in the ascending aorta and, when they do, tend to be associated with diffuse aortic atherosclerosis. Aneurysms in the aortic arch are often contiguous with aneurysms of the ascending or descending aorta. They may be due to atherosclerotic disease, cystic medial degeneration, syphilis, or other infections.The predominant etiology of aneurysms of the descending thoracic aorta is atherosclerosis. These aneurysms tend to originate just distal to the origin of the left subclavian artery and may be either fusiform or saccular. The pathogenesis of such atherosclerotic aneurysms in the thoracic aorta may be similar to that of abdominal aneurysms but has not been extensively examined.

Syphilis

Syphilis was once a common cause of ascending thoracic aortic aneurysm, but today it has become a rarity in most major medical centers as a result of aggressive antibiotic treatment of the disease in its early stages.The latent period from initial spirochetal infection to aortic complications is in the range of 5-40 years, but it is most commonly 10-25 years. During the secondary phase of the disease, spirochetes directly infect the aortic media, most commonly involving the ascending aorta. The muscular and elastic medial elements are destroyed by the infection and inflammatory response and are replaced by fibrous tissue that frequently calcifies.Weakening of the aortic wall from medial destruction results in progressive aneurysmal dilatation. In addition, the infection may spread into the aortic root, and the subsequent root dilatation may result in aortic regurgitation.

Infectious aortitis

This rare cause of aortic aneurysm may result from a primary infection of the aortic wall causing aortic dilatation with the formation of fusiform or saccular aneurysms.More commonly, infected or mycotic aneurysms may arise secondarily from an infection occurring in a preexisting aneurysm of another etiology. When an infected aneurysm involves the ascending aorta, it is often the consequence of direct spread from aortic valve bacterial endocarditis.

Miscellaneous

Other causes of thoracic aortic aneurysms include giant cell arteritis, aortic trauma, and aortic dissection.

Frequency

United States

The population incidence of detected thoracic aortic aneurysms is estimated to be 5.9 new aneurysms per 100,000 person-years. The lifetime probability of rupture in thoracic and thoracoabdominal aneurysms is 75 to 80%, with 5-year untreated survival rates or 10-20%. In nondissecting aneurysms, the median time to rupture has been reported to be 2-3 years.

 

Mortality/Morbidity

In the largest modern series, survival rates for patients with thoracic aortic aneurysms not undergoing surgical repair were as follows: 65% survival at 1 year, 36% survival at 3 years, and 20% survival at 5 years. Aneurysm rupture occurs in 32-68% of patients not treated surgically, with rupture accounting for 32-47% of all deaths. Less than one half of patients with rupture may arrive at the hospital alive. The mortality rates for aneurysmal rupture are 54% at 6 hours and 76% at 24 hours. Hospital mortality rates for primary medical treatment remain relatively high, and a substantial percentage of patients require surgery during initial hospitalization. The main causes of death in both medical and surgical groups are rupture. Improvements in supportive medical care have increased patient survival. In the patients with aortic aneurysm, aortic dissection is the catastrophic event most feared. Rupture of a thoracic aortic aneurysm is more frequent than abdominal aortic rupture. The presence or absence of symptoms is another important predictor. Symptomatic patients have a poorer prognosis than those without symptoms. Onset of new symptoms is frequently a harbinger of rupture or death. Moreover, the high prevalence of additional cardiovascular disease in these patients may have a great impact on mortality. In fact, next to aneurysm rupture, the most common causes of death in persons with aortic aneurysm are other cardiovascular diseases.

Age

The incidence or thoracic aortic aneurysm is more common with increasing age.

Anatomy

Classification

The DeBakey and Stanford classifications are the 2 most widely used methods to describe the type of aortic dissection.In the DeBakey classification, types I, II, and III are based on the origin and extent of the dissecting process.In the Stanford system, type A signifies involvement of the ascending aorta, with or without involvement of the arch or the descending aorta (regardless of the site of the primary intimal tear). Type B represents all others, or dissections that do not involve the ascending aorta.

Dissection and rupture

The aortic layers, beginning at the innermost wall, are the intima, the media, and the adventitia.In aortic dissection, a tear in the intima allows blood to escape from the true lumen of the aorta, rapidly dissecting the inner from the outer layer of the media and expanding the aorta to above normal size. A false channel forms in the outer half of the aortic media, whose walls are exceedingly thin and highly susceptible to rupture.Size is an important predictor of the risk of aneurysm rupture.

Acute versus chronic dissections

Dissections or rupture detected within 14 days of the onset of pain or other initial clinical symptom related to the dissection are classified as acute, and death may occur suddenly or within the first hours or days after onset.Chronic dissection, which is diagnosed 2 weeks after the initial tear, may expand in the weakened aortic wall to develop an aneurysm. As in aortic aneurysm, causative factors are difficult to establish. The risk of rupture within 1 year for aneurysms with diameters of 6 cm is 43%. The risk with diameters of 8 cm and greater is 80%, and the risk for those with diameters smaller than 5 cm is 4%.

Expansion

Dapunt and colleagues monitored 67 patients with thoracic aortic aneurysms by means of serial CT and found a mean rate of expansion of 0.43 cm/y. The only independent predictor of rapid expansion (>0.5 cm/y) was an initial aortic diameter larger than 5 cm.Growth rates were as follows: for aneurysms 5 cm or smaller, 0.17 cm/y, and for aneurysms larger than 5 cm, 0.79 cm/y. Only 1 of 25 aneurysms 4 cm or smaller at baseline showed rapid growth. No aneurysm smaller than 5 cm ruptured during the follow-up period. The only predictor of survival was initial aneurysm size.For dissecting aneurysms, the median time to rupture is approximately 3 days in patients with acute dissection. Patients with aneurysms 5 cm or larger, those with documented aneurysm enlargement, or those with chest or back pain indicating expansion are considered candidates for elective surgery.

Clinical Details

Signs and symptoms associated with aortic dissection are variable and depend on the extent of aortic and branch vessel involvement. Patients with the ultimate diagnosis of aortic dissection are often initially thought to have other conditions such as myocardial ischemia, congestive heart failure, or pulmonary embolus. Several clinical syndromes are particularly suggestive of aortic dissection: pain that progresses over hours or days from chest to neck to arms to abdomen, chest pain with concomitant neurologic deficits, and chest pain with pulse deficits.Approximately 40% of patients with thoracic aortic aneurysms are asymptomatic at the time of diagnosis. Aneurysms are typically discovered as incidental findings on a routine physical examination or chest radiography. When patients experience symptoms, the symptoms tend to reflect either a vascular consequence of the aneurysm or a local mass effect.Vascular consequences include (1) aortic regurgitation from dilatation of the aortic root, which is often associated with secondary congestive heart failure; (2) sinus of Valsalva aneurysms that may rupture into the right side of the heart and cause a continuous murmur and congestive heart failure; and (3) thromboembolism causing stroke, lower extremity ischemia, renal infarction, or mesenteric ischemia.A local mass effect from an ascending or arch aneurysm may cause superior vena cava syndrome as a result of obstruction of venous return via compression of the superior vena cava or innominate veins.Aneurysms of the arch or descending aorta may compress the trachea or main stem bronchus and produce tracheal deviation, wheezing, cough, dyspnea (with symptoms that may be positional), hemoptysis, or recurrent pneumonitis. Compression of the esophagus may produce dysphagia, and compression of the recurrent laryngeal nerve may cause hoarseness.Chest pain occurs in 37% of nondissecting aneurysms and back pain, in 21%. These result from direct compression of other intrathoracic structures or the chest wall or from erosion into adjacent bone. Typically, the pain is steady, deep, boring, and sometimes severe.As with abdominal aortic aneurysms, the most worrisome consequence of thoracic aneurysms is leakage or rupture.

Rupture is accompanied by the dramatic onset of excruciating pain, usually in the region where less severe pain had previously existed. Rupture occurs most commonly into the left intrapleural space or the intrapericardial space and is manifested as hypotension. The third most common site of rupture is from the descending thoracic aorta into the adjacent esophagus (an aortoesophageal fistula), which causes life-threatening hematemesis.Acute aneurysm expansion, which may herald rupture, can cause similar chest or back pain.Thoracic aneurysms may also be accompanied by aortic dissection.

Preferred Examination

Although aortic dissection might be suspected on the basis of history and physical findings, diagnostic imaging is necessary to establish the diagnosis. A clear and efficient imaging strategy is required. The clinical team involved in the diagnosis and treatment of patients with aortic dissection should prospectively agree on a strategy. Their approach should consider the technology available at the institution and the ease of performing each test, especially after hours.The preferred examinations for diagnosis are aortic angiography, MRI, magnetic resonance angiography (MRA), and echocardiography.Aortography has been the criterion standard against which other modalities were measured, but it is rarely used with the advent of transesophageal echocardiography (TEE) and CT, though aortography is still the preferred modality for the preoperative evaluation of thoracic aortic aneurysms and for precise definition of the anatomy of the aneurysm and great vessels.CT is a reliable test for diagnosing aortic dissection, and it is the primary diagnostic test of choice in most institutions. CT scans usually show dilation of the aorta, an intimal flap, and both the false and true lumina. Rapid scanning after an intravenous bolus injection of contrast material allows the detection of differential filling rates in the true and false lumina.TEE is helpful due to the proximity of the esophagus to the aorta and the ability to use higher transducer frequencies help to better delineate the aorta. It is highly sensitive but less specific. TEE, however, is

excellent at detecting pericardial effusion and aortic regurgitation, and can be quickly performed at the patient’s bedside under sedation without radiation or the injection of contrast material. Evaluating the ascending aorta and proximal arch may be difficult.MRI is useful in defining thoracic aortic anatomy and detecting aneurysms and is of particular utility in patients with preexisting aortic disease. MRI is an appealing option in the detection of aortic dissection. Sensitivity and specificity are excellent, but it is time consuming and cumbersome to perform.MRA may prove especially useful in defining the anatomy of aortic branch vessels.Regarding echocardiography, TTE is not accurate for diagnosing thoracic aneurysms, and it is particularly limited in its ability to examine the descending thoracic aorta. TEE is a far more accurate method for assessing the thoracic aorta and has become widely used for detection of aortic dissection. There has been less experience with TEE, however, in the evaluation of nondissecting thoracic aneurysms.Reports have shown high sensitivity for TEE, CT, and MRI for the diagnosis of aortic dissection. However, the specificity of CT and MRI was significantly better than that of TEE.

Aortic Aneurysm Medical Treatment Signs,Symptoms and Diagnosis

An aortic aneurysm is a general term for any swelling (dilatation or aneurysm) of the aorta, usually representing an underlying weakness in the wall of the aorta at that location. While the stretched vessel may occasionally cause discomfort, a greater concern is the risk of rupture, which causes severe pain; massive internal hemorrhage; and, without prompt treatment, results in a quick death.

Classification

Aortic aneurysms are classified by where on the aorta they occur; aneurysms can appear anywhere. An aortic root aneurysm, or aneurysm of sinus of Valsalva, appears on the sinuses of Valsalva or aortic root. Thoracic aortic aneurysms are found on the thoracic aorta; these are further classified as ascending, aortic arch, or descending aneurysms depending on the location on the thoracic aorta involved. Abdominal aortic aneurysms, the most common form of aortic aneurysm, are found on the abdominal aorta, and thoracoabdominal aortic aneuryms involve both the thoracic and abdominal aorta.

Pathology

The physical change in the aortic diameter can occur secondary to trauma, infection, an intrinsic defect in the protein construction of the aortic wall, or due to progressive destruction of aortic proteins by enzymes.

Signs, symptoms and diagnosis

 Most intact aortic aneurysms do not produce symptoms. As they enlarge, symptoms such as abdominal pain and back pain may develop. Compression of nerve roots may cause leg pain or numbness. Untreated, aneurysms tend to become progressively larger, although the rate of enlargement is unpredictable for any individual. Rarely, clotted blood which lines most aortic aneurysms can break off and result in an embolus. They may be found on physical examination. Medical imaging is necessary to confirm the diagnosis. Symptoms may include: anxiety or feeling of stress; nausea and vomiting; clammy skin; rapid heart rate.

Abdominal aortic aneurysm

Abdominal aortic aneurysms, hereafter referred to as AAAs, are the most common type of aortic aneurysm. One reason for this is that elastin, the principal load-bearing protein present in the wall of the aorta, is reduced in the abdominal aorta as compared to the thoracic aorta (nearer the heart). Another is that the abdominal aorta does not possess vasa vasorum, hindering repair. Most are true aneurysms that involve all three layers (tunica intima, tunica media and tunica adventitia), and are generally asymptomatic before rupture.The prevalence of AAAs increases with age, with an average age of 65-70 at the time of diagnosis. AAAs have been attributed to atherosclerosis, though other factors are involved in their formation.

An AAA may remain asymptomatic indefinitely. There is a large risk of rupture once the size has reached 5 cm, though some AAAs may swell to over 15 cm in diameter before rupturing. Before rupture, an AAA may present as a large, pulsatile mass above the umbilicus. A bruit may be heard from the turbulent flow in a severe atherosclerotic aneurysm or if thrombosis occurs. Unfortunately, however, rupture is usually the first hint of AAA. Once an aneurysm has ruptured, it presents with a classic pain-hypotension-mass triad. The pain is classically reported in the abdomen, back or flank. It is usually acute, severe and constant, and may radiate through the abdomen to the back.

The diagnosis of an abdominal aortic aneurysm can be confirmed at the bedside by the use of ultrasound. Rupture could be indicated by the presence of free fluid in potential abdominal spaces, such as Morison’s pouch, the splenorenal space (between the spleen and left kidney), subdiaphragmatic spaces (underneath the diaphragm) and peri-vesical spaces. A contrast-enhanced abdominal CT scan is needed for confirmation.

Only 10-25% of patients survive rupture due to large pre- and post-operative mortality. Annual mortality from ruptured abdominal aneurysms in the United States alone is about 15,000. Another important complication of AAA is formation of a thrombus in the aneurysm.

Medical treatment

 Medical therapy of aortic aneurysms involves strict blood pressure control. This does not treat the aortic aneurysm per se, but control of hypertension within tight blood pressure parameters may decrease the rate of expansion of the aneurysm.

Surgical treatment

The definitive treatment for an aortic aneurysm is surgical repair of the aorta. This typically involves opening up of the dilated portion of the aorta and insertion of a synthetic (Dacron or Gore-tex) patch tube. Once the tube is sewn into the proximal and distal portions of the aorta, the aneurysmal sac is closed around the artificial tube. Instead of sewing, the tube ends, made rigid and expandable by nitinol wireframe, can be much more simply and quickly inserted into the vascular stumps and there permanently fixed by external ligature

The determination of when surgery should be performed is complex and case-specific. The overriding consideration is when the risk of rupture exceeds the risk of surgery. The diameter of the aneurysm, its rate of growth, the presence or absence of Marfan Syndrome or similar connective tissue disorders, and other coexisting medical conditions are all important factors in the determination.

A rapidly expanding aneurysm should be operated on as soon as feasible, since it has a greater chance of rupture. Slowly expanding aortic aneurysms may be followed by routine diagnostic testing (ie: CT scan or ultrasound imaging). If the aortic aneurysm grows at a rate of more than 1 cm/year, surgical treatment should be electively performed.

The current treatment guidelines for abdominal aortic aneurysms suggest elective surgical repair when the diameter of the aneurysm is greater than 5 cm. However, recent data suggests medical management for abdominal aneurysms with a diameter of less than 5.5 cm.

Endovascular treatment of AAA

In the recent years, the endoluminal treatment of Abdominal Aortic Aneurysms has emerged as a minimally invasive alternative to open surgery repair. The first endoluminal exclusion of an aneurysm took place in Argentina by Dr. Parodi and his colleagues in 1991. The endovascular treatment of aortic aneurysms involves the placement of an endo-vascular stent via a percutaneous technique (usually through the femoral arteries) into the diseased portion of the aorta. This technique has been reported to have a lower mortality rate compared to open surgical repair, and is now being widely used in individuals with co-morbid conditions that make them high risk patients for open surgery. Some centers also report very promising results for the specific method in patients that do not constitute a high surgical risk group.

There have also been many reports concerning the endovascular treatment of ruptured Abdominal Aortic Aneurysms, which are usually treated with an open surgery repair due to the patient’s impaired overall condition. Mid-term results have been quite promising.[citation needed] However, according to the latest studies, the EVAR procedure doesn’t carry any overall survival benefit.

Endovascular treatment of other aortic aneurysms

The endoluminal exclusion of aortic aneurysms has seen a real revolution in the very recent years. It is now possible to treat thoracic aortic aneurysms, abdominal aortic aneurysms and other aneurysms in most of the body’s major arteries (such as the iliac and the femoral arteries) using endovascular stents and avoiding big incisions. Still, in most cases the technique is applied in patients at high risk for surgery as more trials are required in order to fully accept this method as the gold standard for the treatment of aneurysms.

 Prevention

Attention to patient’s general blood pressure, smoking and cholesterol risks helps reduce the risk on an individual basis. There have been proposals to introduce ultrasound scans as a screening tool for those most at risk: men over the age of 65.. The tetracycline antibiotic Doxycycline is currently being investigated for use as a potential drug in the prevention of aortic aneurysm due to its metalloproteinase inhibitor and collagen stabilising properties.

 Research

Stanford University is conducting research to gather information on AAA risk factors, and to evaluate the effectiveness of an exercise program at preventing the growth of small AAAs in older individuals.