Acute aortic syndrome presents challenging diagnostic and therapeutic disease. Intramural hematoma (IMH), penetrating aortic ulcer, aortic dissection (AD), and aneurysm can all have a similar clinical presentation with advanced disease. Many of these syndromes have undergone significant revision in terms of classification and therapeutic strategy in the last 30 years. The increasing sophistication and speed of computed tomographic (CT) scan has raised it to a prominent role in the evaluation of this disease. The exact role endovascular intervention will play in the treatment algorithms remains to be determined. Despite the benefit of modern imaging and this collective experience, it remains a highly morbid malady.

The embryonic vascular system begins formation in the third week of gestation. From the primitive aortic sac arise six ventral paired aortic arches. These pass laterally around the primitive gut to terminate in the paired dorsal aortae. Eventually, there is fusion of the paired dorsal aortae. The first pair of aortic arches contributes to formation of maxillary and external carotid arteries. The second pair contributes to formation of the stapedial arteries. The third pair contributes to formation of the common and internal carotid arteries. The left fourth aortic arches contribute to form the aortic arch and the right fourth arch contributes to formation of the right subclavian artery. The fifth pair of aortic arches usually has no anatomic contribution. An association between persistent fifth aortic arch and chromosome 22q11.2 deletion has been described.¹ The left sixth aortic arch contributes to formation of the left pulmonary artery and the ductus arteriosus. The right sixth aortic arch contributes to formation of the right pulmonary artery.

The descending thoracic aorta arises from the aortic arch just after the origin of the left subclavian artery, at the inferior border of the fourth thoracic vertebrae. This point of transition is termed the aortic isthmus. In adults, the average diameter of the descending thoracic aorta is 2.8 cm in men and 2.6 cm in women.² This narrows as it descends into the abdomen. It terminates as it enters the abdomen via the diaphragmatic aortic hiatus, at the 12th intercostal space. The thoracic aorta descends in the posterior mediastinum to the left of the vertebral column and gradually shifts to the midline at the aortic hiatus. It is surrounded by the thoracic aortic plexus. Anteriorly, the left pulmonary hilum crosses with the left main bronchus and left pulmonary artery being closely associated. Continuing inferiorly, the esophagus, pericardium, and diaphragm are also situated at the anterior border of the thoracic aorta. As the thoracic aorta descends, the esophagus crosses anteriorly and then laterally at the diaphragm. Posteriorly, the hemiazygous vein and anterior vertebral column are associated. Laterally, it is closely applied to the inferior lobe of the left lung. Medially, the esophagus, thoracic duct, and azygous vein are closely associated. There are bronchial, esophageal, intercostal, mediastinal, pericardial, subcostal, and superior phrenic branches of the descending thoracic aorta. The main artery supplying the lower spinal cord (artery of Adamkiewicz) typically arises from a left-sided intercostal artery between the 9th and 12th intercostal spaces. The wall of the aorta is composed of thin inner intima, a thicker middle media, and a thinner outer adventitia containing vasa vasorum. During systole, the elastic laminae of the robust media distend and create potential energy, which is transmitted during the diastolic phase.

Epidemiology

The estimated incidence of thoracic aortic aneurysm (TAA) was 0.37/100 000 person-years.³ The incidence has been reported to be rising dramatically in the past 40 years, with a recent estimate of 10.4/100 000 person-years.⁴ The median age of diagnosis ranges from 64.5 to 68.5 years and the mean 58 to 70.5 years. Men tend to have a higher incidence than women with an average ratio of 1.9:1, but the reported range varies from 0.9:1 to 17:1.⁵ Several risk factors have a strong association with TAA formation. Of those having TAA, greater than 70% have hypertension and at least 80% are smokers.⁵ Other associated risks are coronary artery disease, chronic renal failure, cerebrovascular disease, peripheral vascular disease, visceral occlusive disease, chronic obstructive pulmonary disease (COPD), and diabetes mellitus.⁵ A recent study of TAAs, dissections and pedigrees revealed that 21.5% of non-Marfan syndrome patients had an inherited pattern. Seventy-six percent were transmitted autosomally dominant with a varying penitiance.⁶

Pathophysiology

An aneurysm has been defined as a permanent localized dilation of an artery having at least a 50% increase in diameter compared to the expected normal diameter of the artery. The average diameter of the descending thoracic aorta is 2.8 cm in men and 2.6 cm in women.² It is more accurate to define the size of aneurysm based on expected size of age, gender, and body surface area (BSA) matched controls. This data is available in the Pearce et al. paper² (see Figure 29-1). An imbalance of enzymatic degradation as well as genetically defective structural elements has been implicated in aneurysm formation. Deficiency of other proteins, as occurs with Marfan’s syndrome with deficient fibrillin, is associated with aortic aneurysm also. There is also an association between bicuspid aortic valve and TAA.

Turner syndrome is associated with aortic dilation in approximately 6.3% of those affected according to one survey.⁷ The most common congenital anomaly of the heart is bicuspid aortic valve. Aortic root dimensions were found to be significantly larger in a group of young men with biscup aortic valve.⁸ Up to 15% to 20% of TAA or AD maybe familial.⁹ Several specific loci have been identified to be associated with familial syndrome, specifically 16p12.2–p13.3, 5q13–q14, 3p24–p25, and 11q23.2–q24.⁹ Collagen and elastin contribute significant structural support to the aorta. Elastin allows mobility throughout the cardiac cycle. A paucity of elastin is found in the wall of an aneurysm and can be experimentally depleted by elastase, resulting in dilation of the aortic wall.¹⁰ Dilation of the aorta leads to increased wall tension according to La Place’s law T = (P × r)/2t, where T is wall tension, P is distending pressure, r is the radius, and t is the wall thickness. This principle may also explain the well-documented logarithmic growth of TAA. Collagen types I and III are abundant in the wall of the aorta. A deficiency or increased destruction of collagen via collagenase has been implicated in formation of aneurysm.⁴ Cystic medial degeneration is commonly found in TAA.¹¹ Atherosclerosis is typically found in descending TAA.¹² Of TAAs, about 40% involve the descending aorta.¹¹ TAAs are often part of a multifocal aneurysmal disease. In a review of 217 patients operated for TAA by Crawford, 68% of these had multiple aneurysms with the most common association being with infrarenal abdominal aortic aneurysm (AAA). Additionally, of 1076 patients treated for AAA, 12% were found to have additional aneurysms.¹³ Of patients with thoracoabdominal aortic aneurysm (TAAA), aneurysms of the renal artery occurred an average 2.7% and in peripheral arteries (such as iliac, femoral, or popliteal) an average of 4.2%.⁵ These data emphasize both the systemic nature of this disease as well as the importance of adequate screening for associated aneurysmal disease upon discovery of one site.

Natural History

The natural history of descending TAA is one of progressive enlargement with risk of rupture. Both the size of the aneurysm and the rate of expansion have significant impact on development of complications. The natural history of AAAs is more clearly defined than TAA. Part of the reason for this is the heterogeneity in the literature, often included in these series are a mixture of aneurysm extents as well as different acute aortic syndromes. There is a faster expansion of AAA compared with TAA. Masuda found an average expansion of TAA to be 1.3 mm/year compared with 3.9 mm/year of AAA.¹⁴ The rate of expansion is influenced by initial aneurysm size, with aneurysms >5 cm having a higher rate of expansion.¹⁵ The most common cause of death in patients with untreated TAA is aortic rupture.¹⁶ A series by Dapunt et al. estimated change in TAA diameter to be 0.43 cm at 1 year with largest rate of expansion in smokers and aneurysm size >5 cm at presentation.¹⁵ Coady reported annual aneurysm growth rate of 0.12 cm/year of descending TAA to be greater than ascending TAA of 0.09 cm/year, with a mean size of 5.2 cm at initial presentation.¹⁷ While a larger aneurysm carries a higher risk of rupture, smaller TAA (those between 4 and 5 cm) also have a significant risk of rupture and death.

Classification

The extent of involvement of TAAA is defined according to the Crawford classification¹⁸ (see Figure 29-2). Type I extends from above the sixth intercostal space, near the left subclavian artery, to the root of mesenteric vessels but not to the infrarenal aorta. Type II extends from above the sixth intercostal space to the infrarenal aorta and usually to the bifurcation as well. Type III arise below the sixth intercostal space of the thoracic aorta and extend to the infrarenal aorta as type II does. Type IV runs the length of the abdominal aorta from diaphragm to the bifurcation.¹⁸

Risk Stratification

Predicting the risk of rupture has been evaluated by several authors by proposed formula. As a result of review of 67 patients by Dapunt, a formula was proposed to predict the rate of change of maximal diameter.¹⁵

where Y is the change in diameter, a = 0.0167, X is the initial diameter, and b = 2.1.

Juovonen reviewed a series of 114 patients and reported the following factors to be associated with the risk in rupture in descending TAA; maximal aneurysm diameter, older age, uncharacteristic pain and history of COPD.¹⁹ Based on multivariate risk factor analysis, the following formula was proposed to predict 1-year risk of rupture.

where pain and COPD = 1 if present and 0 if absent. Probability of rupture within 1 year = 1 – e1^λ(365).

Descending TAA tend to rupture at a larger size (median diameter 7.2 cm) compared with ascending or arch aneurysms (median diameter 5.9 cm).¹⁷ A review of 165 patients with TAA led to proposal of a similar formula to predict risk of rupture. Risk factors identified included aneurysm size, COPD, age, and uncharacteristic pain. A more rapid growth rate was seen with smokers.

where pain and COPD = 1 if present and 0 if absent. Probability of rupture within 1 year = 1 – e1^λ(365).

An attempt was made to validate the above formulae by Schimada et al.¹⁶ This review of 88 patients with TAA again demonstrate exponential enlargement over time. When this data was compared with the above formulas, the Coady formula underestimated growth by 0.8 mm while the Dapunt formula overestimated growth by 1.5 mm. The decision to operate will be more straightforward in some patients and will obviate the need for calculation as above. Current indications include presence of symptoms, evidence of dissection, accelerated growth rate (>10 mm/year), or diameter 6 to 7 cm.^18,20 Some patients will not be adequately evaluated by the above formula because there are other possible risks such as development of serious cough with associated recurrent laryngeal nerve irritation with rapid aneurysmal expansion. Additionally, comorbidities may dictate the urgency of repair as well. New onset or increasing severity of pain will also lead to earlier repair. Based on recent review of 721 patients with TAA, the mean rate of rupture or dissection of TAA is 2% for aneurysm <5 cm, 3% for 5 to 5.9 cm, and 6.9% for aneurysms 6 cm or larger.²¹ The biggest dilemma about decision to operate arises in a patient who has a relatively asymptomatic TAA. The above formula maybe useful in these patients to help estimate risk of rupture. The decision to operate on TAA should be based on expected morbidity of the proposed intervention compared with the expected morbidity of no operation. A review of 1509 surgical repairs of TAAAs by Crawford reported predictors of complication and complication rates. Neurologic complication such as paraplegia or paraparesis occurred in 16%, neurogenic bladder in 0.5%, renal failure requiring dialysis in 9%, postoperative stroke in 3%, pulmonary embolus in 1%, pulmonary complication in 33%, cardiac complication in 12%, bleeding requiring reoperation in 7%, sepsis in 8%, gastrointestinal complication in 7%, and coagulopathy in 4%.²² The major cause of postoperative morbidity is occurrence of paraplegia or paraparesis. Several protective strategies have been adopted to minimize this occurrence. Heparinization, mild hypothermia, aggressive reattachment of critical intercostal arteries (T8 to L1), and cerebrospinal fluid drainage have been advocated.²³

A recent meta-analysis reports a benefit for cerebrospinal fluid drainage to prevent paraplegia.²⁴ Risk factors for postoperative complication after elective repair of TAA were identified as preoperative renal insufficiency, older age, a symptomatic aneurysm, and type II aneurysm. A formula was created based on review of this series to predict adverse outcome.²³ An adverse outcome was defined as operative death, paraplegia or paraparesis, stroke, or acute renal failure requiring dialysis.

where age is given in years, C₂ = 1 for type 2 aneurysm and 0 for types I, III or IV, symptoms = 1 or 0, renal = 1 or 0 for preoperative renal insufficiency.

This model has not been validated prospectively. It does emphasize known risk factors for complicated disease. The authors say they use this risk calculation and balance it against calculated risk of rupture, and recommend surgery when risk of rupture exceeds risk of adverse outcome.

Evaluation

Diagnostic imaging includes chest radiograph, contrast aortogram, contrast-enhanced computed tomography (CT) scan, magnetic resonance angiography (MRA), and transthoracic and transesophageal echocardiography.¹¹ On supine chest radiograph, widening of the mediastinum (>8 cm at aortic arch) is about 80% sensitive and 50% specific. Widening of the left paraspinal stripe and deviation of the trachea or esophagus to the right are greater than 80% specific but sensitivity is low.²⁵ Thoracic aortography yields sensitivity almost 100% and specificity of 98% for traumatic aortic injury.²⁵ CT scan is 99.3% specific and about 90% sensitive for detecting aneurysm (Figure 29-3). MRA is almost 100% sensitive and specific for detecting aneurysm.(Figure 29-4).²⁵ An important potential pitfall to be aware of when evaluating cross-sectional imaging is transverse slice through torturous area where the aorta is angulated or curving. This is common in the elderly. This can result in a false-positive read for aneurysm.

Plain film chest radiograph is often used as an initial study with chest complaint. Several criteria have been associated with aortic disease on chest radiograph. Widening of the aortic contour, widening of the mediastinal shadow, tracheal shift to the right or distortion of the left main bronchus, displacement of intimal calcification greater than 6 mm into the aortic shadow, kinking or tortuosity of the aorta, opacification of the pulmonary window, and blurring of the aortic contour. A ratio of mediastinal width to chest width exceeding 0.25 has been used although some sources say up to 0.45 is acceptable.²⁶ Chest radiography has a sensitivity of 64% and specificity of 86% for detecting aortic disease, with the percentage being lower for ascending aortic disease.²⁶ Magnetic resonance imaging (MRI) is an accurate method to image patients with suspected aortic aneurysm or dissection. There is high soft tissue contrast and the ability to finely evaluate the aortic lumen and wall. An advantage when compared with the potentially nephrotoxic and allergenic CT contrast media is that MRI gadolinium-based contrast media are safer and have less renal toxicity.²⁷ Specific considerations apply when evaluating aneurysm disease with MRI/MRA. Accurate measurements of the outer aortic diameter are best obtained without angiographic enhancement. Also, the adventitia is not well visualized on MRA. MRI is also highly accurate for the detection of AD. Because of the length of the study, MRI has limitations in the emergent setting and is restrictive in terms of monitoring equipment.

Transesophageal echocardiography (TEE) offers high-resolution images as a result of the proximity of the esophagus and thoracic aorta. The ascending aorta and aortic root are directly anterior to the esophagus. From the level of the aortic arch to the diaphragm, the aorta and esophagus are close in their course. The newer biplane probes allow better imaging of the aorta compared with the monoplane transducer. In the evaluation of AD, reported sensitivity ranges from 97% to 99% and specificity from 77% to 100%.²⁸ The sensitivity is higher for dissections of the ascending aorta, however.²⁹ The finding of an undulating intimal flap in the aorta is the most definitive finding for AD.²⁹ This modality also allows assessment of aortic valve function, cardiac function, blood flow in false and true lumens and thrombus, and frequently visualization of coronary ostia. It is advantageous to be able to evaluate the coronary ostia for operative planning in the case of a proximal dissection. Typically, this has been done with preoperative coronary angiography. Compared with MRI or CT, it is not as good at detecting thrombus in the false lumen as well as evaluating the entire length of the aorta.²⁸ The ascending aorta and proximal arch are incompletely evaluated by TEE.³⁰ TEE is a good adjunct to follow TAA, however, as some areas are incompletely visualized. It is very useful for aortic valve assessment with known TAA, which is an important consideration in surgical decision. TEE avoids the potential risks of aortography, which may include embolus with possible stroke, possible retrograde extension of the dissection, and site complications. There are certain patients who will not tolerate a TEE. It is not suitable for those with known esophageal varices, stricture, or tumors. It may produce vasovagal reaction and there is the risk of perforation.²⁹

Medical Treatment

Medical management consists of minimizing risk factors for aneurysm growth. Beta blockade is the standard recommendation titrated to a systolic blood pressure of 100 to 120 mm Hg. Long-term beta-blockade therapy has been demonstrated to be effective in slowing the rate of aortic dilatation in patients with Marfan’s syndrome.³¹ The foundation of this treatment is derived from experimental data of turkeys prone to spontaneous aortic rupture having improved survival when propranolol was added to the feed.³² In addition to blood pressure control and heart rate control, this also reduces ΔP/Δt, the so-called antiimpulse therapy.²⁵

Surgical Treatment

It is unclear exactly which TAA should be repaired and which should be observed. Distinguishing between the risk of nonoperative and operative treatment is key but often difficult in the absence of clear guidelines for each patient. Careful risk assessment and knowledge of the natural history of TAA will guide this decision. For elective repair, a TAA of 7 cm or larger should be repaired.¹⁴ Crawford recommends repairing TAA and TAAA greater than 5 cm in a good-risk patient and in symptomatic patients.³³ Lobato and colleagues gathered prospective data on 31 patients with TAA of less than 60 mm and deemed high risk for surgery or who had refused surgical therapy.²⁰ They recommend elective repair when the initial anterior–posterior diameter is 5 cm or greater with an annual growth rate of at least 10 mm. However, during their 47-month follow-up, nine patients with initial aneurysm size of 4 to 4.9 cm had rupture.²⁰ An unanswered question remains what is the best treatment for aneurysms 4 to 5 cm, especially those with a smaller growth rate than 10 mm annually. Operative mortality for type I Crawford TAAA is reported to range between 5% and 8%.⁵

Endovascular Treatment

Endovascular repair (EVAR) of the thoracic aorta has seen increasing utilization. The position of the proximal point of the endograft is defined according to four anatomic divisions of the proximal aorta proposed by Ishimaru.³⁴ These zones (Table 29-1) are based anatomically on a line drawn tangent to the distal side of each of the arch branches (see Figure 29-5).

A recent review by Iyer examined their 6-year experience with 70 cases of EVAR of the thoracic aorta. EVAR was used for TAA in 63% and 30-day mortality was 1/70. Other lesions included one aortoesophgeal fistula, seven traumatic rupture, and eight type B dissections. Postoperative endoleak occurred in 23% and endovascular failure in 11%. The majority of these repairs were done with the Talent self-expanding endograft system and five with Zenith system. Graft access was via common femoral artery, iliac artery via retroperitoneal exposure, or directly to the aorta via laparotomy. Systemic heparin was only used in one case and pharmacologic hypotension was used with graft deployment. For arch lesions, extra-anatomic bypass is used to enlarge the proximal landing zone (as with zone 1 or some zone 2 repairs). When more than one graft component is used, a 4 to 5 cm overlap is utilized. Cerebrospinal fluid drain was placed in 70% of patients and there was only one case of peripheral neuropraxia documented at 30 days with no paraplegia. Acute renal failure and conversion to open occurred in 4%. At 5 years, about 70% of the total treated patients were free from secondary endovascular intervention.³⁵ A similar report by Criado examined their 4-year experience of 47 thoracic lesions with 31 TAA and 16 type B dissections. The mean size of TAA treated was 6.8 cm (4.8 to 10.7 cm). A 4% 60-day mortality was reported. There was no paraplegia or stroke. These report as well as several older reports are adding to the growing body of evidence that may make EVAR of thoracic lesions the procedure of choice in the future. It should be recalled however that the reported 30-year experience of Crawford demonstrated a 30-day survival rate of 92% (1386/1509).²² Long-term results of EVAR have yet to be determined but initial reports are promising. What remains to be answered are what lesions will provide the best long-term outcomes with endovascular approach and which should be treated with open repair.

A phenomenon known as postimplantation syndrome has been described after aortic endograft placement.³⁴ This consists of leukocytosis, mild thrombocytopenia, and postoperative fever. This may represent inflammation of the aortic wall to the graft. Conservative treatment with aspirin is recommended and typically resolves spontaneously.

Epidemiology

Incidence of AD has been estimated to be 2.9/100 000/year incidence. Other estimates have ranged from 0.5 to 4.04/100 000/year. Average age at dissection is about 63.4 years with a male-to-female ratio 1.55: 1.^36,37

Natural History

The physician of King George II, Dr Nicholls, first described AD on autopsy in 1760.³⁸ Despite the significant advances in technology, the mortality of AD remains high. Type A dissection carries a higher rate of complication and mortality. After the acute onset of symptoms of AD, the mortality maybe as high as 1% per hour.³⁹ Predictors of in-hospital death include age >69 years, hypotension or cardiac tamponade, renal failure, and pulse deficits.³⁸ A trend toward improved survival with AD has been seen in recent years.⁴⁰ Erbel and colleagues reported survival rates of 52% for type I dissection, 69% for type II dissection, and 70% for type III dissection.⁴¹ The IRAD data reports mortality of type A dissection to be 26% with surgical treatment and 58% without it. Mortality of type B dissection was 10.7% with medical treatment and 31.4% with surgical treatment³⁷ hence, the difference in treatment of ascending versus descending thoracic ADs.