Chapter 37. Thoracic Aortic Aneurysms & Dissections

Essentials of Diagnosis

• Ascending aortic diameter > 4 cm on imaging study.
• Descending aortic diameter > 3.5 cm on imaging study.

General Considerations

In the ascending aorta, aneurysms tend to take on three common patterns, as indicated in Figure 37–1. These include the supracoronary aortic aneurysm, annuloaortic ectasia (marfanoid), and tubular diffuse enlargement.

The most common pattern is that of supracoronary dilatation of the ascending aorta. In this pattern of disease, the short segment of aorta between the aortic annulus and the coronary arteries remains normal in size. Sinuses are “preserved,” meaning that the aorta indents normally, forming a “waist,” near the level of the coronary arteries. For this type of aneurysm, a supracoronary tube graft suffices.

In the second type, annuloaortic ectasia, the aortic annulus itself becomes dilated, giving a shape to the aorta like an Erlenmeyer chemistry flask. In this type of disease, the segment of aorta between the annulus and the coronary arteries is diseased, dilated, and thinned. Sinuses are “effaced,” meaning that the normal indentation, or waist, is lost. When surgery is required, the entire aortic root must be replaced.

In the third type of ascending aortic disease, the configuration is midway between the previous two patterns, that is, there is some dilatation of the annulus and root and some effacement of the sinuses, but these elements are not dramatic. The overall appearance is that of a large tube, rather than a flask. For such aortas, either supracoronary tube grafting or aortic root replacement may be appropriate.

The Crawford classification (Figure 37–2) is used to categorize the appearance of an aneurysm in the descending aorta and thoracoabdominal aorta. This classification is based on the longitudinal location and extent of aortic involvement, has implications for surgical strategy, and affects the risk of perioperative complications.

Type I aneurysms involve most of the thoracic aorta and the upper abdominal aorta. Type II aneurysms, the most extensive and most dangerous to repair, involve the entire descending and abdominal aortas. Type III aneurysms involve the lower thoracic and abdominal aortas. Type IV aneurysms are predominantly abdominal but involve thoracoabdominal exposure because of the proximity of the upper border to the diaphragm.

Etiology

The genetics of Marfan disease, a well-known cause of aneurysms of the thoracic aorta, have been well delineated, with over 600 mutations identified at one locus on the fibrillin gene.

Increasingly, it is being appreciated that patients who do not have Marfan disease also manifest familial clustering of thoracic aortic aneurysms and dissections. Patients with aneurysms often answer one or both of the following questions affirmatively: “Do you have any family members with aneurysms anywhere in their bodies? Did any of your relatives die suddenly or unexpectedly of apparent cardiac causes?” Detailed construction of family trees on over 500 patients with thoracic aneurysm has indicated that 21% of aneurysm probands have a first-degree relative with a known or likely aortic aneurysm. The true number is certainly much higher, as these estimates are based only on family interview and not on head-to-toe imaging of relatives. Figure 37–3 shows the 21 positive family trees of the first 100 families analyzed. The most likely pattern of inheritance appears to be autosomal dominant with incomplete penetrance. A more recent analysis has shown that the location of the proband's aneurysm largely influences the location of the aneurysms in the family members. If the proband has an ascending aneurysm, the likelihood is that the family members have ascending aneurysms. If, however, the proband has a descending aneurysm, it is likely that the family members have abdominal aortic aneurysms. These proband–family member observations are in keeping with the general concept that aneurysm disease divides at the ligamentum arteriosum: Ascending and arch aneurysms represent one disease, largely nonarteriosclerotic, while descending and abdominal aneurysms represent another disease, largely arteriosclerotic.

Application of modern molecular genetic techniques is successfully making progress toward determining the specific genetic aberrations responsible for these family clusterings and for thoracic aneurysms in general. Original progress was made largely by linkage analysis of large families with multiple members affected. Now, as automated genomic analysis has become feasible and affordable, much scientific discovery has been made by direct exome sequencing. Milewicz, Dietz, Loeys, and others have succeeded in identifying specific familial aneurysm patterns and their underlying mutations (Table 37–1). Note that all of these disorders, except one, are transmitted in an autosomal dominant fashion (with decreased penetrance). Also note that the ACTA2 disorder presents with aortic dissection at small diameters. These mutation-specific categorizations will soon allow personalized medicine based on the specific underlying mutation and its accompanying rupture and dissection patterns.

Examination of single nucleotide polymorphisms (SNPs) in the blood of hundreds of patients with thoracic aortic aneurysms via genome-wide surveys using large (> 30,000) SNP libraries has been accomplished. An “RNA signature” in the blood of patients with thoracic aortic aneurysm was found, which can predict with about 85% accuracy from a blood test alone whether the patient harbors a thoracic aortic aneurysm. This “signature” is composed of specific RNAs that are either markedly upregulated or markedly downregulated in aneurysm patients, compared with healthy controls.

Patients who have a genetic predisposition for aneurysm development, specifically those patients with annuloaortic ectasia or ascending dissection, are significantly protected from arteriosclerosis (Figure 37–4). Remarkably, they have less arterial medial thickness and less calcification than normal controls. It appears likely that the same mutations that promote lysis of the aortic wall also prevent plaque build-up.

Accepting that most patients with aneurysms have an underlying genetic predisposition to the condition, how does this genetic programming lead to the development of an aneurysm? Rapid progress is being made in elucidating these mechanisms. Aneurysm formation is currently thought to involve the following processes (Figure 37–5): extracellular matrix proteolysis, chronic inflammation, cytokine activity, and smooth muscle cell loss. The identification of these mechanisms raises the intriguing possibility of interfering pharmacologically with this pathophysiology, so aneurysm formation or progression can be stopped. The importance of the transforming growth factor-β (TGF-β) pathway in aneurysm formation has been demonstrated; the ability of angiotensin receptor–blocking medications (eg, losartan) to interfere with this pathophysiologic mechanism is being tested, and results of randomized, controlled studies will soon be available. At this time, however, it may be said that no specific pharmacologic strategy exists for delaying aneurysm progression. Results of trials of proteolytic antagonists and β-blockers have been underwhelming. The potential roles of statin medications, anti-inflammatory agents (cyclooxygenase [COX]-2 inhibitors), immunosuppressants (sirolimus), and antibiotics (doxycycline) are being investigated.

The proteolytic enzymes called matrix metalloproteinases (MMPs) are receiving extensive attention in aneurysm pathophysiology. These powerful enzymes are found in excess in thoracic aortic aneurysms (Figure 37–6) and are thought to play a major role in destroying the substance of the aortic wall, leading to decreased wall strength and, ultimately, dilatation and rupture.

The biologic changes in the aortic wall discussed earlier are vitally important, but hemodynamic forces need to be considered as well. As the ascending aorta reaches a diameter of 6 cm, the distensibility vanishes, so that the aorta becomes essentially a rigid tube (Figure 37–7). Because of this rigidity, the force of systole can no longer be beneficially dissipated by elastic expansion of the aorta, and this translates into increased wall stress. Especially at high blood pressures, this wall stress becomes excessive, setting the stage for disruption of the aortic wall via rupture or dissection. It is instructive to note how closely this mechanical data dove-tail with the clinical behavior of the aorta: The mechanical properties deteriorate at 6 cm diameter, and that is precisely the hinge point for clinically manifest rupture and dissection.

Clinical Findings

Natural History

The Yale computerized database now contains information on nearly 3000 patients with thoracic aortic aneurysm, including some 9000 tabulated serial imaging studies and 9000 patient-years of follow-up. This database and these methods of analysis have permitted assessment of multiple fundamental topics and questions regarding the natural behavior of the thoracic aorta and have shed light on appropriate criteria for surgical intervention.

How Fast Does the Thoracic Aorta Grow?

Via specifically developed statistical methods designed to account for important potential sources of error, the annual growth rate of an aneurysmal thoracic aorta has been determined to be 0.12 cm on average. The descending aorta grows faster than the ascending aorta, at 0.19 cm/year compared with 0.07 cm/year. Also, the larger the aorta becomes, the faster it grows.

At What Size Does the Aorta Dissect or Rupture?

Critical to decision making in aortic surgery is an understanding of when complications occur in the natural history of unrepaired thoracic aortic aneurysms. In the case of the thoracic aorta, the two complications that are vitally important are rupture and dissection. Knowing when these complications are likely to occur would permit rational decision making regarding elective, preemptive surgical intervention to prevent their occurrence.

Size criteria apply only to asymptomatic aneurysms. We are learning increasingly that the aorta can indeed “communicate” with us—via pain. Symptomatic (painful) aneurysms should be resected regardless of size. For ascending aneurysms, this pain is usually felt anteriorly, under the breastbone. For descending thoracic aneurysms, the pain is usually felt in the interscapular region of the upper back. For thoracoabdominal aneurysms, the pain is usually felt lower in the back and in the left flank. Other symptoms may occasionally be produced by thoracic aortic aneurysms, including bronchial obstruction, esophageal obstruction, and phrenic nerve dysfunction; these symptoms also constitute indications for surgical intervention.

Initial statistical analysis revealed sharp “hinge points” (Figure 37–8) in aortic size at which rupture or dissection occurred. For the ascending thoracic aorta, the hinge point occurs at 6.0 cm. By the time aortas reach this size, 31% have ruptured or dissected. For the descending aorta, the hinge point is located at 7.0 cm. By the time descending aortas reach this size, 43% have ruptured or dissected.

If a surgeon were to wait for the aorta to achieve the median size at time of complications in order to intervene, by definition rupture or dissection would have occurred in half of the patients (Figure 37–9). Accordingly, it is important to intervene before the median value is attained. The following recommendations take this factor into account, permitting preemptive surgical extirpation before rupture or dissection in most patients.

Current recommendations are listed in Table 37–2 and are based on the hinge points noted in Figure 37–8. Specifically, prophylactic extirpation of the aneurysmal ascending aorta is recommended when the aneurysm measures 5.5 cm; for the descending aorta, which does not rupture until a larger size, surgical intervention is recommended when the aneurysm measures 6.5 cm. Application of these criteria will prevent most ruptures and dissections, without prematurely exposing the patient to the risks and inconveniences of surgery. The efficacy of this management algorithm has recently been documented in a large cohort of prospectively followed patients.

It is well-known that patients with Marfan disease are prone to unpredictable dissection at an early size. For this reason, earlier intervention is recommended for patients with Marfan disease as indicated in Table 37–2.

For patients with a positive family history, but without Marfan disease, the same criteria are applied as for Marfan disease, because malignant early behavior of the aneurysm in these patients may be seen as well.

If the patient has a positive family history, or if an afflicted family member has suffered rupture, dissection, or death, preemptive surgical extirpation is carried out earlier than otherwise.

Studies of aortic anatomy increasingly recognize that patients with a bicuspid aortic valve also have inherently deficient aortas. Therefore, lower intervention dimensions are used for patients with bicuspid aortic valve as well. Table 37–3 indicates that a bicuspid aortic valve is actually a more common cause of aortic dissection than Marfan disease. Table 37–3 compares the general incidence of Marfan disease with that of bicuspid aortic valve. Although the incidence of dissection is 5% for bicuspid valve disease, compared with 40% for patients with Marfan disease, bicuspid valve disease is so much more common that it causes more total cases of dissection than Marfan disease. This factor must be taken into account in planning surgical repair of the ascending aorta of the patient with a bicuspid aortic valve when the aorta is still in the aneurysmal stage and not yet a dissection.

What Is the Yearly Rate of Rupture or Dissection for Thoracic Aortic Aneurysms?

The preceding data indicate the cumulative lifetime rates of dissection or rupture by the time the aorta reaches a certain size. Determining the yearly risk of complications from the natural history of thoracic aortic aneurysm is more challenging because it requires extremely robust data. Such data must produce enough hard end points to permit analysis within a year's time for different size strata. Calculations of yearly rates of rupture or other complications based on size of the aorta have been produced. These yearly rates are expressed based simply on the size of the aorta (Figure 37–10).

These data all point to a diameter of 6 cm as a very dangerous size threshold. At or above this size, the yearly risk for rupture is about 4%, the yearly risk of dissection is about 4%, and the risk of death is about 11%. (Death is often directly related to catastrophic complications from the aneurysm.) The chance of any one of these phenomena occurring—rupture, dissection, or death—is 14%/year. As a mnemonic point of reference, a 6-cm aneurysm can be equated to about the diameter of a soft-drink can. When a thoracic aortic aneurysm reaches the diameter of a soda can, it has certainly attained the point where it poses a major risk to the patient.

These analyses should permit accurate decision making when seeing a patient during an office visit and considering preemptive surgical extirpation of thoracic aneurysms. These data allow the physician to form a reasonable estimate of the individual patient's risk of dissection, rupture, or death for each future year of life, if the aorta is not resected. The risk of rupture, dissection, or death based on aortic size is presented graphically in Figure 37–10.

The question arises whether the same surgical intervention criteria should apply for a small woman as for a large man. It is true that a larger individual can be “allowed” a larger aorta, generally speaking. Conversely, even a moderate-sized aneurysm can be quite threatening in an individual of small stature. For this reason, adverse event rates (rupture or dissection) based on aortic size corrected for body surface area (BSA) have been analyzed. By plotting the aneurysm size along the horizontal scale and the BSA along the vertical scale, each particular patient can be classified into low-, medium-, or high-risk categories—thus taking account of the aneurysm size in relation to the patient's physical size.

Symptoms & Signs

Most thoracic aortic aneurysms are asymptomatic and are detected fortuitously during imaging of other thoracic structures. When they are symptomatic, deep visceral pain in the upper anterior chest or interscapular back can occur. This pain differs from angina pectoris because it is not necessarily precipitated by exertion nor relieved by rest or nitroglycerin. Often, it is rather constant and not influenced by body motion or position. All patients with chest pain should have a screening chest radiograph. Rupture of a thoracic aneurysm usually causes excruciating pain, accompanied by profound dyspnea as the chest fills with blood, and quickly results in shock. A large ascending aortic aneurysm occasionally may result in dysphagia or stridor due to esophageal or large airway obstruction. Rarely, a large aneurysm may cause bone pain due to pressure against thoracic skeletal structures.

Physical Examination

Physical examination is usually unremarkable. The presence of a murmur of aortic regurgitation should raise the suspicion of ascending aortic aneurysm, as should features suggestive of Marfan syndrome or related conditions. Rarely, an abnormal pulsation will be felt due to a large aneurysm contacting the chest wall.

Diagnostic Studies

The remarkable strides made in recent decades in three-dimensional body imaging have dramatically advanced the diagnosis and treatment of thoracic aortic aneurysm. Echocardiography (especially transesophageal) and computed tomography (CT) and magnetic resonance imaging (MRI) scans all yield images that clarify the presence, location, size, and extent of aneurysmal disease. An example of the precise imaging afforded by MRI is indicated for a specific, very extensive aneurysm in Figure 37–11.

In this era of specialized three-dimensional imaging, it is important not to forget the chest radiograph, which can often yield significant information about the thoracic aorta. An example is provided in Figure 37–12. Ascending aortic aneurysm presents as a bulge beyond the right hilar border. Arch aneurysm produces enlargement of the aortic knob. Descending thoracic aneurysm is often easily seen as a deviation of the stripe of the descending aorta, which normally runs parallel to and just left of the vertebral column.

Treatment

Risks of Aortic Surgery

It is certainly helpful to know numerically and statistically the cumulative and yearly rates of rupture, dissection, and death imposed by an aortic aneurysm of a specific size. On the other hand, the equation is incomplete without consideration of the risks inherent in elective, prophylactic surgical extirpation of the thoracic aorta. Certainly these are major operations, and the surgical risks most feared include death, stroke, and paraplegia. However, these operations have become safer, reflecting increased surgical experience, improved perfusion techniques, improved (nonporous) grafts, effective antifibrinolytic agents for perioperative use, improved methods of spinal cord preservation, and the advent of centers specializing in aortic care and surgery. A recently published report emphasizes the “safety of thoracic aortic surgery in the present era.” Mortality rates and rates of other complications after aortic surgery are quite low, especially for operations performed electively on stable patients, in whom the safety of ascending aortic and aortic arch surgery is as high as 98%. Table 37–4 shows the pertinent rates of morbidity and mortality.

Indications & Contraindications

By considering the rates of natural rupture, dissection, and death from the thoracic aneurysm itself versus the risks of operation, the physician can make an informed recommendation about elective, preemptive surgery. Once patients and their families are provided the natural history and surgical risk data, they often have strong opinions of their own. Some patients are reluctant to undergo major surgery, with its significant attendant risks, for an asymptomatic problem. Most patients, however, seem to feel they will never be comfortable until the threatening aneurysm is resected.

One more very important general point needs to be considered. Once the aorta has dissected, the prognosis is thereafter adversely affected (Figure 37–13). Patients who required emergency surgery not only had a higher rate of early mortality, but their survival curve was dramatically poorer. Patients who elected for planned, nonemergent procedures showed a survival rate very similar to that of a normal population. The poor long-term outlook for patients who required emergency surgery is due largely to the fact that, even after surgical replacement of portions of the aorta, the remainder of this vital organ will forever remain dissected. Because the aortic wall was deficient to start with, at half-thickness, after dissection, it is rendered even more vulnerable to subsequent enlargement and rupture.

Surgical Techniques

As discussed earlier, the type of operation for the ascending aorta is based on the pattern of aneurysmal pathology. For many patients, a supracoronary tube graft suffices (Figure 37–14A). For others, a composite graft, including both a valve and a graft, with obligate coronary artery reimplantation, is appropriate (Figure 37–14B). New valve-sparing aortic replacement procedures have been developed and are becoming increasingly popular (eg, David procedure).

The main debate regarding the procedure for ascending aortic operations and those on the aortic arch concerns the optimal means of protecting brain function during the time that anastomoses in the vicinity of the aortic arch are performed. Deep hypothermic circulatory arrest—a state of suspended animation, which is generally safe for 30–45 minutes or longer—is preferred by many surgeons, for its simplicity and effectiveness. In a study on 400 patients, the effectiveness of this remarkable technique as a sole means of brain preservation was confirmed. Retrograde cerebral perfusion—via the superior vena cava—has its advocates, although the actual amount of effective brain perfusion achieved by this means has been questioned. Direct perfusion of the head vessels—usually via a cannula in the innominate artery or cannulas in both the innominate and the left carotid artery—also has its supporters, despite its added complexity. Direct perfusion is gaining in popularity, and it does provide a margin of protection, especially for very complex arch reconstructions or for surgical teams relatively inexperienced with arch replacement. No technique has been demonstrated conclusively superior over the others.

For descending and thoracoabdominal operations, the technique of left atrial to femoral artery bypass has become extremely popular. This method takes strain off the heart by diverting blood away from the left ventricle. This approach mitigates the effect of high aortic cross-clamping on cardiac afterload. It also perfuses the lower body, especially the extremely vulnerable spinal cord. Despite decades of concerted attention, paraplegia from descending and thoracoabdominal aortic replacement continues to be a major clinical problem. The cause is multifactorial, with clamp time, air and particulate embolism, and disconnection of critical intercostal branches all playing a role. Besides the benefits of left atrial to femoral artery perfusion, most authorities feel that routine spinal fluid drainage and deliberate maintenance of a strong postoperative blood pressure (to encourage collateral blood flow) are also effective adjuncts against the complication of postoperative paraplegia.

Specific Clinical Scenarios & Issues

Patient with Pain, But Aneurysm Smaller Than Criteria

The answer to whether such an aorta should be replaced is a resounding yes. The dimensional criteria are specifically intended for asymptomatic patients. Any and all symptomatic aneurysms need to be resected because symptoms are a precursor to rupture. Aneurysm pain represents stretching or irritation of the aortic adventitia, the adjacent chest wall, the mediastinal pleura, or some other structure impinged on by the expanding aneurysm. Even an aorta smaller than the criterion can rupture or dissect. Such a patient is of extreme concern, and preemptive resection is needed. In one case, a patient complained of typical pain of an ascending aortic aneurysm. The aorta was 5.0 cm. Because the medical team thought this was too small for resection, they underestimated the symptoms at presentation. The aorta subsequently ruptured, and the patient died within 48 hours. This point cannot be overemphasized: The size criteria are explicitly intended only for asymptomatic patients; all symptomatic aneurysms need to be resected.

Differentiating Aneurysm Pain from Musculoskeletal Pain

This very important point is not always easy to determine, even in the most experienced hands. The patient usually has a good sense of whether the pain is originating from muscles and joints. The clinician usually gets an additional understanding by asking the following questions:

1. Is the pain influenced by motion or position? (If so, it is probably musculoskeletal.)

2. Do you have a history of lumbosacral spine disease or chronic low back pain? (If so, the symptoms may not be aortic in origin.)

3. Do you feel the pain in the interscapular back? (An affirmative answer indicates an almost certain relationship to thoracic aortic aneurysm.)

Presume that the pain is aortic in origin if no other cause can be conclusively established. This is the only approach that can prevent rupture.

Appropriate Interval for Serial Aortic Imaging

Patients with a thoracic aortic aneurysm should be monitored indefinitely. Stable, asymptomatic patients can undergo imaging about once every 2 years, remembering that the aneurysmal aorta grows at a relatively slow 1 mm/year. In case of new onset of symptoms, imaging should be done promptly, regardless of the interval from the prior scan. For new patients, for whom only one size data point is available, imaging should be done at short intervals until the behavior of aorta is understood. Imaging may be done every 3–6 months for new patients with moderately large aortas. Remember to compare the present scan with the patient's first scan, not with the last prior scan. That is the way to detect growth. Many patients have suffered because scans were only compared with the last prior scan, and major growth went undetected.

Choice of Imaging Modality for Serial Follow-Up

Three quality imaging techniques are currently available: echocardiography (ECHO), CT scan, and MRI. If echocardiography is chosen, it is important to remember that a standard transthoracic echocardiogram cannot see the distal ascending aorta, the aortic arch, or the descending aorta with conclusive accuracy because of intervening air-containing lung tissue. Supplement such studies with a periodic CT scan or MRI, which can visualize the entire aorta. The choice between CT and MRI may depend on ease of availability and radiologic expertise in a particular environment. Both modalities can image the entire aorta extremely well. Elevated creatinine or contrast allergy may contraindicate CT and instead favor MRI. The need to evaluate complex aortic lesions in multiple imaging planes would also favor MRI (although very recently, concerns have been raised about the risk to the kidneys of gadolinium contrast agents used for MRI scanning). Of course, indwelling metallic foreign objects, such as pacemakers or metal artifacts from previous surgery, may make CT the necessary choice instead of MRI. The advantages and limitations of echocardiography and CT are shown in Figure 37–15. Both echocardiography and CT are required for complete evaluation of the entire aorta.

Evaluation of Family Members

The data on familial inheritance has become strong enough that the treating physician is obligated to recommend that family members be evaluated. Physicians of family members should be made aware that aneurysm disease has been diagnosed in the family. A CT scan is recommended for adult males and for females beyond childbearing age. For children and for females of childbearing age, echocardiography of the ascending aorta and abdominal aorta is recommended. Investigators hope to identify humoral markers or genetic aberrations that can be used for familial screening of the aneurysm trait in the very near future. Routine genetic testing is controversial. We now offer and encourage screening of patients and family members by “whole exome sequencing,” which is generally covered by most large insurers.

Activity Restrictions

Continuation of any and all aerobic activities, including running, swimming, and bicycling, is recommended. Serious weight lifters, at peaks of exertion, can elevate systolic arterial pressure to 300 mm Hg. This type of instantaneous hypertension is, of course, not prudent for aneurysm patients. Weight lifters should limit themselves to one-half their body weight. The evidence for effort-induced aortic dissection is mounting. Participation in contact sports or those that might produce an abrupt physical impact, such as tackle football, snow skiing, water skiing, and horseback riding, is proscribed.

Role of Stent Grafting

A word of caution is appropriate concerning stent grafts. Stent therapy has become routine for many patients (especially those with abdominal or descending aortic aneurysms). It is certainly less invasive and more easily tolerated than open surgical techniques. Specialists, however, must avoid “irrational exuberance.” Owing to the high need for subsequent conventional surgery after abdominal aneurysm stent placement, the recent large, multicenter Eurostar study questioned the very efficacy and advisability of stent grafting. Endoleak, stent dislodgement, and aneurysm expansion or rupture were disturbingly widespread in medium-term follow-up. It should be remembered that stents were designed to keep tissue from encroaching on the vessel lumen, not to keep the vessel from expanding. One noted authority believes that the aneurysmal aorta essentially “ignores” the stent graft, dilating regardless of the stent, at its own pace (personal communication, Dr. L. Svennson). Also remember that the natural history of the thoracic aorta is that aneurysms grow slowly, and that hard end points (rupture, dissection, and death) take years to be realized. For this reason, short-term stent studies are nearly meaningless. Long-term studies are needed. This newer modality should be approached with enthusiasm tempered by caution. Its advent should not at this point influence the decision about whether or not to intervene for a specific aneurysm; criteria (see earlier in chapter) should be met before any intervention, including stent therapy, is undertaken. Stent therapy appears especially well suited for patients with rupture of the descending aorta, in which setting a stent can be acutely life-saving.

Essentials of Diagnosis

• Usually middle-aged or elderly hypertensive men; occasionally, young patients with history of Marfan syndrome or other connective tissue disorder. Rarely, young women in late pregnancy or labor.
• Acute chest pain, frequently with hemodynamic instability.
• Possible appearance of shock but normal or elevated blood pressure.
• Various neurologic symptoms, such as Horner syndrome, paraplegia, and stroke.
• Absent or unequal peripheral pulses.
• Aortic regurgitation.
• Widened mediastinum on chest radiograph.
• Confirmatory aortic imaging study.

General Considerations

Terminology

Aortic dissection refers to a splitting of the layers of the aortic wall (within the media) permitting longitudinal propagation of a blood-filled space within the aortic wall. Aortic dissection is thought to be the most common cause of death related to the human aorta (Figure 37–16).

Three related but distinct entities—acute aortic transection, rupture of aortic aneurysm, and aortic dissection—are commonly confused, both in substance and in terminology (Figure 37–17). Acute aortic transection is a traumatic phenomenon, with disruption of the wall of the aorta, without a propagating dissection. The aortic wall is intrinsically normal and resistant to the dissection process. Rupture of aortic aneurysm is self-explanatory; however, confusion in terminology may arise if an acute aortic transaction or an acute aortic dissection happens to rupture—a common eventuality. Acute aortic dissection refers to the very specific process of separation of layers of the aortic wall discussed fully in this chapter. For dissection to occur, the aortic wall must nearly always be affected by structural disease of the media.

Recent years have brought recognition of two important variants of aortic dissection: intramural hematoma (IMH) and penetrating aortic ulcer (PAU) (Figure 37–18).

IMH of the aorta differs from typical dissection in that there is no flap defining a true and a false lumen, and the hematoma (on axial imaging) is located circumferentially around the aortic lumen, rather than obliquely oriented across the aortic lumen. Whether the IMH arises from a small intimal tear (not detected radiographically) or from a rupture of a vasa vasorum within the aortic wall remains controversial. The clinical course is variable; the hematoma may persist, reabsorb (returning the aorta to a normal appearance), lead to aneurysm with the possibility of rupture, or convert to dissection. PAU involves a local penetration deep into the wall of the aorta, resembling a penetrating ulcer of the stomach. This lesion disrupts the internal elastic lamina and erodes into the media, which in some cases may mimic or initiate aortic dissection, pseudoaneurysm formation, IMH, or rupture. Extensive arteriosclerosis is a common accompaniment of PAU (see Figure 37–18).

It is important to recognize that IMH and PAU are diseases of advanced age. In addition, it is important point to mention that although branch vessel occlusion is part and parcel of typical aortic dissection, the variants of aortic dissection PAU and IMH never occlude branch vessels.

The general management of these lesions is still a matter of debate. Most authorities believe that descending aortic IMH and PAU can be managed medically, with “anti-impulse” therapy (see Treatment section). However, early (but not immediate) surgical intervention is preferred in suitable operative candidates to preempt rupture due to a high incidence of death from rupture. Some of the discrepancy in recommendations also has to do with regional differences: In Japan, IMH behaves in a more benign fashion than in the Western world, perhaps reflecting fundamental genetic differences in the aortic wall or differences in body size and aortic dimension. Stent therapies are often applied for IMH or PAU.

For ascending variant dissections IMH and PAU, most authorities agree on aggressive immediate surgical intervention, although one recent paper from Japan challenges the need for routine surgery, even in this anatomic location.

Anatomic Classification

Aortic dissections may be ascending (type A) or descending (type B). The two patterns are determined by the location of the inciting intimal tear. Tears occur in two very specific locations: (1) in the ascending aorta, 2–3 cm above the coronary arteries, and (2) in the descending aorta, 1–2 cm beyond the left subclavian artery. The first type of tear produces ascending dissection and the second, descending dissection. Please note that ascending dissections usually go around the aortic arch to involve the descending and abdominal portions of the aorta (Figure 37–19).

Clinical Findings

Symptoms & Signs

Aortic dissection produces intense, severe pain, often described as tearing or shearing in quality. This pain is sudden in onset (a differentiating feature from the pain of myocardial infarction) and very severe in intensity. Most patients describe this as the most intense pain of their lives, more intense even than childbirth or kidney stone. The pain of ascending dissection is felt in the anterior chest, substernally, and that of a descending dissection is felt posteriorly, between the scapulae. The “tearing,” “shearing,” “knife-like” quality of the pain is quite consistent with the pathophysiology. The pain can migrate downward, into the flank or pelvis, as the dissection process propagates distally. Impending aortic rupture should be considered when pain subsides and later recurs. On occasion, painless dissection does occur, perhaps as often as in 15% of patients; this is usually picked up later, on a routine imaging study done for another reason.

Diagnostic Studies

Chest Radiography

Chest radiography is a useful screening test. Many, if not most, aortic dissections occur in the setting of a chronic aortic aneurysm. To the astute observer, chronic thoracic aneurysms can be visualized on a chest radiograph. The enlarged ascending aorta will protrude to the right of the upper mediastinal border, an arch aneurysm will show as an exaggerated aortic knob, and a descending aortic aneurysm will be visible as a left-deviated stripe of the descending aorta. In case of aortic dissection, chest radiography will usually provide additional clues: most commonly, widening of the mediastinal shadow, pleural effusion, or inward displacement of aortic medial calcification.

Three-Dimensional Imaging Methods

Multiple types of three-dimensional imaging modalities are pertinent in aneurysm disease and aortic dissection, and all have excellent sensitivity and specificity: transesophageal echocardiography (TEE), CT scanning, and MRI. Many patients undergo a TEE and either a CT or an MRI. The CT or MRI shows the three-dimensional structure of the entire aorta. The TEE, while partially blinded to the aortic arch and abdominal aorta, provides information about pericardial effusion and tamponade, valve function, and left ventricular function. TEE images both ascending and descending aortas well.

The primary diagnostic criterion for aortic dissection by CT or MRI is demonstration of two contrast-filled lumens separated by an intimal flap. Sensitivity and specificity of CT and MRI for diagnosis of aortic dissection are approaching 100%. TEE does not lag far behind in accuracy. Contrast aortography, once the gold standard, has fallen by the wayside for diagnosis of aortic dissection, being more invasive and not offering nearly the amount of three-dimensional anatomic information afforded by CT, MRI, and TEE.

Differential Diagnosis

The imaging studies discussed earlier provide specific information that rules in or out the presence of aneurysm or dissection. The main diagnostic issue involves (1) maintaining a high index of suspicion for aneurysm disease and (2) being aware of the protean presentations of aneurysm disease. In particular, aortic dissection has been called “the great masquerader” because it can produce symptoms related to virtually any organ. A high index of suspicion is required to establish the diagnosis promptly, since the presentations of aortic dissection are so myriad and mimic a wide array of other diseases. Specifically, all patients admitted with chest pain without obvious cause should have their thoracic aorta imaged. Patients with a ruptured or dissected thoracic aorta often “masquerade” as heart attacks. It is especially important to rule out aortic dissection in patients about to be treated for myocardial infarction, as administration of thrombolytic drugs in patients with acute aortic dissection is associated with a high mortality rate.

Among conditions for which aortic dissection can be confused are myocardial infarction, musculoskeletal chest pain, pericarditis, pleuritis, pneumothorax, pulmonary embolism, cholecystitis, ureteral colic, appendicitis, mesenteric ischemia, pyelonephritis, stroke, transient ischemic attack, and primary limb ischemia. Especially troublesome to clinicians in emergency departments are patients with abdominal symptoms and signs without apparent abdominal cause; aortic dissection must be considered in such patients.

Given the extensive differential diagnosis, aggressive, objective diagnostic testing is necessary when the possibility of aortic dissection is considered. The diagnosis is most strongly suggested when migratory chest and back pain of less than 24 hours in duration arises in a patient with a history of hypertension. The following recommendations are made for the physicians who are the first to evaluate such patients: (1) Keep aortic dissection (and ruptured thoracic or abdominal aneurysm) in the differential diagnosis. (2) Image freely and liberally to rule out aortic pathology. A CT scan can exclude all three major chest diagnoses likely to result in death: coronary artery disease, pulmonary embolism, and aortic aneurysm or dissection. The modern 64-slice scanners are ideal for this purpose. (3) Remember the D-dimer test. A negative D-dimer, which is most commonly applied to rule out pulmonary embolism, also rules out aortic dissection. The clot that forms in the false lumen of an aortic dissection liberates D-dimer quite strikingly. This simple blood test is nearly 100% sensitive in picking up aortic dissection. (4) Remember to look at the aorta, even if the CT scan is ordered for other, especially abdominal, examination.

Attention to these recommendations will also serve to discourage litigation for failure to diagnose aortic dissection.

Treatment

Pharmacotherapy

Most patients will require intensive medical therapy for acute aortic dissection, either as sole treatment or as a stabilizing measure until appropriate surgical therapy is undertaken.

It has been recognized that aortic dissection propagates more vigorously when either blood pressure or force of cardiac contraction are excessive. Accordingly, blood pressure needs to be controlled in patients with acute aortic dissection or other acute aortic syndromes, including aortic rupture or impending rupture. Nitroglycerin and nitroprusside are usually used for this purpose, because of their effectiveness, their rapid onset of action, and their quick cessation of action upon discontinuation. Blood pressure should be reduced as low as possible without producing neurologic dysfunction or oliguria. Usually, the severity of general occlusive vascular disease determines how low the blood pressure can safely be taken. The blood pressure may be lowered to 90–100 mm Hg initially, until the patient's response can be evaluated. For older patients with extensive end-organ vascular disease, lowering the blood pressure pharmacologically to 120–130 mm Hg may need to be satisfactory.

However, to lower blood pressure by afterload reduction alone would actually increase the sheer stress on the aortic wall. It is crucial to decrease the force of cardiac contraction (Figure 37–20). The morphology of the arterial pulse wave must be blunted by decreasing the force of cardiac contraction. The dp/dt, reflected in the upslope of the initial portion of the aortic pulse wave, must be decreased, usually by administering a short-acting β-blocker, such as esmolol. Another approach is administration of the α- and β-adrenergic antagonist labetalol by intravenous infusion. When β-blocking drugs are contraindicated, calcium channel blockers are a reasonable substitute.

Together, the afterload reduction and the β-blockade are referred to as “anti-impulse” therapy for acute aortic dissection. Regardless of whether the dissection is ascending or descending, or whether or not the patient will be taken emergently to the operating room, such therapy must be instituted—to discourage rupture or extension of the dissection.

Anti-impulse therapy is the appropriate initial response once the diagnosis of any type of acute aortic dissection or related process is made. Often, such therapy is undertaken while imaging studies are being performed to confirm the diagnosis of aortic dissection and define the anatomic type, location, and extent of the process. Definitive therapeutic decisions and treatments will follow.

Surgical Treatment

For acute aortic dissection, the following guidelines regarding definitive therapy apply.

Ascending aortic aneurysms require urgent surgery because death from intrapericardial rupture, aortic regurgitation, or myocardial infarction from coronary artery involvement usually occurs in patients who do not undergo surgery. The dissection layers are reapproximated as a “sandwich” between layers of Teflon felt (Figure 37–21). Overall survival at experienced centers is about 85% for patients with acute type A aortic dissection. The exact surgical procedure performed, vis-à-vis the proximal aortic root and the coronary arteries, varies depending on the circumstances. For patients in whom dissection occurs in the setting of Marfan disease or other cause of annuloaortic ectasia (proximal root enlargement), the aortic valve, aortic root, and ascending aorta are replaced with a prefabricated “composite graft” including both a valve and a graft (see Figure 37–14B).

Descending dissections, in the absence of specific vascular complications, do well with medical management (short- and long-term “anti-impulse” therapy with β-blockers and afterload-reducing medication). If a specific complication occurs, this is addressed directly by surgery (“complication-specific” approach to descending aortic dissection). Ninety-one percent of patients survived the initial hospitalization (type B aortic dissection is “milder” than type A dissection), and about 66% had a completely uncomplicated course while receiving anti-impulse medical therapy alone. The majority of complications were related to vascular malperfusion of specific organs. Stent therapy for acute or chronic type B dissection is controversial.

The subacute and chronic stages of aortic dissection are managed differently. Once the patient with type A dissection has been brought safely through surgery, or the type B patient has been stabilized with anti-impulse therapy, the patient is observed closely for the first month, with repeat aortic imaging. After that point, it is uncommon for the dissection to extend, cause symptoms, or rupture in the short-term to mid-term. The patients are then monitored the same as those with chronic aneurysm. Over years, enlargement of the dissected aorta will develop in some patients and require resection. The same dimensional criteria for surgical intervention can be applied as for nondissected aneurysms. It is usually the most proximal portion of the descending aorta, just beyond the subclavian artery, that dilates first and requires surgical replacement.

Prognosis

Aortic dissection is often fatal without early diagnosis and aggressive treatment. The presenting symptoms and signs are so myriad and nonspecific (see earlier in chapter) that dissection may be overlooked initially in up to 40% of cases; in fact, the diagnosis is not made until postmortem examination in a disturbingly large fraction of patients. This can be a frequent cause of litigation, and guidelines have recently been outlined so litigation can be avoided. Few other conditions demand such prompt diagnosis and treatment, since the mortality rate of untreated dissection approaches 1–2% per hour during the first 48 hours, 89% at 14 days, and 90% at 3 months.

Aortic dissection can result in death in four ways: (1) intrapericardial rupture (of an ascending dissection), (2) acute aortic regurgitation (from an ascending aortic dissection), (3) free rupture into the pleural space (of a descending dissection), and (4) occlusion of any branch of the aorta (with consequent organ ischemia).

Branch vessel occlusion comes about via impingement on the true lumen of any branch vessel (coronaries to iliacs) by the distended false lumen (Figure 37–22). The acute aortic regurgitation of an ascending dissection may be very poorly tolerated, compared with chronic aortic regurgitation, because the sudden nature allows no time for cardiac adaptation. Cardiogenic shock may result.

Several general advances promise to mitigate the scourge of aortic diseases on society: Aortic aneurysms are diagnosed more frequently (incidentally) due to widespread application of imaging techniques for other reasons (echocardiography, CT, MRI). Aortic aneurysms are diagnosed more frequently (deliberately) due to increasing recognition that these are inherited diseases, and family members are screened. Advances in genetics are increasingly facilitating the recognition of specific mutations underlying aortic diseases in specific patients and their families. Personalized therapy will soon be possible. Advances in genetics hold promise for effective screening tests for the general population. Surgical therapies have become extremely safe.

Appropriate intervention criteria have been developed that permit evidence-based preemptive surgical intervention to prevent rupture and dissection. Near-normal prognosis is maintained, provided that surgery is carried out before onset of aortic dissection. Public awareness of aneurysm disease has increased.