Vascular Emergencies

Carotid and Vertebral Artery Dissection

Spontaneous dissection of the carotid or vertebral arteries in the absence of trauma is rare and usually occurs in patients with an underlying history of hypertension or fibromuscular dysplasia or an underlying collagen disorder such as Marfan or Ehlers-Danlos syndrome. It can occur at any age. Clinical manifestations are variable, including neck pain or ipsilateral headache, Horner syndrome (i.e., ptosis, myosis, and unilateral anhydrosis), and acute or chronic focal neurologic events either in the form of a transient ischemic attack or as a permanent neurologic deficit. All imaging modalities, including ultrasound, CT, and MRI, can be used to demonstrate the intimal flap, a compressed true lumen, slow flow in the false lumen, or even thrombus formation. Catheter angiography is generally indicated to better assess flow patterns or to confirm the absence of flow (see Fig. 11-2 ). Most patients are successfully treated with anticoagulation, but stent placement in the true lumen may be necessary to maintain or restore cerebral perfusion.

Aortic Emergencies

Acute aortic syndromes encompass a spectrum of aortic emergencies that include traumatic aortic injury (TAI), aortic dissection, penetrating atherosclerotic ulcer of the aorta, intramural hematoma, and aortic aneurysm rupture. Chapter 2 covers the topics of aortic dissection, intramural hematoma, and penetrating ulcer. This section focuses on traumatic aortic injuries.

TAI is a novel term that encompasses a spectrum of injuries characterized by a variable degree of aortic wall laceration and that include intimal tear, intramural hematoma, traumatic dissection, traumatic pseudoaneurysm, and, in the most severe form, aortic transection. TAI may result from rapid decelerations, crush injuries, penetrating wounds, and surgical or angiographic instrumentation. Blunt trauma is the most frequent cause of TAI, usually as the result of shearing, hydrostatic forces, and/or torsion forces applied along the aortic arch during a motor vehicle accident and falls from heights. Less common causes include displaced clavicular and thoracic vertebral fractures with entrapment of the aorta between the anterior chest wall and the spine. The overall mortality at the scene of the accident has been reported to be as high as 80% in autopsy series, with only 10% to 20% of victims surviving the initial trauma. Bleeding from a laceration or rupture can be controlled by the aortic adventitia or the periaortic tissues, which can help the patient survive the initial injury; however, the end result is pseudoaneurysm formation, which, because of its instability, requires prompt diagnosis and treatment ( Fig. 11-3 ). The region of the aorta most susceptible to blunt injury is the isthmus, where the relatively mobile proximal thoracic aorta and arch join the fixed distal arch at the insertion of the ligamentum arteriosum, just distal to the left subclavian artery origin ( Fig. 11-4 ). This area is involved in as many as 90% to 95% of cases. Other common areas of injury are the aortic root and the diaphragmatic hiatus. Injuries to the ascending aorta are uncommon (5% to 9%) and usually lethal because of the lack of surrounding connective tissue, resulting in rapid death as a result of exsanguination or cardiac tamponade. Clinical suspicion, as well as prompt diagnosis and treatment, are of crucial importance in persons with a TAI, given that 30% of the victims will die within 6 hours and 40% to 50% will die within 24 hours.

Plain radiographs are used for the initial imaging screening of trauma patients because they are readily available and have the capacity to quickly assess injuries such as pneumothorax, hemothorax, mediastinal hematoma ( Fig. 11-5 ), and fractures. Clinical signs and symptoms found in patients with TAI include chest pain, back pain, dyspnea, cough, hoarseness, hypotension, pulse discrepancy, shock, and coma; as many as 30% to 50% of these patients may not show external signs of trauma.

A normal upright chest radiograph has a negative predictive value of 98% when evaluating patients for aortic injury; however, such a radiograph rarely can be obtained for patients with severe multitrauma. In such patients a frontal poorly inspired film tends to be obtained with which mediastinal abnormalities frequently cannot be adequately assessed. Indirect signs for possible TAI on a chest radiograph include widening of the mediastinum, tracheal shift, deviation of the nasogastric tube or endotracheal tube to the right of the T3-T4 spinous processes, widening of the left or right paraspinal lines, apical cap, increased density of the aortopulmonary window, hemothorax, depression of the left main stem bronchus, and an ill-defined aortic arch. Mediastinal widening has reported sensitivities ranging between 81% and 100%, with a specificity of 60%; however, significant interreader variability exists when using this sign to predict aortic injury.

A catheter aortogram has long been considered the gold standard for the diagnosis of TAI, with reported sensitivities and specificities approaching 100%. However, MD-CTA has become the preferred method for screening patients with major traumatic and nontraumatic aortic emergencies because of its improved spatial and contrast resolution and supplemental postprocessing techniques (thin sections, multiplanar reformations, and three-dimensional volume-rendered images), with a performance that rivals that of catheter angiography. MD-CTA has helped to better characterize the location and extent of TAI and other vascular injuries, thus allowing for faster diagnosis and treatment planning. In addition, MD-CTA also can recognize a greater number of normal variants of the vascular anatomy and subtle vascular injuries that were less likely to be seen with conventional CT scans ( Fig. 11-6 ). Furthermore, the need to perform aortography to confirm the diagnosis of TAI is eliminated when direct signs of injury are present on CTA. Direct signs of TAI on CTA include active extravasation of contrast material, pseudoaneurysm formation, irregularity of the aortic wall, abrupt change in the caliber of the aorta, aortic dissection, intimal flaps, and filling defects. Indirect signs of possible aortic injury include periaortic hematoma and mediastinal hematoma ( Fig. 11-7 ).

Catheter angiography can accurately demonstrate abnormalities affecting the aortic lumen; however, it is a time-consuming, invasive procedure that may require higher doses of contrast material and is limited in diagnosing concurrent injuries in multitrauma patients, risking a delay in management of other potentially lethal injuries. A minority of patients may still require catheter angiography when MD-CTA examinations are nondiagnostic or show equivocal or indirect signs of TAI. The decision to perform a catheter angiogram for patients with indirect findings depends on the experience of the interpreter, the quality of the scan, and the clinical condition of the patient. Angiographic diagnosis of intimal injury is demonstrated by the presence of intimal irregularity, linear defects, or filling defects caused by an intimal flap. The presence of contrast material outside the lumen is consistent with active extravasation. The contrast material can be contained or free; both presentations are consistent with a transmural laceration and require immediate attention. The current complication rate for angiography is less than 1%, and complications include aortic rupture, acute renal failure, anaphylaxis, and entry site hematoma.

Transesophageal echocardiography (TEE) is another imaging modality that can provide detailed real-time images of the aorta, heart, and pericardium and can be performed at the patient’s bedside. Studies have shown sensitivity ranging between 63% and 100% and specificity ranging between 84% and 100% for the detection of aortic rupture. The wide range of these results is explained by the fact that TEE is operator dependent. Direct signs of aortic injury include an intimal flap, intraluminal thick stripes, a pseudoaneurysm, aortic occlusion, and an aortic wall hematoma. The main limitation of TEE is its “blind spot,” which represents the lack of visualization of a 3- to 5-cm segment in the distal ascending aorta and proximal arch that can be the site for TAI in 10% to 20% of patients. TEE is an alternative for the evaluation of unstable patients. However, negative findings of TEE in the setting of suspicious clinical or radiologic findings warrant further investigation.

Currently MRA is not considered the imaging technique of choice for the evaluation of TAI because of the relatively long acquisition times, the limited access to manage and control hemodynamically unstable patients, and the difficulties in scanning patients with metallic fragments or devices that may preclude the examination.

Treatment

Immediate surgical intervention is the treatment of choice in patients with TAI who are hemodynamically unstable, have persistent bleeding, or have evidence of an expanding hematoma. Surgical repair is usually a major intervention requiring thoracotomy, aortic cross-clamping, partial cardiopulmonary bypass, suturing or prosthetic grafting of the aorta, and treatment of concomitant injuries such as the evacuation of pericardial tamponade or a large hemothorax. The operative mortality of open thoracotomy ranges between 9% and 28%, with a high rate of major morbidities, including paraplegia (up to 20%) as a result of spinal cord ischemia and stroke. Patient triage and selection are essential to perform a successful thoracotomy because open thoracotomy can worsen the clinical condition of severely injured patients who may not be able to tolerate this procedure.

Alternative management of TAI consists of delayed operative intervention or no operative intervention at all while the patient is closely monitored and blood pressure (BP) is controlled to reduce the risk of free rupture. The mainstay of treatment is to maintain systolic BP below 120 mm Hg (mean arterial pressure below 80 mm Hg) and allow patients who have sustained associated severe trauma to stabilize before undergoing surgical repair. This approach is indicated in hemodynamically stable patients with small aortic tears or those with associated injuries such as significant head, cardiac, or pulmonary trauma, large body surface burns, contaminated wounds, large retroperitoneal hematomas, or other high-risk medical comorbidities. Some of these aortic injuries may develop into a chronic pseudoaneurysm ( Fig. 11-8 ), and others may even resolve during the period of observation. Close clinical and imaging follow-up is imperative to detect injury progression or aortic rupture.

More recently, endovascular treatment of TAI using stent grafts has been introduced as an alternative in the management of hemodynamically stable patients and patients with contraindications to cardiopulmonary bypass, such as severe coagulopathy, extensive additional injuries, and severe underlying cardiac or pulmonary disease. Mortality and morbidity with use of stent grafts appear to be lower than with surgery, most likely because of the less invasive nature of the procedure, a shorter operation time, and lack of surgery-related comorbidities. The stent grafts are delivered via a common femoral artery cut-down procedure and positioned with angiographic and fluoroscopic guidance. The left subclavian artery origin often must be covered with the endograft to provide adequate proximal support without significant clinical consequence in many patients. A significant number of patients with TAI are young, and the aortas of young patients are small; however, most stent grafts available today are made for the treatment of larger aortas affected by degenerative aortic aneurysms. Although it is usually recommended that the size of the stent graft be increased by 10% to 20% to ensure an adequate seal, an excessive increase in the size of the stent graft has been associated with type I endoleaks ( Table 11-1 ) and stent graft collapse. Complications related to the endovascular repair include hematomas in the groin, iliofemoral dissection, endoleaks, graft migration, and arterial rupture. The overall morbidity rate has been found to be approximately 12%, with a mortality rate of 4%. Clinical and imaging follow-up of patients who receive either surgical and endovascular treatment is required to ensure early identification of posttreatment complications.

TABLE 11-1

Endoleak Classification

Endoleak type I	Inadequate seal around the proximal or distal end of the stent graft
Endoleak type II	Retrograde flow from branch vessels (intercostal, lumbar, inferior mesenteric, and gonadal arteries) resulting in persistent filling of aneurysm
Endoleak type III	Stent graft laceration or fabric tears and dislodgment of modular graft devices
Endoleak type IV	Porosity of the graft fabric and diffuse leakage through interstices
Endotension	Excluded sac continues to enlarge without apparent endoleak formation

Minor Aortic Injury

With the increased utilization and improved sensitivity of MDCT in evaluating patients with blunt thoracic trauma, a greater number of minor aortic injuries (MAIs) are being diagnosed. An MAI has been described as a posttraumatic abnormality of the internal contour of the aorta wall, an intimal flap, an intraluminal filling defect, or an intramural hematoma. In a retrospective review of 81 patients by Gunn and colleagues, MAI occurred in approximately 28% of cases of blunt TAI. The majority of MAI cases in this series occurred in the descending thoracic aorta (69%) and were treated conservatively (83%) with antihypertensive drugs, anticoagulant agents, or both. The patients in this series who were treated without operative or endovascular repair were followed up for a mean of 466 days, with no reports of death or complications as a result of MAI. In another retrospective review of 115 patients, Forman and colleagues included in their definition of MAI small pseudoaneuryms of less than 10% of the normal aortic diameter as measured at the level of the pseudoaneurysm. In their study, in about one third of patients with MDCT-diagnosed blunt TAI, the injuries were minor. Although the absence of a mediastinal hematoma nearly excluded the possibility of major TAI, approximately 21% of cases of MAI occurred without evidence of a mediastinal hematoma. About 62% of patients in this series who met the definition of MAI were treated with medication and did not show evidence of progression of TAI at the last imaging follow-up. These studies indicate that MAI is identified by MDCT in more than a quarter of the cases of blunt TAI, MAI has a low mortality, and most of the time MAI can be managed conservatively with excellent outcomes.

Penetrating Aortic Injury

Penetrating injuries to the intrathoracic great vessels are uncommon, with an incidence of 1%; they have a high mortality rate that ranges between 50% and 85%, despite advances in trauma care and prehospital resuscitation. These types of injuries can be caused by gunshot wounds (GSWs) or stab wounds that traverse the chest or the base of the neck ( Fig. 11-9 ). Overall, GSWs are more common and more lethal than stab wounds. Penetrating injuries to the thoracic aorta often occur along the ascending aorta and the arch branches, with a documented low incidence of descending thoracic injuries. These types of lesions are commonly associated with coexisting lethal intrathoracic injuries, which worsen the patient’s prognosis. Although the aorta is protected by osseous structures, a laceration in this large-caliber vessel with high intraluminal pressure can cause rapid exsanguination. Thoracic aortic injuries have a worse prognosis than injuries to the abdominal aorta, probably because of the retroperitoneal location of the abdominal aorta, which can slow the rate of exsanguination. Thoracic aortic injuries usually manifest as hemorrhage into the mediastinum or pleural cavity presenting as hemothorax, cardiac tamponade, or mediastinal hematoma.

A portable supine chest radiograph continues to be the best initial imaging modality to screen for chest trauma. Radiopaque markers should be placed at the entry and exit sites of the wounds to provide a guide to the possible wound trajectory and the organs that might have been injured. Penetrating vascular lesions are suggested by a large hemothorax, widened mediastinum, foreign bodies in the proximity of the great vessels, or a bullet in a position different from its predicted course, suggesting bullet embolism, also known as “missing missile.”

MDCT/CTA has become an integral part of the assessment of aortic injury resulting from blunt trauma, and its role in penetrating injuries to the aorta continues to expand because of its availability, short acquisition time, and ability to identify vascular lesions. MDCT/CTA is also able to locate bullets and fragments, document the wound tract and bullet path, visualize associated fractures, and evaluate internal organ injuries. MDCT/CTA continues to have a major role in hemodynamically stable patients who have GSWs that are suspected to have transversed the mediastinum, because MDCT/CTA can define the wound tract and its relationship with vascular and aerodigestive structures, thus helping plan further treatment.

Aortic angiography remains the gold standard for the diagnosis of vascular lesions because of its ability to show pseudoaneurysm formation, active extravasation, and arteriovenous fistulas. However, fewer catheter aortograms are being performed because of the previously mentioned benefits of noninvasive imaging with MD-CTA. In the case of a penetrating injury to the major vessels, lesions in the central venous structures such as the innominate veins can be simultaneously present in up to one fourth of patients, and they account for nearly 22% of fatalities in penetrating chest trauma. When catheter angiography is performed, a venous phase should be carried out in all arteriograms, and if venous injury is suspected, use of conventional venograms should be considered.

Penetrating injuries to the thoracic aorta are usually treated with urgent surgical repair via an anterolateral thoracotomy. Endovascular repair with stent grafts may be possible for penetrating injuries in some patients (see the section on TAI).

Thoracic Aortic Aneurysms

Thoracic aortic aneurysms (TAAs) are often the result of atherosclerotic disease or cystic medial degeneration with subsequent weakening and dilatation of the aortic wall. The development of TAAs is usually a silent process, it is seen less commonly than abdominal aortic aneurysms, and it generally occurs in older men in their sixth and seventh decades. Based on their structure, aneurysms are classified as either fusiform aneurysms, which involve the full circumference of the vessel wall, or saccular aneurysms that involve only a focal portion. Thoracic aneurysms occur in the ascending aorta in 40% to 60% of cases, in the descending aorta in 30% to 40% of cases, and in the aortic arch and thoracoabdominal aorta in 10% of cases. The incidence and prevalence of thoracic aortic disease, including TAA, are steadily increasing because of the aging population and improved diagnostic techniques. The reported incidence of TAA ranges from 6 to 10 cases per 100,000 patients per year. The natural history of untreated TAA is progressive expansion of the aneurysm and, ultimately, rupture ( Fig. 11-10 ).

Given that most patients are asymptomatic, TAAs are commonly diagnosed as an incidental finding on imaging studies. However, they also can present with signs and symptoms related to compression of adjacent structures. Ascending aortic aneurysms can compress the coronary arteries or the origins of the great vessels, causing myocardial or cerebral ischemia. Together with arch aneurysms, they can erode into the mediastinum and compress the left recurrent laryngeal nerve, causing hoarseness, or the phrenic nerve, leading to hemidiaphragmatic paralysis. They also can press on (1) the tracheobronchial tree, causing wheezing, dyspnea, cough, hemoptysis, and pneumonitis; (2) the esophagus, resulting in dysphagia; and (3) the superior vena cava (SVC), causing SVC syndrome. Mural thrombus from the aneurysms can be a source of emboli and cause strokes, as well as renal, mesenteric, or limb ischemia. Heart failure can result from aortic root aneurysms, leading to aortic regurgitation, or from the rupture of a sinus of Valsalva aneurysm into the right side of the heart. Chest and back pain are rare symptoms associated with compression of intrathoracic structures or erosion into adjacent bones ( Fig. 11-11 ).

The most feared complications of TAAs are aneurysm dissection and rupture. Dissection can lead to arterial occlusion and end-organ ischemia, and rupture can cause massive hemorrhage that usually cannot be contained by adjacent structures and therefore is considered a surgical emergency. Patients present with hypotension and acute onset of chest pain, abdominal pain, back pain, or neck pain. Rupture occurs more commonly into the left pleural space, but it also can occur into the pericardium, causing pericardial tamponade ( Fig. 11-12 ), or into the esophagus, resulting in an aortoesophageal fistula and dramatic upper gastrointestinal (GI) bleeding.

The risk of rupture and dissection of TAAs increases with aneurysm size. The mean rate of rupture or dissection for small aneurysms is around 2%; it increases to 3% for aneurysms measuring 5.0 to 5.9 cm and to 7% in patients with aneurysms 6 cm in diameter or larger. The mean aortic growth rate has been estimated to be 0.1 cm per year. Greater growth rates are seen in patients with Marfan syndrome, aneurysms of the descending aorta, and dissecting aneurysms. Although men are more prone to the development of thoracic aneurysms, women have a higher likelihood of rupture and dissection.

Multiple causes have been associated with thoracic aortic aneurysms, and they differ depending on their location within the thoracic aorta. Aneurysms of the ascending aorta and aortic arch are more commonly seen in elderly patients as a result of atherosclerosis or aortic valve stenosis, leading to poststenotic dilatation. The younger patient population presenting with thoracic aneurysms in the ascending aorta and aortic arch ( Tables 11-2 and 11-3 ) usually have a connective tissue disorder such as Marfan syndrome (mutation of the fibrillin-1 gene), Ehlers-Danlos syndrome (a defect in type III collagen), Loeys-Dietz syndrome, or Turner syndrome (which is associated with bicuspid aortic valves). Other infrequent causes include mycotic and syphilitic aneurysms. Aneurysms of the sinus of Valsalva can be congenital, infectious, or postsurgical in origin and are seen as dilatations in connection with the aortic root. Vasculitis can affect all portions of the thoracic aorta. One of the most striking entities is Takayasu arteritis, which is characterized by a chronic inflammatory process of unknown cause that affects the aorta, its major branches, and the pulmonary arteries. Takayasu arteritis affects the vessel wall, causing obliterative luminal changes, occlusion, or dilatation. Aneurysm formation can be a fatal complication of this disease because it may lead to heart failure as a result of aortic valve regurgitation or aortic rupture.

Aneurysms of the descending thoracic aorta are also most commonly degenerative in origin; they are caused by atherosclerosis and usually are associated with hypertension, hypercholesterolemia, and smoking. These aneurysms are generally fusiform with significant intimal calcifications and demonstrate associated tortuosity and mural thrombus. Other causes include chronic aortic dissection (Stanford type A or B), which can dilate over time as the wall of the false lumen weakens, creating a dissecting aneurysm with a high risk of rupture. Injuries to the descending thoracic aorta associated with blunt trauma can result in the formation of pseudoaneurysms, which are characteristically located near the aortic isthmus distal to the subclavian artery and have a high risk of rupture (these pseudoaneurysms are discussed in the TAI section). As with ascending thoracic aneurysms, younger patients with descending thoracic aortic aneurysms usually have an underlying connective tissue disorder ( Table 11-4 ) or vasculitis. Mycotic aneurysms also can affect this segment.

Aneurysms located in the thoracoabdominal aorta appear less frequently than abdominal or thoracic aneurysms, which is fortunate, because these lesions are difficult to treat as a result of the numerous visceral branches that arise along this segment. Aneurysms located in the thoracoabdominal aorta are divided into four types according to the Crawford classification ( Table 11-5 ). Types II and III are the more complex aneurysms, with type II having the highest risk of treatment-induced spinal cord infarction and renal failure.

Diagnosis

Thoracic aortic aneurysms are usually seen on a routine chest radiograph as an incidental finding. Common findings include a widened mediastinum, enlarged aortic knob, tracheal deviation, aortic kinking, and blunted aortopulmonary window. However, a chest radiograph is limited in the diagnosis of thoracic aneurysms because it cannot accurately differentiate a tortuous aorta from an aortic aneurysm, and small aortic aneurysms can easily be missed.

CTA provides sufficient information to diagnose and follow up on the progression of a TAA; angiographic sequences allow for precise delineation of the extension of the aneurysm, involvement of the great vessels, and other associated thoracic disease (see Fig. 11-11 ). Unenhanced CT is used to identify areas of aortic calcification, mural thrombus, acute wall hematoma (circumferential or crescentic), and recent hemorrhage. Administration of contrast material helps differentiate the patent lumen from a mural thrombus and demonstrates an intimal flap and a false lumen in areas of aortic dissection. CT also shows the relationship of the aneurysm to the adjacent structures and helps correlate associated patient symptomatology with imaging findings. Mycotic aneurysms can affect any portion of the aorta and are seen as saccular dilatations with multilobulated contours ( Fig. 11-13 ). Cross-sectional CT imaging features include a perianeurysmal soft tissue mass, fluid collection, and occasionally gas-forming inflammation. Syphilitic aortic aneurysms are rarely seen these days, but when they are encountered, they usually demonstrate extensive aortic calcification or linear calcifications with longitudinal wrinkling of the wall, causing a “shaggy tree bark” pattern; approximately 75% of the cases exhibit a saccular structure.

Both CTA and MRA can be used successfully as preprocedural imaging techniques to plan surgical or endovascular repair, because they are both able to measure the dimensions of the aneurysm and detect the involved vessels. Accurate measurements are crucial when selecting the appropriate stent graft diameter for endovascular procedures to minimize stent-related complications such as endoleak, stent migration, and branch occlusion. CTA with multiplanar reconstruction and digital subtraction angiography are considered the most useful techniques to depict structural characteristics of the aneurysm, as well as to detect graft complications.

Contrast-enhanced MRA (CE-MRA) can provide exquisite detail of the aneurysm and associated dissection or branch involvement. Sometimes it also can visualize small vascular structures in greater detail, such as the Adamkiewicz artery, providing valuable information in the planning of surgical repair and helping to avoid postoperative neurologic deficits. However, CE-MRA cannot visualize aortic calcification, which can be important during treatment planning. These two modalities continue to complement each other, because one may show certain characteristics that cannot be shown by the other.

In either case, serial imaging studies are usually required in this patient population to monitor aneurysm size. A repeat study can be obtained 6 months after the initial diagnosis, and if the aneurysm is stable, imaging follow-up can be obtained annually.

Abdominal Aortic Aneurysms

Abdominal aortic aneurysms (AAAs) are fusiform or saccular dilatations of the abdominal aorta that often form as a consequence of degeneration of the media from atherosclerotic disease, which causes weakening of the wall and widening of the luminal diameter. Degenerative aneurysms are more frequent in the elderly population, whereas in younger patients, abdominal aneurysms are usually a result of inheritable diseases such as Marfan syndrome and Ehlers-Danlos syndrome. Predisposing factors that have been associated with AAA include advanced age, family history, male gender, tobacco use, and white race, with a lower incidence found in patients of Asian descent. Patients with AAA have a higher incidence of hypertension, atherosclerosis, myocardial infarction, heart failure, and peripheral vascular disease than do control subjects matched for age and gender.

Other types of AAA include inflammatory and infectious aneurysms. Inflammatory aneurysms, as their name indicates, show an increased inflammatory reaction in the aneurysm wall and surrounding tissues and present with a triad of chronic abdominal pain, weight loss, and elevated erythrocyte sedimentation rate. Infectious or mycotic aneurysms are more frequently associated with transient bacteremia with Staphylococcus or Salmonella , which leads to infection of the vessel wall or an atherosclerotic plaque by hematogenous spread. Rarely, aneurysms caused by tuberculosis infection have been reported.

Patients with AAA are usually asymptomatic, and aneurysms often are detected as an incidental finding on radiologic studies. Sometimes clinical examination can detect an AAA if a pulsatile abdominal mass or an abdominal bruit is present; unfortunately, physical examination sensitivity among studies varies widely, and confirmatory imaging is usually required. Symptomatic patients usually report lower back pain or abdominal pain that can be nonspecific and easily confused with other disease processes. Patients with acute rupture present with abrupt-onset pain in the lower back or abdomen that can be associated with a pulsatile abdominal mass and hypotension. The natural history of abdominal aortic aneurysms is usually characterized by gradual expansion of the aneurysm sac with mural thrombus formation lining the inner surface. The aneurysm may then cause compression of adjacent structures, thromboembolic events, erosion into neighboring structures (e.g., the duodenum or iliac vein), and, ultimately, rupture.

AAA rupture is a catastrophic complication with a reported mortality of up to 90%. It usually occurs in the posterolateral aspect of the aorta, causing retroperitoneal hemorrhage. Less commonly, it ruptures in the anterolateral aspect, causing intraperitoneal bleeding and quick exsanguination. Contained ruptures in the form of retroperitoneal hematomas may be associated with ecchymosis of the flanks, otherwise known as Grey-Turner syndrome . In general, the larger the aneurysm, the higher the risk for spontaneous rupture, and the faster it tends to expand. The average expansion rate for aortic aneurysms 4.0 cm in diameter is 1 to 4 mm per year, followed by 4 to 5 mm per year for aneurysms measuring 4.0 to 6.0 cm, and 7 to 8 mm per year in larger aneurysms. The lifetime risk of rupture of abdominal aneurysms larger than 5.0 cm in diameter is 20%; it then increases to 40% for aneurysms that measure more than 6.0 cm, and to more than 50% for aneurysms that exceed 7.0 cm. The corresponding estimated annual rupture rates are 4%, 7%, and 20%, respectively.

Other AAA complications that may require immediate treatment are those related to compression or erosion of adjacent structures, as seen in large degenerative AAA, or inflammatory or mycotic aneurysms. Some examples include gastric outlet syndrome as a result of compression of the duodenum, aortoenteric fistula formation causing massive upper GI bleeding, and venous fistula formation in the inferior vena cava (IVC), the left renal vein, or the common iliac vein. These fistulae can have high flow rates and result in acute congestive heart failure or in hematuria and flank pain.

Diagnosis

Early AAA detection is imperative to reduce patient morbidity and mortality as a result of this silent disease. The most common site of arterial aneurysms is the abdominal aorta, especially along its infrarenal segment. Abdominal aneurysms are diagnosed when the aorta is 1.5 times greater that the diameter of the normal aorta or when the minimum anteroposterior diameter is greater than 3.0 cm, irrespective of age and gender.

Imaging techniques are currently relied on for the diagnosis and follow-up of aortic aneurysms. Real-time ultrasonography is a noninvasive and cost-effective modality useful for screening for AAA, with an accuracy that approaches 100% in the diagnosis of infrarenal aortic aneurysms. Its accuracy in measuring the aortic diameter below the level of the renal arteries has been shown to correlate well with direct intraoperative measurements. The size of the aneurysm is calculated by measuring the maximum anteroposterior aortic diameter or the largest transverse diameter measured in a plane perpendicular to the luminal arterial axis to avoid overestimation of the aneurysm size. AAA is seen as a dilated vessel with irregular lumen and eccentric echogenic thrombus material. Although ultrasound is a convenient procedure, it is limited by the patient’s body habitus and the interposition of bowel gas that obscures visualization of the deeper structures. Likewise, it is not as reliable as CTA for detecting complications such as rupture, aneurysm extension into the suprarenal aorta, and detection of posttherapeutic complications such as endoleaks.

MD-CTA has emerged as the new gold standard for the diagnosis of AAAs, replacing catheter arteriography. MD-CTA offers high-resolution imaging and shorter scan times, allowing the detection and characterization of aneurysms and related complications, including impending rupture and contained or complete rupture. Several signs of impending rupture have been identified on CT (see Box 11-3 ), including increased aneurysm size, a low thrombus-to-lumen ratio, and mural thrombus hemorrhage that is usually identified as high-attenuation crescents in the wall of the aortic aneurysm on unenhanced CT images ( Fig. 11-14 ). Various signs suggest a contained AAA rupture, including loss of definition of the posterior aortic wall, the presence of an organized hematoma with contour abnormalities of the vessel wall, interruption of a continuous ring of aortic wall calcification, and a posterior wall of the aorta following the contour of the vertebra with or without associated vertebral erosion (a sign known as a “draped” aorta). A ruptured AAA is determined by the presence of hemorrhage contiguous to the aorta that almost always involves the retroperitoneal space and rarely involves the iliopsoas compartment. Periaortic blood might be seen in the pararenal space, perirenal space, or both; a hematocrit sign (cellular-fluid level) and rectus sheath bleeds are rarer but are helpful when they are present and tend to be associated with coagulopathic hemorrhage ( Fig. 11-15 ).

CT is also useful in differentiating between different types of aortic aneurysms. Mycotic aneurysms are usually seen as saccular-shaped collections with a lobulated contour; other features may include a periaortic soft tissue mass with stranding, retroperitoneal paraaortic fluid collection, vertebral erosion, gas-forming inflammation around the aneurysm, intraaortic air pockets, and thrombus formation within a false lumen after aneurysmal rupture (see Fig. 11-13 ). In inflammatory aneurysms, CT and MRI can detect the cuff of soft tissue inflammation surrounding the aneurysm, thickening of the aneurysm wall, perianeurysmal and retroperitoneal fibrosis, and adherence of the anterior aneurysm wall to adjacent structures ( Fig. 11-16 ). On precontrast CT images, the thickened aortic wall has soft tissue attenuation that enhances after intravenous administration of contrast material. The arterial wall may become indistinguishable from periaortic fibrosis. Periaortic fibrotic tissue can adhere to the ureters, small bowel, duodenum, and IVC, causing entrapment of these structures and further complicating surgical repair. Preoperative imaging is of great importance in the planning of surgical and endovascular treatment of patients with AAA. Important preoperative features and measurements performed on CTA include the maximum transverse aneurysm diameter, relation of the aneurysm to the renal arteries, presence of a proximal neck (renals to aneurysm distance), presence of a distal neck (aneurysm to aortic bifurcation distance), extension of the aneurysm into the iliac arteries, identification of concomitant aneurysms, and appearance of the access pathway, including femoral and iliac arteries. Some conditions that limit the delivery of an endograft include a very large AAA, a markedly tortuous aorta, pelvic arteries with abrupt angles, and extensive calcification causing luminal narrowing. Any of these factors can represent an exclusion criterion depending on the specific anatomy and the degrees of angulation and stenosis. They determine the suitability or unsuitability for endovascular treatment, and cautious analysis of the images in conjunction with the surgical/interventional team is crucial to decrease the risk of endoleaks, attachment failures, graft migration, and conversion to open repairs. MRA has shown properties similar to those of CTA for diagnosing and performing preoperative planning for AAA; it provides precise information about the location and extent of the aneurysm. Conventional arteriography is currently used as an adjunct imaging modality for evaluating patients with complex vascular anatomy, for evaluating aneurysms that could not be fully characterized with CTA and MRA, and for intraprocedural graft positioning.

Treatment

Treatment is usually recommended for AAAs measuring 5.5 cm or larger to eliminate the risk of rupture and for symptomatic AAAs regardless of their diameter. Ruptured aneurysms with hemodynamic compromise require immediate surgical repair. In select cases, endovascular intervention might be indicated for the management of ruptured aneurysms that remain hemodynamically stable. Patients with aneurysms 4.0 to 5.4 cm in diameter should undergo imaging follow-up every 6 to 12 months for early detection of expansion. Currently, elective repair is not recommended in patients with asymptomatic aneurysms that measure less than 5.0 cm in men, or less than 4.5 cm in women. Higher mortality rates are seen in patients who have emergent repair rather than elective surgery, and for this reason, early detection and surveillance of high-risk populations remain crucial in the treatment of AAAs.

Open surgical treatment is usually offered to hemodynamically unstable patients and to patients who want an elective surgical repair. Such treatment involves a midline transabdominal incision or left retroperitoneal flank access with subsequent clamping of the aorta, excision of the aneurysm, and placement of a synthetic graft. Aortic clamping can be linked to the development of significant morbidities such as ischemia of the lower extremities or bowel or paraplegia.

Postoperative complications related to an elective open surgical approach vary between 0.4% and 10% and include pseudoaneurysm formation, graft infection, enteric fistulas, and graft limb occlusion.

Treatment of pararenal and suprarenal aortic aneurysms is more complex because it requires cross-clamping of the aorta above the visceral arteries and sometimes branch vessel reimplantation, thus increasing operative morbidity and mortality, with up to 15% of patients requiring temporary dialysis and 5% experiencing permanent renal failure. The overall 5-year survival rate among these patients is 40% to 50%.

Endovascular aortic aneurysm repair (EVR) offers a less invasive approach to reduce the operative morbidity and mortality. Blood losses are significantly reduced because the graft is placed intravascularly via femoral access, and likewise, the risk for lower limb and visceral ischemia is lower because aortic clamping is not performed. EVR is offered to patients who are undergoing repair for asymptomatic AAA or symptomatic nonruptured AAA or to patients who are at high risk surgically and have significant comorbidities. The role of the endograft is to exclude the aneurysmal sac from the arterial circulation and decrease the mechanical stress over the vessel wall ( Fig. 11-17 ). Studies have shown benefits in the perioperative mortality and a 30-day morbidity and mortality rate of less than 3% with EVR. However, despite its short-term advantages, long-term survival and quality-adjusted life expectancy do not seem to vary significantly when compared with open surgery repair.

A common complication related to stent graft placement is the development of endoleaks ( Fig. 11-18 ). An endoleak is defined as an incomplete exclusion of the aneurysmal sac. The estimated incidence oscillates in the 10% to 45% range, and currently lifelong imaging follow-up is recommended to detect endoleaks and other graft-related complications. Delayed rupture is rare (0.1% to 1% per year) and has been associated with type I and type III endoleaks, graft migration, and endograft kinking. When type I or type III endoleaks are recognized, immediate treatment is indicated through graft extension, reintervention, or conversion to open repair. Other serious complications include graft infection and occlusion ( Fig. 11-19 ). Occlusion is usually due to distortion of one of the graft limbs and can cause lower extremity ischemia if it is not promptly treated. Continuous aortic expansion at the “neck” can cause endograft migration and delayed type I endoleak, but this occurrence is rare because endografts are oversized at the time of placement by up to 20% to ensure an adequate seal.