Abdominal Aortic Aneurysm Rupture

Disponível em: http://emedicine.medscape.com/article/756735-overview#aw2aab6b3

Epidemiology United States statistics

Ruptured abdominal aortic aneurysm (AAA) causes an estimated 15,000 deaths per year. The frequency of rupture is 4.4 cases per 100,000 persons. The reported incidence of rupture varies from 1-21 cases per 100,000 person-years.

International statistics

The frequency of rupture is 6.9 cases per 100,000 persons in Sweden, 4.8 cases per 100,000 persons in Finland, and 13 cases per 100,000 persons in the United Kingdom.

Prognosis

The prognosis is guarded in patients who suffer AAA rupture prehospital. More than 50% do not survive to the ED; of those who do, survival rate drops by about 1% per minute. However, survival rate is good in the subset of patients who are not in severe shock and who receive timely, expert surgical intervention.

In 1988, 40,000 surgical reconstructions for abdominal aortic aneurysm (AAA) were performed in the US, with substantial mortality differences between elective versus emergency operations. As elective aneurysm repair has a mortality rate drastically lower than that associated with rupture, the emphasis must be on early detection and repair free from complications.

The long-term prognosis is related to associated comorbidities. Long-term survival is shortened by chronic heart failure and chronic obstructive pulmonary disease. Rupture of associated thoracic aneurysms is also an important cause of late death. Overall, AAA repair is very durable, with few long-term complications (< 5% false aneurysm). In general, the survival rate of patients with successful aortic aneurysm repair is comparable to that of people in the age-matched population at large who have never had an aneurysm.

Pathophysiology

Aneurysm diameter is an important risk factor for rupture. In general, abdominal aortic aneurysms (AAAs) gradually enlarge (0.2-0.8 mm/y) and eventually rupture. Hemodynamics play an important role. Areas of high stress have been found in AAAs and appear to correlate with the site of rupture. Computer-generated geometric factors have demonstrated that aneurysm volume is a better predictor of areas of peak wall stress than aneurysm diameter. This may have implications in determining which AAAs require surgical repair.

AAA rupture is believed to occur when the mechanical stress acting on the wall exceeds the strength of the wall tissue. Wall tension can be calculated using the Laplace Law for wall tension: P × R/W, where P = mean arterial pressure (MAP), R = radius of the vessel, and W = wall thickness of the vessel.

AAA wall tension is a significant predictor of pending rupture. The actual tension in the AAA wall appears to be a more sensitive predictor of rupture than aneurysm diameter alone. For these reasons, the clinician may wish to achieve acute blood pressure control in patients with AAA and elevated blood pressure.

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Clinical Presentation

Persons with abdominal aortic aneurysms (AAAs) that have ruptured may present in many ways. The most typical manifestation of rupture is abdominal or back pain with a pulsatile abdominal mass. However, the symptoms may be vague, and the abdominal mass may be missed. Symptoms may include groin pain, syncope, paralysis, or flank mass. The diagnosis may be confused with renal calculus, diverticulitis, incarcerated hernia, or lumbar spine disease.

Transient hypotension should prompt consideration of rupture because this finding can progress to frank shock over a period of hours. Temporary loss of consciousness is also a potential symptom of rupture.

Patients with a ruptured AAA may present in frank shock as evidenced by cyanosis, mottling, altered mental status, tachycardia, and hypotension. At least 65% of patients with ruptured AAA die from sudden cardiovascular collapse before arriving at a hospital.

It is important to note progressive symptoms (eg, abdominal or back pain, vomiting, syncope, claudication). These should alert the clinician to the possibility of expansion with imminent rupture.

Peripheral emboli and claudication

Atheroemboli from small AAAs produce livedo reticularis of the feet or blue toe syndrome (see the image below). Occasionally, small AAAs thrombose, producing acute claudication.

Atheroemboli from small abdominal aortic aneurysms produce livedo reticularis of the feet (ie, blue toe syndrome).

Aortocaval fistulae

AAAs may rupture into the vena cava, producing large arteriovenous fistulae. In this case, symptoms include tachycardia, congestive heart failure (CHF), leg swelling, abdominal thrill, machinery-type abdominal bruit, renal failure, and peripheral ischemia.

Aortoduodenal fistulae

Finally, an AAA may rupture into the fourth portion of the duodenum. These patients may present with a herald upper gastrointestinal bleed followed by an exsanguinating hemorrhage.

Indications

Even patients who do not have symptoms from their abdominal aortic aneurysms (AAAs) may eventually require surgical intervention because the result of medical management in this population is a mortality rate of 100% over time due to rupture. In addition, these patients have a high likelihood of limb loss from peripheral embolization.

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The decision to treat an unruptured abdominal aortic aneurysm (AAA) is based on operative risk, the risk of rupture, and the patient’s estimated life expectancy. In 2003, the Society for Vascular Surgery (SVS) published a series of guidelines for the treatment of AAAs based on these principles.[4] The operative risk is based on patients’ comorbidities and hospital factors.

Abdominal ultrasonography can provide a preliminary determination of aneurysm presence, size, and extent. Rupture risk is in part indicated by the size of the aneurysm (see Table 1, below).

Table 1. Abdominal Aortic Aneurysm Size and Estimated Annual Risk of Rupture

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With AAAs smaller than 5.5 cm, elective repair has not been shown to improve survival. [6] Prospective studies have concluded that following aneurysms larger than 5.5 cm with serial ultrasounds or CT scans is safe. A slightly higher rupture rate in women exists, and this threshold may be lower.

Thus, the decision to repair an AAA is a complex one in which the patient must play an important role. In some very elderly patients or patients with limited life expectancy, aneurysm repair may not be appropriate. In these patients, the consequences of rupture should be frankly discussed. If rupture occurs, no intervention should be performed.

Contraindications

Contraindications for operative intervention of abdominal aortic aneurysms (AAAs) include severe chronic obstructive pulmonary disease (COPD), severe cardiac disease, active infection, and medical problems that preclude operative intervention. These patients may benefit best from endovascular stenting of the aneurysm.

In many patients, the decision to operate is a balance between risks and benefits. In an elderly patient (>80 y) with significant comorbidities, surgical repair may not be indicated. However, the decision to intervene should not be based on age alone, even with rupture. The decision is best based on the patient's overall physical status, including a positive attitude toward the surgery.

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Patients with known cancer that has an indolent course (eg, prostate cancer) may merit aneurysm repair if their estimated survival is 2 years or longer.

Laboratory studies

A complete blood count with differential is used to assess transfusion requirements and the possibility of infection. A metabolic panel (including kidney and liver function tests) is indicated for ascertaining the integrity of renal and hepatic function, in order to assess operative risk and guide postoperative management.

Type and crossmatch blood to prepare for the possibility of transfusion, including clotting factors and platelets.

Because synthetic material is used in the intervention, assess and eliminate potential foci of infection preoperatively by urinalysis.

Assessment of pulmonary function is part of the preoperative workup, to determine operative risk and postoperative care. Patients who can climb a flight of stairs without excessive shortness of breath generally do well. If the patient's pulmonary status is in question, blood gas measurement and pulmonary function tests are helpful.

Chest radiography

Chest radiography is used to gain a preliminary assessment of the status of the heart and lungs. Concurrent pulmonary or cardiac disease may need to be addressed prior to treating the aneurysm.

Computed tomography

Preoperative CT scanning helps more clearly define the anatomy of the aneurysm and other intra-abdominal pathologies. Nonenhanced CT scanning is used to size aneurysms. [7] Although sizing the aneurysm is important, the anatomic relationships important to surgery are also determined. These include the location of the renal arteries, length of the aortic neck, condition of the iliac arteries, and anatomic variants such as a retroaortic left renal vein or horseshoe kidney.

Enhanced spiral CT scanning of the abdomen and pelvis with multiplanar reconstruction and CT angiography is the test of choice for preoperative evaluation for open and endovascular repair (see the image below).

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Of AAA cases, 10-20% have focal outpouchings or blebs visible on CT scans that are thought to contribute to the potential for rupture. The wall of the aneurysm becomes laminated with thrombus as the blebs enlarge. This can give the appearance of a relatively normal intraluminal diameter in spite of a large extraluminal size.

Magnetic resonance angiography

Magnetic resonance angiography (MRA) is quickly replacing the traditional angiographic assessment of aneurysms. The study provides excellent anatomical definition and 3-dimensional assessment of the problem. Gadolinium-enhanced MRA can provide excellent images, even though regional variations in quality are reported.

Conventional angiography

Angiography remains the criterion standard for the diagnosis of AAA, and it is indicated in the presence of associated renal or visceral involvement, peripheral occlusive disease, or aneurysmal disease. Angiography is also essential with any renal abnormality (eg, horseshoe kidney, pelvic kidney). (See the image below.)

Angiography is used to diagnose the renal area. In this instance, an endoleak represented continued pressurization of the sac.

Echocardiography

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Because of the fluid shift involved during the operative repair of AAA, cardiac function should be assessed using echocardiography. By ascertaining the ejection fraction of the patient, the operative intervention can be planned and cardiac protective measures can be instituted as needed. This study is particularly indicated in patients with a history of CHF or known cardiac enlargement.

Pulmonary assessment

Assessment of pulmonary function is of paramount importance in these patients. Because surgical intervention requires an abdominal incision, preoperative assessment of the patient's pulmonary status allows for tailored postoperative care.

Cardiac assessment

Assess cardiac status in all patients with vascular disease. If one vascular bed is involved with an atherosclerotic process, then consider that others also may be involved. Electrocardiography findings provide a baseline assessment of cardiac rhythm and old disease processes.

A stress test can be performed to uncover unsuspected cardiac ischemia. Significant coronary disease may need to be addressed before the AAA can be repaired.

Treatment & Management

Abdominal aortic aneurysms (AAAs) are typically repaired by an operative intervention. The possible approaches are the traditional open laparotomy, newer minimally invasive methodologies, or by the placement of endovascular stents.

Preoperative details

Preoperatively, obtain a careful history and perform a physical examination and laboratory assessment. These basic assessments provide the information for estimating perioperative risk and life expectancy after the proposed procedure.

Carefully consider whether the patient's current quality of life is sufficient to justify the operative intervention. In the case of elderly persons who may be debilitated or may have mental deterioration, this decision is made in conjunction with the patient and family.

Once the decision is made, identify comorbidities and risk factors that increase the operative risk or decrease survival. Ascertain the patient's activity level, stamina, and stability of health. Perform a thorough cardiac assessment tailored to the patient's history, symptoms, and results from preliminary screening tests such as the electrocardiogram and stress test.

Because COPD is an independent predictor of operative mortality, assess lung function by performing a room-air arterial blood gas measurement and pulmonary function tests. In patients with abnormal test results, preoperative intervention in the form of bronchodilators and pulmonary toilet often can reduce operative risks and postoperative complications.

Preoperative intravenous antibiotics (usually a cephalosporin) are administered to reduce the risk of infection. Arranging for appropriate intravenous accesses to accommodate blood loss, arterial pressure

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monitoring through an arterial line, and Foley catheter placement to monitor urine output are routine preparations for surgery.

For patients at high risk because of cardiac compromise, a Swan-Ganz catheter is placed to assist with cardiac monitoring and volume assessment. Transesophageal echocardiography can be useful to monitor ventricular volume and cardiac wall motion and to provide a guide with respect to fluid replacement and pressor use.

Prepare for blood replacement. The patient should have blood available for transfusion. Intraoperative Cell Saver use and preoperative autologous blood donation have become popular.

Maintain a normal body temperature during the operative intervention to prevent coagulopathy and maintain normal metabolic function. To prevent hypothermia, place a recirculating, warm forced-air blanket on the patient and warm any intravenous fluids and blood before administration.

In summary, the following are standard preoperatively:

Type and crossmatch blood Administer prophylactic antibiotics (cefazolin , 1 g intravenous piggyback) Insert a Foley catheter Establish large-bore intravenous access Monitor central venous pressure or establish Swan-Ganz catheterization (if indicated) Prepare the skin from the nipples to the mid thigh Administer general anesthesia (with or without epidural anesthesia) Cell Saver use has become popular Insert a nasogastric tube

Intraoperative details

The aorta may be approached either transabdominally or through the retroperitoneal space. Approach juxtarenal and suprarenal aortic aneurysms from the left retroperitoneal space.

Self-retaining retractors are used. Keep the bowel warm and, if possible, not exteriorized. The abdomen is explored for abnormalities (eg, gallstones, associated intestinal or pancreatic malignancy).

Depending on the patient's anatomy, the aorta can be reconstructed with a tube graft, an aortic iliac bifurcation graft, or an aortofemoral bypass.

For proximal infrarenal control, first identify the left renal vein. Occasionally (< 5% of cases), patients may have a retroaortic vein. In this situation, take care when placing the proximal clamp. Division of the left renal vein is usually required to clamp above the renal arteries.

Regarding pelvic outflow, in most instances, the inferior mesenteric artery is sacrificed. Therefore, to prevent colon ischemia, make every attempt to restore at least one hypogastric (internal iliac) artery perfusion. If the hypogastric arteries are sacrificed (associated aneurysms), reimplant the inferior mesenteric artery.

For supraceliac aortic control, first divide the ligaments to the left lateral segment of the liver and then retract the segment. The crura of the diaphragm are separated, and the aorta is bluntly dissected. Supraceliac control is recommended for inflammatory aneurysms.

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The aorta is reconstructed from within using PTFE or Dacron. The aneurysm sac is closed, and the graft is put into the duodenum to prevent erosion.

Special considerations

Inflammatory aneurysms require supraceliac control, minimal dissection of the duodenum, and balloon occlusion of the iliac arteries. In patients with inflammatory aneurysms or large iliac artery aneurysms, identify the ureters; occasionally, ureteral stents are recommended in patients with inflammatory aneurysms.

Prevention of distal embolization

The patient is heparinized (5000 U intravenously) prior to aortic cross-clamping. If significant intraluminal debris, juxtarenal thrombus, or prior peripheral embolization is present, the distal arteries are clamped first, followed by aortic clamping.

Before restoring lower extremity blood flow, both forward flow (aortic) and back flow (iliac) are allowed to remove debris. The graft is also irrigated to flush out debris.

The colon is inspected prior to closure, and the femoral arteries are palpated. Before the patient leaves the operating room, determine lower extremity circulation. If a clot was dislodged at the time of aortic clamping, it can be removed with a Fogarty embolectomy catheter. Heparin reversal is not usually required.

Postoperative Details

Fluid shifts are common following aortic surgery. Fluid requirements may be high in the first 12 hours, depending on the amount of blood loss and fluid resuscitation in the operating room. Monitor the patient in the surgical intensive care unit for hemodynamic stability, bleeding, urine output, and peripheral pulses. A postoperative electrocardiogram and chest radiograph are needed. Prophylactic antibiotics (eg, cefazolin at 1 g) are administered for 24 hours.

The patient is seen in 1-2 weeks for suture or skin staple removal, then yearly thereafter.

Complications

The following are potential complications of abdominal aortic aneurysms:

Death - 1.8-5% if elective and 50% if ruptured Pneumonia - 5% Myocardial infarction - 2-5% Groin infection - Less than 5% Graft infection - Less than 1% Colon ischemia - Less than 1% if elective and 15-20% if ruptured Renal failure related to preoperative creatinine level, intraoperative cholesterol embolization, and

hypotension Incisional hernia - 10-20% Bowel obstruction Amputation from major arterial occlusion Blue toe syndrome and cholesterol embolization to feet

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Impotence in males - Erectile dysfunction and retrograde ejaculation (>30%) Paresthesias in thighs from femoral exposure (rare) Lymphocele in groin - Approximately 2% Late graft enteric fistula

Endovascular Stent GraftsEndovascular stent grafts for the treatment of abdominal aortic aneurysm (AAA) are a less invasive form of treatment. Patients are discharged 1-2 days following surgery. The graft is placed through 2 small incisions. In September 2000, 2 grafts were approved by the US Food and Drug Administration (FDA). Since then, several more devices have received FDA approval.[8] Recently, the FDA has recommended careful follow-up because of persistent endoleaks and late ruptures.

In some instances, endoleaks represent continued pressurization of the sac (see image below). Aneurysm sacs may also demonstrate elevated pressure despite the absence of a demonstrable endoleak. This has been described as "endotension."

Persistently elevated aneurysm sac pressure, whether secondary to endoleak or endotension, is worrisome because it may progress to AAA rupture. Early data demonstrated a need for secondary interventions, via endovascular techniques, in as many as 10% of patients per year following endovascular aneurysm repair, compared with 2% in the first 5 years for open repair. Improvement has been made in the rate of secondary interventions following endovascular repair, but long-term durability has yet to be determined.

Angiography is used to diagnose the renal area. In this instance, an endoleak represented continued pressurization of the sac.

Informing the patient about these potential problems is important prior to implanting these grafts. In addition, patients with endografts require follow-up evaluation with serial CT scanning on a schedule that demands more office visits than are required for patients who receive conventional grafts.

Currently, endovascular repair is advocated for patients at increased risk for open aneurysm repair, but until results from randomized controlled trials are available, patient preference is the strongest determinant in deciding between endovascular and open aneurysm repair.

IMAGES:

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Endoaneurysmorrhaphy.

Endovascular grafts

Aneurysm with retroperitoneal fibrosis and adhesion of the duodenum and fibrosis.

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Radiograph shows calcification of the abdominal aorta. The left wall is clearly depicted and appears aneurysmal; however, the right wall overlies the spine.

The lateral view clearly shows calcification of both walls. Abdominal aortic aneurysm can be diagnosed with certainty.

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CT demonstrates an abdominal aortic aneurysm. The aneurysm was noted during workup for back pain, and CT was ordered after the abdominal aortic aneurysm was identified on radiographs. No evidence of rupture is seen.

Arteriogram demonstrates an infrarenal abdominal aortic aneurysm. This arteriogram was obtained in preparation of an endovascular repair of the aneurysm.

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Lateral arteriogram demonstrates an infrarenal abdominal aortic aneurysm. Demonstration of the superior mesenteric artery, inferior mesenteric artery, and celiac artery on the lateral arteriogram in important to completely evaluate the extent of the aneurysm.

Arteriogram after successful endovascular repair of an abdominal aortic aneurysm.

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Ultrasonogram of a patient with an abdominal aortic aneurysm. This aneurysm was best visualized on a transverse or axial image. This patient underwent a conventional abdominal aortic aneurysm repair.

MRI of a 77-year-old man with leg pain believed to be secondary to degenerative disk disease. During evaluation, an abdominal aortic aneurysm was discovered.

Abdominal Aortic Aneurysm Rupture Imaging

Disponível em: http://emedicine.medscape.com/article/416397-overview#a19

Overview

Abdominal aortic aneurysms (AAAs) are segmental dilatations of the aortic wall that cause the vessel to be larger than 1.5 times its normal diameter or that cause the distal aorta to exceed 3 cm. These can continue to expand and rupture spontaneously, exsanguinate, and cause death. AAA rupture is an important cause of unheralded deaths in people older than 55 years. (See also Abdominal Aortic Aneurysm Imaging and Abdominal Aortic Aneurysm.)

See the AAA image below.

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Contrast-enhanced abdominal CT in an elderly patient who presented with severe back pain but was hemodynamically stable. CT reveals an abdominal aortic aneurysm (AAA) with eccentric mural thrombus. A disruption of the calcific rim of the AAA toward the left quadrant appears with adjacent isoattenuating soft tissue anterior to the left psoas muscle. Clinical and radiologic findings are consistent with a diagnosis of contained AAA rupture with left retroperitoneal hematoma.

Guidelines and recommendations

The following are screening recommendations by Kaiser Permanente for AAA by ultrasonography in the general population[1] :

One-time screening for AAA by ultrasonography is recommended in men aged 65 to 75 years.

It is an option to limit AAA screening to men aged 65 to 75 years who have never smoked.

Routine screening for AAA in women is not recommended.

The following are screening recommendations by Kaiser Permanente for AAA in adults with a family history of AAA[1] :

For men aged 50 and older with a known positive family history of aortic aneurysm in a first-degree relative, AAA screening is recommended.

The guideline development team makes no recommendation for or against screening women with a positive family history of AAA.

Systematically collecting information on aortic aneurysm family history is not recommended.

Preferred examination

Ideally, in a hemodynamically stable patient, nonenhanced and enhanced helical or spiral CT of the thorax, abdomen, and pelvis should be expeditiously performed. This examination provides key information about the extent of aneurysmal disease, and it can be used to confirm and localize the site of rupture, as shown in the image below.

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Contrast-enhanced abdominal CT in an elderly patient who presented with severe back pain but was hemodynamically stable. CT reveals an abdominal aortic aneurysm (AAA) with eccentric mural thrombus. A disruption of the calcific rim of the AAA toward the left quadrant appears with adjacent isoattenuating soft tissue anterior to the left psoas muscle. Clinical and radiologic findings are consistent with a diagnosis of contained AAA rupture with left retroperitoneal hematoma.

In the patient with an unstable presentation, an emergency operation is indicated. Time may permit only rapid bedside ultrasonography (US) and Doppler study of abdominal aorta and iliac arteries to confirm the presence of aneurysms.

The maximal aneurysm diameter is adequately assessed by using B-mode ultrasonography, CT scanning, and MRI.{{Rref20}[2, 3, 4] Aortography reveals only the lumen of the abdominal aortic aneurysm because laminated clot obscures the outer limit of the aneurysm wall. Therefore, it often causes underestimation of the true aortic diameter.

Screening

The U.S. Preventive Services Task Force (USPSTF) recommends one-time screening for AAA by ultrasonography in men aged 65 to 75 who have ever smoked (B recommendation; at least fair evidence). American College of Cardiology/American Heart Association guidelines include a class I, level B recommendation that patients with infrarenal or juxtarenal AAAs measuring 5.5 cm or larger undergo repair to eliminate the risk of rupture, and a class I, level A recommendation that patients with infrarenal or juxtarenal AAAs measuring 4.0 to 5.4 cm in diameter be monitored by ultrasound or CT scans every 6 to 12 months to detect expansion. [5] For women, the USPSTF issued a grade D recommendationagainstroutine screening for AAA, since the available data show a low prevalence of large AAAs and no reduction in mortality in females.[6]

In the Cardiovascular Health Study, aneurysm dilatation of 3 cm or greater on a single screening ultrasound exam identified 68% of all AAA repairs over the next 10 years and 6 of the 10 AAA-related deaths in 4% of the total population; dilatation of 2.5 cm or more identified 91% of all AAA repairs and 9 of the 10 deaths in 10% of the total population.[7]

Postoperative follow-up

Little objective information is available to guide long-term surveillance after successful emergency repair of AAA rupture. There are data from the prospective Canadian Aneurysm Study in the 1990s, which found that in approximately 15% of patients who underwent elective open repairs, another thoracic or abdominal aneurysm was subsequently detected; a similar percentage of patients had significant iliac aneurysms (following tube AAA graft).[8] These findings led the investigators to recommend routine CT from thorax to pelvis after 5 years.

Patients who have undergone endovascular aneurysm repair require more frequent and lifelong follow-up. Imaging studies are typically performed 1, 6, and 12 months after EVAR and annually thereafter. [9] CT angiography is most commonly used.[10]However, studies have found contrast-enhanced ultrasound to be comparable to CT angiography.[11, 12] MRI and magnetic resonance angiography have also been used for surveillance, as has digital subtraction angiography, although this technique is now not as necessary. [10, 13] Intermittent surveillance of the thoracic aorta should be performed, particularly in patients with preexisting aortic ectasia or dilatation elsewhere or for patients (or their siblings) with connective tissue disease (eg, Marfan or Ehlers-Danlos syndromes), bicuspid aortic valve, or familial aortopathies.

Limitation of techniques

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CT or MRI can rapidly provide detailed information about the blood vessels and their surrounding structures for treatment planning; the choice between them can be based on which is faster locally. Occasionally, however, these examinations may require too much time for them to be suitable for use in patients whose condition is unstable. Also, when contrast material is used in conjunction with CT to delineate blood-filled structures, it poses a risk of acute renal failure, particularly in hypovolemic elderly patients who may already have baseline nephrosclerosis or diabetic nephropathy.

Sonography is a quick and convenient modality, but it is much less sensitive and specific for the diagnosis of aneurysmal rupture. The absence of sonographic evidence of rupture does not rule out this entity if clinical suspicion is high.

Patient education

For patient education information, see the Circulatory Problems Center , as well asAortic Aneurysm .

Radiography

A curvilinear calcified rim, often to the left of the midline, is apparent on some plain abdominal radiographs. This may be the first clue to abdominal aortic aneurysms in patients who present with unexplained abdominal pain but are temporarily hemodynamically stable. Lateral radiography can depict large AAAs on the basis of calcification in the wall. This finding can be seen in more than half of patients.

Calcification identified on lateral instant verterbral assessment images has been identified as a risk factor for rupture of an abdominal aortic aneurysm.[14]

The degree of confidence is low. Suspicious findings on the abdominal radiograph should be confirmed by using another imaging modality.

Mural calcification can be radiographically inapparent and lead to a false-negative finding in as many as half of small AAAs. Magnification errors are also possible, leading to overestimation or underestimation of AAA diameter.

Computed Tomography

General findings

Unlike angiography, CT and MRI provide information about the wall of the aorta, and they delineate the presence of thrombus.[15] They provide detail about surrounding abdominal structures and their relationship to the abdominal aortic aneurysms. Perianeurysmal fibrosis, venous anomalies (eg, retroaortic left renal vein, circumaortic venous collar), and horseshoe kidney are reliably demonstrated. However, these modalities (particularly MRI) are currently time and labor intensive, and they are not suitable for use in patients in unstable condition.

The degree of confidence is high. Soft tissue attenuation outside of the abdominal aorta can be related to changes secondary to an inflammatory process. These findings may possibly be related to pathology from other abdominal viscera. The sensitivity and specificity for the detection of AAA rupture decreases if a nonenhanced study is performed.

Helical or spiral CT angiography (CTA) allows reasonable visualization of branches of the aorta in the context of surrounding structures. Postprocessing of images and 3-dimensional reconstruction are possible.

Magnetic resonance angiography (MRA) has the advantage of eliminating the need for potentially nephrotoxic contrast agents and ionizing radiation, but its acquisition speed, cost, and image quality still need to be improved. Ferromagnetic implants or severe claustrophobia also can preclude use of MRI and/or MRA.

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Findings that indicate possible AAA rupture include soft tissue hyperintensity outside the aortic wall, an indistinct aortic wall, thinning or fracture of a calcified aortic wall segment, penetration of a hematoma into the leaves of the mesentery, or contrast extravasation into the psoas muscle or retroperitoneum.

The nonenhanced study shows a mass or collection that extends into the perirenal spaces or, less commonly, into the pararenal spaces. The AAA is often obscured or anteriorly displaced. Other possible findings include a focally indistinct aortic margin (usually the site of rupture), enlargement or obscuration of the psoas muscle, and anterior displacement of the kidney.

Patients with relatively small AAAs may experience a sealed rupture. Characteristic findings in such cases are a draped aorta and adjacent vertebral erosion.[16]

Apter et al determined that a sealed rupture of an AAA can occur in relatively small aneurysms and that a draped aorta and adjacent vertebral erosion are characteristic CT signs of such a rupture. [17] They reviewed the CT scans of 6 patients with a sealed rupture of an AAA and those reported in the literature for aneurysm size, presence of a draped aorta, and adjacent vertebral erosion.

CT-specific findings

Freshly extravasated blood typically has a high CT attenuation value, whereas an isoattenuating or hypoattenuating hematoma signifies that a leak is days or weeks old.

When intraluminal blood dissects into a thrombus and comes in contact with a weakened aortic wall, the risk of rupture is increased. The crescent sign is of high density within the AAA mural thrombus or wall, which is thought to represent an intramural hematoma.[18] The attenuation should be greater than the aortic lumen on unenhanced scans and should be greater than the psoas muscle on enhanced scans. This was found to be a sign of acute or impending rupture in a retrospective series, with a sensitivity, specificity, and positive predictive value of 77%, 93%, and 53%, respectively.[19] In addition, thrombus fissuration can be seen from the same process. This sign can be observed on enhanced CT studies as linear contrast infiltration from the aortic lumen into intramural thrombus[20] .

Periaortic fibrosis outside of the subintimal calcification is suggestive of inflammatory aneurysm.

Postoperative findings

Follow-up imaging after EVAR is used to monitor the status of the repair. Endoleak (persistent blood flow within the aneurysmal sac) is the most common complication of EVAR, occurring in approximately one quarter of patients. Endoleaks are classified into 5 types, according to the source of blood flow into the sac[10] :

Type I: from incomplete or ineffective sealing at the graft attachment site; this is usually an early complication.

Type II: from retrograde blood flow from aortic collateral vessels.

Type III: from disruption of the graft.

Type IV: from porosity of the graft.

Type V: expansion of the aneurysm without an obvious endoleak, termed endotension.

Techniques

In a study of multidetector-row CTA in patients with aortic aneurysms, Kubo et al found that optimal contrast enhancement was produced by using 75 mL of contrast medium followed by 20 mL of saline flush.

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Iezzi et al found that in patients undergoing multidetector-row CTA for surveillance after EVAR, the delayed phase should be acquired 300 seconds after injection of contrast medium.

In contrast, Hong et al reported that endoleaks detected only in the delayed phase of CT resolved spontaneously; they therefore suggested eliminating the delayed phase of acquisition, so as to minimize radiation exposure.[21]

Magnetic Resonance Imaging

MRI is a valuable alternative to CT in patients with renal insufficiency in whom contrast material–induced nephropathy is a concern.[22] MRI is also helpful in further delineating the aorta in the context of a large retroperitoneal collection that obscures the borders between adjacent structures, as well as laminated clot or atherosclerotic debris on the aneurysmal wall.

Gadolinium-based contrast agents have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans. NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.

The degree of confidence is high. Soft tissue intensity changes outside of the abdominal aorta can be related to changes secondary to an inflammatory process. These findings might actually be related to pathology from other abdominal viscera.

Ultrasonography

Unlike most other modalities (eg, aortography, CT, MRI), abdominal US can be performed expeditiously and at the bedside.[23] For partially encapsulated hematomas, a hypoechoic or anechoic para-aortic space-occupying lesion may be detected.

Color-flow Doppler can aid in detecting the site of leak or extravasation, although adjustment to low-velocity scales may be necessary to register leaks with low flow rates. Contrast-enhanced US has been advocated for surveillance after EVAR; the recommended dose of contrast medium for such studies is 2.4 mL.[11, 12]

The degree of confidence is high for the detection of abdominal aortic aneurysms and low for the detection of AAA rupture. Although duplex US is competitive with CT or MRI for the detection of an AAA (>95% sensitivity), the visualization of surrounding structures is poor, and the sensitivity and specificity for rupture are low. For detection of endoleak after aneurysm repair, contrast-enhanced US imaging was shown to be significantly more accurate than unenhanced US studies (89.3% vs 63.1%).[11]

Conditions that limit the detection and measurement of AAAs or its branches include excessive bowel gas, obesity, and recent abdominal surgery. If a ruptured AAA is clinically suspected, other diagnostic modalities should be pursued even if the sonographic results are negative.

Angiography

Digital subtraction angiography (DSA) provides high-spatial-resolution images of the lumen of the vascular tree and permits the quantification of significant stenoses in renal, mesenteric, and iliofemoral arteries. [24] CT also depicts aberrant vessels well.

Angiographic findings of abdominal aortic aneurysms rupture include the following:

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Contained rupture: Circumscribed extraluminal contrast enhancement is present. Additional views (oblique or lateral) may be required to detect this finding.

Leaking aneurysm: Frank extravasation of contrast material with poor washout is observed. This is rarely demonstrated because the patients are typically in unstable condition and are transported directly to the operating room.

Displacement of the visceral arteries or kidneys and ureters: Sometimes, this finding can be evident in larger collections of blood.

Contrast enhancement: Contrast material may flow into these structures from the AAA in the rare circumstance of rupture into the GI tract or IVC.

The degree of confidence is low. Angiography is rarely used in the setting of suspected rupture and is relatively contraindicated except when endovascular stent grafting is planned because this is an experimental procedure. However, unusual manifestations of AAA rupture, such as into the IVC or GI tract (aortoenteric fistula) is demonstrated well with angiography.

The absence of extraluminal contrast cannot be used to totally rule out a small leak or stabilized rupture. If the degree of suspicion is high, a tomographic imaging modality should be performed.

Laminated or mural thrombus may give the false arteriographic impression that no AAA is present. A calcified aortic shell and the absence of lumbar arteries are clues to the presence of an aneurysm.

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