Aneurysms are diagnosed when the aorta has an abnormal segment with a diameter greater than 50% of the adjacent normal aorta. More commonly, aneurysms are diagnosed when the thoracic aorta exceeds 5
cm in diameter and the abdominal aorta exceeds 3
cm [
1].
AD occurs when an intimal tear develops, allowing blood to penetrate the aortic wall, dissects longitudinally through the media, and forms a false lumen [
6]. Systemic hypertension is regarded as the major predspsing factor to the development of AD [
7]. Another important potential etiology is cystic medial necrosis which can be seen with Marfan's disease. In fact, Marfan's disease accounts for the majority of dissections in patients younger than 40 [
7]. Other associations with AD include chromosomal aberrations (Turner's or Noonan's syndrome), bicuspid aortic valve, Ehlers-Danlos syndrome, aortic coarctation, pregnancy, cocaine use, and prior catheterization [
8]. Atherosclerosis is not an independent risk factor for AD.
AADs are characterized by symptoms that are present for less than 14 days; in chronic dissections, the symptoms are present for a longer period [
9].
The Stanford classification system for dissections is based on the need for surgical intervention. Stanford type A dissection involves the ascending thoracic aorta and the dissection flap may extend into the descending aorta. Type A dissections account for 60%–70% of cases [
10] and typically require urgent surgical intervention to prevent extension into the aortic root, pericardium, or coronary arteries [
11]. If untreated, type A dissections are associated with a mortality rate of over 50% within 48 hours [
12].
Stanford type B dissection involves the descending thoracic aorta distal to the left subclavian artery and accounts for 30%–40% of cases [
10]. Management requires medical treatment of hypertension, unless there are complications due to extension of the dissection (e.g., end-organ ischemia or persistent pain) that would necessitate surgical intervention.
Patients typically present with complains of an abrupt onset of chest pain. The location of the pain can migrate as the dissection increases in size. Physical examination findings are limited in many dissection cases. Aortic murmurs can result from proximal dissection. If the dissection migrates proximally into the pericardium, resulting symptoms of tamponade can occur. The dissection can also lead to occlusion of the coronary arteries that can lead to acute myocardial infarctions. These do routinely occur in approximately 5% of Stanford type A dissections [
13].
Chest X-rays are useful in the initial evaluation but are not specific for diagnosis. Abnormalities on X-ray occur in 60%–90% of ADs, but if they are not present, the diagnosis cannot be excluded [
14]. A widened mediastinum can be present in both Stanford type A and B dissections.
Adding plasma d-dimer measurement to the clinical assessment allows for confident exclusion of AD. Elevated d-dimer, a breakdown product of cross-linked fibrin, is a highly sensitive but nonspecific marker for AD, present in virtually all cases [
15]. Electrocardiographic (ECG) findings may be normal unless there has been compromise of the coronary arteries giving rise to an acute coronary syndrome with associated findings on the ECG tracing.
CT is highly sensitive and specific for the detection and characterization of the AD. On unenhanced images, thickening of the aortic wall and displacement of intimal calcifications may be seen. CT findings of the AD center on the detection of intimomedial flap which requires intravenous contrast. When any form of intravascular treatment is planned, distinguishing the true lumen from the false lumen becomes important. In most cases, the true lumen is the one that is continuous with the unondissected portion of the aorta [
16]. A larger lumen size on axial imaging is a feature suggestive of the false lumen. Additional CT signs that may be helpful include the beak sign and the aortic cobweb sign. The beak sign refers to an acute angle with the intimomedial flap and the vessel wall and is a feature of the false lumen. The cobweb sign refers to strands of medial tissue that may be seen within the false lumen. When multiple phases of contrast enhancement are obtained, the false lumen tends to enhance late and washes out in a delayed fashion.
CT is the most frequently used diagnostic imaging modality for the initial evaluation of patients with suspected AD [
17]. Multidetector row CT is the most rapid diagnostic test for AD, with data acquisition accomplished in less than 30 seconds [
18]. Reported sensitivities and specificities range from 79% to 100% for CT, but early studies should be interpreted with caution as this technology is evolving rapidly [
18,
19]. Angiography and echocardiography are often implemented to characterize the lesion in more details. Other imaging modalities can include magnetic resonance imaging (MRI) and ultrasound (US), but they are not as useful in the initial evaluation of aortic dissection. Modern multisection CT allows rapid image acquisition and data reconstruction and aids in treatment planning. It helps differentiate type A from type B dissection, may localize the intimal entry site and helps assess branch-vessel involvement and compromise and the relationship of the branch vessels to the true or false lumen. This information aids in planning treatment with either root replacement, intravascular stent placement, or fenestration [
20].
Triple-rule-out CT is a new protocol used to assess the aorta, coronary arteries, pulmonary arteries, and the middle and lower portions of the lungs during a single scan with use of several optimally timed boluses of contrast material and ECG gating in patients who are at low risk for acute coronary syndrome. The aim is to minimize the contrast material dose and radiation exposure while achieving optimal image quality, providing coronary artery image quality equivalent to that of dedicated coronary CT angiography; pulmonary artery image quality equivalent to that of dedicated pulmonary CT angiography and high quality images of the thoracic aorta without pulsation artifact. In an appropriately selected emergency department patient population, triple-rule-out CT can safely eliminate the need for further diagnostic testing in over 75% of patients [
21].
