Aortic Dissection: A Double-Barreled Threat
The sudden onset of tearing chest pain may be the initial presentation of an acute aortic dissection. A small tear in the aortic intima can quickly split the aortic wall (yellow arrow) and create a swiftly expanding false lumen, which may be rapidly fatal even with prompt medical treatment.
In the United States, aortic dissection occurs most frequently in patients with hypertension.[1,2] The incidence of acute aortic dissection has been estimated to range from 3.5 to 16.3 per 100,000 person-years, with a higher incidence among older individuals.[1,2] The true prevalence of aortic dissection is difficult to determine, however, with most estimates having been based on autopsy studies, in which the condition was found in 1-3% of cadavers.[3] Approximately 2000 new cases are reported each year in the United States.[2]
Aortic Dissection: A Double-Barreled Threat
The aorta is composed of three layers: the intima (black arrow), the media (yellow bar), and the adventitia (red arrow). These layers contain variable quantities of collagen, elastin, and smooth-muscle cells. In a process called cystic medial necrosis, these components break down with age and are replaced with basophilic ground substance. These changes place the aorta at increased risk for developing tears that can propagate into dissections.
Cystic medial necrosis was first described in 1929; although the name has generally been retained, research has demonstrated that the process behind it is neither cystic nor necrotic. As a result, it is sometimes referred to as cystic medial degeneration.[4] Systemic disease processes that weaken the vascular wall, such as systemic lupus erythematosus (SLE), will increase the risk of aortic dissection.[5] In the above image, a plane of dissection is noted in the middle of the media (green arrow), with hematoma filling the separated layer.
Aortic Dissection: A Double-Barreled Threat
Aortic dissections develop via a tear in the intimal wall of the aorta (shown), which allows for formation and propagation of a subintimal hematoma. This results in the existence of a true lumen and a false lumen within the aorta, which is colloquially called a double-barreled aorta. The total cross-sectional area occupied by the two lumina is unchanged, such that as the false lumen grows, a decreasing amount of blood is able to flow through the true lumen. If there is a tear in both the proximal and distal portions of the dissection, then blood may be able to flow through both the true and the false lumen, though the flow itself will not be laminar. If the dissection occurs within the pericardial space, then cardiac tamponade may develop.
Aortic Dissection: A Double-Barreled Threat
There are two major classification schemes for aortic dissections: DeBakey and Stanford (the newer of the two). Classification, as follows, is based on the distribution of the ascending and descending aortic arch components of the dissection[6-8]:
- Image A: Stanford A or DeBakey I
- Image B: Stanford A or DeBakey II
- Image C: Stanford B or DeBakey III
- Image D: Stanford A or DeBakey I, with an additional entry tear in the descending thoracic aorta
Note that a primary arch dissection does not fit neatly into either classification scheme. Differentiating ascending from descending involvement helps to dictate operative versus nonoperative management. As a result, the Stanford classification scheme is more commonly used.[9] However, the Society for Vascular Surgery recommends additional anatomic descriptions based on the extent of the acute aortic pathology (ie, location of the entry tear).[10]
Aortic Dissection: A Double-Barreled Threat
"...spontaneous tear of the arterial coats is associated with atrocious pain, with symptoms, indeed, in the case of the aorta of angina pectoris and many instances have been mistaken for it." (William Osler, 1910)
The classic presentation of an acute aortic dissection is the sudden onset of severe, tearing chest pain.[11] Unfortunately, no single sign or symptom can positively identify a dissection, and roughly one third of dissections are missed on initial evaluation. Common presenting signs and symptoms include the following:
- Anterior chest pain
- Jaw pain
- Interscapular ripping pain
- Syncope
- Stroke symptoms
- Altered mental status
- Nonspecific neurologic symptoms
- Dyspnea
- Dysphagia
- Orthopnea
- Hemoptysis
In rare cases, aortic dissection may be painless, and this is associated with a higher mortality.[12] Careful elucidation of historical causes and risk factors (as listed above) must be undertaken. Physical examination findings include the following:
- Hypertension or hypotension
- Blood pressure differential between arms
- Neurologic deficits
- New cardiac murmurs
- Left-side decreased breath sounds
- Signs of cardiac tamponade
Aortic Dissection: A Double-Barreled Threat
All patients with a suspected aortic dissection should undergo electrocardiography (ECG), given the symptomatic overlap with myocardial infarction (MI). Dissections affecting the aortic root may interrupt blood flow to the coronary arteries. This can lead to ST-segment elevations, which must be carefully distinguished from those of acute MI to ensure that thrombolytics are not administered.[13,14] However, the most common ECG abnormality in aortic dissection is ST depression (arrows). The pattern of ECG changes is dependent on which coronary artery is interrupted.
Aortic Dissection: A Double-Barreled Threat
Chest radiography is frequently the first imaging modality used in the diagnosis of aortic dissection. Although it is neither sensitive nor specific, radiographs can be used to rule out other causes of chest pain (eg, pneumothorax). A chest radiograph is the most appropriate initial imaging study for suspected aortic dissection, provided that it can be performed quickly and without delaying definitive computed tomography (CT) scanning or magnetic resonance imaging (MRI), according to the American College of Radiology Appropriateness Criteria.[15] A widened mediastinum (yellow double-headed arrow) is the most common finding, especially in patients with an ascending aortic dissection. Additional findings include the following:
- Double aortic knob sign
- Inward displacement of aortic-wall calcification
- Tracheal displacement to the right
- Pericardial effusion
- Cardiac enlargement
- Left apical opacity
- Irregular aortic contour
- Pleural effusion
Alternative causes of mediastinal widening are lymphoma, a tumor, adenopathy, and an enlarged thyroid among others, but the time course of development is very different than that of a dissection.
Aortic Dissection: A Double-Barreled Threat
If an aortic dissection ruptures, blood can extravasate into the ipsilateral pleural space, causing hemothorax. Massive hemothorax will result in nearly complete opacification of the involved thorax on a chest radiograph (shown). This usually occurs in the left hemithorax after an acute type B dissection.[16] This is a rare late-stage finding that usually indicates that treatment will be unsuccessful, with the immediate goals being to stop bleeding and to stabilize the patient's hemodynamic status.[17]
Aortic Dissection: A Double-Barreled Threat
Ultrasonography (US) with Doppler evaluation allows discrimination between true (red arrow) and false (yellow arrow) lumina on the basis of the presence or absence of blood flow. US also allows functional evaluation of cardiac strain and identification of the site of intimal tear, extension of dissection, pericardial effusion, and aortic incompetence, especially in patients with ascending dissections. Transthoracic echocardiography can also assist in operative planning and identify patients with the highest complication risk.[18] The downside of US is its user-dependent nature, which may limit its availability in emergency settings.[15]
Aortic Dissection: A Double-Barreled Threat
The CT image shown demonstrates an aortic dissection of the mid-descending aorta with a dissection flap (white arrow). The right kidney shows normal perfusion (green arrow), whereas the left kidney is poorly perfused (yellow arrow).
CT, which provides another excellent means of rapidly and accurately diagnosing aortic dissections, has a sensitivity and specificity of over 90%.[11,19] A CT scan with contrast will show the extent of the dissection, identify the intimal flap separating the two aortic channels, and reveal offshoot vessels no longer receiving blood flow.[20]
Given its widespread availability and ready access in emergency departments, CT scanning is often the modality used to make definitive diagnosis. Specific CT protocols for suspected aortic dissection allows imaging during the early arterial phase when the aorta is maximally enhancing.[21]
Aortic Dissection: A Double-Barreled Threat
The above three-dimensional (3-D) color reconstruction of a CT angiogram (CTA) demonstrates a descending thoracic aortic dissection with severance of the left renal artery. A small stub of the renal artery can be seen at the yellow arrow. A cross-section of the aortic lumen is seen in the bottom right of the image.
A key advantage of CT is the ability to rapidly reformat the images into three dimensions. Once the domain of specialized workstations, reformatting software capability is now routinely included in many picture archiving and communication systems (PACS). This allows identification of secondary findings, such as occluded branch vessels, and facilitates surgical/interventional planning. Serial examinations can be used to observe patients after surgical treatment and evaluate for postoperative complications.
Aortic Dissection: A Double-Barreled Threat
The sagittal gradient-echo MRI shown here, obtained in early systole, demonstrates a jet of blood flowing through the intimal tear from the smaller anterior true lumen into the larger posterior false lumen (red arrow). The intimal flap (yellow arrow) is recognized as the linear structure of medium signal intensity that divides the true and false lumina.
MRI is another cross-sectional imaging modality that provides greater than 90% sensitivity and specificity in the detection of aortic dissection.[22] MRI can accurately identify the site of intimal tear, the extent of dissection, and the presence of aortic insufficiency. MRI can be performed without contrast material for patients with coexisting renal insufficiency. It is the preferred technique for serial evaluation of patients with chronic dissections and can provide excellent dynamic information to aid in diagnostic and therapeutic decision-making processes.[23] Unfortunately, MRI is not always available in the acute care setting. Additionally, getting patients to lie still for image acquisition can be difficult when they are experiencing severe chest pain.
Aortic Dissection: A Double-Barreled Threat
Angiography is the classic reference standard for the diagnosis of aortic dissection. A double-barrel aorta sign can be clearly seen with a true (yellow arrow) and false lumen (green arrow). Angiography allows real-time visualization and provides excellent information for surgical planning because the vessels coming off the aortic arch are easily assessed. In addition, the true and false lumina can be directly interrogated.
Angiography is highly operator-dependent, however; worsening of the existing dissection is a very real complication of the procedure. Currently, with the advent of modern CT and MRI scanners, the use of angiography is mostly limited to patients for whom nonsurgical interventions are indicated.
Aortic Dissection: A Double-Barreled Threat
This CT scan shows a curved planar reformat in a patient who underwent an aortic root graft repair (red arrows) for a chronic fenestrated aortic dissection (blue arrow).
Treatment of aortic dissection begins with aggressive blood pressure control to reduce the shearing forces of myocardial contractility, which cause progressive intimal tearing and dissection propagation. Nitroprusside, labetalol, and diltiazem are commonly used agents for managing blood pressure.[7] Narcotics and opiates are the preferred agents for controlling pain, thus reducing adrenergic stimulation.
Medical management is the treatment of choice for uncomplicated descending aortic dissections.[24] Emergency surgery is warranted for ascending or arch dissections, typically with the use of a Dacron graft to seal off the false lumen.[25] Operative mortality is generally around 10%. Endovascular therapy is an emerging and acceptable treatment for descending aortic dissection.[7,26]
Aortic Dissection: A Double-Barreled Threat
The CT images above were obtained from a patient with Marfan syndrome. Left: Contrast-enhanced scan showing the intimal flap due to dissection from the aortic root to the ascending aorta, innominate artery, and subclavian artery. Right: Enhanced reconstructed scan showing the path of dissection.
Because of the high operative mortality, research has been directed toward earlier detection and prevention of aortic dissections. Multiple serum biomarkers—including smooth-muscle myosin heavy chain, soluble elastin fragments, polycystin 1, and D-dimer—have shown promise for the early detection of acute aortic dissection. Research indicates that a combination of any two of these markers has a sensitivity of 94% and a specificity of 85%.[27]
Other research efforts are focusing on identifying patients with a genetic predisposition to aortic dissection, such as those with Ehlers-Danlos or Marfan syndrome. Patients with heritable risk factors for aortic dissection will benefit from supplemental screening and risk management.[28,29]
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