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Aortic Dissection

Lars Grimm, MD, MHS | October 30, 2017 | Contributor Information

image from Medscape.

An aortic dissection (arrows) is demonstrated above.

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 and create a swiftly expanding false lumen, which may be rapidly fatal even with prompt medical treatment.

Image from Medscape.

A false lumen (yellow arrow) is seen in the above aortic dissection.

In the United States, aortic dissection occurs most frequently in hypertensive patients. The incidence of acute aortic dissection has been estimated to be approximately 3 per 100,000 persons, but more recently the rate has increased to 6 per 100,000; the rate is up to 15 per 100,000 persons 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] Aortic dissection is the cause of an estimated one in 10,000 hospital admissions; [4] approximately 2000 new cases are reported each year in the United States. [5]

Image from Medscape.

The aorta is composed of 3 layers: the intima (black arrow), media (yellow bar), and adventitia (red arrow), which 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. [6] In this image, a plane of dissection is noted in the middle of the media (green arrow), with hematoma filling the separated layer.

Image from Medscape.

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 in such cases is colloquially called a double-barreled aorta. The total cross-sectional area occupied by the two lumens is unchanged, so 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 false lumen, although the flow itself will not be laminar. If the dissection involves the pericardial space, then cardiac tamponade may develop.

Image from Medscape.

There are two major classification schemes for aortic dissections: DeBakey and Stanford. Classification, as follows, is based on the distribution of the ascending and descending aortic arch components of the dissection [7,8,9]:

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 newer, Stanford classification scheme is more commonly used. [10]

...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 one sign or symptom can positively identify a dissection, and roughly one third are missed on initial evaluation. Common presenting signs and symptoms include anterior chest pain, jaw pain, interscapular ripping pain, syncope, stroke symptoms, altered mental status, nonspecific neurologic symptoms, dyspnea, dysphagia, orthopnea, and hemoptysis. Careful elucidation of historical causes and risk factors (as listed above) must be undertaken. Physical examination findings include hypertension or hypotension, a blood pressure differential between arms, neurologic deficits, new cardiac murmurs, left-sided decreased breath sounds, and signs of cardiac tamponade.

Image from Medscape.

All patients with a suspected aortic dissection should undergo electrocardiography (ECG) given the symptomatic overlap with myocardial infarction. 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 myocardial infarction to ensure that thrombolytics are not administered. [12,13] 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.

Image courtesy of Lars Grimm.

Chest radiographs are frequently the first imaging modality used in the diagnosis of aortic dissection; although they are neither sensitive nor specific, they can be used to rule out other causes of chest pain such as pneumothorax. A widened mediastinum(yellow, double-headed arrow) is the most common finding, especially in patients with an ascending aortic dissection. Additional findings include a 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, and pleural effusion. Other things to consider with mediastinal widening are lymphoma, a tumor, adenopathy, and an enlarged thyroid.

Image from Medscape.

If an aortic dissection ruptures, blood can extravasate into the ipsilateral pleural space, causing hemothorax. Massive hemothorax will cause nearly complete opacification of the involved thorax on a chest radiograph, as shown. This is a rare, late stage finding that usually indicates treatment will be unsuccessful.

Image from Wikimedia Commons | Ksheka.

Ultrasonography with Doppler evaluation allows for discrimination between the true (red arrow) and false (yellow arrow) lumens based on the presence or absence of blood flow. Ultrasonography also allows for the functional evaluation of cardiac strain and the 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. [14]

Image courtesy of Lars Grimm.

Computed tomography (CT) scanning, which provides another excellent means of rapidly and accurately diagnosing aortic dissections, has a sensitivity and specificity of greater than 90%. [11] A CT scan with contrast will show the extent of the dissection, identify the intimal flap separating the two aortic channels (arrow), and reveal offshoot vessels no longer receiving blood flow. [15]The collected images can be reformed in three dimensions on specialized workstations to aid in surgical planning. Serial examinations can be used to observe postoperative patients and evaluate for postsurgical complications. With the widespread adoption of advanced CT scanners by emergency departments, most dissections will be confirmed on contrast-enhanced CT images.

Image courtesy of Joel L. Fishman, MD.

The sagittal gradient-echo magnetic resonance imaging (MRI) scan 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 lumens.

MRI is another cross-sectional imaging modality that provides greater than 90% sensitivity and specificity in the detection of aortic dissection. MRI can accurately identify the site of intimal tear, the extent of dissection, and the presence of aortic insufficiency. MRI does not require ionizing radiation, and some sequences can be performed without contrast material. It is the preferred technique for the serial evaluation of patients with chronic dissections and can provide excellent dynamic information to aid in diagnostic and therapeutic decision-making processes. [16] Unfortunately, MRI is not always available in the acute care setting. Additionally, getting patients to lie still for image acquisition can be difficult in patients with severe chest pain.

Image from Medscape.

Angiography is the classic reference standard for the diagnosis of aortic dissection (shown). It allows for real-time visualization and provides excellent information for surgical planning because the vessels off of the aortic arch are easily assessed. In addition, the true and false lumens can be directly interrogated. Angiography is highly operator dependent, however, as worsening of the existing dissection is a very real complication of the procedure. With the advent of modern CT and MRI scanners, the use of angiography has become mostly limited to patients for whom nonsurgical interventions are indicated.

Image courtesy of Lars Grimm.

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).

The 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. [8] Narcotics and opiates are the preferred agents to control pain, thus reducing adrenergic stimulation. Medical management is the treatment of choice for uncomplicated descending aortic dissections.[17] Emergent surgery is used for ascending or arch dissections, typically with the use of a Dacron graft to seal off the false lumen. Operative mortality is typically around 10%. Endovascular therapy is an emerging and acceptable treatment for descending aortic dissection. [8,18]

Image from Wikimedia Commons | David S. Goodsell of The Scripps Research Institute.

Due to high operative mortality, research has been directed toward earlier detection and prevention of aortic dissections. Multiple serum biomarkers—including smooth muscle myosin heavy chain (shown), 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%. [19] Other research efforts are focusing on identifying patients with a genetic predisposition to aortic dissection, such as those with Ehlers-Danlos or Marfan syndrome.

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