Aortic dissection

Aortic dissection is the prototype and most common form of acute aortic syndromes and a type of arterial dissection. It occurs when blood enters the medial layer of the aortic wall through a tear or penetrating ulcer in the intima and tracks longitudinally along with the media, forming a second blood-filled channel (false lumen) within the vessel wall.

Epidemiology

The majority of aortic dissections are seen in elderly hypertensive patients. In a very small minority, an underlying connective tissue disorder may be present. Other conditions or predisposing factors may also be encountered, in which case they will be reflected in the demographics. Examples include ⁵:

• atherosclerosis

• structural aortic abnormalities

  • bicuspid aortic valve

  • aortic coarctation

  • abnormal connective tissues

    • Marfan syndrome

    • Ehlers-Danlos syndrome

    • Loeys-Dietz syndrome

• Turner syndrome

  • ​under-recognized, common cause of dissection in young women

• pregnancy

• prior intervention ²²

  • cardiac surgery

  • aortic surgery

  • intra-aortic balloon pumps ⁷

• ciprofloxacin use (unclear if class effect for fluoroquinolone agents) ²⁰

  • fluoroquinolones seem to promote loss of extracellular matrix integrity, by several mechanisms

  • in the UK caution is now advised in using these agents in high-risk patients ²¹

Interestingly, there is a reported seasonal pattern of disease, with great risk in winter and during morning hours ²².

Clinical presentation

The duration of aortic dissection is arbitrarily categorized into three phases ^18,19:

• acute: within 14 days of first symptom onset

• subacute: between 14 days to 3 months

• chronic: more than 3 months from the initial onset of symptoms

Patients are often hypertensive (although they may be normotensive or hypotensive) and present with anterior or posterior chest pain and a tearing sensation in the chest. There may be a difference in blood pressure between the two arms depending on where the dissection occurs.

Depending on the extent of dissection and occlusion of aortic branches, end-organ ischemia may also be present (seen in up to 27% of cases) ⁵, including:

• abdominal organ ischemia

• limb ischemia

• ischemic or embolic stroke

• paraplegia: involvement of the artery of Adamkiewicz

If the aortic dissection involves the aortic root it may result in involvement of the coronary arteries and can present similarly to ST-elevation myocardial infarction on an ECG. However, treating these patients with antiplatelets/anticoagulation could be disastrous in aortic dissection.

Some cases of aortic dissection may result in rupture, causing collapse and often death. Signs of cardiac tamponade (Beck's triad) may also be encountered if rupture occurs in the pericardial space.

There have been efforts to construct a clinical decision rule to stratify the risk of acute aortic dissection and avoid over-investigation. The aortic dissection detection risk score (ADD-RS) combined with a negative D-dimer test has been demonstrated to be effective in reducing unnecessary exams, however, it has not been widely accepted into clinical practice and requires further validation ^13,14.

Pathology

The normal lumen lined by intima is called the true lumen and the blood-filled channel in the media is termed the false lumen. In most cases the vessel wall is abnormal. Causes include:

• hypertension (pathogenesis: medial degeneration)

• inherited connective tissue disorders (pathogenesis: medial degeneration)

  • Marfan syndrome

  • Ehlers-Danlos syndrome

  • Loeys-Dietz syndrome

• atherosclerosis (pathogenesis: penetrating ulcer)

• vasculitis (pathogenesis: inflammation)

• pregnancy (pathogenesis: unknown)

• iatrogenic: aortic catheterization, intra-aortic balloon pump

The false lumen typically has higher and less pulsatile pressure, thought to be due to reduced distal outflow. The true lumen is often smaller due to compression by the false lumen ²².

Radiographic features

Imaging is essential in delineating the morphology and extent of the dissection as well as allowing for classification (which dictates management). Two classification systems are in common usage, both of which divide dissections according to the involvement of the ascending aorta:

1. Stanford classification

2. DeBakey classification

The Stanford classification has gained favor with cardiothoracic surgeons, and approximately, 60% of dissections involve the ascending aorta (Stanford A or DeBakey I and II) ⁵.

Aortic dissection may sometimes be classified as communicating versus non-communicating ^16,17.

A classification system was proposed that utilizes the acronym DISSECT (duration, intimal tear, size of the dissected aorta, segmental extent of involvement, clinical complications, thrombosis of the false lumen) ¹⁸.

Pleural effusions are commonly seen which increase in size during the acute phase of dissection ²².

Plain radiograph

Chest radiography may be normal or demonstrate several suggestive findings, including:

• widened mediastinum: >8.0-8.8 cm at the level of the aortic knob on portable anteroposterior chest radiographs ^9,10, although this upper limit of normal varies (may be significantly larger) depending on projection, FFD (film-focus distance) and x-ray cassette positioning ¹⁵

• double aortic contour

• irregular aortic contour

• inward displacement of atherosclerotic calcification (>1 cm from the aortic margin) ^9,11

Depending on etiology, there may be signs of periaortic or mediastinal hematoma which include:

• obscuration of the aortic knob

• opacification of the AP window

• deviation of mediastinal structures

  • esophagus or nasogastric tube to the right

  • trachea to the right

  • left main bronchus inferiorly (decreased angle from the horizontal)

• increased thickness of the left and/or right paratracheal stripe

• apical capping, particularly on the left

CT

CT, especially with arterial contrast enhancement and specifically computed tomography angiography (CTA) of the aorta is the investigation of choice, able not only to diagnose and classify the dissection but also to evaluate for distal complications. It has reported sensitivity and specificity of nearly 100% ^3,5.

Thoracic aortic dissection can extend distally into the abdominal aorta and iliac arteries. Simultaneous CT imaging of the abdomen and pelvis is therefore often performed to identify concomitant mesenteric or abdominal visceral malperfusion.

Non-contrast CT may only demonstrate subtle findings. However, a high-density mural hematoma is often visible. Displacement of atherosclerotic calcification into the lumen is also a frequently identified finding.

Dissections involving the aortic root should ideally be assessed with ECG-gated CTA which nearly eliminates pulsation artifact. Pulsation artifacts can mimic dissection, which is very common and seen in up to 92% of non-gated CTA studies ⁸.

Contrast-enhanced CT (preferably CTA) gives excellent detail. Findings include ^1-3,5:

• intimal flap

• double-lumen representing the true and false lumens

• dilatation of the aorta due to aortic insufficiency ²³

• an atypical variant that may be seen is an aortic intramural hematoma

• Mercedes-Benz sign in the case of a "triple-barreled" dissection

• windsock sign

• imaging features of any of the complications of aortic dissection (see below)

An essential part of the assessment of aortic dissection is identifying the true lumen, as the placement of an endoluminal stent graft in the false lumen can have dire consequences. Distinguishing between the two is often straightforward, but in some instances, no clear continuation of one lumen with a normal artery can be identified. In such instances, a number of features are helpful ^3,23:

• true lumen

  • often compressed by the higher-pressure false lumen and the smaller of the two

  • has outer wall calcifications (helpful in acute dissections)

  • early and homogenous enhancement

  • often contiguous with the aortic root

  • the origins of the celiac trunk, SMA (superior mesenteric artery) and right renal artery usually arise from the true lumen

• false lumen

  • often larger lumen size due to higher false luminal pressures (but size can be influenced by phase of the cardiac cycle)

  • often of lower contrast density due to delayed opacification

  • at risk for rupture due to reduced elastic recoil and dilation

  • typical location:

    • right anterolateral aspect of the ascending aorta

    • left posterolateral aspect of the descending aorta

  • beak sign

  • cobweb sign (as slender linear areas of low attenuation specific to the false lumen due to residual ribbons of media that have incompletely sheared away during the dissection process) ³

  • maybe thrombosed and seen as mural low density only (more common in chronic dissections)

  • the left renal artery usually arises from the false lumen

  • surrounds the true lumen in Stanford type A 

Chronic dissection flaps are often thicker and straighter than those seen in acute dissections ³.

The CTA report should include ²⁷:

• proximal and distal extent of dissection

• location of the intimo-medial tear(s) (entry site +/- re-entry site)

• associated with other forms of acute aortic syndrome

• aortic size (largest orthogonal measurement) and assessment of aortic valve competancy

• involvement and supply (from true or false lumen) of aortic branches including the coronary ostia

• presence of thrombosis in the false lumen

• signs of organ ischemia or vessel occlusion

• presence or absence of extravasated blood in mediastinal, pleural, or pericardial spaces

Transesophageal echocardiography

Transesophageal echocardiography (TOE) has very high sensitivity and specificity for assessment of acute aortic dissection, but due to limited access and its invasive nature, it has largely been replaced by CTA (or MRA in some instances) ⁵.

MRI

Although in general MRA has been reserved for follow-up examinations, rapid non-contrast imaging techniques (e.g. true FISP) may see MRI having a larger role to play in the acute diagnosis, particularly in patients with impaired renal function ⁴. It has similar sensitivity and specificity to CTA and TOE ⁵ but suffers from limited availability and the difficulties inherent in performing MRI on acutely unwell patients.

Angiography (DSA)

Conventional digital subtraction angiography has historically been the gold standard of investigation. CTA has now replaced it as the first-line investigation, not only due to it being non-invasive but also on account of better delineation of the poorly opacifying false lumen, intramural hematoma and end-organ ischemia. 

Angiography still is required for endoluminal repair.

Risks of angiography include general risks of angiography plus the risk of catheterizing the false lumen and causing aortic rupture.

Treatment and prognosis

• aggressive blood pressure control with beta-blockers as they reduce both blood pressure and also heart rate thereby reducing extra pressure on the aortic wall

• immediate surgical repair (for type A dissection or complicated type B dissection)

Complications

Complications of all types of aortic dissection include:

• dissection and occlusion of branch vessels

  • involvement can be static or dynamic ²³

  • abdominal organ ischemia eg kidneys (left kidney most common), bowel, spleen

  • limb ischemia

  • ischemic stroke

  • paraplegia: involvement of the artery of Adamkiewicz

• distal thromboembolism

• aneurysmal dilatation: this is an indication for endovascular or surgical intervention ⁶

• aortic rupture

• hemothorax ²⁵

Specifically, Stanford type A dissection may also result in:

• coronary artery occlusion (right coronary artery more common ²²)

• aortic incompetence

• rupture into the pericardial sac causing hemopericardium with resulting cardiac tamponade

• pulmonary artery intramural hematoma due to bleeding into the shared aortopulmonary adventitial sheath

• acute obstructive right heart failure due to compression of the pulmonary arteries by hematoma

Although the combination of blood pressure control and surgical intervention has significantly lowered in-hospital mortality, it remains significant, at 10-35%. Over the 10 years following diagnosis another 15-30% of patients require surgery for life-threatening complications ⁵.

Specifically, Stanford type B dissection of the false lumen may ²³:

• resolve

• thrombose (partially or completely)

• remain stable

• progress in the caudocranial direction

Differential diagnosis

The differential on chest x-ray is that of a dilated thoracic aorta.

On CT, a number of entities that can mimic a dissection should be considered ⁵:

• pseudodissection due to aortic pulsation motion artifact (typically left anterior and right posterior aspects of the ascending aorta)

• pseudodissection due to contrast streaks

• mural thrombus

• intramural hematoma: an atypical type of aortic dissection and part of the acute aortic syndrome

• penetrating atherosclerotic ulcer which is part of the acute aortic syndrome

• adjacent atelectasis

• minimal aortic injury

On cardiac MRI:

• susceptibility artifact can give a "pseudoflap-like" appearance on certain sequences ²⁴

Clinically, a number of causes of acute chest pain are often considered:

• acute coronary syndrome

• acute pulmonary embolism

• pneumonia

• Bornholm disease: this is a diagnosis of exclusion, rarely thought about - CT is usually normal, or occasionally non-specific pleural inflammation and/or infiltrates