Aortic dissection is the most common form of the 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 along the media, forming a second blood-filled channel within the wall.
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:
- structural aortic abnormalities
- Turner syndrome
- intra-aortic balloon pumps 7
Aortic dissection is arbitrarily divided into:
- acute: within 14 days of first symptom onset
- chronic: after 14 days
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.
Depending on the extent of dissection and occlusion of aortic branches, end-organ ischaemia may also be present (seen in up to 27% of cases) 5, including:
- abdominal organ ischaemia
- limb ischaemia
- ischaemic or embolic stroke
- paraplegia: involvement of the artery of Adamkiewicz
There have been efforts to construct a clinical decision rule stratify 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.
The normal lumen lined by intima is called the true lumen and the blood-filled channel in the media is called the false lumen. In most cases the vessel wall is abnormal. Causes include:
- hypertension (pathogenesis: medial degeneration)
- inherited connective tissue disorders (pathogenesis: medial degeneration)
- atherosclerosis (pathogenesis: penetrating ulcer)
- vasculitis (pathogenesis: inflammation)
- pregnancy (pathogenesis: unknown)
- iatrogenic: aortic catheterisation, intra-aortic balloon pump
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:
In recent years, the Stanford classification has gained favour with cardiothoracic surgeons. Approximately 60% of dissections involve the ascending aorta (Stanford A or DeBakey I and II) 5.
Aortic dissection are also sometimes classified as communicating versus non-communicating 16-17.
Chest radiography may be normal or demonstrate a number of 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 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
CT, especially with arterial contrast enhancement (CTA) 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.
Non-contrast CT may demonstrate only subtle findings; however, a high-density mural haematoma 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 totally eliminates pulsation artefact. Pulsation artefact can mimic dissection, 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
- dilatation of the aorta
- complications (see below)
- an atypical variant that may be seen is an aortic intramural haematoma
- Mercedes-Benz sign in the case of a "triple-barreled" dissection
- windsock sign
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 normal artery can be identified. In such instances, a number of features are helpful 3:
- often compressed by the false lumen and the smaller of the two
- outer wall calcifications (helpful in acute dissections)
- origin of coeliac trunk, SMA and right renal artery usually from true lumen
- often larger lumen size due to higher false luminal pressures
- at risk for rupture due to reduced elastic recoil and dilation
- 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
- often of lower contrast density due to delayed opacification
- maybe thrombosed and seen as mural low density only (more common in chronic dissections)
- origin of left renal artery usually from false lumen
- surrounds true lumen in Stanford type A
Chronic dissection flaps are often thicker and straighter than those seen in acute dissections 3.
Transoesophageal 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.
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.
DSA - angiography
Conventional digital subtraction angiography has historically been the gold standard 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 haematoma and end-organ ischaemia.
Angiography still is required for endoluminal repair.
Risks of angiography include general risks of angiography plus the risk of catheterising 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 hence reduce extra pressure on the aortic wall
- immediate surgical repair (for type A dissection or complicated type B dissection)
Complications of all types of aortic dissection include:
- dissection and occlusion of branch vessels
- abdominal organ ischaemia
- limb ischaemia
- ischaemic stroke
- paraplegia: involvement of artery of Adamkiewicz
- distal thromboembolism
- aneurysmal dilatation: this is an indication for endovascular or surgical intervention 6
- aortic rupture
A type A dissection may also result in:
- coronary artery occlusion
- aortic incompetence
- rupture into pericardial sac with resulting cardiac tamponade
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.
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 artefact (typically left anterior and right posterior aspects of the ascending aorta)
- pseudodissection due to contrast streaks
- mural thrombus
- intramural haematoma: really 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
Clinically, a number of causes of acute chest pain are often considered:
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- acute aortic syndrome
- thoracic aortic aneurysm
- abdominal aortic aneurysm
- endovascular aneurysm repair (EVAR)
- reporting tips for aortic aneurysms
- aortic coarctation
- aortic pseudocoarctation
- cervical aortic arch
- interrupted aortic arch
- transposition of the great arteries
- variant anatomy of the aortic arch
- traumatic aortic injury