Aortic Dissection

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AORTIC DISSECTION


Contents

Definition

Aortic dissection is defined as separation of the layers within the aortic wall. Tears in the intimal layer result in the propagation of dissection (proximally or distally) secondary to blood entering the intima-media space.


Classification

The classification based on the duration of onset of symptoms prior to presentation and prognosis [1][2][3]

I. An acute aortic dissection (< 2 weeks) is associated with high morbidity and mortality rates.

II. Patients with chronic aortic dissection (>2 weeks or occasionally found) have a better prognosis. The aortic dissection mortality rate is still high despite advancements in diagnostic and therapeutic modalities


Anatomical classifications[4]

Three major classification systems are used to define the location and extent of aortic involvement:

De Bakey classification

  • Type I – Originates in ascending aorta, propagates at least to the aortic arch and often beyond it distally.
  • Type II – Originates in and is confined to the ascending aorta.
  • Type III – Originates in descending aorta, rarely extends proximally but will extend distally.

The Stanford classification

The Stanford classification has 2 types, as follows:

  • Type A: All dissections involving the ascending aorta, regardless of the site of origin (DeBakey types I and II)
  • Type B: All dissection not involving the ascending aorta (DeBakey type III)

This system also helps delineate treatment. Type A dissections usually require surgery, whereas type B dissections are managed medically under most conditions .[5]


Descriptive classification:

  • Proximal : Includes DeBakey types I and II or STanford Type A
  • Distal: Includes De Bakey type III or STanford type B

European Society of Cardiology Guidelines Classification (that includes "atypical" forms of aortic dissection)[3]

Class 1: classical aortic dissection with an intimal flap between true and false lumen

Class 2: medial disruption with formation of intramural haematoma/haemorrhage

Class 3: discrete/subtle dissection without haematoma, eccentric bulge at tear site

Class 4: plaque rupture leading to aortic ulceration, penetrating aortic atherosclerotic ulcer with surrounding haematoma, usually subadventitial

Class 5: iatrogenic and traumatic dissection


Class 1–5 represent a subdivision to the Stanford or De Bakey classification


Imaging techniques

Imaging techniques for aortic dissection must achieve the following diagnostic goals[3]:


  • Confirm diagnosis
  • Classify the dissection/delineate the extent
  • Differentiate true and false lumen
  • Localize intimal tears
  • Distinguish between communicating and non-communicating
  • dissection
  • Assess side branch involvement (including coronary arteries)
  • Detect and grade aortic regurgitation
  • Detect extravasation (periaortic or mediastinal haematoma,
  • pleural or pericardial effusion)

Diagnosis methods [3]

The following methods can confirm the diagnosis with high accuracy:

  • transthoracic echocardiography (TTE)
  • transoesophageal echocardiography (TEE)
  • computed tomography (CT)
  • magnetic resonance imaging (MRI)
  • angiography
  • intravascular ultrasound

The decision for a specific technique depends on two major factors:

  • Availability in emergency situations
  • Experience of the emergency room and imaging staff

Titlu- Echocardiography


Aortic dissection and its variants, included in the ESC classification of aortic dissection [3] can be correctly diagnosed by echocardiography.


Transthoracic echocardiography followed by transoesophageal echocardiography is class I recommendation for diagnostic imaging in acute aortic dissection and class IIb for chronic aortic dissection (MRI is class I indication in this case)[3]


Technical considerations [3]

The aorta and the major aortic side branches can be studied using different scanning fields. It is important to use all scanning planes in order to provide complete information about the type and extension of aortic dissection. M-mode and two-dimensional echocardiography are taken in standard cross-sectional planes: parasternal, apical, subcostal scan fields. The ascending aorta is visualized in left parasternal projections. In aortic ectasia the right parasternal view and the apical view are useful. The aortic arch is best visualized via the suprasternal approach. Orthogonal and longitudinal scan planes can provide information concerning extension of dissection into the innominate artery, left common carotid or left subclavian artery. The descending aorta can occasionally be vizualized in the left parasternal cross-sectional image and via specialized paraspinal scan fields obtained by imaging the patient through the back in the right decubitus position or by using abdominal scan planes

Echocardiographic criteria of aortic dissection [3][6]

TEE Colour Doppler Asc Ao dissection:intimal flap,entry tear
TEE 2D, Colour Doppler Asc Ao dissection intimal flap,secondary communication
TOE: Descending aorta chronic dissection, with the intimal flap (arrow) that separates the two lumina and a reentry tear

The diagnosis of aortic dissection

The diagnosis of aortic dissection is confirmed when two lumina separated by an intimal flap can be visualized within the aorta.

Positive criteria are :

-complete obstruction of a false lumen

-central displacement of intimal calcification

-separation of intimal layers from the thrombus

-shearing of different wall layers during aortic pulsation

Tear definition

A tear is defined as a disruption of the flap continuity with fluttering of the ruptured intimal

borders. Smaller intimal tears can be detected by colour Doppler, registering jets transversing the

dissection membrane. The number of tears is recorded and the location as wall as the distance from the teeth to the tip of the probe is documented.


The differentiation in the true from the false lumen

The differentiation in the true from the false lumen is based on M-mode, two-dimensional, and Doppler echocardiographic signs.


Criteria for identifying the true lumen are :

  • systolic expansion and diastolic collapse
  • the absence or low intensity of spontaneous echocardiographic contrast
  • systolic jets directed away from the lumen
  • systolic forward flow.

Criteria for the false lumen are :

  • diastolic diameter increase
  • spontaneous echocardiographic contrast
  • reversed, delayed, or absent flow
  • thrombus formation.

Flow signals within the false lumen represent signs of communication, whereas the absence of flow signals in multiple cross-sections means no communication.

If the only communication is found distal to the beginning of the dissection, retrograde dissection can be assumed, which can be further differentiated into retrograde dissection with or without involvement of the ascending aorta.


A thrombus is diagnosed when a mass separate from the intimal flap and the aortic wall is imaged in

the free space of the false or rarely the true lumen. Because thrombus formation depends on flow velocity, it may be graded to provide an estimate of the degree of

communication.

Transthoracic echocardiography (TTE) [3] [6]

Transthoracic echocardiography (TTE) may be used as the initial imaging modality when aortic dissection is clinically suspected in the emergency room given its availability, rapidity, and additional information on cardiac status. Contrast may improve its accuracy . The low negative predictive value of TTE does not permit dissection to be ruled out, and further tests will be required if the TTE exam is negative. The value of TTE is also limited in patients with abnormal chest wall configuration, obesity, pulmonary emphysema, and in those on mechanical ventilation. These limitations may prevent adequate decision-making in some cases, but have been overcome by transoesophageal echocardiography (TOE).


In patients with acute chest pain, special attention should be paid during the TTE exam to:

  • aortic root dilatation
  • aortic regurgitation
  • pericardial effusion,

since these findings should raise the suspicion of acute aortic syndrome.

If a dissection cannot be directly visualized, other imaging techniques are mandatory [6]


Transoesophageal echocardiography (TOE)

TOE has constituted a decisive advance in the diagnosis of aortic dissection. It can image the entire thoracic aorta except for a small portion of the distal ascending aorta near the proximal arch. The proximity of the oesophagus to the aorta, without interference from the chest wall or lung, permits high-quality images to be obtained [6]


TOE is clearly superior to TTE in the diagnosis of intramural haematoma and aortic ulcers.


Intramural haematoma

TOE, just above the aortic ring : anterior ascending aorta wall haematoma(arrow), 0,8 cm thick.
TOE: Ascending aorta, before the junction with the aortic arch - Intimal ulceration in the depth of the haematoma

Intramural haematoma is characterized by circular or crescentic thickening of the aortic wall (>5 mm).

Echocardiographic findings of intramural haematoma are circular or crescentic thickening (>5 mm)

of the aortic wall and there should be no flow within.

Diagnosis is straightforward in typical cases, but the haematoma may sometimes be mistaken for an intraluminal thrombus or a dissection with thrombosed false lumen. Displacement of intimal calcification caused by accumulation of blood within the aortic media is useful for the differential diagnosis.

Penetrating aortic ulcer

Penetrating aortic ulcer presents as an image of crater-like outpouching with jagged edges in the aortic wall, generally associated with extensive aortic atheromas[6].


Echocardiography in aortic dissection complications diagnosis

1.Echo-free spaces resulting from the presence of free fluid around the aorta are judged to be signs of penetration due to periaortic haematoma. Additional mediastinal haematoma is assumed when the distance from the oesophagus to the left atrium or the aorta is increased (>1 cm) and pleural effusion is found.


2. Pericardial effusion is considered to exist when an echofree space between the epicardium and pericardium is present.


3. Aortic regurgitation is diagnosed when disturbed diastolic flow signals are detected in the left ventricular outflow tract. Grading is performed by taking Doppler and colour Doppler echocardiographic criteria into account [3].


Mechanisms of significant aortic regurgitation in aortic dissection

Mechanisms of significant aortic regurgitation in aortic dissection [6]:

  • Dilatation of the aortic annulus secondary to dilatation of the ascending aorta
  • Rupture of the annular support and tear in the implantation of one of the valvular leaflets
  • Asymmetrical dissections, the haematoma itself may displace a sigmoid below coaptation level
  • Prolapse of the intima in the outward tract of the left ventricle through the valvular orifice
  • Previous aortic valve disease

Artifacts

Artifacts due to reverberation within the lumen of the ascending aorta can be observed. These artifacts may present a problem to non-experienced observers and may be the cause of the low sensitivity in some reports.


Conclusion [3]

Transoesophageal echocardiography in addition to transthoracic echocardiography can be used for decision making in the emergency room or even operating theatre in acute aortic dissection with high accuracy. Pitfalls have to be taken into account.

The high resolution enables the diagnosis also of intramural haematoma, plaque ulceration, as well as traumatic aortic injury.

Only experienced examiners should interpret echocardiographic studies.

When more spatial resolution is necessary, CT or MRI are used in addition.

Thoracic CT: descending aorta chronic dissection, with an intimal flap and two lumina (the true lumen-gray arrow) unequally opacified, the false lumen (white arrow) partially thrombosed
Thoracic CT: ascending aorta haematoma (arrow)
Thoracic CT: ulceration (gray arrow) in the depth of haematoma (white arrow), which opacifies concomitant with the aortic lumina

References

  1. Hagan PG, Nienaber CA, Isselbacher EM, et al. The International Registry of Acute Aortic Dissection (IRAD): new insi‏ts into an old disease. JAMA. Feb 16 2000;283(7):897-903. Medline.
  2. Patel PD, Arora RR. Pathophysiology, diagnosis, and management of aortic dissection. Ther Adv Cardiovasc Dis. Dec 2008;2(6):439-68. Medline
  3. 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 Diagnosis and management of aortic dissection, Recommendations of the Task Force on Aortic Dissection, European Society of Cardiology, R. Erbel (Chairman), F. Alfonso, C. Boileau, O. Dirsch, B. Eber, A. Haverich, H. Rakowski, J. Struyven, K. Radegran, U. Sechtem, J. Taylor, Ch. Zollikofer, European Heart Journal (2001) 22, 1642–1681
  4. Isselbacher, Diseases of aorta in Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, Single Volume, 8e ISBN-13: 978-1416041061 pg 1470]
  5. Clouse WD, Hallett JW Jr, Schaff HV, et al. Acute aortic dissection: population-based incidence compared with degenerative aortic aneurysm rupture. Mayo Clin Proc. Feb 2004;79(2):176-80. [Medline].
  6. 6.0 6.1 6.2 6.3 6.4 6.5 European Journal of Echocardiography (2010) 11, 645–658 Echocardiography in aortic diseases: EAE recommendations for clinical practice
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