Thoracic aorta

11 Thoracic aorta


The prevalence of aortic pathology is steadily increasing as a result of:


• the ageing population, because ageing is accompanied by progressive modifications of the cardiovascular system

• the high incidence of arterial hypertension, which increases progressively with age

• a better understanding of aortic pathology due to improvements in diagnostic techniques.

Transoesophageal echocardiography (TEE) has become a complementary technique to transthoracic echocardiography (TTE), and one that is almost indispensable in studying aortic pathologies such as dissection, haematoma, aneurysm and atheroma of the thoracic aorta.


However, despite significant progress in echo Doppler technology, the diversity of the aetiologies and the complexity of the aortic lesions are sometimes bewildering to the echocardiographer.



AORTIC DISSECTION


Aortic dissection is a medico-surgical emergency that requires a precise diagnosis as early as possible.


TEE is a highly effective first-line diagnostic method, and is easily carried out at the patient’s bedside. It can be used to explore almost the entire thoracic aorta. However, it requires an experienced echocardiographer and, in case of any doubt, recourse to another imaging technique. In cardiological practice, TEE is always preceded by TTE, which may already suggest the diagnosis.


The limitations and pitfalls of TEE relate to:


• the identification of the aortic dissection

• the precise location of the dissection

• the detection of complications associated with the dissection.

Given the clinical gravity of aortic dissection, and particularly in emergency cases, good familiarity with these pitfalls is crucial.



Pitfalls when diagnosing dissection


These pitfalls are due to:


• reduced specificity of the echocardiographic signs of dissection

• the sometimes difficult distinction between the true and false lumen

• imprecise visualization of the dissection point of entry.


Specificity of the echocardiographic signs


The echocardiographic diagnosis of aortic dissection is based on the visualization of the mobile intimal flap, floating between the true and false lumen (Figs 11.1 and 11.2).


image

Figure 11.1 Schematic appearance of an aortic dissection with a true lumen (T), false lumen (F), intimal flap (fl) and entry point (arrow). Transverse view of the aorta.


image

Figure 11.2 Dissection of the thoracic aorta in TEE (transverse view). (a) Intimal flap with the entry point separating the aortic lumen into two channels. (b) Dissection entry point detected using colour Doppler.


The identification of this intimal flap is pathognomonic for the diagnosis of dissection, but there are diagnostic pitfalls of overdiagnosis (false-positive results) and underdiagnosis (false-negative results), which should be guarded against.



Overdiagnosis of aortic dissection


False-positive diagnoses of aortic dissection (Box 11.1) are dominated by:


• so-called linear artefacts located in the aortic lumen and simulating an intimal flap (Fig. 11.3)

• intra-aortic reverberations from thick aortic walls or a calcified aortic ring may be falsely interpreted as a dissection.


Box 11.1 Causes of false-positive diagnosis of aortic dissection when using TEE



• Ultrasound artefacts (linear or mirrored)

• Ultrasound reverberations due to thickened aortic walls, a calcified aortic annulus or a mediastinal extracardiac structure (haematoma, abscess, tumour, fat)

• Annular ectasia of the aorta with abnormally long aortic sigmoid valves, which may be confused with an intimal flap

• An aneurysm of the descending thoracic aorta, with a mural thrombus simulating a thrombotic false lumen

• A false aortic aneurysm, which may give the appearance of a localized pseudodissection

• An isthmic rupture

• An abscess of the aortic cuff

• A detached atheromatous plaque, called a ‘floating’ plaque, within the aortic lumen

• An intra-aortic clapper-shaped thrombus, sometimes floating

• A pleural effusion

• A vascular superposition (innominate venous trunk, hemizygotic vein)

• Theile transverse sinus

• The spinal canal

image

Figure 11.3 Artefactual images of aortic dissection in TEE. (a) Linear artefacts within the ascending aorta; anterior artefact with amplitude and movement double that of the posterior wall of the aorta. (b) Mirror-image-type artefact at the level of the descending thoracic aorta observed in colour Doppler.


Rights were not granted to include this figure in electronic media. Please refer to the printed book.


(Images by Prof. R. Roudaut.)


Linear artefacts, which are located principally on the ascending aorta, are frequently observed in monoplanar TEE. The use of multiplanar TEE has reduced the incidence of these aretefacts, but has not resolved the problem: the artefactual images are often found in different planes. Linear artefacts stem from multiple reflections of the ultrasound off the walls of the left atrium in particular. They arise in the presence of a wide aorta, with a diameter greater than that of the left atrium, with the ultrasound probe at a distance equal to or double the atrial diameter (Fig. 11.4). In fact, an aortic diameter of > 50 mm and a left atrium/ aorta ratio of < 0.6 are excellent predictive factors for the creation of artefacts.


image

Figure 11.4 Creation of a type A linear artefact in the ascending aorta (AO) that is larger than the left atrium (LA). The artefact (dashed line) is projected into the aortic lumen at a distance equal to or double the atrial diameter (A = 2B). P, ultrasound probe.


Three principal types of artefact have been described that are due to the reflection of the ultrasound from either the posterior wall of the left atrium (type A), the posterior wall of the right pulmonary artery (type C) or the interaction between the two (type B). Type A linear artefacts, due to the reflection by the aorta/left atrium interface, are the most common. They may be easily identified by their particular echocardiographic appearance, especially when using M-mode (Box 11.2), which makes it possible to avoid an erroneous diagnosis of aortic dissection.



Box 11.2 Echocardiographic signs that suggest an ultrasound artefact simulating aortic dissection



• Linear artefactual images that are > 2.5 mm thick, of low mobility and have blurred contours

• The displacement of the artefact parallels that of the posterior wall of the aorta

• The distance between the posterior wall of the ascending aorta and the artefact is identical to the distance between the probe and the posterior wall of the aorta

• The absence of broad oscillations of the artefact (as opposed to a highly mobile dissection flap, with expansion of the true lumen during systole)

• The absence of an identifiable entry point to the dissection (no aliasing phenomenon in colour Doppler, which may indicate the existence of an entry point)

• The absence of a double lumen appearance, with different flow velocities in pulsed and colour Doppler (identical velocities on either part of the artefact)

• The possible continuation of the artefact beyond the aortic walls

Related posts:

  1. Other technical pitfalls
  2. The physical limitations of echocardiography
  3. Cardiac valves
  4. Systolic and diastolic function of the ventricles

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Jun 4, 2016 | Posted by in CARDIOLOGY | Comments Off on Thoracic aorta

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