Thoracic aortic aneurysms

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Definition

Aortic aneurysm is a pathological dilatation of the normal aortic lumen involving one or several segments . The most used criterion defines aneurysm as a permanent dilatation at least 1,5 times that of expected normal diameter of that given aortic segment [1], although no definition is universally accepted.


Aneurysms are described in terms of location, size, morphological appearance and origin. The morphology of an aortic aneurysm is :

  • fusiform - the most common shape, with symmetrical dilatation of the full circumference of the aortic wall
  • saccular - a more localized dilatation that involves only a portion of the aortic wall [1]


The presence of an aortic aneurysm may be a marker of more diffuse aortic disease; 25-28% of patients with thoracic aortic aneurysms have concomitant abdominal aortic aneurysms . Thus, a patients in whom an aortic aneurysm is discovered should undergo examination of the entire aorta for the possible presence of other aneurysms [1].


Thoracic aortic aneurysms are much less common than aneurysms of the abdominal aorta.

The main classification of thoracic aortic aneurysms is anatomical (ascending aorta, aortic arch or descending aorta), because the etiology, natural history and treatment differ for each of these aortic segments involved [1].

Aneurysms of ascending aorta occur most commonly , followed by aneurysms of descending aorta, whereas aortic arch or thoracoabdominal aneurysms occur less often [1].


Etiology

Aneurysms of ascending thoracic aorta often result from cystic medial necrosis, leading to a weakening of the aortic wall, which results in formation of fusiform aneurysms. Such aneurysms often involve the aortic root and may consequently result in aortic regurgitation; this association is called annuloaortic ectasia.

Cystic medial necrosis occurs :

  1. at younger ages in association with
    1. conective tissue disorders (Marfan, Ehlers-Danlos syndrome)
    2. familial thoracic aortic aneurysm syndrome
    3. bicuspid aortic valve
  2. in older ages, accelerated by hypertension[1]


Atherosclerosis is the main cause of descending aorta aneurysms and infrequently leads to ascending aorta aneurysms (only when is associated with diffuse aortic atherosclerosis)[1]. Aneurysms in aortic arch are contiguous with those of ascending or descending aorta and have the same etiology (cystic medial necrosis or atherosclerosis). Syphilis and infectious aortitis are nowadays very rare causes of ascending aorta aneurysms [1].

Aortic aneurysms are an important cause of cardiovascular morbidity and mortality. Except when complications are life-threatening, such as acute aortic syndrome or aortic rupture, most aortic aneurysms are asymptomatic and without abnormalities on physical examination; thus, diagnosis and follow-up depend exclusively on imaging techniques.

Echocardiography

Echocardiography has become the most used imaging test and plays an important role in the diagnosis and follow-up of aortic diseases [2].

Transthoracic echocardiography

Transthoracic echocardiography is one of the techniques most used to measure proximal aortic segments in clinical practice.

Using different windows, the proximal ascending aorta is visualized in the left and right parasternal long-axis views and, to a lesser extent, in basal short-axis views.

In all patients with suspected aortic disease, the right parasternal view is recommended for estimating the true size of the ascending aorta.

The ascending aorta is also visualized in the apical long-axis and modified apical five- chamber views; however, in these views, the aortic walls are seen with suboptimal lateral resolution.

Modified subcostal views may in some cases (more frequently in children) be helpful, but here the ascending aorta is far from the transducer.

All these views also permit assessment of the aortic valve, which is often involved in aneurysms of the ascending aorta .

Suprasternal view primarily depicts the aortic arch and the three major supra-aortic vessels (innominate, left carotid, and left subclavian arteries), with variable lengths of the descending and, to a lesser degree, ascending aorta. Although this view may be obstructed, particularly in patients with emphysema or short, wide necks, it should be systematically sought if aortic disease is evaluated.


The entire thoracic descending aorta is not well visualized by transthoracic echocardiography .

A short-axis view of the descending aorta can be imaged posteriorly to the left atrium in the parasternal long-axis view.

From the apical window, a short-axis cross-section of the descending aorta is seen lateral to the left atrium in the four-chamber view and a long-axis stretch in the two-chamber view. By 90 grd transducer, a rotation long-axis view is obtained and a mid part of the descending thoracic aorta may be visualized


In summary, although transthoracic echocardiography is not the ideal tool for visualizing all aortic segments, important information can always be gained by careful use of all echo windows . [2]

Transoesophageal echocardiography

Transoesophageal echocardiography is the ultrasound technique of choice in thoracic aorta assessment and provides high-resolution images of the entire thoracic aorta except for a small portion of the distal ascending aorta near the innominate artery. Transoesophageal echocardiography overcomes limitations encountered by transthoracic echocardiography. Transthoracic echocardiography and transoesophageal echocardiography should be used in a complementary manner.


The most important transoesophageal views of the ascending aorta, aortic root and aortic valve are the high transoesophageal long-axis and short-axis views. A short segment of the distal ascending aorta, just before the innominate artery, remains unvisualized owing to interposition of the right bronchus and trachea (blind spot). Images of the ascending aorta often contain artefacts due to reverberations from the posterior wall of the ascending aorta or the posterior wall of the right pulmonary artery, presenting as aortic intraluminal linear horizontal lines moving in parallel with the reverberating structures, as can be ascertained on M-mode tracings. The descending aorta is easily visualized in short-axis and long-axis views from the coeliac trunk to the left subclavian artery. Further withdrawal of the probe shows the aortic arch, where the inner curvature and anterior arch wall are usually well seen all the way to the ascending aorta. In the distal part of the arch, the origin of the subclavian artery is easily visualized. However, in awake patients, the origin of innominate and left carotid arteries is not clearly visualized, although in anaesthetized patients, it is possible to identify the origin of these supra-aortic arteries. One of the limitations of transoesophageal echocardiography is to locate the exact level of a given abnormality in the descending aorta. When no reference vessels, such as subclavian artery or coeliac trunk, are visualized, this limitation can be overcome by rotating the transducer and identifying the level of the descending aorta in comparison with the structures of the heart or great vessels (anterior structures). Like the ascending aorta, the descending aorta often produces an artefactual pseudo-aorta located posteriorly to the true aorta (‘double-barrel aorta’). [2]

Aorta size[2]

Measurements of aortic diameter by echocardiography are accurate and reproducible.

Standard measurement conventions established the leading edge-to-leading edge diameter in end-diastole.

Two-dimensional (2D) aortic measurements are preferable to M-mode, as cyclic motion of the heart and resultant changes in M-mode cursor location result in systematic underestimation by 1– 2 mm of aortic diameter by M-mode in comparison with the 2D aortic diameter (as ilustrated in the images below).

Parasternal long axis view, M Mode, ascending aorta aneurysm - severe aortic dilation (70mm)
2D Parasternal long axis view, thoracic aorta aneurysm - severe Valsalva sinuses and ascending aorta dilation (72/71 mm)

Standard diameter measurements are at the aortic annulus, at the level of the sinuses of Valsalva and at the sinotubular junction .

Aortic annular diameter is measured between the hinge points of the aortic valve leaflets (inner edge–inner edge) in the left parasternal long-axis view, during systole, which reveal the largest aortic annular diameter.

In a normal ascending aorta, the diameter at sinus level is the largest, followed by the sinotubular junction and the aortic annulus. If aortic dilatation is detected at any level, its maximum diameter should be measured and reported. [2]

Normal values [2]

Aorta size is related most strongly to body surface area and age. Therefore, body surface area may be used to predict aortic root diameter in several age intervals. Roman et al. [3] considered three age strata: younger than 20 years, 20–40 years, and older than 40 years by published equations. These normal values have been accepted to date as the reference values. Some groups have suggested indexing by height to avoid the influence of overweight on body surface area. Nevertheless, large series defining normal ranges of this index are lacking.

Aortic root dilatation at the sinuses of Valsalva is defined as an aortic root diameter above the upper limit of the 95% confidence interval of the distribution in a large reference population. In adults, a diameter of 2.1 cm/m2 has been considered the upper normal range in ascending aorta[4].


Transthoracic echocardiography suffices to quantify maximum aortic root and proximal ascending aorta diameters when the acoustic window is adequate. Nevertheless, the technique is more limited for measuring the remaining aortic segments. Transoesophageal echocardiography overcomes part of these transthoracic echocardiography limitations by affording better measurement of aortic arch and descending thoracic aorta size. Transoesophageal echocardiography may make oblique measurements when the descending aorta is elongated or tortuous. To avoid this overestimation, aortic diameter measurement by transoesophageal echocardiography should be attempted only when circular sections are obtained.

Measurements of descending thoracic aorta in short axis and of the aortic arch in long axis are recommended. The absolute and indexed normal values of the various aortic segments are shown in the table below. [2]

Table:Normal size of thoracic aortic segments [2]

Absolute size Indexed size (on body surface area)
Aortic annulus 20-31 mm 13 ± 1 mm/m2
Sinuses of Valsalva 29-45 mm 19 ± 1 mm/m2
Sinotubular junction 22-36 mm 15 ± 1 mm/m2
Ascending aorta 22-36 mm 15 ± 2 mm/m2
Aortic arch 22-36 mm
Descending aorta 20-30 mm

Transthoracic echocardiography is an excellent modality for imaging aortic root dilatation,[3][4][5] which is important for patients with annuloaortic ectasia, Marfan syndrome, or bicuspid aortic valve. Since the predominant sites of dilatation are in the proximal aorta, transthoracic echocardiography often suffices for screening . In ascending aorta dilatation, some echocardiographic features play an important role in the assessment of the mechanisms of functional aortic regurgitation.


Transthoracic echocardiography suffices in the assessment of proximal ascending aorta when the acoustic window is adequate. However, transoesophageal echocardiography is clearly superior to transthoracic echocardiography for assessing aneurysms located in the aortic arch and descending thoracic aorta. However, transoesophageal echocardiography is limited in tortuous aortas, since in these cases, the aorta may be separated from the oesophagus, resulting in inability to image these aorta [2] segments. Thus, the modalities of choice are MRI and CT [2]

Recommendation

In aortic root aneurysms, the accurate measurement of diameters by transthoracic echocardiography or transoesophageal echocardiography is crucial for surgical indication and surgical management strategies. In the arch and descending aorta, other imaging modalities with better reproducibility and larger field of view, such as CT or MRI, may be more suitable. [2]

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Isselbacher, Diseases of aorta in Braunwald's Heart Disease: A Textbook of Cardiovascular Medicine, Single Volume, 8e ISBN-13: 978-1416041061 pg 1458, 1464
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 European Journal of Echocardiography (2010) 11, 645–658 Echocardiography in aortic diseases: EAE recommendations for clinical practice
  3. 3.0 3.1 Roman MJ, Devereux RB, Kramer-Fox R, O’Loghlin J. Two-dimensional echocardiographic aortic root dimensions in normal children and adults. Am J Cardiol 1989;64:507 –12.
  4. 4.0 4.1 Drexler M, Erbel R, Muller U, Wittlich N, Mohr-Kahaly S, Meyer J. Measurement of intracardiac dimensions and structures in normal young adult subjects by transesophageal echocardiography. Am J Cardiol 1990;65:1491 – 6.
  5. Schaefer BM, Lewin MB, Stout KK, Gill E, Prueitt A, Byers PH et al. The bicuspid aortic valve: an integrated phenotypic classification of leaflet morphology and aortic root shape. Heart 2008;94:1634 – 8.
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