Using The Descending Aortic Wall Thickness Measured In Transesophageal Echocardiography As A Risk Marker For Aortic Dissection

Abstract

Objective

The aim of this study is to estimate whether aortic wall thickness is increased in patients with Aortic dissection (AD) compared to low risk control group and can be used in addition to aortic diameter as a risk marker of AD.

Background

AD occurs due to pathologies that may increase thickness of the aortic wall. Transesophageal echocardiography (TEE) has the ability to visualize both the thoracic aortic wall and lumen. Aortic diameter has been used to predict aortic dissection and timing of surgery, but it is not always predictive of that risk.

Methods

In 48 patients with AD who underwent TEE were examined retrospectively and compared to 48 control patients with patent foramen ovale (PFO). We measured aortic diameter at different levels, intimal/medial thickness (IMT) and complete wall thickness (CMT). Demographic data and cardiovascular risk factors were reviewed. The data was analyzed using ANOVA and student t test.

Results

(AD) patients were older [mean age 66 AD vs. 51 PFO], had more hypertension, diabetes, hyperlipidemia and Coronary artery disease. Both IMT and CMT in the descending aorta were increased in AD group [(1.85 vs. 1.43 mm; P=0.03 and 2.93 vs. 2.46 mm; p=0.01). As expected the diameter of ascending aorta was also greater in AD (4.61 vs. 2.92 cm; P=0.004).

Conclusions

CMT and IMT in the descending aorta detected by TEE is greater in patients with AD when compared to control and may add prognostic data to that of aortic diameter.

Introduction

Aortic dissection (AD) is a catastrophic, and often unpredictable disorder that affects approximately 2.6 to 3.5 per 100,000 person-years. (1, 2) Risk factors for AD includes hypertension, aortic aneurysm, atherosclerosis, cystic medial necrosis as well as many connective tissue disorders. Historically, when aortic aneurysm exists, aortic dimensions were used as a marker to estimate the risk of dissection and therefore suggest a time frame for possible repair.(3) However, there is a subpopulation of patients who develop thoracic aortic dissection despite relatively normal size. (4) Therefore, it is prudent to identify other parameters predictive of dissection.

Increased aortic wall thickness may be an important marker predictive of dissection. (5–7) In thoracic aortic dissections, the pathologic processes that lead to degeneration of the aortic media, also known as cystic medial necrosis, are complex. They involve smooth muscle cells apoptosis and disarray, destruction of the elastic fibers, and accumulation of proteoglycan in the aortic media. (7, 8) Ultrasound is a useful tool for measuring vascular wall thickness. (9) Carotid intimal- medial thickness (IMT), as measured by B-mode ultrasound, serves as a marker of cerebral vascular disease, and correlates with risks of future vascular events. (10) In addition, using transthoracic echocardiography, Gradus-Pizlo and colleagues are able to detect wall thickness as small as 0.9 ± 0.1 mm accurately. (11) Transthoracic ultrasound using transducer frequency 5–7 MHz yields an axial resolution of 0.2–0.3 mm at the depth of 5 cm. Given that transesophageal ultrasound commonly used to image the aorta utilizes similar or higher transducer frequency at shallower depth, we expect the axial resolution to be superior at < 0.2 mm and therefore appropriate to assess aortic wall thickness.

Materials and methods

Two independent observers obtained off-line, repeated measurements of the aorta diameter in the following locations: sinus of Valsalva, sinotubular junction, ascending and thoracic descending aorta in both transverse and longitudinal planes. When possible, both IMT and CMT measurements were obtained in the descending aorta. Care was taken to avoid measurements of sites with intramural hematoma or dissection flaps. The data was obtained from 48 patients with AD and compared with aortic measurements obtained from other 48 controls that had TEE done for PFO repair. Demographic data and cardiovascular risk factors of both groups were reviewed. Single and multiple variable analysis was performed, using Student’s t-test for continuous variables and χ2 test for categorical variables to compare the baseline characteristic as well as the different aortic diameter and wall thickness between the two groups. All tests were two tailed with P = 0.05 considered significant. We used SPSS for Windows version 14.0 (SPSS Inc., Chicago, IL, USA).

Results

The baseline characteristics of AD and PFO are displayed in Table-1. AD patients were older (66 ± 9 vs. 51± 10; P<0.001), had more hypertension, hyperlipidemia, diabetes and coronary disease. Giving the extension of the aortic dissection, consistent measurement of the IMT and CMT was technically feasible only in parts of the descending aorta. Combined IMT thickness at the level of 40cm beyond the incisors was greater in (AD) compared to controls (1.85 ± 0.5 mm vs. 1.43± 0.6 mm; P<0.001). The total aortic wall thickness was greater in (AD) patients compared with controls (2.93 ± 0.4 mm vs. 2.46± 0.5 mm; P<0.001). Figure-1 shows the comparison of different wall thickness measurements between both groups. As expected the diameter of the ascending aorta was greater in (AD) compared to controls (4.61± 1.5 cm vs. 2.92± 1.4 cm; P<0.001). Similarly the diameters were greater in AD group at the level of sinus of Valsalva (4.58 ± 1.6 cm vs. 2.89 ± 1.5 cm; P<0.001) and the sinotubular junction (4.21± 1.4 cm vs. 2.61± 1.3 cm; P<0.001). Figure-2 shows the comparison of different aortic diameters measurements between both groups.

Table 1.

Baseline Characteristics of Aortic Dissection Vs. PFO Patients

Characteristic	Aortic Dissection (N = 48)	PFO Patients (N = 48)	P-value
Demographics
Age (yrs)	66 ± 9	51 ± 11	<0.001
Female	32%	26%	0.183
African American	81%	76%	0.230
Hispanic	7%	10%	0.457
Past Medical History
Hyperlipidemia	43%	11%	<0.001
Hypertension	100%	65%	0.01
Diabetes	28%	14%	0.005
CAD	33%	8%	<0.001
Other Medications Ordered
Aspirin	84%	57%	<0.001
Clopidogrel	23%	5%	<0.001
Beta Blockers	63%	9%	<0.001
Angiotensin-converting enzyme inhibitors	72%	24%	<0.001

Figure-1.

Comparison of Intimal-Medial Thickness (IMT) and Complete Wall Thickness (CMT) between AD and PFO

Figure-2.

Comparison of Lumen Diameter at Different levels between AD and PFO Patients

Discussion

Aortic dissection is relatively rare, but devastating and unpredictable compared with other causes of cardiovascular death. It progresses rapidly in most patients from severe intense chest pain to instability due to any of the potential associated complications including acute aortic insufficiency, congestive heart failure, acute coronary occlusion, pericardial effusion and tamponade, stroke, syncope, limb ischemia, renal insufficiency, shock, rupture, and death. (3) Despite better diagnostic imaging methods and newer surgical techniques for treatment, the mortality of type A aortic dissection ranges from 14% to 30% and still averages ≈25%, which in part can be attributed to the inability to predict those at risk and provide preventive measures. (3)

Recognizing patients at risk for aortic dissection is difficult. The most commonly established risk factors are either clinical and very common hypertension, or imaging dependent as aortic dilation or aneurysm. Even special patients, which are known to be at risk for dissection as Marfan syndrome, Ehlers-Danlos syndrome, familial aortic aneurysm, and congenitally bicuspid aortic valve are often recognized after they present with an acute aortic syndrome. (3)

Historically, aortic dimensions have been used to determine timing of aneurysm repair because increased aortic size has been correlated with risk of dissection. However, there is a subpopulation of patients who develop thoracic aortic dissection despite relatively normal size. (4) Data from the International Registry of Acute Aortic Dissection (IRAD) that included 591 patients with type A dissection showed that the majority of patients had an ascending aorta diameter <5.5 cm and more than 40% had a diameter less than 5.0 cm at the time of dissection. (3) In our study the mean ascending aorta diameter was 4.6 cm with more than 70% of patient had a diameter less than 5 cm. Therefore, it is prudent to identify other parameters predictive of dissection.

Another potential predictor that can be promising in predicting those for risk for aortic dissection is aortic wall thickness. (5–7) Degeneration of the aortic media, known as cystic medial necrosis is a complex process that has been implicated in the pathogenesis of both aortic dissection and aneurysm. This process involve smooth muscle cells apoptosis and disarray, destruction of the elastic fibers, and accumulation of proteoglycan in the aortic media. (5, 7) There is growing evidence that many familial aortic dissection syndromes can be attributed to mutations encoding for contractile proteins. (8, 12, 13) In all patients with these mutations, there are typical histologic findings of medial degeneration of the aorta including focal areas of increased smooth muscle cells, as well as focal fibromuscular dysplasia of the vasa vasorum, characterized by lumen narrowing. (8, 12, 13) In addition, there are also increase deposition of mucopolysacccharide in the extracellular matrix, and smooth muscle cell hyperplasia in the aortic media, that may lead to diffuse thickening of the aortic wall. (8, 12, 13)

Carotid IMT serves as a marker of cerebral vascular disease, and correlates with risks of future vascular events. (10) There was evidence that carotid IMT is increased in patients with AD and abdominal aortic aneurysm. (14, 15)Therefore, direct aortic IMT may be even a better marker for predicting those at risk for aortic dissection. Trans-thorathic echocardiography transducers were used to detect the coronary wall thickness as small as 0.9 ± 0.1 mm. Transthoracic ultrasound transducers use frequency of 5–7 MHz yields an axial resolution of 0.2–0.3 mm at the depth of 5 cm. Given that transesophageal ultrasound commonly used to image the aorta utilizes similar or higher transducer frequency at shallower depth, we expect the axial resolution to be superior at < 0.2 mm and therefore it will be sufficient to be used for IMT and complete wall thickness measurements.

Our study has many limitations. First, we were unable to measure aortic IMT at the level of aortic arch secondary to intimal tear at that level. Second, we were not able to detect a specific IMT or aortic wall thickness that can set the threshold for predicting the occurrence of the aortic dissection and suggest a timeline for proposed protective surgery. Despite these limitations, our study shows that aortic wall IMT and full thickness can add an additional echocardiography predictor for those at risk for dissection and may help to do earlier interventions to prevent this devastating illness.

Conclusions

The aortic wall thickness and intimal/medial thickness detected by TEE at the level of 40cm beyond incisors is greater in patients with aortic dissection compared with PFO patients independent of aortic diameter. This may be useful in identifying patients with an increase risk of developing aortic dissection.