How Big Is Too Big: When To Repair An Ascending Aortic Aneurysm
John S. Ikonomidis MD, PhD
Professor and Chief, Cardiothoracic Surgery, Medical University of South Carolina
Ascending thoracic aortic aneurysms are potentially lethal conditions which often require operation in
order to prevent rupture, dissection, and death. The decision to operate on the thoracic aorta is relatively
straightforward for patients without significant operative contraindications who present with very large, rapidly
expanding, or symptomatic ascending thoracic aortic aneurysms. In contrast, determining whether surgery is
indicated is more complex for asymptomatic patients with a gradually expanding, moderate-sized thoracic aorta.
In deciding when to operate, the surgeon must consider both the benefits of resection and the operative risks:
the ultimate objective is to select patients for whom the operative risks are justified. While much of the decision
making in this regard relies on the experience and judgement of the surgeon, more tools are becoming available
to assist in selection of the most appropriate time for operative repair. The oral presentation and this brief
review will outline the contemporary thinking with regard to size indications for ascending aortic replacement.
Normal aortic size
The normal diameter of the ascending aorta may be influenced by gender and BMI but is independently
associated with age.1 Hannuksela et al.1 used radiographic data from a normal population to develop a formula
that allows for calculation of the upper limit of normal diameter of the ascending aorta (D) relative to patient
age:
D(mm) = 31 + 0.16 × age (in years)
A modification (body mass index, BMI) is added in case of extreme weight:
D(mm) = 21 + 0.14 × age (in years) + (0.41 × BMI)
According to these equations, an ascending aortic diameter of 34mm is still “normal” in a 20-year-old
patient (average diameter 27 mm). In an 80-year-old patient (average diameter 37mm) a diameter of 44mm is
still classified “normal”. Adhering to the classic definition that an aortic diameter increase of 50% marks the
borderline between ectasia and aneurysm, the thresholds at which a dilated ascending aorta/root should be
considered an aneurysm are thus, 40mm in a 20-year-old, 45mm in a 40-year-old, 50mm in a 60-year-old and
55mm in an 80-year old. However, it is important to keep in mind that the above benchmarks may not predict
freedom from complications and do not take into account other factors such as body size and presence of
congenital bicuspid aortic valve (BAV) or other connective tissue disorders such as Marfan Syndrome (MFS).
Guidelines for Operation
The classic, widely quoted Yale study2 of the natural history of thoracic aortic aneurysm disease yielded
the following results:
As shown above, complications begin to occur in ascending aortic aneurysms after 5 cm diameter, with
a dramatic and highly statistically significant “hinge point” occurring at 6 cm. Accordingly, current guidelines
recommend that for degenerative aneurysms of the ascending aorta, open replacement should be undertaken
when the aortic diameter exceeds 5.0 to 5.5 cm.3 For connective tissue disorders such as MFS, Ehlers-Danlos
syndrome, BAV disease and Turner’s syndrome, the size limit is reduced to 4.5-5.0 cm.3-5 In addition, any
aneurysm that does not meet size criteria for surgery but which has a growth rate greater than 0.5 cm/year
should be considered for surgery.
Symptomatic aneurysms of any size should undergo surgery, as should
aneurysms associated with acute aortic dissection.3
Indices Used to Risk Stratify Patients with Ascending Aortic Aneurysms
There is evidence to suggest that the mechanical characteristics of the aortic wall are related to size or
body mass. Natural history studies have shown, for example that aortic aneurysms have a tendency to rupture at
smaller sizes in smaller people. Accordingly, a variety of “biometric indices” have been developed which allow
for risk stratification by body size. In 2006, Davies et al.6 proposed an aortic size index, plotting body surface
area (BSA) versus aortic diameter for risk stratification and surgical indication in patients with degenerative
thoracic aortic aneurysms:
Thus, a patient with a BSA of 1.30 m2 and an aortic diameter of 4.0 cm can be considered to have the
same rupture risk (approximately 8%/year) as a person with a BSA of 2.50 m2 and an aortic diameter of 7.0 cm.
Further, Svensson et al. implemented a ratio to calculate operative risk, by using the following formula
including aortic width (r), cross-section area, and patient height:
r2 × π (cm2)
height (m)
This ratio has been applied to patients with MFS4 and BAV5 disease and it has been recommended that
ratios greater than 10 be considered for surgery for both disease states.
With regard to BAV disease, cusp fusion pattern has been linked with variations in protease activity
within the aneurysm wall such that the left-right (L-R) fusion pattern (most common, about 70% of BAV) is
associated with the most aggressive activity.7 Thus, surgeons operating on a patient with a BAV who encounter
a borderline aneurysmal ascending aortic may be swayed to replace the aorta if the valve cusp fusion pattern is
L-R.7
More recently, stress relationships has been successfully mapped within ascending aortic aneurysms,
allowing identification of areas within the aorta which are at increased risk for dissection and rupture:8
Shown above is a stress map of a patient with BAV associated ascending aortic aneurysm disease. One
can see from the images that the area of highest stress is concentrated on the lesser curvature of the aorta just
above the sino-tubular junction – a very typical location for the primary intimal tears associated with Stanford
type A dissections.
Finally progress is being made with regard to risk prediction of aneurysm etiology and size based upon
candidate biomarkers,9 some of which are measurable in plasma.10 These may ultimately provide a desktop tool
allowing individual patient-specific screening for and risk stratification of ascending aortic aneurysm disease.
References
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2006;40:175–178.
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thoracic aortic aneurysms: a study of growth rates and complications. Ann Thorac Surg 1999;67:1922–
1926.
3. Hiratzka, LF, Bakris GL, Beckman JA, et al. 2010 ACCF/AHA/AATS/ACR/ASA/SCA/SCAI/SIR/
STS/SVM Guidelines for the diagnosis and management of patients with thoracic aortic disease:
executive summary. Circulation 2010;121:1544-1579.
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patients with Marfan syndrome. J Thorac Cardiovasc Surg 2002;123:360-361.
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Gorman JH III, Gorman RC, Spinale FG, Jones JA. Aortic dilatation with bicuspid aortic valves: cusp
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9. Jones JA, Stroud RE, O’Quinn EC, Black LE, Elefteriades JA, Bavaria JE, Gorman JH III, Gorman RC,
Spinale FG, Ikonomidis JS. Selective microRNA suppression in human thoracic aneurysms: relation to
aortic size and proteolytic induction. Circ Cardiovasc Genet 2011;1:605-613.
10. Ikonomidis JS, Ivey CR, Wheeler JB, Akerman AW, Rice A, Patel RK, Stroud RE, Shah AA, Hughes
CG, Ferrari G, Mukherjee R, Jones JA. Plasma biomarkers for distinguishing etiological subtypes of
thoracic aortic aneurysm disease. J Thorac Cardiovasc Surg 2013;145:1326-1333.