Overview of Proximal Aortic Aneurysms

Mr Saleem Jahangeer, Consultant Cardiac and Aortic Surgeon, St James’s Hospital Dublin aorta@stjames.ie

Anatomy of the proximal aorta

An understanding of the anatomy of the proximal aorta is essential when considering aortic aneurysms. The proximal aorta is divided into several segments:

Root: extends from Left ventricular outflow tract up to the sinotubular junction(STJ). Contains the coronary ostia, aortic valve and sinuses of Valsalva.

Ascending Aorta: above the STJ up to the level of the innominate artery

Arch: contains the epi-aortic vessels – The innominate artery, left common carotid artery and left subclavian artery.

Descending Aorta: Extending from the left subclavian artery to the level of the diaphragm.

Proximal aortic aneurysms

An aneurysm is defined as a dilatation of more than 1.5 times the size of the normal aorta. In cases of aortic aneurysm, approximately 60% occurs in the Root/Ascending aorta, 10% in the arch, 40% in the descending aorta, and 10% in the thoracoabdominal aorta; however, an aneurysm may involve multiple aortic segments. Very rarely, the aneurysmal process can involve the whole aorta; from aortic root to the iliac bifurcation.

Thoracic aortic aneurysms are asymptomatic in more than 95% of cases until diagnosed. The annual incidence of TAAs has been assessed at 6 to 10 cases/100,000 patient-years. Interestingly, the incidence of TAAs appears to be increasing, most probably due to an aging population and more frequent imaging.

Risk Factors for Aortic aneurysms:

A. Age/Atherosclerosis

B. High Blood Pressure/Bicuspid aortic valve

C. Connective tissue disorders – e.g. Marfan’s Syndrome

D. Dissections / Cystic media degeneration

E. Trauma, aortitis, infection, syphilis

F. Family history

Natural history of aortic aneurysms

The normal aorta expands by 0.9mm/10 years in male and 0.7mm/10 years in female. In the presence of an aneurysmal wall, expansion tends to occur faster in larger aneurysms (Law of Laplace). In dimensions > 5cm, the rate of expansion can be around 0.8cm per year. Patients with familial aortopathy have faster growth rates of 0.21 cm/year compared with patients with sporadic TAAs (0.16 cm/year) and thus present earlier. Marfan’s aortas grow at 0.1 cm/ year, Loeys-Dietz syndrome can grow faster than 1.0 cm/year, resulting in mean age of death at 26 years.

The main complications of an enlarging aortic aneurysm are: Aortic dissection and Aortic rupture, both of which can be fatal. Other complications would include mass effect on adjacent structures (e.g. tracheal compression) and aortic valve regurgitation due to stretching of the valve commissural posts. The annual risk of rupture or dissection is ∼2% for aneurysms between 4.0 and 4.9 cm, and reaches nearly 7% for sizes >6.0 cm. There is a drastic increase in the risk of rupture or dissection beyond a diameter of 6.0cm (Figure 2). Other independent risk factors for rupture include pain, age, smoking history and COPD.

Bicuspid Aortopathy

It is important to consider bicuspid aortic valve when discussing aortic aneurysm. The incidence of aortic dilation and acute complications (rupture and dissection) is higher in patients with BAV.

BAV has only 2 leaflets (functional or anatomical) as compared to the three leaflets of a normal aortic valve.

BAV are classified according to Siever’s Classification

1. Type 0: No raphe

2. Type 1: 1 raphe

3. Type 2: 2 raphes

The most common types of leaflet fusions are Right/Left fusion (70%), Right/Non-Coronary fusion (10-20%) and Left/Non-Coronary fusion (5-10%).

A Right/Left fusion tend to generate a right-handed helical flow in the aorta, resulting in dilatation of the Ascending +/- Root of the aorta. A Right/NonCoronary fusion generates a lefthanded helical flow which almost always only result in the dilation of the ascending aorta shows a 4-D flow MRI indicating the wall stress with bicuspid R/L fusion compared to a trileaflet aortic valve.

The combination of aortic coarctation and BAV is assumed to be associated with a higher risk of ascending aortic dilatation and dissection, and current European Society of Cardiology (ESC) guidelines advise therefore to operate at a lower threshold in the presence of a coarctation in BAV patients.

Diagnosis of aortic aneurysms

As mentioned, the vast majority of aortic aneurysms are clinically silent. Most aneurysms are incidentally diagnosed clinically, whether it is on a chest X-ray or CT scan done for another indication. Aortic aneurysms can also be incidentally picked up on echocardiogram. However, the smaller field of view with echocardiogram usually limits the diagnosis to the aortic root and ascending aorta only. The gold standard diagnostic tool for aortic aneurysms remains a CT aortogram with contrast. A gated CT scan is preferred to avoid motion artefact at the aortic root for accurate measurements. Magnetic Resonance Angiography (MRA) also provides excellent diagnostic images and avoids exposure to ionising radiation.

MANAGEMENT OF AORTIC ANEURYSMS

Medical Therapy

The aim of medical therapy is to decrease the shear stress on the aortic wall by decreasing blood pressure and cardiac contractility. B-Blockers are the first line anti-hypertensive agent with a target systolic BP of no more than 120mmHg. In patients with Marfan’s syndrome, prophylactic use of beta-blockers, angiotensinconverting enzyme (ACE) inhibitor, and angiotensin II receptor blocker have been shown to reduce either the progression of the aortic dilation or the occurrence of complications. Most patients with aortic diseases have comorbidities such as coronary artery disease, chronic kidney disease, diabetes mellitus, dyslipidaemia and hypertension. It is important that treatment and prevention strategies must be similar to those indicated for the above diseases. To prevent blood pressure spikes, competitive sports should be avoided in patients with an enlarged aorta. Isometric exercise with a high static load (e.g. weightlifting) should be discouraged.

Surgical Intervention

Surgical intervention provides definitive repair of the aneurysmal aortic segment. The main principle of surgery is to prevent aortic dissection or risk of rupture by replacing the aneurysmal aorta and restoring the aortic dimensions back to normal.

The type and extent of surgery will depend on which segments of the aorta are involved. Surgery for thoracic aortic aneurysm can range from a simple interposition graft to more complex procedures such as aortic root replacements, arch replacements and Frozen elephant trunk procedures

Indications for surgical intervention

Indications for surgery are based mainly on the aortic dimensions and derived from findings on natural history regarding the risk of complications weighed against the risk of elective surgery. Other than absolute size, other factors such as connective tissue disease, rate of expansion, family history, concomitant aortic valve surgery and symptoms are important to consider when deciding the timing of surgery. Generally speaking, in patients with no elastopathy, surgery should be considered for ascending aorta/root/arch aneurysms measuring 55mm or more. In patients with connective tissue disorders such as Marfan’s, a lower threshold of 50mm is generally indicated. If high risk factors are present along with Marfan’s, surgery is indicated at a size threshold of >45mm.

In bicuspid valve patients, surgery should be performed when the maximal aortic diameter is ≥55 mm; these face a lower risk of complications than in Marfan’s patients. A lower threshold of 50 mm can be considered in bicuspid patients with additional risk factors, such as family history, systemic hypertension, coarctation of the aorta, or rapid increase in rate of expansion. In patients undergoing aortic valve surgery who also have a dilated aorta, the aorta is replaced as well if the diameter is >45mm.

The latest guidelines from the ESC are summarised

Surveillance

Patients with aortic disease usually require life-long surveillance, regardless of the initial treatment strategy (medical or surgical). This surveillance consists of clinical evaluation, reassessment of a patient’s medical therapies and treatment goals, as well as imaging of the aorta. These patients should be followed up in a dedicated surveillance aortic clinic. Given the silent nature of aortic aneurysms, dedicated surveillance is vital to tackle this silent killer.

In patients with aneurysm that have yet to reach threshold for surgical intervention, surveillance is particularly important to monitor rate of expansion of the aneurysm and plan surgical intervention to avoid any potential dissection or rupture. In patients who had surgery, it is important to monitor the remaining segments of aorta over time, as it is not uncommon for these patients to develop aneurysmal segments of the remaining native aortic segments over time. We have recently set up a surveillance clinic dedicated for proximal aortic aneurysms here at St James’s Hospital.

Any referrals of suspected proximal aortic aneurysms can be sent by email: aorta@stjames.ie

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