An aneurysm is defined as a persistent, abnormal dilatation of an artery to 1.5 times its normal diameter.

A thoracic aortic aneurysm can involve the ascending aorta or aortic root (60%), aortic arch (10%), descending aorta (40%), or thoracoabdominal aorta (10%) segments*.

Whilst thoracic aneurysms are less common than abdominal aortic aneurysms, they are associated with high mortality. They have an incidence of 6 in 100,000 person-years, with an increasing prevalence with age.

*This classification is important because aneurysms in different locations present with different symptoms, and have different levels of surgical risk.

Aetiology

Thoracic aortic aneurysms develop due to degradation of the tunica media, the layer of the artery which provides tensile strength and elasticity to the wall. As a result, the artery loses structural integrity and dilates, and as the diameter increases, the wall tension rises and further increases the diameter in a vicious cycle.

By TeachMeSeries Ltd (2019)

Figure 1 – Lateral view of the thoracic aorta, including the intercostal branches

The main causes of thoracic aneurysm are:

• Connective tissue diseases
  • Marfan’s syndrome or Ehlers-Danlos syndrome.
• Bicuspid aortic valve
  • Increased risk of thoracic aortic aneurysms is seen in patients with Turners syndrome (from increased risk of bicuspid aortic valve)
• Trauma
• Aortic dissection
• Aortic arteritis, e.g. Takayasu Arteritis
• Tertiary syphilis

Thoracic aortic aneurysms grow at a mean rate of 1-2mm/year. This rate is higher in those with Marfan’s syndrome, descending aneurysms (compared to ascending aneurysms), and a dissected aneurysm (compared to a non-dissected).

Risk Factors

The main risk factors for developing a thoracic aortic aneurysm include family history*, hypertension, atherosclerosis (specifically descending aneurysms), smoking, high BMI, male gender, and advancing age.

*19% of patients have a positive family history

Clinical Features

Typically thoracic aneurysms are asymptomatic and are found incidentally.

In those that are symptomatic, the most common presenting complaint is pain, with the location of the pain potentially localising the aneurysm:

Table 1 – Site of Thoracic Aneurysm vs. Typical Associated Location of Pain

Other symptoms of thoracic aneurysms include:

• Back pain – secondary to spinal compression by descending or thoracoabdominal aneurysm
• Hoarse voice – from damage to the left recurrent laryngeal nerve in arch aneurysms
• Distended neck veins – from SVC compression
• Symptoms of heart failure – from involvement of the aortic valve
• Dyspnoea or cough – secondary to tracheal or bronchial compression

Clinical signs are not commonly found on examination, however chronic disease may present with the signs of aortic root disease or heart failure.

Thoracic aneurysms have a risk of rupture or dissection, which are potentially lethal. An acute aortic syndrome will present with sudden onset pain in the back, chest, neck, and/or abdomen

Differential Diagnosis

As thoracic aneurysms are mostly found incidentally on imaging, other differential diagnosis are rarely considered before the definitive diagnosis is determined.

In symptomatic patients, the scope of symptoms for a thoracic aneurysm is wide. However for those presenting with chest or back pain (the most common presentation), diagnoses of ACS, PE, pneumothorax, and aortic dissection should all be considered.

Investigations

Thoracic aneurysms are diagnosed through imaging. However, initial in-patient and pre-operative testing should include routine bloods (FBC, U&Es, clotting) with a group and save, ECG, and CXR performed.

Imaging

Many thoracic aneurysms are first identified as incidental findings on imaging.

A thoracic aneurysm can be seen on CXR demonstrating a widened mediastinal silhouette, an enlarged aortic knob, and possible tracheal deviation. However, a radiograph is not sensitive enough to make the definitive diagnosis and further imaging is required

A CT chest scan with contrast is the preferred imaging modality for thoracic aneurysms, providing sufficient detail to ascertain the level and the size of the aneurysm*.

Transoesophageal echocardiography (TOE) can be used to good effect to further detect any concurrent aortic insufficiency or dissection; TOE should form part of the routine assessment of patients with Marfan’s disease and suspected thoracic aortic disease.

*Due to the potentially tortuous nature of the thoracic aorta and the use of axial images in CT scans, care must be taken to ensure the actual aortic diameter is being measured, not simply measuring through the aorta off-axis; reconstructing axial imaging into 3D images via CT angiography or MR angiography can overcome this issue

Adapted from work by James Heilman, MD (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Figure 2 – X-ray of a thoracic aortic aneurysm. A – Arrow marks the lateral border of the aorta. B – after endovascular repair.

Management

Despite limited evidence, patients with a confirmed thoracic aneurysm should be started on medical management*, alongside any concurrent further investigations and definitive management planned.

These patients are at increased cardiovascular risk, therefore should be initiated on statin and antiplatelet therapy to decrease the risk of myocardial infarction. Blood pressure should be controlled and smoking cessation is imperative.

*There is some evidence that suggests beta-blocker therapy in patients with a Marfan’s related thoracic aneurysm can slow the rate of growth, but this has not yet been extrapolated into otherwise healthy patients.

Surgical Management 

Surgical management is dependent on the location of the aneurysm, with the threshold for surgery varying according to the health of the patient. Patients suffering from Marfan’s syndrome or having a previous thoracic dissection have a greater risk of dissection and rupture, and threshold levels for intervention are often lower

Prognosis

Development of a second aneurysm is not uncommon in these patients post-intervention, so ongoing imaging studies as an outpatient are required following surgery, either with CT or MRI.

Recent studies demonstrate a perioperative mortality of between 2-17%, with arch aneurysms having the highest mortality, at 25% mortality. Mortality is significantly lower in those undergoing endovascular repair.

In patients who are monitored prior to surgery, the annual risk of rupture or dissection is 2% when <5cm, 3% for 5-5.9cm, and 7% for those 6cm or above.

By James Heilman, MD (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Figure 3 – Contrast CT scan demonstrating a 7cm thoracic aortic aneurysm (black arrow), which has ruptured. The white arrow demonstrates blood in the thorax.