Thoracic Aortic Aneurysm
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. This classification is important because aneurysms in different locations present with different symptoms, and have different levels of surgical risk.
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.
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.
The major 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)
- Tertiary syphilis
- Aortic dissection
- Aortic arteritis – e.g. Takyasau Arteritis
Thoracic aortic aneurysms grow at a mean rate of 1mm/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).
The main risk factors for developing a thoracic aortic aneurysm include family history (19% patients have positive family history), hypertension, atherosclerosis (mainly influencing descending thoracic aortic aneurysm development), smoking, high BMI, male and advancing age.
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:
|Site of Aneurysm||Location of Pain|
|Ascending aorta||Anterior chest|
|Descending aorta||Between the scapulae|
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 is potentially lethal. An acute aortic syndrome will present with sudden onset pain in the back, chest, neck, and/or abdomen.
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.
Thoracic aneurysms are diagnosed through imaging. However, initial in-patient and pre-operative testing should include FBC, U&Es, clotting, and a group and save, alongside ECG and CXR performed as a surgical workup.
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, x-ray is not sensitive enough to make the definitive diagnosis and further imaging is required
A contrast CT scan 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
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 also 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 is dependent on the location of the aneurysm, with the threshold for surgery varying according to the health of the patient.
Surgical Intervention Criteria
Whilst cut off criteria will vary between countries and centres, a guide to thoracic aneurysm surgical interventions* is:
Treated when the diameter >5.5cm, the affected region of the aorta is excised and replaced with a dacron graft. If the aortic root is involved a Bentall procedure is performed, which uses a graft that contains a prosthetic aortic valve.
Threshold varies significantly according to the performance of the patient, but once an aneurysm is over 5.5cm surgery should be considered. The affected aorta is replaced with a multi-limbed graft, allowing for the branching of the great vessels (these procedures have a high risk of cerebral ischaemia from embolisation). Deep hypothermic circulatory arrest, with anterograde/retrograde cerebral perfusion is used to protect the brain. An alternative approach is to perform sequentially bypasses from the right to left carotid, and left carotid to left subclavian, followed by endovascular stenting of the arch.
Intervention is indicated when the diameter exceeds 6.5cm. These can be repaired open, or with endovascular techniques. There is a risk of paraplegia, secondary to spinal cord ischaemia during this surgery. Endovascular techniques have been shown to produce fewer postoperative complications, and to have a lower mortality.
*Patients suffering from Marfan’s have a greater risk of dissection and rupture, and threshold levels for intervention are often lower
Development of a second aneurysm is not uncommon in these patients, so ongoing imaging studies as an outpatient are required following surgery.
Recent studies demonstrate a perioperative mortality of between 2-17% overall, with arch aneurysms having a 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.