Aortic arch aneurysm

Aortic arch aneurysm

Characteristics

  1. Permanent localised dilatation of the thoracic aorta. The average diameter of the normal thoracic aorta is <4.5 cm. This is the commonest mediastinal vascular abnormality. Most are fusiform dilatations (some are saccular), associated with degenerative atherosclerosis with a mean age at diagnosis of 65.
  2. Dissecting aortic aneurysms or intramural haematomas are a specific form of thoracic aneurysm. Again, associated with hypertension and degenerative atherosclerosis, a split in the aortic wall allows blood to track between the intimal and adventitial layers of the aorta. They can occur following trauma. This can produce widening of the aorta and a very high risk of rupture. Slow flow in the false lumen can result in ischaemia and infarction to end organs supplied by the thoracic and ultimately the abdominal aorta. It can be graded by the Stanford classification into type A (ascending aorta and arch – 2/3) and type B descending aorta distal to major vessels (1/3).
  3. Other rarer causes include congenital causes, infection (mycotic aneurysms, e.g. bacteria or syphilis), connective tissue disorders (e.g. cystic medial necrosis in Marfan’s syndrome), inflammatory diseases (e.g. Takayasu’s) and dilatation post aortic valvular stenosis. Theseoccur in a younger age group.
  4. The size of the aneurysm increases with age.
  5. The risk of rupture increases with aneurysm size.

Clinical features

  1. This is commonly found incidentally on routine CXRs in asymptomatic patients.
  2. The patients may present with substernal, back and/or shoulder pains which can often be severe.
  3. Rarely patients may present with stridor, hoarse voice or dysphagia from the aneurysm compressing local mediastinal structures.
  4. Aortic dissection is associated with aortic regurgitation and cardiac failure, heart murmurs and differential blood pressure measurements in the arms. In addition dissecting aneurysms can produce ischaemia and infarction to end organs (e.g. stroke, renal failure, ischaemic bowel).
  5. Rupture of the aneurysm is almost always fatal with patients presenting with collapse and hypotension from hypovolaemic shock.

Radiological features

  1. CXR – a soft tissue mediastinal mass in the region of the aorta, measuring 4–10 cm. Wide tortuous aorta >4.5 cm. Curvilinear calcifications outlining the aortic wall. Left pleural effusions, left apical cap or left lower lobe collapse.
  2. CT – above findings _ extensive mural thrombus present within the aortic wall. In early aortic dissections the aortic wall may be thickened and of slightly increased attenuation. A dissection flap may be demonstrated with a double channel to the aorta. High attenuation lies within the false lumen, which is usually present in the superior aspect of the aortic arch. Contrast-enhanced CT demonstrates the differential flow within the two or more lumens. There may also be evidence of a haemopericardium with retrograde dissection back to the heart.
  3. MRI – contrast-enhanced MRA is a very good alternative to characterising the site and extent of aortic aneurysm, particularly dissecting aneurysms.
  4. Transoesophageal echocardiography – very sensitive in characterising aortic aneurysms and in particular the cardiac involvement in dissecting aneurysms.
  5. Angiography – the true and false lumens demonstrated may be of normal, reduced or enlarged calibre.
  6. Rupture may be associated with high attenuation fluid in the mediastinum and pleural space.

Differential diagnosis

  1. The differential diagnosis, other than the different types of aortic aneurysm already described, is chronic aortic pseudoaneurysm. This occurs in 2.5% of patients who survive the initial trauma of acute aortic transection. There is focal aortic dilatation with disruption of the aortic wall. Blood is contained by adventia and connective tissue only. The pseudoaneurysm increases in size with time and is at risk of rupture.

Management

  1. Aneurysm repair is considered in all patients when the aneurysmal size increases beyond 6 cm.
  2. Both surgical and endovascular stent grafting are successful treatment options.
  3. Surgical mortality is as high as 10%.
  4. Control of risk factors such as hypertension.
  5. Surveillance of aneurysms>5 cm.
  6. Dissecting aneurysms are a surgical emergency. ABC first line, then assessment and grading of the dissection. In particular 3D CT reformats are very helpful for completely assessing the extent and branch artery involvement. Early surgery considered in type A dissections.
  7. Non-surgical survival rates are lower than 10%.

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