A 68 year old man was admitted to hospital for an ischaemic stroke. His medical history revealed hypertension, diabetes mellitus, chronic atrial fibrillation, and a blunt trauma sustained during a motor vehicle accident. Routine chest x ray revealed an enlarged superior mediastinal silhouette (left panel). Subsequently, magnetic resonance imaging (MRI) showed a thoracic saccular aneurysm of the aortic arch with involvement of the branches (right panel).

Transverse arch aneurysms are relatively uncommon, comprising only 11% of all aortic aneurysms. Saccular thoracic aneurysms are frequently post-traumatic (high speed deceleration accidents); other causes include syphilis, autoimmune disorders, and atherosclerosis. The most frequently involved location is the relatively immobile isthmus just distal to the origin of the left subclavian artery, adjacent to the attachment of the ligamentum arteriosus.

Many thoracic aneurysms are visible on chest x ray and are characterised by widening of the mediastinal silhouette, enlargement of the aortic knob or displacement of the trachea of the midline.

However, saccular aneurysms may not be evident on the chest x ray.

MRI is useful in detecting aneurysms and especially magnetic resonance angiography may prove to be useful in defining the anatomy of aortic branch vessels. The aneurysm in this patient may be caused by atherosclerotic disease and/or his deceleration trauma.