Isolated left innominate vein aneurysm: a rare cause of mediastinal widening.

Mediastinal widening in an adult is mostly nodal in origin. Occasionally vascular aneurysms may be the underlying cause, in which case the aorta or its branches are most frequently involved. Thoracic venous aneurysms, on the other hand, have been reported only in anecdotes, with fusiform aneurysm of the superior vena cava being the commonest. Isolated aneurysms involving the brachiocephalic/innominate vein are extremely rare. We describe detection of a saccular aneurysm of the innominate vein, as the underlying cause of mediastinal widening seen on a chest radiograph in a 42-year-old asymptomatic woman. The characteristic radiological findings of thoracic venous aneurysms are described with particular reference to the importance of multiplanar computed tomography in such settings. Also discussed is the role of imaging in the diagnosis and guiding the management of this rare entity.