Unilateral absence of ventilation and perfusion due to thoracic aortic aneurysm.

An elderly man presented with progressive atelectasis of the left lung, a mediastinal mass, and dilatation of the thoracic aorta. Ventilation and perfusion radionuclide lung scans revealed the complete absence of ventilation in the left lung and near-complete absence of perfusion. Evaluation by MRI and angiography showed an obstructing mediastinal "mass" to be a saccular aortic aneurysm. A thoracic aortic aneurysm should be a consideration in the differential diagnosis for unilateral decrease or absence of ventilation and perfusion with a history or findings of mediastinal mass or aneurysm.