Findings: inspiration film shows increased lucency of right lung, a smaller left lung and displacement of the mediastinum towards the left. Lucency of right lung disappears on expiration and the mediastinum shifts towards the midline. The left hemidiaphragm does not move (it is superposed to the 9th rib in inspiration and expiration).
This appearance is typical of obstruction of the left bronchus: on inspiration air enters in the right (normal) lung, which becomes more lucent and displaces the mediastinum towards the opposite (abnormal) left lung. On expiration, the right lung empties, attracting the mediastinum towards the right side.
The patient has extrinsic asthma. CT shows obstruction of left bronchus by thick mucus (arrows). She was treated with mucolytics and bronchodilators and the appearance of the chest returned to normal (Fig 3).
Final diagnosis: obstruction of left bronchus by thick mucus in a patient with extrinsic asthma.
Teaching point: remember that in unilateral bronchial obstruction the mediastinum shifts towards the abnormal lung on inspiration and towards the normal lung on expiration.