A pulmonary bulla may simulate a pneumothorax: mistaken chest tube placement can result in a bronchopleural fistula. Stability on old films is helpful to confirm a bulla. In addition, bullae tend to be bilateral and often have internal "cobwebs". Bullae and pneumothoraces have different shapes: a bulla is like a bubble in the upper lung, displacing normal lung inferiorly, please see the diagram above. Unfortunately, bullae occasionally rupture and cause a pneumothorax which results in a confusing appearance.