Total collapse of a lung occurs when the obstruction is within the main stem bronchus. Once again a carcinoma is the commonest cause in adults.
The appearance is one of total opacification of the affected hemithorax. The volume loss causes deviation of the trachea and shift of the mediastinum to the affected side. Sometimes the opposite lung may herniate across the midline, giving rise to the impression that some lung remains aerated. This is particularily so at the apices, at the site of the anterior and posterior junctional lines.
The main differential diagnoses for this appearance are massive pleural effusion and pneumonectomy. In the latter there may be evidence of the surgery, either a rib abnormality or a visible metallic suture line at the hilum. An effusion will produce midline shift in the opposite direction. However collapse and effusion often coexist, in which case there may be minimal shift.
That concludes this tutorial. Feedback is always welcome.