The chest x-ray shows a large right-sided hydropneumothorax. There is a right-sided intercostal chest drain in situ. Subcutaneous emphysema can be seen in the right chest wall, as well as the neck.

Causes of hydropneumothorax:

• Complication of thoracocentesis (which is the case in this particular patient)
• Bronchopleural fistula
• Oesophageal rupture
• Post-pneumonectomy
• Chest trauma

How to differentiate a pleural effusion from a subtle hydropneumothorax on an erect chest x-ray?

A pleural effusion on an erect frontal chest x-ray will exhibit a meniscus sign, whereas hydropneumothorax will be evidenced by a horizontal fluid level due to the loss of surface tension.