Pneumothorax is diagnosed by the presence of a white visceral pleural line on the chest X-ray, with no pulmonary vessels visible beyond this.

Look carefully at the lung parenchyma for evidence of underlying disease.

Contralateral shift of the trachea and mediastinum with depression of the hemi-diaphragm are typical features of tension pneumothorax although may not always be evident.

Do not mistake an emphysematous bulla for a small pneumothorax. Points in favour of a bulla are:

• Adhesions between the lung and the parietal pleura
• A scallop-shaped edge to the cavity
• Faint markings over the lucency caused by the lung enfolding the bulla
• The presence of other bullae

If you suspect a pneumothorax, but the chest X-ray appears normal, recheck the lung apices and the right border of the heart. In the supine patient look for:

• Unusually sharp appearance of the cardiac border or diaphragm, with increased transradiancy of the adjacent parts of the thorax and abdomen
• A vertical line parallel to the chest wall (caused by retraction of the middle lobe from the chest wall)
• A diagonal line from the heart to the costophrenic angle