Intrathoracic herniation of abdominal contents occurs through congenital defects in the muscle, through traumatic tears or, most commonly, through acquired areas of weakness at the central oesophageal hiatus. Congenital hernias presenting in childhood are discussed elsewhere. When the defect is small it may not come to attention until adulthood, when it usually presents as an incidental abnormality on the chest radiograph. Bochdalek defects through the pleuroperitoneal canal occur along the posterior aspect of the diaphragm and the hernia usually contains retroperitoneal fat or a portion of kidney or spleen (Fig. 1). The majority occur on the left. A well-defined, dome-shaped, soft-tissue opacity is seen midway between the spine and lateral chest wall on the frontal view and above the posterior costophrenic recess on the lateral view. It may appear to 'come and go' on serial PA radiographs because of varying degrees of inspiration and differences in transdiaphragmatic pressure. It has been shown that asymptomatic small Bochdalek hernias are present in 6% of otherwise normal adults. These hernias appear on a lateral radiograph as a focal bulge centred approximately 4–5 cm anterior to the posterior diaphragmatic insertion. On CT and MRI the diagnosis can be made when a soft tissue or fatty mass is seen protruding through a small defect in the posteromedial aspect of either hemidiaphragm.

FIGURE 1 ■ Bochdalek hernia. (A) Lateral chest radiograph shows a focal bulge on the diaphragmatic contour just above the posterior costophrenic recess. (B) CT shows a fatty mass abutting the defect in the posteromedial aspect of the left hemidiaphragm.

A Morgagni hernia presents in adulthood as an anterior opacity at the right cardiophrenic angle. It frequently contains omentum and may contain bowel. Its smooth, well-defined margin and soft-tissue radiodensity usually allow its differentiation from the much more common fat pad collection at this site. It is more difficult to differentiate from a low-lying pericardial cyst. Morgagni hernias containing gut can be diagnosed using barium, but the diagnosis is more simply established by means of CT or MRI. Hernias through the oesophageal hiatus are extremely common, particularly in the elderly in whom they may be an incidental finding on CT.

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