Interstitial pulmonary emphysema. Interstitial pulmonary emphysema is the dissection by air around bronchovascular bundles and along interlobular septa as the result of rupture of alveoli, usually in association with mechanical ventilation. It may also be seen in patients dying due to acute asthma, as a result of cardiopulmonary resuscitation, and in association with a variety of infectious diseases. It may be acute or persistent (characterised by a foreign body giant cell reaction to the air) and may be localised to a single lobe or distributed diffusely through all of the lobes [23, 28]. Some of the air may track into lymphatics (but not the pleural lymphatics), raising the differential diagnosis of congenital pulmonary lymphangiectasia, which also involves the pleural lymphatics. Immunohistochemical staining for endothelial cells is helpful in localising the cysts. Infantile lobar emphysema. Infantile lobar emphysema refers to an overexpansion of a pulmonary lobe occurring in an infant, leading to pulmonary compromise. Pathologically, there is a spectrum of morphological changes, with the two most common patterns being overdistension of normal-appearing lung (classic pattern) and increased complexity of alveoli with increased radial alveolar counts (polyalveolar lobe or hyperplasia of lung figure 13 shows the classic pattern and a polyalveolar lobe at the same power). These two morphological patterns may occur due to the different timing of an inciting lesion leading to anatomical or functional bronchial abnormality, and the degree of obstruction of the bronchus, with a relatively closed bronchus and fluid retention resulting in hyperplasia. Conversely, a weak bronchial wall without fluid retention would result in relatively normal lung development in utero, with the bronchus collapsing only after birth, leading to overdistension of alveoli [29]. Fig. 13. – Infantile lobar emphysema. a) Classic pattern of overdistension of alveoli, and b) polyalveolar lobe with complex alveolar architecture but relatively normally sized alveoli (haematoxylin and eosin stain). CLASSIFICATION OF CHILDHOOD CYSTIC LUNG LESIONS 17