- Can have morphological similarity to emphysema (below), however, it occurs in different situations with differing aetiology and implications
- Described as: "a condition of the lung characterised by an increase beyond normal in size of air spaces distal to the terminal bronchiole without destructive changes in the alveolar walls"
- It can arise in two ways :
- Obstructive
- Follows a "check-valve" type lesion in bronchi or bronchioles - the size of the overinflated area will obviously depend on the location of the obstruction, e.g. M. apri in pigs
- Non-obstructive
- Occurs when other lung units are decreased in size e.g. in enzootic pneumonia