Tracheomalacia is defined as tracheomegaly (increased tracheal size > 3 cm) and increased compliance (tendency of the airway to collapse on expiration), best assessed on a biphasic CT with images of the trachea obtained during inspiration and expiration.
Tracheomalacia could be secondary to congenital causes, including Mounier-Kuhn syndrome, as in this case, or other acquired causes.