Paratracheal air cysts are not an uncommon incidental finding in routine thoracic imaging. They characteristically occur on the right side, in the region of the thoracic inlet. Occasionally they may mimic pneumomediastinum, so-called pseudopneumomediastinum.
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Terminology
Paratracheal air cysts encompass a number of entities including 9:
There is overlap in the use and description of the terms paratracheal air cyst and tracheal diverticulum in the literature, and they often seem to be synonyms for the same entity when in fact they are not 9,10.
Epidemiology
They are present in 3-4% of the population. They may be seen in children and adults with an increased female predilection 4.
Associations
chronic obstructive pulmonary disease (COPD): for acquired type 8
Pathology
Their exact cause is unclear and may be either a congenital or acquired phenomenon. The acquired form is thought to be due to prolonged increased intraluminal pressure, e.g. due to a chronic cough, COPD.
The cysts may be unilocular or multilocular and do not communicate with the trachea. A direct connection with the trachea is visible on CT in ~35% of cases 10, in which case the term tracheal diverticulum may be used.
Location
Paratracheal air cysts project posteriorly where the cartilage rings are deficient and usually lie to the right where there is no esophagus supporting the paratracheal tissue. They usually occur at the level of the thoracic inlet.
They can occasionally present in other locations such as the subcarinal region: subcarinal air cysts
Radiographic features
CT
Paratracheal air cysts are typically located on the right side at the level of the thoracic inlet. Most measure 3-10 mm and are elongated in shape 5.
Treatment and prognosis
Paratracheal air cysts are an incidental finding and no treatment is required. The biggest point is to not mistake them for something more sinister.
Complications
Although usually asymptomatic, a paratracheal air cyst may accumulate respiratory secretions that become infected and lead to coughing or tracheobronchitis.
Differential diagnosis
Imaging differential considerations include:
pneumomediastinum: gas usually encircles the trachea
apical pneumothorax: look for other sites of intrapleural gas
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