The axial images in this case demonstrates initially relatively normal appearing lungs and airways. More careful inspection, however, demonstrates that there is an additional airway arising medially from the proximal bronchus intermedius. This is the typical location of a blind ending supernumerary bronchus termed the accessory cardiac bronchus. This is considered the only true supernumerary bronchus and is pretty rare occurring in 0.1 to 0.5% of cases. This bronchus may extend 1-5 cm and is usually as in this case blind ending and asymptomatic. However, the blind ending nature impedes clearance of mucus and secretions and in some cases this may result in recurrent inflammation or even haemoptysis. In these cases surgical intervention may be required. A small minority of cases are not blind ending and can be associated with a small ventilated area of lung although typically this is somewhat malformed. The abnormality may be associated with other anomalies of bronchial branching and isolated reports of other congenital variations such as abnormal drainage of the right pulmonary artery directly into the left atrium are described in the literature.
The clue to the diagnosis is the highly typical location. Nothing should arise medial to the bronchus intermedius. The findings are easily confirmed on coronal oblique reformats, minimum intensity projections or volume rendered images below. Reporting the finding can be helpful for unexplained symptoms and to alert bronchoscopists who may be unfamiliar with the findings due to their rarity. Remember the average radiologist sees many more airways than the average bronchoscopist, a rare finding for us is super-rare for the bronchoscopist!