Posterior wall of bronchus intermedius

The posterior wall of the bronchus intermedius (PWBI) is a thin, well-defined anatomical structure that forms the posterior boundary of the bronchus intermedius - the segment of the right bronchial tree that extends from the takeoff of the right upper lobe bronchus to the origin of the right middle lobe bronchus and right lower lobe bronchus.

Anatomical Definition

The bronchus intermedius is unique in the tracheobronchial tree because its posterior wall is exceptionally thin, bordered posteriorly only by aerated lung (the superior segment of the right lower lobe). This creates a distinctive interface: the PWBI is outlined on its luminal side by air within the bronchus and on its external side by air in the adjacent lung parenchyma. This air-tissue-air interface makes the PWBI a clearly identifiable structure on both imaging and bronchoscopy.

As a Radiologic Landmark

On lateral chest radiographs, the PWBI appears as a thin, well-defined stripe. Its normal thickness has been established at ≤3 mm; thickening beyond this threshold or an increase of ≥1.5 mm compared to prior studies is considered abnormal.

Thickening of the PWBI serves as a radiographic sign of several pathologic processes, including congestive heart failure (the most common cause, producing uniform thickening), neoplastic infiltration or hilar lymphadenopathy (which may produce uniform or lobulated thickening), sarcoidosis, and right lung pneumonia.

On CT, the PWBI is similarly well visualized and allows more precise measurement and characterization of thickening. Focal nodularity of the PWBI on CT is seen in approximately 5% of normal subjects and is most commonly caused by a draining pulmonary vein (from the posterior segment of the right upper lobe or the superior segment of the right lower lobe), a normal variant that should be distinguished from pathologic thickening.