Bronchiectasis: current clinical and imaging concepts.

B RONCHIECTASIS should not be considered a single diagnostic entity. Rather, it should be treated as a morphologic abnormality representing the final common pathway of a variety of conditions that cause long-standing inflammation or infection of the bronchial tree.1 Pathologically, bronchiectasis is manifest as dilation of one or more branches of the bronchial tree; usually the dilation is such that multiple bronchi can be followed to within 1 cm of the pleural surface (as opposed to normal bronchi, which usually cannot be seen grossly within 2 cm of the pleura). 2 Historically the gross appearance of bronchiectatic airways has been divided into three different categories: cylindrical, saccular, and varicose (Fig 1). 3 Cylindrical bronchiectasis represents relatively uniform dilation of segments of the bronchial tree, usually involving sixthto eighthgeneration branches. Saccular bronchiectasis indicates balloon-like dilation of bronchial branches and usually involves thirdto fourth-generation branches. Finally, varicose bronchiectasis indicates an intermediate form between cylindrical and saccular bronchiectasis in which focally dilated bronchial segments are interposed between normal or narrowed bronchial segments. The dilation of the bronchial tree seen in bronchiectasis is essentially due to the destruction of the elastic and muscular layers of the bronchial wall. Occasionally a single prolonged infection, in conjunction with the severe inflammatory response that it elicits, may be sufficient to cause the bronchial injury necessary to elicit bronchiectasis. More typically, however, bronchiectasis is caused by a vicious cycle of recurrent infection and chronic inflammation. 4 The pathway to bronchiectasis classically begins in a host with impaired pulmonary clearance mechanisms; this impairment may be in-born and generalized as in patients with cystic fibrosis, or focal and acquired as in patients with bronchial obstruction. The subsequent stagnation of secretions within the tracheobronchial tree increases endoluminal pressure and predisposes to infection. Once an endobronchial infection begins, pathogens tend to remain within the bronchial tree for prolonged periods of time, leading to an exaggerated inflammatory response. The combination of prolonged infection and inflammation initially leads to destruction of the ciliated respiratory epithelium and submucosa before the infection is cleared. This mucosal and submucosal injury increases the risk of the host for future infection; thus the entire cycle will usually repeat itself, causing subsequent destruction of the elastic, muscular, and cartilaginous layers of the bronchial tree. This progressive bronchial injury eventually leads to bronchial dilation. 5 Table 1 lists selected predisposing factors for bronchiectasis. Although the list is long, it is far from comprehensive. However, the most common causes of bronchiectasis can be easily remembered through a systematic approach. The best method for approaching the differential diagnosis for bronchiectasis separates the list into two basic categories: causes of focal bronchiectasis and causes of diffuse bronchiectasis. The primary cause of focal bronchiectasis, especially in developing countries and among the disadvantaged in the United States, remains recurrent childhood infections. This includes undertreated bacterial infections as well as viral pneumonias caused by viral organisms, such as adenovirus and respiratory syncystial virus. Measles, whooping cough, and tuberculosis, although historically common causes of bronchiectasis, have decreased in importance with the advent of immunizations and chemoprophylaxis (Fig 2). 6 After childhood infection, endobronchial obstruction is probably the next most frequent cause of focal bronchiectasis on a worldwide basis. This includes obstruction from an endobronchial mass, such as a carcinoid or foreign body, intrinsic obstruction due to an airway stenosis or congenital defect, and extrinsic compression of the airway due to adenopathy or a large lung mass. 7 Aspiration should also be considered whenever one identifies focal bronchiectasis, especially if obstruction and childhood infection have been ruled out. On the other hand, diffuse bronchiectasis is usually due to a congenital or acquired deficiency