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DEFINITIONS

A bulla is an air-containing space within the lung parenchyma that arises from destruction, dilation, and confluence of airspaces distal to terminal bronchioles and is larger than 1 cm in diameter (Fig. 50-1).1 Its walls are composed of attenuated and compressed parenchyma. Bullae occur in various clinical contexts: (1) with emphysema (“bullous emphysema”); (2) with pulmonary fibrosis, as in the late stages of sarcoidosis, complicated pneumoconiosis, or COVID-19;2 (3) in “vanishing lung,” in which the parenchyma is rapidly replaced by multiple bullae; and (4) in lungs that are otherwise normal (“bullous lung disease”) and, therefore, likely secondary to a mechanism different from that of bullae occurring in conjunction with emphysema (Table 50-1).3,4 Distinctions are drawn between bullae, blebs, and cysts (Table 50-2).3 A bleb is an accumulation of air between the two layers of the visceral pleura that arises when the thin covering of the bleb ruptures and permits entry of air (Fig. 50-1). Cysts are epithelial-lined cavities that may resemble bullae on radiographs.4 Many fall into the category of developmental anomalies and include mixtures of mesenchymal and epithelial components that are normally present in the lung. The pathologic nature of these cystic lesions is reflected in their names: “cystic adenomatoid malformations,” “peripheral bronchogenic cysts,” “congenital polycystic disease,” and “atypical bronchopulmonary sequestration.”1

Figure 50-1

Blebs and bullae. A. Development of a bleb. A bleb is an accumulation of air within the pleura that is not confined by connective tissue septa within the lung. Air that escapes within the substance of the lungs makes its way to the surface, separating the internal from the external elastic layers on the visceral pleura. B. Different types of bullae. In contrast to a bleb, a bulla is confined by connective tissue septa of the lung and is deep to the internal elastic layer of the visceral pleura. Three different types of bullae are shown arising from a lung that has been removed from within the chest wall. A type I bulla is shown at the apex, a type II is in the middle zone, and a type III is arising at the base. The short dark lines denote connective tissue septa. Panacinar emphysematous parenchyma is present within the types II and III bullae. (Adapted with permission from Reid L. The Pathology of Emphysema. Chicago, IL: Year Book; 1967.)

TABLE 50-1Characteristics of Bullae

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