Blebs and Bullae

forces to the lung parenchyma and the laceration of the lung parenchyma. If a family history of recurrent pneumothoraces is elicited, a diagnosis of BirtÐHoggÐDube syndrome may be suggested [43]. This condition is associated with pneumothoraces, renal cell carcinomas, and skin Þbrofolliculomas.

Key Points for Differential Diagnosis

1.When assessing pulmonary cysts on HRCT, the most important feature that allows differentiation among diseases is their distribution. After then, careful observation of cyst shape and size, as well as for the presence of any ancillary Þndings are needed to reÞne the differential diagnosis.

2.Although it is not always possible to distinguish a bulla from a lung cyst or a pneumatocele, bullae are uncommon as isolated Þndings, except in the lung apices, and are usually associated with evidence of extensive centrilobular or paraseptal emphysema. The distinction between bleb and bulla is difÞcult on HRCT, but is of little practical signiÞcance.

3.The combination of nodules, cavitary nodules, and bizarre shaped, uneven-sized cysts with upper and middle lung zone predominance in a smoker allows a conÞdent diagnosis of LCH [35].

4.Although diffusely distributed multiple lung cysts are the characteristic features of lymphangioleiomyomatosis, a few scattered cysts in a woman who presents with a pneumothorax may signify an early form of lymphangioleiomyomatosis.

5.Predominant CT Þndings of lymphocytic interstitial pneumonia (ground-glass opacity and poorly deÞned centrilobular nodules) and combinations of interlobular septal thickening, thickening of bronchovas-

cular bundles, and scattered distribution of cysts may be helpful in differentiating lymphocytic interstitial pneumonia from other cystic diseases [35].

6.A combination of lack of a perceptible cyst wall and areas of ground-glass opacity around the cysts in individuals exposed to cigarette smoking is a characteristic feature of desquamative interstitial pneumonia [35].

7.The presence of multiple thin-walled, complex cystic masses usually allows the diagnosis of CCAM or pulmonary sequestration. The essential feature in the diagnosis of pulmonary sequestration is demonstration of the systemic arterial supply to the abnormal lung [44].

8.The identiÞcation of cystic lesions in an HIVpositive patient is highly suggestive of Pneumocystis jirovecii pneumonia. Pneumocystis pneumonia cysts are usually accompanied by various degrees of ground-glass opacity.

9.BirtÐHoggÐDube syndrome may manifest as a spontaneous pneumothorax or as lung cysts found at the lung bases in patients with a solid renal mass. The cysts are usually randomly scattered and have lower lung zone predominance. The large dominant cysts tend to be located in the lung bases and may be multiseptated [45].

Blebs and Bullae

Pathology and Pathogenesis

Bulla is a pathological entity caused by the conßuence of two or more of the terminal elements of the bronchial tree. They may be found subsequent to any condition that interferes with normal respiratory mechanism that produces increased intra-alveolar pressure. A single bulla is a rarity;