The direction in which a cyst develops and its final shape depend largely upon its location within the lung relative to adjacent anatomical structures. Thus, a centrally located cyst tends to remain round, whereas others near the periphery may become polycyclic, lobulated, or umbilicated because of pressure from nearby vessels or bronchi (Fig. 3.83A,B), or from adjacent cysts. Where there are multiple cysts, fusion is prevented by the host-derived pericyst around the hydatids.

Pulmonary hydatids vary from 1 cm in diameter to an enormous cyst of 20 cm or more, obliterating an entire lung and displacing the heart and mediastinum to the opposite side. Cysts may cause sufficient compression to cause death from asphyxia. Usually, however, the patient with an uncomplicated cyst will have few or no clinical symptoms and the cyst may be found accidentally on routine chest radiography. Only when the hydatid is of sufficient size will there be signs of compression, such as dyspnea, bronchitis, pleurisy, or distention of the thoracic wall. Other symptoms develop when the cyst becomes complicated by rupturing into either the bronchial tree or pleura. The coughing up of "grape skins" is pathognomonic of rupture of a hydatid cyst into a bronchus and may effect a natural cure or result in secondary hydatidosis via bronchial spread. If the cyst ruptures into the pleura, a hydropneumothorax, pyopneumothorax, empyema, or bronchopleural fistula may result. Residual pleural thickening and fibrosis may ensue, or multiple secondary pleural cysts may develop.

As the pulmonary hydatid becomes older and larger, its sharp contour on chest radiographs may become hazy as the fibrous pericyst grows thicker and less regular. Localized compression atelectasis may further complicate its differentiation from other circumscribed pulmonary lesions. The correct diagnosis may be suggested radiographically only if there is evidence of umbilication (which can occur also with some primary or metastatic lung neoplasms) (Fig. 3.83A,B) or if hydatid cysts are noted elsewhere within the liver or other organs. In prior decades, bronchography would demonstrate displacement of bronchi around the cyst with poor filling of the smaller bronchi because of compression (Fig. 3.92C), but these findings were nonspecific and seen in other benign lung tumors or cysts.