Intra-abdominal infections, especially subphrenic abscesses, can occur through the diaphragm to cause empyemas. Uncommonly, esophageal rupture can cause spread of infection from the mediastinum to the pleural space. Finally, hematogenous seeding is an infrequent mechanism of empyema formation. At present, empyema is much less common, and a change in bacterial etiology has occurred. S. aureus is the most common cause of empyema, and various gram-negative bacilli, including P. aeruginosa, K. pneumoniae , and E. coli , are next in frequency.
Many infections are mixed.
Anaerobes have been recognized in 25 to 76 percent of empyemas and may occur in pure culture or in combination with aerobic or facultative organisms. Bacteroides species, Fusobacterium , and anaerobic gram-positive cocci are the anaerobes most often seen. M. tuberculosis has become a relatively uncommon cause of pleural space infections, and fungi are implicated only rarely. Transdiaphragmatic rupture of a liver abscess occasionally produces amebic empyema. In the majority of patients, the clinical presentation of empyemas includes fever, dyspnea, chest pain, and cough.