Infection of the pleural space leading to empyema formation, and the importance of clearing infection and pus from this space, has been recognized since ancient times. Historically, empyema was associated with pneumococcal pneumonia, with Streptococcus pneumoniae causing up to 70% of pleural space infections. With effective antibiotic treatment for pneumonia, the incidence of empyema has decreased markedly, and the spectrum of causative organisms has widened, with S. pneumoniae now accounting for as few as 10% to 20%. However, parapneumonic effusions occur in 30% to 60% of pneumonia cases, and, when empyema occurs, it is associated with an overall mortality of 20%.

Parapneumonic effusions are classified as simple or uncomplicated, complicated, and empyema, based on the appearance and biochemical characteristics of aspirated fluid, which supports the clinical impression of a continuum of disease (Table 35.1).

This classification also has clinical utility in that, during the early acute phase, with free flowing fluid, treatment is simpler than in the more chronic fibropurulent stage associated with multiple loculations and the need for greater interventional therapy. Empyema may be defined as the presence of organisms and numerous host defense cells, neutrophils, in the pleural fluid, or, more narrowly, as pus apparent to the naked eye. Bronchopleural fistula (BPF) may be caused by an empyema or may be associated with empyema following surgery, penetrating lung injuries, or a lung abscess.