Empyema is caused by infected parapneumonic effusions, which begin as sterile fluid collections. The first phase is the exudative phase, with free-flowing, nonviscous fluid, which overlaps with parapneumonic effusion; the second is the fibrinopurulent phase (Fig. 95.1), with viscous fluid, loculations, and pleural peels; and finally there is the organizing phase, with fibrosis (Figs. 95.2 and 95.3).² The causative organism or organisms are generally that of the adjacent pneumonia and reflect the clinical setting (Chapter 38).

Empyema may also result from hematogenous or direct spread of bacterial infections caused by osteomyelitis, subdiaphragmatic abscesses, surgery, trauma, and gastropleural fistulas, in which cases causative organisms are varied.^2,3,4,5

Pneumonias that result in either effusions or empyema occur in about 1 in 1,000 children.⁶ Community-acquired pneumonia in adults is complicated by empyema in 0.7% of patients.¹ Patients who are immunocompromised are at increased risk for empyema following pneumonia.^7,8,9

Most commonly, an empyema develops from similar pathogens that cause a pneumonia.² In community-acquired pneumonias, streptococci are the most frequent causes of pneumonia and empyema, with a propensity for pneumonias caused by Streptococcus viridans group. Less common organisms include Staphylococcus aureus, Pseudomonas aeruginosa, Haemophilus influenzae, and Mycoplasma pneumoniae.^7,10,11 In immunocompromised patients, the bacterial etiology can be quite diverse and includes more frequent Gramnegative rods.⁹

Bacteria are isolated from blood, sputum, or pleural fluid. Because many pleural samples are culture negative, molecular studies are useful in determining the organism and species and serotype of streptococcus.¹⁰

Spontaneous bacterial empyema denotes bacterial infection of a previously sterile pleural effusion, in the absence of contiguous pneumonia. It usually occurs in patients with cirrhosis, at a rate of 1% to 2%.¹²

Grossly, empyema cavities are generally seen at autopsy at relatively advanced stages and consist of loculated masses with hemorrhagic fluid and frank pus that are generally within the pleural cavity. There may be adjacent abscesses within the lung parenchyma.

Histologically, the findings that are seen in pleural decortications or pleural “peels” include fibrinous pleuritis, acute and chronic inflammation, and granulation tissue. In later stages, there is pleural fibrosis (Fig. 95.4). Occasionally, there is reactive mesothelial hyperplasia that rarely raises the concern for malignancy.