Empyema

Epidemiology

In the United States, empyema is seen in approximately 60,000 patients annually, with a mortality rate of 15%. Approximately 50% of empyemas in the United States today result from pneumonia. Pleural infection may also result from lung surgery, trauma, esophageal perforation, or transdiaphragmatic spread of an intra-abdominal infection. At least 40% of all patients hospitalized with pneumonia have an ipsilateral pleural effusion. The average length of stay for patients with parapneumonic empyema in the United States was approximately 15 days and is shorter for children than adults. The in-hospital fatality rate was 7.2% in the NIS database and was lower for children (0.4%) than for adults older than 65 years (16.1%).

Historical Review

Empyema is most often used to refer to collections of pus in the space around the lungs (pleural cavity). The word is derived from the Greek word empyein, meaning “pus-producing.” Hippocrates first described the serious nature of thoracic empyema more than 2400 years ago. Even then, the need for surgical intervention was recognized, and open drainage was essentially the only treatment available until the late 19th century. Gotthard Bülau was among the first to use a closed method of drainage of infected pleural fluid, which he reported in 1891. It was not, however, a widely recognized treatment option until the findings of the U.S. Army Empyema Commission led by Graham and Bell in 1918. The influenza epidemic of 1917-1919 resulted in a dramatic rise in the incidence of streptococcal empyema. During the early period of the epidemic, open thoracic drainage was commonly used in military hospitals to treat parapneumonic fluid. Often this was done early in the acute empyema phase, prior to the development of adhesions, and resulted in lung collapse and respiratory compromise. The early mortality rate from such a condition was 30.2% in this group of otherwise healthy, young men. A much better understanding of the pathology and evolution of empyema was obtained as a result of the Empyema Commission. As a result, with the use of closed drainage, the mortality rate for empyema dropped to 4.3%. The principles outlined by Graham, consisting of avoidance of open drainage during the acute phase and sterilization and obliteration of the infected space, constitute the fundamentals of modern therapy. With the discovery of effective antibiotics in the mid-20th century, the incidence of empyema was significantly reduced. The introduction of antibiotics, however, led to the emergence of resistant infections, polymicrobial infection, and partially treated empyemas. The incidence of streptococcal empyema decreased markedly with the introduction of antibiotics, and Staphylococcus aureus emerged as the predominant pathogen. More recently, gram-negative and anaerobic bacteria have become important pathogens. In spite of current medical advances, thoracic empyema continues to be a challenging problem, associated with significant morbidity and mortality.

Bacteriology

The bacteria traditionally associated with parapneumonic empyema are Streptococcus pneumonia, Streptococcus pyogenes, Staphylococcus aureus, and, more recently, Streptococcus anginosus (formerly Streptococcus milleri ). Anaerobes have been identified as sole or coexisting pathogens in 25% to 76% of cases. A recent review of the NIS database between 1996 and 2008 characterized the changing epidemiology of parapneumonic empyema in community hospitals across the United States. The largest increase in relative incidence of empyema occurred in young adults. The most serious cases of empyema were associated with staphylococcal infections. For patients older than 40 years, staphylococcal empyema had a significantly higher in-hospital mortality rate than those associated with other pathogens. The mortality rate was 8.9 for patients aged 40 to 64 with staphylococcal empyema compared to 4.4% to 6.1% for other pathogens. For individuals older than 65 years, the mortality rate for staphylococcal empyema was 21.8% versus 12% to 16% for other pathogens.

Most empyemas were categorized as having unknown pathogens. Among the pathogen specified cases, staphylococcal empyema accounted for most of the increased incidence of empyema, and these patients had the longest lengths of stay and highest mortality rate. This increase in staphylococcal empyema has also been noted in previous studies involving tertiary care facilities.

The incidence of pneumococcal empyema has remained relatively stable in both children and adults. Infant vaccination with a seven-valent pneumococcal conjugate vaccine (PCV7) started in 2000. Studies documented significant reductions in the incidence of pneumonia hospitalizations after the introduction of this vaccine. Overall, national rates of childhood pneumococcal empyema remained relatively stable during the study period. The introduction of a 13-valent pneumococcal conjugate vaccine in 2010 in the United States may provide protection against several pneumococcal serotypes commonly associated with empyema.

The number of parapneumonic empyemas classified as being caused by unknown pathogens has increased significantly. Defining the etiology of empyema is difficult, and it is unclear whether this increase is related to truly unknown organisms, to imperfect laboratory testing, or to the increased use of antibiotics prior to hospitalization. Some studies using molecular techniques have suggested that a significant percentage of culture-negative empyema may be caused by pneumococci, mainly serotype 1. Although recent studies suggest Streptococcus anginosus may be a leading cause of empyema, it can be difficult to distinguish between streptococcal species. Mixed bacterial infections are found in some parapneumonic empyemas.

The cause of the observed increase in the incidence of empyema is not clear. Part of the increase may be related to the increased incidence of antibiotic-resistant pathogens. The increase in cases of staphylococcal empyema seems to correlate with the increase in methicillin-resistant organisms. Streptococcus pneumoniae is an exception—the incidence of resistant pneumococcal disease decreased markedly following the introduction of PCV7.