Pleural Infection and Empyema

Pleural infection is infection of the pleural space, which most often develops as a complication of pneumonia and progresses along a spectrum from an uncomplicated parapneumonic effusion to a complicated effusion and, at its most severe, an empyema — frank pus in the pleural cavity. It is a serious condition because infected pleural fluid tends to become loculated and fibrous, and clearing it can be difficult.

Definition

Pleural infection is the presence of infection within the pleural space; an empyema is its advanced form, defined by the presence of pus, or of organisms on Gram stain or culture, in the pleural fluid. A complicated parapneumonic effusion is an infected effusion (often with low pleural-fluid pH and glucose and high LDH) that generally requires drainage even before frank pus is present.

Scope

This topic covers what pleural infection and empyema are, the parapneumonic-to-empyema spectrum, the microbiology and pleural-fluid features that mark infection, and the conceptual basis of drainage. It is a reference and educational entry, not a protocol for antibiotic choice, drainage timing, or surgery in an individual patient.

Core questions

• Where on the parapneumonic-to-empyema spectrum does the effusion lie?
• Do the pleural fluid appearance, pH, and microbiology indicate infection requiring drainage?
• What organisms are involved, and how do community- and hospital-acquired infections differ?
• How do loculation and fibrosis influence the approach to clearing the pleural space?

Key concepts

• Parapneumonic effusion
• Complicated parapneumonic effusion
• Empyema (pus in the pleural space)
• Pleural fluid pH, glucose, and LDH in infection
• Loculation and fibrinous septation
• Community- versus hospital-acquired pleural infection
• Chest tube drainage
• Intrapleural fibrinolytic therapy

Mechanisms

Most pleural infection begins when pneumonia provokes a sterile, exudative parapneumonic effusion; if bacteria invade the pleural space, the effusion becomes infected and enters a fibrinopurulent phase in which fibrin is deposited, the fluid becomes loculated into separate pockets, and pleural-fluid pH and glucose fall while LDH rises as bacteria and neutrophils consume substrate. Untreated, the process can advance to an organising phase in which a thick fibrous peel forms over the lung and restricts its expansion. The microbiology differs from pneumonia and from community- to hospital-acquired disease, frequently including streptococci and anaerobes in community-acquired infection and staphylococci and Gram-negative organisms in hospital-acquired infection. Because loculated, infected fluid drains poorly, breaking down fibrin septations is a central problem in clearing an empyema.

Clinical relevance

Pleural infection is an important complication of pneumonia that carries appreciable mortality and frequently requires prolonged treatment, and recognising the parapneumonic-to-empyema spectrum is part of understanding how respiratory infection is evaluated. This entry describes the concept and its evidence base for reference and education; it does not provide antibiotic regimens, drainage thresholds, or surgical indications for an individual patient.

Epidemiology

Pleural infection complicates a substantial minority of pneumonia admissions and its incidence has been reported to be rising in some settings, affecting both children and adults, with higher risk at the extremes of age and in people with comorbidity. Outcomes remain serious, with a meaningful proportion of adults requiring surgical drainage or dying despite treatment.

Evidence & guidelines

The British Thoracic Society pleural disease guideline on pleural infection (Davies et al., 2010) frames the parapneumonic-to-empyema spectrum and the rationale for drainage. Randomised trials shaped the role of intrapleural agents: the MIST1 trial (Maskell et al., 2005) found that intrapleural streptokinase alone did not improve outcomes, while the MIST2 trial (Rahman et al., 2011) showed that the combination of intrapleural tissue plasminogen activator and DNase improved pleural fluid drainage. Pleural-fluid analysis to identify infected effusions builds on the exudate framework of Light et al. (1972). Guidance changes over time and is summarised here only for orientation.

History

Drainage of pus from the chest is among the oldest recorded surgical interventions, described in the Hippocratic corpus. The modern era brought systematic pleural-fluid analysis, the recognition of a graded parapneumonic-to-empyema spectrum, and controlled trials of intrapleural therapy that clarified which agents help clear infected, loculated fluid, culminating in society guidelines that structure contemporary assessment.

Debates

What is the role of intrapleural fibrinolytic and enzyme therapy?

After a single fibrinolytic agent failed to improve outcomes in MIST1, the MIST2 trial showed that combined tissue plasminogen activator and DNase improved drainage; the place of this combination relative to early surgery, and patient selection for it, remain matters of ongoing discussion.

Key figures

• Helen E. Davies
• Nick A. Maskell
• Najib M. Rahman
• Robert J. O. Davies
• Richard W. Light

Seminal works

• davies-2010
• maskell-2005
• rahman-2011

Frequently asked questions

What is the difference between a parapneumonic effusion and an empyema?

A parapneumonic effusion is pleural fluid that forms alongside pneumonia and may be sterile and uncomplicated; an empyema is the advanced, infected end of that spectrum, defined by frank pus or organisms in the pleural space, with a complicated parapneumonic effusion lying in between.

Why is empyema difficult to treat?

Infected pleural fluid tends to become loculated by fibrin into separate pockets and can leave a thick fibrous peel over the lung, so it drains poorly; clearing it often requires effective drainage and, in some cases, measures to break down the loculations or surgery.