1

What is a pleural empyema (Figure 1)?

A pleural empyema represents an accumulation of pus within the pleural cavity which may be localised or involve the entire pleural space.

Pleural empyema is one of the oldest surgical diseases, described by Hippocrates between the 5th and 2nd centuries BC and is also known as empyema thoracis or pyothorax.

2

What is the difference between a pleural empyema and a lung abscess (Figure 2)?

An empyema involves the pleural space, whereas a lung abscess is an infected cavity within necrotic lung parenchyma and is entirely surrounded by lung tissue.

An empyema may arise as a complication of a lung abscess either by lymphatic spread of the infection into the pleural space or by direct rupture of the abscess into the pleura.

3

What is the aetiology of an empyema (Figure 3)?

An empyema most commonly develops following bacterial pneumonia (70%).

Less common causes include:

a)

lung surgery, thoracocentesis or chest drain insertion;

b)

penetrating chest trauma;

c)

oesophageal rupture;

d)

mediastinal infection, including descending necrotising fasciitis;

e)

extension of a subdiaphragmatic or paravertebral abscess.

Rarely, an empyema may develop by haematogenous spread associated with systemic sepsis, particularly in an immunocompromised patient.

4

What are the most common organisms associated with empyema?

Bacterial pneumonia is the commonest cause of parapneumonic effusions and empyema, with the following organisms most frequently isolated:

a)

Gram-positive aerobic organisms, including:

i)

Streptococcus species (S. pneumoniae, S. pyogenes, and S. milleri);

ii)

Staphylococcus aureus;

b)

Gram-negative aerobic organisms, including:

i)

Klebsiella;

ii)

Pseudomonas;

iii)

Haemophilus species;

c)

anaerobic organisms, including:

i)

Bacteroides;

ii)

Peptostreptococcus.

Empyema following cardiothoracic surgery is usually secondary to Staphylococcus aureus or Gram-negative bacterial infection.

Empyema associated with aspiration pneumonia often involves mixed flora, containing both aerobic and anaerobic bacteria.

5

What is the pathophysiology of an empyema?

Although there are many different causes of an empyema, the majority develop as a complication following bacterial pneumonia. The pathological transition from parapneumonic effusion to empyema usually occurs through three clearly defined stages:

a)

exudative stage (uncomplicated or simple parapneumonic effusion) – where an exudative neutrophil-rich effusion develops as a result of increased pulmonary interstitial fluid and increased capillary permeability in response to inflammation associated with pneumonia. The fluid may be clear or slightly cloudy, with no organisms found on Gram stain or culture. This stage occurs approximately 2-5 days from the onset of the pneumonia;

b)

fibrinopurulent stage (complicated parapneumonic effusion) – where bacterial infection of the pleural fluid occurs (5-10% of patients). Fibrin deposition occurs within the pleural space causing loculation and pleural thickening, associated with the pleural fluid becoming more viscous and even purulent. This fluid is usually cloudy but may remain sterile on culture because of rapid bacterial clearance from the pleural space. This stage usually develops 5-10 days after the onset of the pneumonia;

c)

organising stage (pleural empyema) – where there is an accumulation of thick, viscous pus within the pleural space and the development of a thick fibrinous rind (pleural peel) over the visceral and parietal pleura, which prevents full lung expansion. This occurs over 2-3 weeks.

The clinical progression of a patient through these stages, however, is less clearly defined.

Empyema necessitans refers to the extension of pleural infection outside the pleural cavity into the chest wall and overlying soft tissues.

6

What is the epidemiology of parapneumonic effusions and empyema?

Although a parapneumonic effusion occurs in 35-40% of hospitalised cases of pneumonia, empyema only develops in up to 20% of these patients.

The incidence of parapneumonic effusion depends upon the causative organism:

a)

pneumococcal pneumonia – 60% of cases have parapneumonic effusions;

b)

other bacterial pneumonia – 40%;

c)

viral pneumonia – 20%.

Risk factors for the development of an empyema include:

a)

age (children and the elderly);

b)

comorbidities, including bronchiectasis, rheumatoid arthritis and diabetes;

c)

immunosuppression, including corticosteroid use;

d)

pneumonia requiring hospitalisation;

e)

intravenous drug abuse and alcoholism;

f)

debilitation and malnutrition.

Poor oral hygiene, alcohol misuse, gastro-oesophageal reflux disease (GORD) and other factors associated with aspiration are particular risk factors for anaerobic infection.

7

What are the clinical features of an empyema?

Most cases of parapneumonic effusion and empyema present on a background of pneumonia, with typical symptoms including productive cough, pleuritic chest pain, breathlessness and fever.

The symptoms of parapneumonic effusion and empyema can vary depending upon the causative organism, with aerobic infections usually presenting more acutely, whilst anaerobic infections may present with non-specific symptoms.

The persistence of fever for longer than 3 days following the initiation of antibiotic therapy may indicate progression to pleural involvement of the infection (complicated parapneumonic effusion or empyema).

The signs of empyema include fever, tachypnoea and tachycardia, in combination with the findings of a pleural effusion (decreased tactile fremitus, dullness to percussion and decreased breath sounds).

If there is extensive underlying pulmonary consolidation, signs of pneumonia may also be present, including coarse crackles or bronchial breath sounds.

8

What are the complications of an empyema?

Direct extension of infection into the:

a)

chest wall (empyema necessitans);

b)

bronchus (bronchopleural fistula);

c)

pericardium (pyopericardium);

d)

ribs (osteomyelitis);

e)

mediastinum (mediastinitis);

f)

bloodstream (generalised sepsis).

Fibrothorax – fibrosis and contraction of the chest cavity.

9

What are the haematological and microbiological features of a parapneumonic effusion or empyema?

Full blood count (FBC) – may show a leucocytosis and left shift in the ratio of immature to mature neutrophils.

C-reactive protein (CRP) – can be used as a sensitive marker of progression of the pneumonia, with failure of the CRP level to fall by 50% being associated with an adverse outcome.

Aerobic and anaerobic blood cultures – should be sent in all patients.

Sputum Gram stain, culture and sensitivity – are used to help guide initial antibiotic therapy.

All patients with a pleural effusion ≥10mm depth in association with sepsis or a pneumonic illness should undergo diagnostic pleural fluid analysis. The appearance of the initial aspirate can indicate the stage of progression:

a)

clear yellow liquid – uncomplicated parapneumonic effusion;

b)

opaque turbid fluid – complicated parapneumonic effusion;

c)

thick, viscous, foul-smelling purulent fluid – established empyema.

Certain findings of the pleural fluid are suggestive of a complicated parapneumonic effusion or empyema that will require chest drain insertion for resolution:

a)

grossly purulent fluid;

b)

pH <7.2 (with normal peripheral blood pH);

c)

lactate dehydrogenase (LDH) >1,000 IU/L;

d)

glucose <3.4mmol/L;

e)

positive Gram stain or culture;

f)

white cell count >50,000 cells/μL.

Pleural fluid should also be sent for cytology and acid-alcohol-fast bacilli analysis for mycobacteria, if clinically indicated.

10

What are the chest radiological features of a parapneumonic effusion and empyema?

Uncomplicated parapneumonic effusions (Figure 4) are characterised by:

a)

pulmonary consolidation or infiltrates;

b)

free pleural fluid – which can be identified as:

i)

blunting of the costophrenic angle (which represents >200mL fluid);

ii)

dense linear shadow layering between the lung and the chest wall.