| Appearance | Biochemistry and bacteriology | Risk category for poor outcome | |
|---|---|---|---|
| Simple/uncomplicated parapneumonic | Clear fluid | pH >7.2 LDH ≤1000 IU/L Glucose >3.3 mmol/L Negative Gram smear or culture | 1 and 2 Very low or low |
| Complicated parapneumonic | Clear fluid or turbid | pH ≤7.2 LDH >1000 IU/L Glucose ≤3.3 mmol/L Positive Gram smear or culture likely | 3 Moderate |
| Empyema | Frank pus | Positive Gram smear or culture likely Biochemistry unnecessary | 4 High |
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 formation following surgery, penetrating lung injuries, or a lung abscess.
Etiology
Empyema occurs most commonly in association with bacterial pneumonia, either in a community- or hospital-acquired setting. In a study in 434 pleural infections in the United Kingdom using standard culture and nucleic acid amplification techniques a causative organism was identified in 74%. Of the 336 isolates in the community-acquired setting 52% were of the genus Streptococcus, approximately 20% were anaerobes, 10% were staphylococcal, and 10% were gram-negative organisms. In the hospital-acquired infections (60 isolates) Staphylococcus was the major genus isolated (35%) of which 71% were methicillin-resistant forms; 23% were gram-negative organisms; 18% were Streptococcus; and 8% were anaerobes. Other organisms isolated included Actinomyces spp., Enterococcus spp., and Mycobacterium tuberculosis. This large study supports smaller studies suggesting the spectrum of organisms in pleural infections differs from that in pneumonia. Local events such as thoracic surgery, rupture of the esophagus, hepatic or subphrenic abscesses, and all penetrating injuries may introduce organisms, especially gram-negative or anaerobic organisms, into the pleural space. Ameba may enter the pleural space from an amebic abscess in the liver. Tuberculous empyema is a modest problem in the developed world, but is still seen in reactivation of tuberculosis in the elderly. However, in developing countries, with rapid urbanization, continued population increase, and greater levels of human immunodeficiency virus infection, there is an increasing incidence of tuberculous empyema. Pleural space infections may cause a BPF or may occur secondary to a BPF. Lung resection surgery remains the major cause of BPF, occurring in 3% to 5% of these operations.
Clinical features
There are no specific clinical features to differentiate simple from uncomplicated parapneumonic effusions. The main features are fever, chest pain, sputum production, appropriate physical signs of an effusion, and peripheral blood leukocytosis. Progression to an empyema is usually indicated clinically by the persistence or recurrence of fever and features of systemic upset with a lack of resolution of physical signs, because differentiation of consolidation from a small- to medium-volume effusion may not be possible. Other physical features include dyspnea with large effusions, rapid-onset finger clubbing, lethargy, and marked weight loss. Purulent sputum may indicate the development of a BPF. However, more insidious onset may occur with the presentation occurring over weeks to months after the original pneumonia or injury.
Investigations
A chest radiograph usually shows collection of fluid, although a localized, loculated collection may resemble an intrapulmonary mass. Differentiation between these possibilities may be resolved by the addition of a lateral chest radiograph to the standard posteroanterior film or by ultrasound or computed tomographic (CT) scanning. Ultrasonography is superior to CT in identifying septations in a loculated collection and also defines pleural thickening. Increasingly bedside ultrasound is used to guide a percutaneous diagnostic aspiration, increasing the diagnostic yield and patient safety. However, it is occasionally difficult to distinguish between empyema with a BPF and a lung abscess. In this setting, the use of CT scanning may guide both investigation and management approaches.
Aspirated material should be collected under anaerobic conditions and a portion submitted for anaerobic culture and a sample in a blood culture bottle can increase the anaerobic yield. Routine bacterial and mycobacterial culture should be undertaken with cytologic examination. If appropriate, fungi and parasites should be sought. Other investigations including pH, glucose concentration, and lactate dehydrogenase (LDH) activity of the fluid may be of use if there is little evidence of purulence to the naked eye. A meta-analysis of pleural fluid biochemistry, based on user characteristics, demonstrated that pH, especially ≤7.2, was a guide to the need for tube drainage and that glucose or LDH determination conferred no extra benefit; however, if pH assessment is unavailable, a glucose ≤
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