Lung abscess is a circumscribed collection of pus within the lung, and potentially life threatening. They are often complicated to manage and difficult to treat.
As a result of the widespread availability of antibiotics, the incidence of lung abscesses has dramatically reduced. Similarly, mortality has reduced. The elderly, immunocompromised, malnourished, debilitated and of course those who do not have access to antibiotics are particularly susceptible and have the worst prognosis 6. Particularly due to increased number of immunocompromised (secondary to HIV/AIDS and iatrogenic immunosuppression) the rate has once more increased 7.
Lung abscesses are divided according to their duration into acute (< 6 weeks) and chronic (> 6 weeks) 7. Presentation is usually non-specific and generally similar to a non-cavitating chest infection. Symptoms include fever, cough and shortness of breath. Peripheral abscesses may also cause pleuritic chest pain 7.
If chronic, symptoms are more indolent and include weight loss and constitutional symptoms. In some cases erosion into a bronchial vessel may result in sudden and potentially life threatening massive haemoptysis.
It is convenient to divide lung abscesses into primary and secondary as they differ not only in aetiology, but also microbiology and prognosis.
A primary abscess is one which develops as a result of primary infection of the lung. They most commonly arise from aspiration, necrotising pneumonia or chronic pneumonia, e.g. pulmonary tuberculosis 7.
In patients who develop abscesses as a result of aspiration, mixed infections are most common, including anaerobes.
Some organisms are particularly prone to causes significant necrotising pneumonia resulting in cavitation and abscess formation. These include 1:
- Staphylococcus aureus
- Klebsiella sp: Klebsiella pneumonia
- Pseudomonas sp
- Proteus sp
In immunocompromised patients additional organisms may also be implicated including 7:
- Candida albicans: pulmonary candidiasis
- Legionella micdadei & Legionella pneumophila: Legionella pneumonia
- Pneumocystis carinii (uncommon): pneumocystis jirovecii pneumonia
A secondary abscess is one which develops as a result of another condition. Examples include:
- bronchial obstruction: bronchogenic carcinoma, inhaled foreign body
- haematogeneous spread: bacterial endocarditis, IVDU
- direct extension from adjacent infection: mediastinum, subphrenic, chest wall
Also sometimes grouped with secondary abscesses are colonisation of pre-existing cavities with organisms 7.
As aspiration is the most common cause of pulmonary abscesses it is no surprise that the superior segment of the right lower lobe is the most common site of infection 6.
The classical appearance of a pulmonary abscess is a cavity containing an air-fluid level. In general abscesses are round in shape, and appear similar in both frontal and lateral projections. Additionally all margins are equally well seen, although adjacent consolidation may make assessment of this difficult. These features are helpful in distinguishing a pulmonary abscess from an empyema (see empyema vs pulmonary abscess)
CT is the most sensitive and specific imaging modality to diagnose a lung abscess. Contrast should be administered, as this enables the identification of the abscess margins, which can otherwise blend with surrounding consolidated lung.
Abscesses vary in size, and are generally rounded in shape. The may contain only fluid or have an air-fluid level. Typically there is surrounding consolidation, although with treatment the cavity will persist longer than consolidation.
The wall of the abscess is typically thick and the luminal surface irregular.
Bronchial vessels and bronchi can be traced as far as the wall of the abscess, whereupon they are truncated.
Ultrasound does not play a routine role in the assessment of lung abscesses as any aerated intervening lung will prevent visualisation. Peripheral abscesses abutting the pleura or with only compressed or consolidated lung may however be visible, and should not be mistaken for an empyema 4. Consolidated lung may mimic a fluid collection with low level echoes.
Treatment and prognosis
Lung abscesses are usually managed with prolonged antibiotics and physiotherapy with postural drainage 2. Bronchoscopy may be beneficial in establishing bronchial patency to improve drainage 3. In cases that are refractory to conservative management, or those complicated by haemoptysis, empyema or suspected malignancy, surgical resection is the 'traditional' definitive treatment 5. Percutaneous drainage under CT guidance has also been advocated in selected cases (e.g. patients refractory to conventional therapy) 3-9.
Larger abscesses (> 4 cm in diameter) are less likely to be cured with medical management only and have a higher mortality irrespective of treatment 3,6.
Complications of surgery or percutaneous drainage include :
Despite treatment abscesses continue to have high mortality (15-20%) 3, 6. This is particularly the case in nosocomial infections, which account for the majority of deaths, presumably due to the combined effect of pre-existent illness and higher prevalence virulent of antibiotic resistant strains, particularly P. aeruginosa (mortality rate of 83%), S. aureus (50%) and Klebsiella pneumoniae (44%) 6.
General imaging differential considerations include
- empyema (see empyema vs pulmonary abscess)
- bronchogenic carcinoma (cavitating)
- pulmonary metastasis: with necrosis
- pulmonary cavitating granulomatous disease (e.g granulomatosis with polyangiitis)
- large infected pneumatocoele: infected emphysematous bulla
- cavitating pneumonia / necrotising pneumonia
Other considerations on plain film include
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