Lung abscess

Lung abscess is usually polymicrobial and associated with aspiration. Streptococci and anaerobes are the leading causes of community acquired infection.[7]

Anaerobic bacteria are abundant in the gingival crevices, particularly in the presence of periodontal disease, with Peptostreptococcus, Bacteroides, Prevotella species, and Fusobacterium species the most frequently isolated.[10][13] Causative aerobes include Streptococcus milleri, S. aureus, Klebsiella species, Streptococcus pyogenes, and Haemophilus influenzae, with micro-aerophilic streptococci and viridans streptococci being important aetiological agents in immunocompetent patients.[10][14]

In patients with diabetes mellitus, consider Klebsiella pneumoniae and anaerobes, and in patients with multiple lung abscesses, consider Panton-Valentine leukocidin-positive Staphylococcus aureus.[5][15][16] Gemella, Actinomyces and Nocardia species are other important pathogens.[7]

When occurring as a complication of pneumonia, abscesses are mostly monomicrobial and caused by aerobic bacteria, such as S aureus, K pneumoniae, Haemophilus influenzae, Pseudomonas aeruginosa, Escherichia coli, Pasteurella multocida, Burkholderia, Legionella species, Streptococcus pneumoniae, and group A streptococci.[17][18]S aureus and P aeruginosa are frequently isolated in hospital-acquired lung abscess or necrotising pneumonia after oropharyngeal colonisation.

Lung abscesses in immunocompromised patients are usually caused by multiple pathogens (anaerobes are not usually isolated).[19] In patients with impaired cell-mediated immunity, lung abscess results from opportunistic pathogens such as mycobacteria, Nocardia, Aspergillus, and Rhodococcus. In leukopenic patients, aerobic bacteria (S aureus, P aeruginosa, and Haemophilus species) and fungi (e.g., Aspergillus and zygomycetes) are important pathogens.

Gram-negative organisms, such as P aeruginosa, are expected in lung abscess occurring secondary to pneumonia or immunosuppression, hospital-acquired lung abscess, or necrotising pneumonia after oropharyngeal colonisation with these organisms.[20]

Lung abscess usually occurs in patients with a predisposition to gastric content aspiration because of an altered consciousness or dysphagia associated with neurological or oesophageal disease.[4] Common causes of gastric content aspiration include alcoholic stupor, seizures, stroke, neurological bulbar dysfunction, drug overdose, and general anaesthesia. Other causes include dental or oropharyngeal surgery (especially tonsillectomy in the sitting position) and oesophageal disease (stricture, malignancy, and reflux). Nasogastric and endotracheal tubes that interfere with normal anatomical barriers also predispose to the aspiration of oropharyngeal fluid. Aspirating contaminated oropharyngeal secretions causes a necrotising infection that follows a segmental distribution limited by the pleura (expansion to the pleural space is uncommon). The resultant cavity is usually solitary and has a thick fibrous wall.

Aspiration-related lung abscesses are usually found in the right lung and in the dependent portions (e.g., posterior segment of the right upper lobe and superior segments of both lower lobes).

Other processes that may cause lung abscess include infection distal to an obstructing tumour or foreign body, infection secondary to a pulmonary embolic infarct, septic embolisation from right-sided (e.g., tricuspid valve) bacterial endocarditis or septic embolisation from peripheral septic thrombophlebitis.[21] Abscesses due to septic embolisation typically involve multiple non-contiguous lung areas.[22] Almost 50% of lung abscesses in adults aged >50 years are associated with lung tumours and 17% to 40% are associated with preceding pneumonia.[9][18]

Lemierre's syndrome is an acute oropharyngeal infection due to Fusobacterium species that usually affects young healthy people and is complicated by jugular vein thrombophlebitis and metastatic septic embolisation.[23] Variants of this syndrome include multiple lung abscesses after inferior vena cava thrombosis due to a soft tissue leg abscess.[24] Finally, direct expansion through the diaphragm of a liver amoebic abscess may result in an amoebic lung abscess, which typically occurs in the right lower lobe.

Clinical classification

Lung abscesses are classified as primary or secondary and by the duration of symptoms prior to diagnosis as acute, sub-acute, or chronic.

Acute lung abscess

Symptoms present for <2 weeks, and usually only for a few days.

Sub-acute lung abscess

Symptoms present for ≥2 weeks.

Chronic lung abscess

Symptoms present for >1 month.

Primary lung abscess

Occurs in the absence of underlying medical conditions
Usually develops in people prone to aspiration of gastric contents or those in relatively good health
Accounts for almost 80% of lung abscesses.

Secondary lung abscess

Associated with comorbidities such as obstructive airway disease, neoplasms, immunosuppression, diabetes mellitus, and extra-pulmonary infection or sepsis.[4]

Pathophysiology

Classification

Clinical classification