Lung Abscess

Definition and classification

A lung abscess is a localised collection of pus/necrotic debris within the lung parenchyma, forming a cavity (classically >2 cm) — once a bronchopulmonary fistula develops, it shows an air–fluid level on imaging. Classified by duration as acute (resolves in <6 weeks) or chronic (persists >6 weeks), and by mechanism as:

• Primary — from aspiration of oropharyngeal secretions (dental/ periodontal infection, sinusitis, altered consciousness, swallowing disorders, GORD, frequent vomiting) — generally solitary, in otherwise healthy patients.
• Secondary — from a pulmonary cause: bronchial obstruction (tumour, foreign body, enlarged lymph nodes) or existing lung disease (bronchiectasis, cystic fibrosis, bullous emphysema, infected pulmonary infarcts) — often multiple abscesses, and a worse prognosis since it depends on treating the underlying condition too.
• Haematogenous — spread from an extrapulmonary source.

Overview

Lung abscesses are classically caused by aspiration of oral secretions in patients with poor dental hygiene. Poor dental hygiene is itself an independent risk factor for the condition. Patients may have a history of an aspiration event, halitosis, and foul-tasting sputum. Other aspiration risk factors include seizures, bulbar dysfunction, alcohol intoxication/alcohol use disorder, and cognitive impairment; immunocompromise (HIV/AIDS, post-transplant, prolonged immunosuppressive therapy) is also predisposing. Often a subacute course over several weeks: fevers, night sweats, productive cough, dyspnoea — the cough is often initially non-productive, becoming productive (the hallmark sign) once bronchial communication develops, and may include haemoptysis.

Most cases are polymicrobial (anaerobes — Bacteroides, Prevotella, Peptostreptococcus, Fusobacterium, streptococci); a monomicrobial abscess is typically Staph aureus, Klebsiella pneumoniae, Strep pyogenes, Burkholderia pseudomallei (melioidosis), Haemophilus influenzae type b, Nocardia or Actinomyces. Because aspiration classically occurs supine, abscesses favour the posterior segment of the right upper lobe and the superior segment of the right lower lobe.

Lung abscess can also form via haematogenous spread (e.g. secondary to IV drug use) or right-sided infective endocarditis — other haematogenous sources include abdominal sepsis, infected catheters, and septic thromboembolism; in these cases, blood cultures and echocardiography help guide management, alongside treating the underlying source.

Cavitary lesion clues: in a patient with pneumonia, a cavitary lesion on CXR suggests Staph aureus as the cause; patients with Strep pneumoniae pneumonia are typically very unwell. CT is more sensitive than CXR for characterising the cavity and distinguishing it from pleural empyema. Sputum culture and, where the diagnosis or organism remains unclear, bronchoscopy (also useful for excluding an obstructing tumour/foreign body) help confirm the causative organism.

Treatment (non-severe, without chest wall pain)

If the cause is unknown, treat as though oral aspiration has occurred (i.e. cover gram-negatives):

• Amoxicillin 1 g TDS + metronidazole 400 mg BD, for 3–4 weeks, or
• Augmentin 875/125 BD for 3–4 weeks (consider adding amoxicillin 1 g TDS if a large abscess or Strep pneumoniae is confirmed).

Consider MRSA cover if there has been healthcare exposure in the preceding 3 months. Clindamycin is no longer favoured as empiric therapy given the risk of Clostridioides difficile infection, but remains an option for penicillin-allergic patients. Switch to oral therapy once the patient is afebrile, stable, and tolerating an oral diet — amoxicillin-clavulanate (Augmentin) is the oral drug of choice; metronidazole alone is not considered adequate, given the typically polymicrobial flora.

Large abscesses may need longer therapy — consider following serial CRP. Abscesses >6 cm are unlikely to resolve on antibiotics alone and often need surgical (lobectomy/pneumonectomy) or percutaneous/endoscopic drainage (the latter preferred in poor surgical candidates). Check for radiographic resolution; if not resolving on antibiotics, refer to respiratory for consideration of bronchoscopic drainage.

Differential for cavitary lung lesions

• Consolidation surrounding bullae in a patient with emphysema
• Tumour (excavating bronchial carcinoma) — favoured by thicker, more irregular cavity walls and the absence of fever/purulent sputum/ leukocytosis
• Vasculitis (granulomatosis with polyangiitis)
• Tuberculosis (excavating) or other mycoses
• Localised pleural empyema, infected emphysematous bullae, pulmonary hematoma, cavitary pulmonary infarct, hiatal hernia
• Melioidosis — a tropical disease common in northern Australia, classically in diabetics, causing abscesses anywhere but especially the lungs

Prognosis and complications

Primary lung abscesses treated with appropriate antibiotics have a good prognosis (~90% cure rate); secondary abscesses carry a much poorer prognosis (~75% mortality reported), driven largely by the underlying comorbidity, immunocompromise or neoplasm rather than the abscess itself. Poor prognostic factors include older age, severe comorbidities, immunosuppression, bronchial obstruction, and an underlying neoplasm. Complications from under-recognised or inadequately treated disease include pyopneumothorax, pleural empyema, bronchopleural fistula, pleural fibrosis, trapped lung, and respiratory failure. Preventive measures — moderating alcohol intake, dental care, head-of-bed elevation in at-risk patients — target the aspiration risk factors that drive most primary cases.