Lung Abscess

Lung abscess is a form of liquefactive necrosis within the lung tissue, leading to the formation of cavities larger than 2 cm, filled with necrotic debris or fluid due to microbial infection.

This condition often arises from aspiration events, which can occur during anaesthesia, sedation, or unconsciousness following injury. Alcoholism is a major predisposing factor for lung abscesses.

Signs and Symptoms

The onset of symptoms is typically gradual, although in cases of necrotising staphylococcal or gram-negative bacillary pneumonias, patients might present acutely ill. Common symptoms include cough, fever with shivering, and night sweats. The cough may produce foul-smelling, purulent mucus in approximately 70% of cases, and less frequently, blood in about one-third of cases. Additional symptoms include chest pain, shortness of breath, lethargy, and signs of chronic illness.

Patients with lung abscess often appear cachectic. Finger clubbing is observed in one-third of patients, and dental decay is common, particularly among alcoholics and children. Physical examination typically reveals features of lung consolidation such as localised dullness on percussion and bronchial breath sounds.

Complications

Although rare today, complications can include the spread of infection to other lung segments, bronchiectasis, empyema, and bacteraemia with potential metastatic infections such as brain abscess. Other complications include rupture into the pleural space, pleural fibrosis, trapped lung, respiratory failure, bronchopleural fistula, and pleurocutaneous fistula.

Causes

Several conditions can contribute to lung abscess formation:

• Aspiration of oropharyngeal or gastric secretions
• Septic emboli
• Necrotising pneumonia
• Vasculitis, such as granulomatosis with polyangiitis
• Necrotising tumours, with primary squamous carcinoma of the lung being the most common

Organisms

The pattern of microbial causation has shifted in the post-antibiotic era. Anaerobic bacteria, previously found in up to 90% of cases, are now less frequent. Causative organisms include:

• Anaerobic bacteria: Actinomyces, Peptostreptococcus, Bacteroides, Fusobacterium, Parvimonas micra, Streptococcus milleri
• Aerobic bacteria: Staphylococcus, Klebsiella, Haemophilus, Pseudomonas, Nocardia, Escherichia coli, Streptococcus, Mycobacterium
• Fungi: Candida, Aspergillus, Histoplasma, Blastomyces, Coccidioides
• Parasites: Entamoeba histolytica, Paragonimus

Diagnosis

Imaging Studies

Lung abscesses are often unilateral and typically involve the posterior segments of the upper lobes or the apical segments of the lower lobes, as these areas are gravity-dependent when a patient is lying down. The presence of air-fluid levels suggests rupture into the bronchial tree or, less commonly, the growth of gas-forming organisms.

Laboratory Studies

Raised inflammatory markers such as high ESR and CRP are common but nonspecific. Examination of expectorated mucus often reveals mixed bacterial flora. Cultures from transtracheal or transbronchial aspirates obtained via bronchoscopy can provide more specific information. Fibre optic bronchoscopy is useful for excluding obstructive lesions and aiding in bronchial drainage of pus.

Management

The cornerstone of treatment for lung abscess is broad-spectrum antibiotics to cover mixed flora. Pulmonary physiotherapy and postural drainage also play very important roles. Surgical intervention may be necessary for drainage or pulmonary resection in selected patients.

Acute Cases

Antibiotics:

• Anaerobic: Metronidazole or clindamycin
• Aerobic: Beta-lactams, cephalosporins
• MRSA or Staphylococcus infection: Vancomycin or linezolid

Additional Treatments:

• Postural drainage and chest physiotherapy
• Bronchoscopy for aspiration or antibiotic instillation, or to investigate atypical presentations for underlying foreign bodies or malignancies

Prognosis

Most cases of lung abscess respond well to antibiotics, and the prognosis is generally excellent unless there is an underlying debilitating condition. The mortality rate from lung abscess alone is approximately 5% and continues to improve with advancements in medical care.

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Self-assessment MCQs (single best answer)

What is the primary form of necrosis in a lung abscess?

Which predisposing factor is most commonly associated with lung abscesses?

What is a common symptom of lung abscess?

Which of the following is a complication of lung abscess?

Which segments of the lungs are most often involved in lung abscesses?

Which organism is NOT commonly associated with lung abscess?

Which diagnostic imaging is often used to identify lung abscesses?

What is a key feature seen in imaging studies of lung abscesses?

Which antibiotic is typically used to treat anaerobic bacteria in lung abscesses?

What is the approximate mortality rate for lung abscess alone?