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132 Lung Abscess

Chapter 132 | Lung Abscess

KEY CLINICAL POINTS

  • Lung abscess is a necrotizing lung infection characterized by cavitation >2 cm in diameter, often caused by aspiration of oral anaerobes.
  • Primary abscesses arise from aspiration (common in middle-aged men with risk factors like alcoholism or neuromuscular disease), while secondary abscesses result from obstructive processes or immunocompromise.
  • Diagnosis relies on imaging (CT for cavitation with air-fluid levels) and microbiological studies, with clindamycin or amoxicillin-clavulanate as first-line antibiotics.
  • Complications include empyema, bronchopleural fistula, and septic emboli; treatment duration ranges from 3–14 weeks.
  • Putrid-smelling sputum and foul breath are diagnostic of anaerobic infection, while non-anaerobic abscesses present with more fulminant symptoms.

1. DEFINITION & OVERVIEW

Lung abscess represents necrosis and cavitation of the lung following microbial infection. Abscesses are typically single, >2 cm in diameter, and often associated with aspiration or underlying conditions. Primary abscesses arise from aspiration of oral anaerobes, while secondary abscesses result from obstructive processes, immunocompromise, or septic emboli.

Table 132-1 Examples of Microbial Pathogens That Can Cause Lung Abscesses

CLINICAL CONDITION PATHOGENS
Primary lung abscess (often with risk factors for aspiration) Anaerobes (e.g., Peptostreptococcus spp., Prevotella spp., Bacteroides spp., milleri group streptococci), microaerophilic streptococci
Secondary lung abscess Staphylococcus aureus, gram-negative rods (e.g., Pseudomonas aeruginosa, Enterobacteriaceae), Nocardia spp., Aspergillus spp., Cryptococcus spp., Legionella spp., Rhodococcus equi, Pneumocystis jirovecii
Embolic lesions Staphylococcus aureus (endocarditis), Fusobacterium necrophorum (Lemierre’s syndrome)
Endemic infections (with or without underlying immunocompromise) Mycobacterium tuberculosis, Coccidioides spp., Histoplasma capsulatum, Blastomyces spp., parasites (e.g., Entamoeba histolytica, Paragonimus westermani, Strongyloides stercoralis)
Miscellaneous Bacterial pathogens (often S. aureus) after influenza/viral infection, Actinomyces spp.

1.1 Pathogenesis

Primary abscesses develop from aspiration of oral anaerobes (e.g., Peptostreptococcus, Bacteroides) into lung parenchyma, leading to polymicrobial infection and necrosis. Secondary abscesses arise from obstructive lesions (e.g., tumors, foreign bodies) or systemic conditions (e.g., immunosuppression), allowing bacterial proliferation and cavitation.

1.2 Classification

Abscesses are classified as primary (aspiration-related) or secondary (obstructive/immunocompromised). Acute abscesses (<4–6 weeks) vs. chronic (~40% of cases).

2. EPIDEMIOLOGY

Primary lung abscesses are more common in middle-aged men with risk factors for aspiration (e.g., alcoholism, neuromuscular disease, esophageal dysmotility). Incidence decreased with antibiotic use and improved oral surgery techniques. Secondary abscesses occur in immunocompromised patients (e.g., post-transplant, HIV) or those with obstructive lung disease.

2.1 Risk Factors

Aspiration (alcoholism, seizures, bulbar dysfunction), esophageal lesions, gastric reflux, and immunosuppression. Oral hygiene (gingivitis/periodontal disease) increases risk of anaerobic colonization.

3. ETIOLOGY & PATHOPHYSIOLOGY

Primary abscesses are caused by aspiration of oral anaerobes and microaerophilic streptococci. Secondary abscesses result from obstructive processes (e.g., tumors, foreign bodies) or systemic conditions (e.g., immunosuppression). Polymicrobial infections with synergistic virulence factors lead to extensive necrosis.

3.1 Primary Abscesses

Aspiration of oral flora into lung parenchyma causes polymicrobial infection. Anaerobes produce more tissue necrosis in synergistic infections. Common pathogens include Peptostreptococcus, Bacteroides, and microaerophilic streptococci.

3.2 Secondary Abscesses

Obstructive lesions (e.g., tumors, foreign bodies) or systemic conditions (e.g., immunosuppression) prevent clearance of oropharyngeal secretions. Pathogens include S. aureus, gram-negative rods, and opportunistic organisms (e.g., fungi, mycobacteria).

4. CLINICAL FEATURES

Initial symptoms resemble pneumonia (fever, cough, chest pain). Anaerobic abscesses present with chronic symptoms (night sweats, fatigue, anemia). Putrid-smelling sputum and foul breath are diagnostic of anaerobic infection. Non-anaerobic abscesses (e.g., S. aureus) present with more fulminant fever and rapid progression.

4.1 Physical Findings

Fever, poor dentition, gingival disease, amphoric breath sounds. Digital clubbing and absent gag reflex may be present.

5. DIFFERENTIAL DIAGNOSIS

Cavitary lung lesions include non-infectious processes (e.g., lung infarction, malignancy, sequestration), cryptogenic organizing pneumonia, sarcoidosis, vasculitides, and septic emboli (e.g., from tricuspid endocarditis). Pulmonary manifestations of systemic diseases (e.g., inflammatory bowel disease, pyoderma gangrenosum) must also be considered.

6. INVESTIGATIONS & DIAGNOSIS

Chest imaging (CT preferred) detects cavitation with air-fluid levels. Sputum analysis (Gram stain, culture) may identify pathogens, though anaerobes are often missed. Molecular techniques (16S RNA amplification) improve detection. Blood cultures and serologic tests for opportunistic pathogens (e.g., fungi, viruses) are used in immunocompromised patients. Bronchoscopy or CT-guided aspiration may be required for definitive diagnosis.

6.1 Diagnostic Algorithm

  1. Chest CT to confirm cavitation with air-fluid levels.
  2. Sputum Gram stain/culture (non-invasive, but may miss anaerobes).
  3. Blood cultures and serologic tests for opportunistic pathogens.
  4. Bronchoscopy/CT-guided aspiration for refractory cases.
  5. Molecular diagnostics (e.g., 16S RNA) for rapid pathogen identification.

7. MANAGEMENT & TREATMENT

Antibiotic therapy is first-line. For primary abscesses: clindamycin (600 mg IV q8h → 300 mg PO q6h after improvement) or amoxicillin-clavulanate (4 g/day). Duration: 3–14 weeks until imaging shows resolution. Surgical drainage is required for complications (e.g., empyema, bronchopleural fistula).

7.1 Antibiotic Regimens

Clindamycin (600 mg IV q8h; switch to 300 mg PO q6h after fever resolution) or amoxicillin-clavulanate (4 g/day). For immunocompromised patients, broader coverage (e.g., vancomycin, carbapenems) may be needed.

7.2 Surgical Interventions

Drainage for empyema, bronchopleural fistula, or persistent abscess. Percutaneous catheter drainage is preferred over thoracotomy. Surgical resection may be required for large abscesses or complications.

8. PROGNOSIS & COMPLICATIONS

Mortality is ~1–5% with appropriate treatment. Complications include empyema, bronchopleural fistula, septic emboli (Lemierre’s syndrome), and recurrent infection. Poor outcomes are associated with immunosuppression, delayed treatment, or multidrug-resistant organisms.

9. SPECIAL CONSIDERATIONS

Pregnancy: Avoid certain antibiotics (e.g., tetracyclines). Pediatrics: Monitor for aspiration risk and growth. Elderly: Consider comorbidities and drug interactions. Immunocompromised patients require broader antimicrobial coverage and prolonged treatment.

10. KEY POINTS & CLINICAL PEARLS

  1. Aspiration is the leading cause of primary lung abscess; anaerobes are the main pathogens.
  2. Putrid-smelling sputum and foul breath are diagnostic of anaerobic infection.
  3. CT is essential for diagnosis, while sputum culture may miss anaerobes.
  4. Clindamycin is preferred over penicillin for anaerobic coverage.
  5. Treatment duration ranges from 3–14 weeks, with imaging-guided discontinuation.
  6. Surgical drainage is required for complications like empyema or bronchopleural fistula.