Description

• Pulmonary abscess or lung abscess is a microbial infection of the lung that results in necrosis and destruction of the normal pulmonary parenchyma resulting in cavitation with or without central necrosis surrounded by thick wall (1).
• Usually >2 cm in diameter (1,2)
• Cavitations are filled with purulent material.
• Necrotizing pneumonia and lung gangrene, which are on the same clinical spectrum, have multiple areas affected parenchyma.
• If infected tissue involves visceral pleura, it will develop pyopneumothorax or pleural empyema (3).

Epidemiology

Incidence

• Constitute an uncommon infection due to access to antibiotics and advances in oral and dental hygiene; exact incidence is unknown.
• Mortality ranges from 2% to 38.2% (1,3) but can go as high as 75% if not recognized and treated in timely manner (2,3).
• Middle-aged men have more predisposition.

Etiology and Pathophysiology

• Commonly presents secondary to aspiration from oral bacteria from the gingival crevice, leading to inflammation cascade and eventual liquefactive necrosis of tissue
• Less commonly, septic emboli from endocarditis can seed and predispose lung abscesses (2).
• Classification
  • By etiology: primary (i.e., aspiration of secretions, necrotizing pneumonia) and secondary (i.e., bronchial obstruction, hematogenic dissemination) (3)
  • By time of development in acute (<6 weeks) versus chronic (>6 weeks) (3)
  • By routes of spreading: bronchogenic (i.e., aspiration, bronchial obstruction by tumor or foreign body) and hematogenic (i.e., endocarditis, septic emboli)
• Microbiology
  • Preantibiotic era abscesses were caused by one type of bacteria, whereas more recently is cause by polymicrobial flora 90% of cases (3).
  • Immunocompetent hosts: anaerobic bacteria (i.e., Bacteroides fragilis, Fusobacterium capsulatum and Fusobacterium necrophorum, Peptostreptococcus, and microaerophilic streptococci) and aerobic bacteria (i.e., Staphylococcus aureus, including methicillin-resistant S. aureus [MRSA]; Streptococcus pyogenes and Streptococcus pneumoniae, Klebsiella pneumoniae, Pseudomonas aeruginosa, Haemophilus influenzae type B, Acinetobacter spp., Escherichia coli, and Legionella)
  • Immunocompromised patients: gram-negative rods in addition to those mentioned above. Consider atypical organisms like fungi, Nocardia, Rhodococcus, Mycobacterium tuberculosis, and Actinomyces.
  • Rarely, amoebic liver abscesses can rupture through the diaphragm and cause lung abscess; consider if there is history of prolonged stay in an endemic region.

Genetics
No specific genetic predisposition has been described.

Risk Factors

• Aspiration risk factors
  • Alcohol intoxication
  • Use of CNS depressant drugs (i.e., opiates)
  • Seizures
  • General anesthesia with surgery
  • Dysphagia from muscular dysfunction either local or systemic
  • Nasogastric tube and severe gastroesophageal reflux disease (GERD)
  • Gingivitis
  • Diabetes, altered mental status, coma
• Airway obstruction (3)
  • Neoplasia
  • External compression from lymph nodes
  • Endobronchial foreign bodies
  • Underlying abnormal parenchyma (bullous emphysema, bronchiectasis)
  • Congenital malformation (i.e., bronchoesophageal fistula)
• Immunosuppression
  • Neutropenia
  • Chemotherapy
  • HIV/AIDS
  • Chronic steroid use
  • Cystic fibrosis

General Prevention

• Treatment of predisposing conditions
• Aspiration precautions
• Treatment of periodontal diseases and improving oral hygiene
• Optimization of underlying comorbidities
• Pulmonary physical therapy
• Alcohol cessation counseling
• Identifying underlying predispositions and correcting them

Pediatric Considerations

• Lung abscess in children are rare and represents 0.7/100,000 admissions per year (1).
• Pediatric population has toxic presentation that ranges from only loss of appetite to fever with altered mental status.
• Lung abscesses after resolution have less morbidity when compared to adult presentations (1).
• Commonly occurs in children with underlying conditions such as immunodeficiency syndromes, immunosuppression states, neurologic conditions which can predispose to aspiration (1)
• S. aureus is the most common isolated etiologic pathogen in children (3).