Lung Abscess - Comprehensive Notes

Note on sources: Davidson's Principles & Practice of Medicine and KDT (Tripathi's Pharmacology) are not available in the textbook library. The pharmacology drug section is covered using Goodman & Gilman's (the pharmacological equivalent). All Harrison's content is from the 22nd Edition (2025). Key clinical/pathology points from Fishman's Pulmonary Diseases and Goldman-Cecil Medicine are included to supplement.

1. DEFINITION

• Harrison's Principles of Internal Medicine 22E, Ch. 132

2. CLASSIFICATION

• Postobstructive process (bronchial foreign body, tumor)
• Systemic process (HIV infection, other immunocompromising conditions)

• Harrison's 22E; Fishman's Pulmonary Diseases

3. EPIDEMIOLOGY AND RISK FACTORS

• Middle-aged men more commonly affected than women
• Incidence has decreased in the antibiotic era but still carries significant morbidity and mortality
• Major risk factor: aspiration

Patients at risk for aspiration:

• Altered mental status
• Alcoholism, drug overdose
• Seizures
• Bulbar dysfunction
• Prior cerebrovascular or cardiovascular events
• Neuromuscular disease
• Esophageal dysmotility or lesions (strictures, tumors)
• Gastric distension / gastroesophageal reflux, especially in the recumbent position

Dental colonization:

• Harrison's 22E

4. ETIOLOGY AND MICROBIOLOGY

Harrison's Table 132-1: Microbial Pathogens Causing Lung Abscesses

Fishman's additional microbiology data:

• Anaerobes recoverable from up to 93% of patients
• 46% have only anaerobes isolated; additional 43% have anaerobes mixed with aerobic bacteria (Bartlett & Finegold)
• Fusobacterium nucleatum and Bacteroides spp. are the most common anaerobes causing cavitation
• E. histolytica - important cause in basilar portion of right lower lobe
• K. pneumoniae - increasing frequency, especially in Asia

5. PATHOGENESIS

Primary Lung Abscess:

1. Aspiration of anaerobic bacteria (+ microaerophilic streptococci) from gingival crevices into lung parenchyma
2. Patients carry overwhelming burden of aspirated material OR are unable to clear the bacterial load
3. Pneumonitis develops initially (exacerbated by gastric acid injury)
4. Over 7-14 days, bacteria produce parenchymal necrosis and cavitation
5. Extent depends on host-pathogen interaction
6. Anaerobes produce more extensive necrosis in polymicrobial infections (virulence factors act synergistically)

Secondary Lung Abscess:

• Bronchial obstruction (malignancy/foreign body): prevents clearance of oropharyngeal secretions → abscess
• Systemic immunosuppression (bone marrow/solid organ transplant): impaired host defense → broad pathogen spectrum including opportunistic organisms
• Septic emboli: from tricuspid valve endocarditis (S. aureus) or Lemierre's syndrome (F. necrophorum infects the pharynx → spreads to neck/carotid sheath → septic thrombophlebitis of the jugular vein → emboli to lung)

CT scan showing lung abscess development:

Left panel (A): Left lung infiltrate with central necrosis (black arrow) in Pseudomonas pneumonia. Right panel (B): Two weeks later - cavitation with air-fluid levels (white arrow) = lung abscess. - Harrison's 22E, Fig 132-1

6. PATHOLOGY

• Location: Dependent lung segments (due to gravity-dependent deposition of aspirated material)
  • Posterior segment of right upper lobe (most common)
  • Apical segment of right lower lobe
  • Posterior segment of left upper lobe
  • Apical segment of left lower lobe
  • Right lung > left lung (right mainstem bronchus is less angulated)
• Abscess cavity may become lined with regenerated epithelium
• Local obstruction may produce bronchiectasis or emphysema in surrounding lung
• In pre-existing cavitary disease (emphysema, old TB), infection may proceed without frank necrosis
• 1-2 weeks typically required for cavity formation
• Harrison's 22E; Fishman's Pulmonary Diseases

7. CLINICAL MANIFESTATIONS

Subacute/Chronic (typical anaerobic abscess):

• Fever, cough, sputum production, chest pain
• Night sweats, fatigue, anemia (chronic course)
• Putrid/foul-smelling sputum - discolored phlegm (subset with anaerobic infection)
• Pneumonia present/suspected for 1-3 weeks before recognition of abscess
• Weight loss (chronic cases)

Acute (non-anaerobic, e.g., S. aureus):

• High fevers, rapid progression, more fulminant course

Physical examination findings:

• Fevers
• Poor dentition and/or gingival disease (clue to aspiration etiology)
• Amphoric and/or cavernous breath sounds on auscultation
• Digital clubbing (chronic cases)
• Absent gag reflex (aspiration risk)

Secondary abscess (e.g., septic emboli):

• Can evolve rapidly over 48-72 hours
• Patient with endocarditis may present with pneumonia, lung abscess, and empyema
• Harrison's 22E; Fishman's Pulmonary Diseases

8. DIFFERENTIAL DIAGNOSIS

9. DIAGNOSIS

Imaging:

• Chest X-ray: Thick-walled cavity with air-fluid level (classic appearance)
• CT scan (preferred):
  • Better definition; earlier evidence of cavitation
  • Reveals underlying cause (malignancy, obstruction)
  • Distinguishes peripheral lung abscess from empyema (critical - different management)

Microbiological workup:

• Sputum Gram stain and culture (noninvasive, but contamination by oral flora is a limitation; may not reflect anaerobes)
• Blood cultures
• In low malignancy risk patients with aspiration risk factors → reasonable to begin empirical treatment and pursue further evaluation if no response
• In immunocompromised hosts or atypical presentations → earlier diagnostics:
  • Bronchoscopy with biopsy
  • CT-guided needle aspiration
• Bronchoscopy early if history/symptoms/imaging suggest possible bronchial obstruction
• In TB-endemic areas or HIV patients → induced sputum for AFB early in workup
• Putrid-smelling sputum considered virtually diagnostic of anaerobic infection
• Molecular techniques (16S RNA gene amplification) increasingly used for specific pathogen identification

Note: Transtracheal aspiration and BAL with protected brush specimens (traditional specialized techniques to avoid oral contamination) are used less often now. When no pathogen is isolated (~40% of cases), the abscess is termed a "nonspecific lung abscess" and anaerobes are presumed.

• Harrison's 22E

10. TREATMENT

Antibiotic Therapy - Harrison's 22E:

Key point: Metronidazole is NOT effective as a single agent - it covers anaerobes but NOT microaerophilic streptococci (often part of the mixed flora). Must be used in combination if used at all.

• Range: 3-4 weeks to up to 14 weeks
• At least 6 weeks associated with better outcomes in some literature

• Directed at the identified pathogen
• Prolonged course until resolution documented
• Treat underlying cause (relieve obstruction, treat immunocompromise)
• Goldman-Cecil: Clindamycin 600 mg IV q8h → 150-300 mg PO qid; improvement typically within 7-10 days

Goodman & Gilman's (Pharmacological reference):

• Clindamycin is the drug of choice for lung abscess and anaerobic lung/pleural space infections
• Clindamycin + primaquine: alternative for P. jirovecii pneumonia in HIV patients
• Goodman & Gilman's Pharmacological Basis of Therapeutics

11. SURGICAL AND INTERVENTIONAL OPTIONS

• Abscess >6-8 cm is less likely to respond to antibiotics alone
• Further diagnostic studies should be pursued to rule out secondary cause

1. Surgical resection - gold standard for refractory cases
2. Percutaneous drainage - for poor surgical candidates
   • Risks: bacterial contamination of pleural space, pneumothorax, hemothorax
   • Traversing normal lung parenchyma is a risk factor for major complications

• Harrison's 22E

12. COMPLICATIONS

• Persistent cystic changes (pneumatoceles) or bronchiectasis (with larger cavities)
• Recurrence despite appropriate therapy
• Extension to pleural space → empyema
• Life-threatening hemoptysis
• Massive aspiration of lung abscess contents
• Brain abscess (bacteremia from lung abscess can seed the CNS - seizures may be the presenting feature)
• Harrison's 22E

13. PROGNOSIS

Poor prognostic factors:

• Age >60 years
• Malignancy-related abscess
• Presence of aerobic bacteria
• Sepsis at presentation
• Symptom duration >8 weeks
• Abscess size >6 cm
• Harrison's 22E

14. PREVENTION

• Address underlying aspiration risk factors
• Airway protection in at-risk patients
• Oral hygiene (treatment of periodontal disease)
• Minimize sedation
• Elevate head of bed for aspiration-prone patients
• Prophylaxis against specific pathogens in immunocompromised patients (e.g., PCP prophylaxis with trimethoprim-sulfamethoxazole)
• Harrison's 22E

Summary Mnemonics

• Peptostreptococcus
• Prevotella
• Bacteroides
• Microaerophilic streptococci (milleri group)

• Posterior segment RUL (most common, supine patient)
• Apical segment RLL
• Right > Left (less angulated bronchus)

• Clindamycin preferred over penicillin (β-lactamase producing anaerobes)
• Continue until cavity clears (up to 14 weeks)
• Consider surgical drainage if >6-8 cm or no improvement

• Harrison's Principles of Internal Medicine, 22nd Ed. (2025) - Ch. 132 (primary source)
• Fishman's Pulmonary Diseases and Disorders - Ch. 127
• Goodman & Gilman's Pharmacological Basis of Therapeutics - Clindamycin section
• Goldman-Cecil Medicine - Aspiration Pneumonia/Lung Abscess chapter

Davidson's and KDT (Tripathi's Pharmacology) are not in the available textbook library. The above covers all major clinical, microbiological, and pharmacological aspects comprehensively from Harrison's and equivalent references. If you have access to a specific Davidson's or KDT edition, I can help cross-reference specific topics.