Lung Abscess and Pulmonary Gangrene

LUNG ABSCESS

Definition

Classification

Etiology & Microbiology

• Peptostreptococcus spp.
• Bacteroides spp.
• Fusobacterium nucleatum
• Prevotella melaninogenica

• Staphylococcus aureus (can cause abscess without anaerobic component)
• Klebsiella pneumoniae (classic in alcoholics — "currant jelly" sputum)
• Pseudomonas aeruginosa
• Streptococcus milleri group
• Nocardia, fungi (Aspergillus, Histoplasma) in immunocompromised patients

Predisposing Factors / Pathophysiology

1. Microaspiration of bacteria into dependent lung segments
2. Local pneumonitis → tissue necrosis → cavity formation
3. Communication with bronchus → air-fluid level develops

• Altered consciousness (alcohol, seizures, general anesthesia, drug overdose)
• Dysphagia, esophageal disorders
• Poor dental hygiene / periodontal disease (antecedent dental infection is common)
• Bronchial obstruction (tumor, foreign body)
• Septic emboli (IV drug use, right-sided endocarditis)
• Immunosuppression (HIV, chemotherapy, transplant)

Anatomical Distribution

• Right lower lobe — posterior segment (most common when supine)
• Right upper lobe — posterior segment
• Left lower lobe — posterior segment
• Upper lobe involvement suggests hematogenous spread or specific organisms (e.g., Klebsiella)

Clinical Features

• Constitutional: fever, malaise, night sweats, weight loss
• Respiratory: cough, chest pain, dyspnea
• Hallmark: foul-smelling (putrid) sputum — pathognomonic of anaerobic infection
• Hemoptysis (variable)
• Finger clubbing (in chronic cases)
• Signs of consolidation on examination

Diagnosis

Imaging

• Cavitary lesion with a thick wall and air-fluid level
• Usually in dependent segments
• Surrounding consolidation/infiltrate

• Defines cavity morphology, wall thickness, and multilocularity
• Distinguishes abscess from empyema with bronchopleural fistula
• Key CT features: thick irregular enhancing wall, air-fluid level, surrounding parenchymal consolidation

Differential diagnosis of cavitary lung lesion includes: necrotizing pneumonia, empyema with bronchopleural fistula, pneumatocele, cavitating malignancy, tuberculosis, fungal infection, Wegener's granulomatosis (Harrison's, p. 5087)

Laboratory

• CBC: leukocytosis with left shift
• Elevated CRP, ESR
• Blood cultures (positive in hematogenous cases)
• Sputum Gram stain & culture (anaerobic culture requires proper technique)
• Bronchoscopy with BAL/protected specimen brush — if obstruction suspected or no response to treatment
• Serology / fungal antigens if immunocompromised

Management

Antibiotic Therapy

• Duration: typically 4–6 weeks (until radiological resolution)
• IV initially → step down to oral when clinically improving

Postural Drainage

• Physiotherapy and postural drainage to promote expectoration of pus
• Positioning the patient with the affected segment dependent

Bronchoscopy

• Therapeutic: relieve obstruction (foreign body, tumor)
• Diagnostic: obtain cultures, exclude malignancy

Surgical / Interventional

• Failure to respond to antibiotics after 4–6 weeks
• Massive hemoptysis
• Abscess > 6 cm diameter
• Suspected underlying malignancy
• Tension cavity
• Bronchopleural fistula

Prognosis & Complications

• Cure rate ~90% with appropriate antibiotic therapy
• Poor prognostic factors: large cavity (>6 cm), immunosuppression, aerobic/nosocomial organisms, obstruction by tumor
• Complications:
  • Empyema / bronchopleural fistula
  • Massive hemoptysis
  • Spread to contralateral lung
  • Septicemia
  • Pulmonary gangrene (most severe complication — see below)

PULMONARY GANGRENE

Definition

Pathophysiology

1. Virulent organisms or impaired host defenses lead to extensive pneumonitis
2. Vascular thrombosis of pulmonary vessels (both arterial and venous) within the affected area → ischemic necrosis
3. Superimposed septic and putrefactive processes → massive tissue destruction
4. Sloughing of necrotic lung tissue → pulmonary sequestrum (detached necrotic tissue floating in a cavity — "sequestrum sign" on imaging)

Etiology

• Klebsiella pneumoniae (classical cause, especially in diabetics/alcoholics)
• Staphylococcus aureus (including MRSA)
• Streptococcus pneumoniae (type III, uncommon but recognized)
• Pseudomonas aeruginosa
• Anaerobes
• Aspergillus (in severely immunocompromised)

Clinical Features

• Extremely ill, toxic patient
• High-grade fever, rigors
• Severe chest pain
• Massive foul-smelling / blood-stained sputum (expectoration of necrotic lung tissue)
• Rapidly progressive respiratory failure
• Septic shock
• Hemoptysis (can be massive and life-threatening)

Imaging

• Extensive consolidation of a lobe or lung
• Large, irregular cavities
• Sequestrum: a sloughed piece of necrotic lung floating within the cavity

• Extensive necrosis with multiple irregular cavities
• Vascular occlusion / absent enhancement of involved parenchyma
• Sequestrum sign: detached lung parenchyma within a fluid-filled cavity
• Contralateral spread may be seen

Comparison: Lung Abscess vs. Pulmonary Gangrene

Management of Pulmonary Gangrene

1. Resuscitation — ICU admission, aggressive fluid resuscitation, vasopressors for septic shock
2. Broad-spectrum antibiotics (IV)
   • Cover anaerobes + Gram-negatives + MRSA
   • E.g., Piperacillin-tazobactam + vancomycin ± metronidazole
   • Antifungals if immunocompromised
3. Drainage of pus (bronchoscopic or CT-guided)
4. Bronchoscopic debridement of necrotic material
5. Surgical resection
   • Often required due to massive necrosis and failure of conservative therapy
   • Pneumonectomy or lobectomy may be necessary
   • High operative mortality — surgery reserved for life-threatening hemorrhage or failure to improve
6. Supportive care: mechanical ventilation, nutritional support, management of complications

Prognosis

• Mortality remains high (up to 20–50% in some series)
• Early aggressive treatment improves survival
• Survivors may have significant residual pulmonary dysfunction

Summary