Atypical Pneumonia

Overview

Etiology

Clinical Presentation

• Gradual onset (days to weeks vs. acute in typical pneumonia)
• Non-productive or minimally productive cough
• Low-grade fever, headache, malaise, myalgias (flu-like prodrome)
• Absence of rigors (common in pneumococcal pneumonia)
• Extra-pulmonary features: pharyngitis, earache (bullous myringitis with Mycoplasma), GI symptoms, rash, hemolytic anemia (cold agglutinins with Mycoplasma)
• Legionella can cause more severe disease with high fever, confusion, hyponatremia, and diarrhea

Diagnosis

• Chest X-ray: patchy, bilateral, interstitial or reticulonodular infiltrates — often more diffuse than expected clinically
• Chest CT: bilateral ground-glass opacities (GGOs), subpleural/peripheral distribution, lower lobe predominance (see image below)
• Lab findings: Normal or mildly elevated WBC (leukocytosis less pronounced than in typical bacterial pneumonia), elevated cold agglutinins (Mycoplasma)
• Microbiological workup:
  • Urinary antigen tests: Legionella (serogroup 1), S. pneumoniae
  • Serology: paired titres for Mycoplasma, Chlamydia
  • PCR (nasopharyngeal): respiratory viruses, Mycoplasma, Chlamydia
  • Sputum culture (low yield for atypical organisms)

PA chest X-ray (A) showing bilateral peripheral opacities; axial CT (B) showing subpleural ground-glass opacities with small areas of consolidation — classic findings of atypical pneumonia.

Management

Empirical Antibiotic Therapy

• Macrolides and tetracyclines (doxycycline) cover atypical organisms; β-lactams alone do not
• IDSA/ATS recommends a minimum of 5 days of antibiotics, with clinical stability as the endpoint (resolution of vital sign abnormalities, ability to eat, normal mentation)
• Shorter courses supported by meta-analyses and RCTs

Supportive Care

• Adequate hydration, antipyretics, rest
• Supplemental oxygen if hypoxic
• ICU admission criteria: severe CAP (CURB-65 or PSI scoring)

Key Distinguishing Features vs. Typical Pneumonia

Complications

• Respiratory failure (especially with Legionella, viral pneumonia)
• Pleural effusion
• Hemolytic anemia (Mycoplasma — cold agglutinins)
• Neurological complications (Mycoplasma — encephalitis, Guillain-Barré)
• Myocarditis, pericarditis (rare, Mycoplasma)

Special Populations

• Young adults/college students: Mycoplasma predominates ("walking pneumonia")
• Elderly/immunocompromised: RSV, metapneumovirus, and Legionella more severe
• Epidemic/cluster setting: Legionella (water source exposure), adenovirus (military)
• Immunocompromised: Broader differential including PCP (Pneumocystis jirovecii), CMV, fungal

Lung Abscess & Pulmonary Gangrene

LUNG ABSCESS

Definition

Etiology

• Altered consciousness (alcoholism, seizures, general anesthesia, drug overdose)
• Dysphagia (neurological disease, esophageal disorders)
• Poor dental hygiene / periodontal disease
• Oropharyngeal/GI malignancy
• Bronchial obstruction (endobronchial tumor, foreign body)
• Immunocompromised states (HIV, transplant, corticosteroid use)
• Septic emboli (IV drug use, right-sided endocarditis)
• Hematogenous spread from distant infection

Pathophysiology

1. Aspiration of oropharyngeal contents (anaerobe-rich) → organisms seed dependent lung segments
2. Initial pneumonitis develops in the dependent zones (posterior segments of upper lobes, superior segments of lower lobes — favored in recumbent position)
3. Tissue necrosis from bacterial enzymes, toxins, and host inflammatory response
4. Liquefaction of necrotic tissue → abscess cavity forms
5. Communication with a bronchus → air enters the cavity, creating the classic air-fluid level on imaging
6. Surrounding wall of granulation tissue and fibrosis forms over time
7. If drainage is inadequate → chronic abscess with thick fibrotic wall

Clinical Presentation

• Cough — initially dry, then productive of foul-smelling, purulent sputum (pathognomonic of anaerobic infection)
• Hemoptysis (can be massive)
• Fever, chills, night sweats
• Pleuritic chest pain
• Anorexia, weight loss (especially chronic abscess)
• Fatigue, malaise

• Decreased breath sounds over the affected area
• Amphoric/cavernous breath sounds (hollow, blowing quality over large cavity)
• Dullness to percussion (surrounding consolidation/fluid)
• Clubbing — may be present in chronic abscess
• Foul breath (halitosis) in anaerobic infections
• Fever and tachycardia

Diagnosis

• Leukocytosis with left shift (neutrophilia)
• Elevated CRP, ESR
• Anemia (chronic disease in prolonged cases)
• Blood cultures (positive in hematogenous spread)

• Sputum Gram stain and culture (limited by oropharyngeal contamination)
• Bronchoscopic samples (BAL, protected specimen brush) — preferred for anaerobes
• Transthoracic needle aspiration (if peripheral)
• Pleural fluid culture (if effusion present)

• Chest X-ray: thick-walled cavitary lesion with air-fluid level — the classic finding
• CT chest (preferred): better delineates cavity wall thickness, satellite lesions, underlying endobronchial obstruction or malignancy; distinguishes lung abscess from empyema

Panel A: PA chest X-ray showing a large cavitary lesion in the right lung with a horizontal air-fluid level. Panel B: Axial CECT confirming a thick, enhancing peripheral wall — classic for lung abscess. CT is critical to distinguish abscess from empyema and exclude underlying malignancy.

Differential Diagnosis

Management

1. Antibiotic Therapy (Cornerstone of Treatment)

2. Postural Drainage & Physiotherapy

• Position patient to facilitate drainage through the relevant bronchus
• Chest physiotherapy to aid expectoration
• Avoid in patients with large cavities at risk of aspiration of purulent contents

3. Bronchoscopy

• Indications: exclude endobronchial obstruction (foreign body, tumor), obtain microbiological samples, facilitate drainage
• NOT used for therapeutic drainage of abscess itself (risk of soiling other lung segments)

4. Percutaneous Drainage (CT-guided)

• Indicated for: large abscess (>6 cm), failure of antibiotic therapy after 4–6 weeks, organisms resistant to antibiotics
• Increasingly preferred over surgery in poor surgical candidates

5. Surgery

• Reserved for failure of medical therapy (10–15% of cases)
• Indications:
  • No response to antibiotics after 6–8 weeks
  • Massive hemoptysis
  • Suspected underlying malignancy
  • Bronchopleural fistula
  • Abscess >6 cm unresponsive to drainage
• Procedure: lobectomy or segmentectomy (pneumonectomy reserved for pulmonary gangrene)

Complications

• Bronchopleural fistula — communication between abscess and pleural space
• Empyema — most common complication; requires drainage
• Massive hemoptysis — erosion of pulmonary vessels; life-threatening
• Pyopneumothorax — abscess ruptures into pleural space
• Septicemia / bacteremia — hematogenous dissemination
• Brain abscess — haematogenous spread (especially anaerobes)
• Pulmonary gangrene — extension of necrosis to a large segment/lobe
• Chronic lung abscess — fibrotic thick-walled cavity, persistent infection
• Amyloidosis — in very chronic cases (rare)

PULMONARY GANGRENE

Definition

Pneumonia → Necrotizing pneumonia → Lung abscess → Pulmonary gangrene

Etiology

Pathophysiology

1. Virulent organism infects lung parenchyma → intense inflammatory response
2. Vascular invasion and thrombosis of pulmonary vasculature (arteries and veins) → ischemia
3. Loss of blood supply → massive coagulative necrosis of entire segments or lobes
4. Liquefaction of necrotic tissue → large irregular cavities with sloughed, devitalized lung tissue
5. The necrotic lung fragment detaches and floats in the fluid-filled cavity — the hallmark "pulmonary sequestrum" or "floating island" sign on CT
6. Secondary infection by mixed flora accelerates destruction
7. May extend to pleural space → pyopneumothorax, bronchopleural fistula

Clinical Presentation

• Acute, severely ill patient — unlike the subacute course of simple lung abscess
• High-grade fever, rigors, sepsis
• Productive cough with foul-smelling, often bloody-purulent sputum
• Massive hemoptysis (common; can be life-threatening)
• Severe pleuritic chest pain
• Dyspnea, tachypnea, cyanosis
• Rapid deterioration → respiratory failure, septic shock
• Examination: signs of consolidation/pleural effusion; toxic appearance

Diagnosis

• Chest X-ray: large area of consolidation progressing to a large, irregular cavitary lesion; may see a dense shadow within the cavity (sloughed lung)
• CT chest (diagnostic of choice):
  • Large, irregular cavitary lesion with thick walls
  • "Floating island" / pulmonary sequestrum sign: sloughed necrotic lung fragment floating within the fluid-filled cavity
  • Extensive surrounding consolidation, pleural effusion
  • Vascular cut-off or thrombosis in adjacent vessels (CT angiography)
  • Pyopneumothorax if bronchopleural fistula develops

• Severe leukocytosis or leukopenia (ominous)
• Elevated procalcitonin, CRP
• Blood cultures — frequently positive
• Sputum culture, BAL
• Consider bronchoscopy for sampling

Differential Diagnosis

Management

1. ICU Admission & Supportive Care

• High-flow oxygen / mechanical ventilation if respiratory failure
• Vasopressors for septic shock
• Nutritional support (often severely malnourished)

2. Aggressive Antibiotic Therapy

• Broad-spectrum from the outset; narrowed based on cultures
• Cover gram-negatives, S. aureus (MRSA), and anaerobes:
  • Piperacillin-tazobactam or carbapenem (meropenem/imipenem) PLUS
  • Vancomycin or linezolid (MRSA cover) PLUS
  • Metronidazole (additional anaerobic cover if not already included)
• Duration: prolonged (6–8 weeks minimum), guided by clinical and radiological response

3. Drainage Procedures

• Chest tube (tube thoracostomy): mandatory if empyema or pyopneumothorax
• CT-guided percutaneous drainage: if accessible collection
• Bronchoscopy: to clear secretions and obtain cultures

4. Surgery (Definitive for Non-Responders)

• Often unavoidable in pulmonary gangrene
• Pneumonectomy (removal of entire lung) is frequently required due to extent of destruction
• Lobectomy if gangrene is confined to a single lobe
• High surgical mortality in the acute phase — timing is critical (ideally after stabilization)
• Decortication if organized empyema present

5. Adjunctive Measures

• Antifungal therapy if fungal co-infection suspected
• Immunoglobulin (IVIG) in severe S. aureus PVL-toxin-mediated disease (off-label)
• Hyperbaric oxygen — limited evidence, used in some centers

Complications

• Massive, life-threatening hemoptysis
• Bronchopleural fistula — very common; prolongs hospital stay
• Pyopneumothorax — rupture into pleural space
• ARDS (acute respiratory distress syndrome)
• Septic shock and multi-organ failure
• Mediastinitis — if infection spreads
• Post-pneumonectomy complications: empyema, respiratory failure
• Death — mortality 30–60% even with aggressive management

Comparison: Lung Abscess vs. Pulmonary Gangrene