Lung abscess full details for md exam answer 15 marks from fishman and Murray

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Lung Abscess - Comprehensive MD Exam Answer (15 Marks)

Sources: Fishman's Pulmonary Diseases and Disorders (Chapter 127) & Murray & Nadel's Textbook of Respiratory Medicine (Chapter 50)

Definition and Classification

A lung abscess is a localized area of suppurative necrosis of the lung parenchyma resulting in a cavity containing pus and necrotic debris, usually with an air-fluid level on imaging. It most commonly arises from polymicrobial anaerobic infection originating in the oral cavity.

Classification (Fishman's, p. 2242):

Category	Description
Acute	Symptoms < 4-6 weeks
Chronic	Symptoms > 4-6 weeks (often > 1 month before seeking care)
Primary	Aspiration or pneumonia in an immunocompetent host
Secondary	Pre-existing condition (obstruction, immunodeficiency, spread from extrapulmonary site, bronchiectasis)
Putrid abscess	Foul-smelling; caused by anaerobic organisms
Nonspecific	No pathogen identified on culture

Pathophysiology and Predisposing Conditions

Most lung abscesses result from aspiration of anaerobic bacteria from the gingival crevices of the oral cavity (Murray & Nadel, Ch. 50). The sequence is:

Aspiration of oropharyngeal contents
Pneumonitis develops over 7-14 days
Tissue necrosis follows, creating a cavity
Cavity may become lined by regenerated epithelium; local obstruction can produce bronchiectasis or emphysema in surrounding lung

Key predisposing conditions:

Periodontal disease / poor dentition (major risk factor)
Altered consciousness: alcoholism, sedating drugs, head trauma, seizures, general anesthesia
Dysphagia or pharyngeal dysfunction
Defective cough reflex
Obstruction (neoplasm, foreign body)
Immunocompromise (HIV, chemotherapy, transplant)
Pulmonary embolism, bronchiectasis
Previous cavitary disease (emphysema, old TB) - infection may proceed without frank necrosis

Edentulous patients rarely develop primary lung abscess; its occurrence in them should prompt a search for bronchial obstruction, pulmonary embolus, or septic embolus (Fishman's, p. 2242).

Anatomical Location

Because aspiration is the dominant mechanism, abscesses preferentially involve the most dependent lung segments:

Posterior segment of the right upper lobe (most common)
Apical segments of the lower lobes
Less commonly: posterior segment of the left upper lobe

The right lung is more often affected because the right main bronchus is shorter, wider, and more vertical than the left, favoring aspiration into the right side (Murray & Nadel, Ch. 50).

Microbiology

Anaerobes are recoverable in up to 93% of patients with lung abscess (Fishman's, p. 2242):

46% have only anaerobes isolated
43% have a mixed anaerobic + aerobic infection (Bartlett and Finegold data)

Common anaerobes: Fusobacterium nucleatum, Prevotella species (denticola, intermedia, oris, buccae), Bacteroides fragilis, Peptostreptococcus species, microaerophilic streptococci

Aerobic/facultative organisms also implicated:

Staphylococcus aureus (classically post-influenza; also in IV drug users)
Klebsiella pneumoniae (especially upper lobe, in alcoholics; "currant jelly" sputum)
Pseudomonas aeruginosa, Streptococcus pyogenes
Nocardia asteroides, Rhodococcus equi (immunocompromised)
Actinomyces species

Nosocomial lung abscess: Gram-negative bacteria with hospital-acquired resistance patterns predominate.

Uncommon but important causes (Fishman's, p. 2242):

Legionella species, Burkholderia pseudomallei (melioidosis), Burkholderia mallei (glanders)
Entamoeba histolytica - basilar portion of right lower lobe (almost always)
Mycobacteria: M. tuberculosis, MAC, M. kansasii
Fungi: Aspergillus, zygomycetes (Mucoraceae), Histoplasma, Blastomyces, Coccidioides - in diabetic/immunocompromised hosts

Clinical Features

Symptoms are typically subacute and indolent when caused by anaerobes (Murray & Nadel, Ch. 50):

Fever, cough, sputum production (develops over weeks to months)
Night sweats and weight loss
Pleuritic chest pain
Hemoptysis (may be minor or life-threatening)
Foul-smelling / putrid sputum - highly suggestive of anaerobic infection
Bad breath (halitosis)

Note: Absence of foul odor does NOT exclude anaerobes - some organisms do not produce malodorous metabolic products.

Secondary lung abscesses (e.g., from septic pulmonary emboli) can evolve rapidly over 48-72 hours, unlike the typical indolent course of primary abscesses (Fishman's, p. 2243).

Physical examination:

Fever, possible decreased level of consciousness
Poor dentition (important clue)
Crackles, egophony, dullness to percussion (if pleural effusion present)
Amphoric (cavernous) breath sounds over the cavity

Radiological Diagnosis

Chest X-ray classic appearance:

Irregularly shaped cavity with an air-fluid level within a consolidation/infiltrate
Location: posterior segments of upper lobes, superior segments of lower lobes

CT chest (preferred investigation) - Murray & Nadel, Ch. 50:

Features favoring lung abscess:

Air-fluid level
Surrounding parenchymal opacity (ground-glass or consolidation)
Irregular ("shaggy") inner wall
Round shape with similar dimensions on different views
Does NOT compress adjacent lung

Features distinguishing from empyema:

Empyema: oblong shape, smooth inner lining, compresses surrounding lung
Abscess: round shape, ragged inner lining, no lung compression

Features suggesting malignancy:

Nodular inner wall
Wall thickness ≥ 16 mm
Lymphadenopathy, pleural effusion

Septic emboli: Multiple distinct nodules (1-3 cm) with cavities in different stages, subpleural wedge-shaped infiltrates.

CT scan showing right apical lung abscess with air-fluid level and surrounding ground glass opacity (Murray & Nadel, Fig. 50.6)

Figure: Axial CT showing a complex, thick-walled right apical cavity consistent with a pyogenic abscess; arrows indicate surrounding ground-glass opacity from hemorrhage (Murray & Nadel, Fig. 50.6)

Differential Diagnosis

Infectious cavities (Fishman's, Table 127-6):

Bacterial: S. aureus, Enterobacteriaceae, Pseudomonas, Nocardia, Actinomyces
Mycobacterial (often multifocal): TB, MAC, M. kansasii
Fungal: Aspergillus, zygomycetes, Histoplasma, Coccidioides, Cryptococcus, PCP
Parasitic: Entamoeba histolytica, Paragonimus westermani, Strongyloides

Non-infectious cavitating conditions:

Bronchogenic carcinoma (cavitating squamous cell carcinoma)
Septic embolism (endocarditis, IV drug use)
Granulomatosis with polyangiitis (Wegener's)
Pulmonary infarction
Bronchiectasis, pulmonary sequestration
Fluid-filled cysts (traumatic, hydatid)

Diagnostic Workup

Chest X-ray - initial study; may need serial films to track evolution
CT chest - preferred; defines extent, loculations, relationship to pleura, mediastinal structures; guides drainage
Sputum - Gram stain and culture (aerobic, anaerobic, AFB, fungal); sputum anaerobic culture is often unreliable due to upper airway contamination
Blood cultures - particularly in immunocompromised or septic patients
Bronchoscopy - essential role when:
- Failure to respond to treatment
- Suspicion of obstruction (tumor, foreign body)
- Need for protected-brush specimen or BAL for culture
- To facilitate drainage (cautiously)
Sputum cytology - to exclude malignancy
Thoracentesis - if pleural effusion present (rule out empyema)
CBC, ESR, CRP - markers of infection and response to treatment
HIV testing - in appropriate clinical context

Treatment

1. Antimicrobial Therapy

Empirical therapy must cover strict anaerobes and facultative anaerobic streptococci (Murray & Nadel, Ch. 50):

First-line options	Rationale
Beta-lactam + beta-lactamase inhibitor (amoxicillin-clavulanate, ampicillin-sulbactam, piperacillin-tazobactam)	Broad coverage; preferred first-line
Clindamycin (600 mg IV q8h, then 300 mg oral q6h)	Excellent anaerobic coverage; reserved for beta-lactam allergy due to C. difficile risk
Carbapenem (e.g., imipenem, meropenem + streptococcal coverage)	For resistant organisms; note meropenem monotherapy lacks streptococcal coverage

Duration: Typically 4-6 weeks (or longer until cavity resolves). Clinical improvement expected within 3-4 days, defervescence within 7-10 days.

MRSA abscess (post-influenza):

MSSA: cefazolin, nafcillin, or oxacillin
MRSA: linezolid (preferred for lung penetration) or vancomycin
Avoid daptomycin - poor bioavailability in lungs

2. Postural Drainage and Physiotherapy

Position patient to drain affected segment
Chest physiotherapy to facilitate expectoration
Standard conservative management for most abscesses

3. Percutaneous (CT-guided) Catheter Drainage

Indications (Murray & Nadel, Ch. 50):

Abscesses not communicating with airway (no air-fluid level on imaging)
Failure of antibiotic therapy
Abscess > 4 cm (higher failure rate with antibiotics alone)

Technique:

Seldinger technique with pigtail catheter (12-14 French)
Position patient with affected side up to prevent spillage to unaffected lung
Avoid decubitus position with unaffected side dependent
Success rate ~84% across published series; complication rate ~16%
Complications: pneumothorax, hemothorax, hemoptysis, catheter clogging, bronchopleural fistula

4. Bronchoscopic Drainage

Limited role; risk of spilling pus to healthy lung
Useful for diagnosing/relieving obstruction, sampling for culture

5. Surgical Resection

Indications (Fishman's, p. 2244):

Failure to respond to antibiotic therapy (no improvement by 7-10 days)
Large abscesses (> 5-6 cm)
Ventilation-perfusion scans showing little residual function in necrotic region
Airway obstruction (tumor, foreign body) limiting drainage
Rapidly progressive necrotizing infection
Infarcted lung
Life-threatening hemoptysis
Bronchopleural fistula or empyema not responding to drainage

Procedure: Lobectomy (most common), rarely pneumonectomy in extensive disease.

Patients poor surgical risks: Percutaneous drainage as bridge to surgery; avoid if pleural space not protected (risk of catastrophic empyema from abscess contents).

Complications

(Murray & Nadel, Ch. 50; Fishman's, p. 2244)

Hemoptysis - bronchial arteries supply blood to the abscess wall; may range from minor to massive and life-threatening
Bronchopleural fistula - abscess erodes into pleural space
Empyema - spread of infection to pleural cavity; can also occur as surgical complication
Mediastinitis - rare but life-threatening
Metastatic abscess - bacteria/infected clots bypass the pulmonary filter to reach systemic circulation:
- Brain abscess: lung abscess/bronchiectasis is associated with brain abscess by adjusted OR of 8.2 (95% CI 3.6-18.5) in population-based studies (Murray & Nadel, Ch. 50)
- Stroke - paradoxical embolism via pulmonary venous drainage

Prognosis and Mortality

(Fishman's, p. 2244; Murray & Nadel, Ch. 50)

Pre-antibiotic era: mortality 32-34% regardless of treatment modality
With antibiotics: mortality reduced to 15-20%
High-risk features carrying mortality up to 75% despite treatment:
- Immunocompromise
- Bronchial obstruction
- Large abscesses (> 6 cm)
- Aerobic organisms (especially K. pneumoniae, S. aureus, P. aeruginosa)
- Advanced age
- Malnutrition / systemic debility
- Major delay in seeking medical attention
- Progressive pulmonary necrosis

Summary Table: Primary vs. Secondary Lung Abscess

Feature	Primary	Secondary
Host	Immunocompetent	Immunocompromised / structural lung disease
Cause	Aspiration, community-acquired pneumonia	Obstruction, bronchiectasis, hematogenous spread
Onset	Subacute (1-3 weeks)	Can be acute (48-72 h for septic emboli)
Organisms	Predominantly anaerobes	Mixed; aerobic organisms more common
Location	Dependent segments (right upper lobe posterior, lower lobe apical)	Variable
Prognosis	Better	Worse

References: Fishman's Pulmonary Diseases and Disorders, Chapter 127 (pp. 2242-2244); Murray & Nadel's Textbook of Respiratory Medicine, Chapter 50 (pp. 1154-1157)

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