Lung abscess

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A lung abscess is a collection of pus within the lung parenchyma. While aspiration of oropharyngeal secretions is the most common cause, lung abscesses can also occur secondary to bronchial obstruction (e.g., malignancy, foreign body aspiration), hematogenous spread (e.g., infectious endocarditis, IV drug use), contiguous spread (e.g., liver abscess), and as a complication of necrotizing pneumonia. The most common causative pathogens are anaerobic bacteria, although aerobic bacteria and, in rare cases, fungi and parasites can also cause lung abscesses. Typical symptoms of a pyogenic lung abscess include fever and cough with production of foul-smelling sputum; they may take weeks or months to develop and may initially be attributed to pneumonia or pulmonary tuberculosis. Lung abscesses typically appear as spherical intraparenchymal cavitary lesions with an air-fluid level on chest imaging. Chest CT is useful for detecting smaller abscesses as well as differentiating abscesses from other cavitary pulmonary lesions. Empiric antibiotic therapy for pyogenic lung abscesses should be started immediately after obtaining relevant cultures and continued for several weeks. Occasionally, percutaneous or bronchoscopic drainage or surgical resection is required. The underlying cause should be evaluated for and treated in all patients.

• Lung abscess: a localized collection of pus and necrotic tissue within lung parenchyma caused by microbial infection
• Primary lung abscess: abscess in normal lung parenchyma, typically due to aspiration (∼ 80%)
• Secondary lung abscess: abscess in a patient with immunocompromise or preexisting disease (e.g., lung neoplasm), or due to hematogenous spread (e.g., septic emboli, IV drug use)

Reference: ^[2]

Causative pathogens ^[2][3][4]

• Bacterial (pyogenic lung abscess)
  • Most commonly caused by anaerobic bacteria that colonize the oral cavity (e.g., Peptostreptococcus spp., Prevotella spp., Bacteroides spp., Fusobacterium spp.)
  • Less commonly caused by aerobic bacteria, such as:
    • Microaerophilic streptococci and viridans streptococci in polymicrobial lung abscesses
    • Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, group A Streptococcus, Streptococcus anginosus, or Haemophilus influenzae in monomicrobial lung abscesses
• Parasitic: e.g., Entamoeba histolytica and Paragonimus westermani
• Fungal: e.g., Aspergillus spp., Cryptococcus spp., Coccidioides spp., and Histoplasma spp.

One of the most severe complications of influenza in children and adults is a secondary bacterial infection with Staphylococcus aureus.

Risk factors for lung abscess ^[2][3][4]

• Risk factors for aspiration, such as:
  • Impaired consciousness
  • Impaired swallow in neurological disorders and vocal cord paralysis
• Increased oropharyngeal bacterial growth (e.g., periodontal disease, dental abscesses, tonsillitis)
• Bronchial obstruction (e.g., lung cancer, foreign body aspiration, bronchial stenosis)
• Immunocompromised state
• Pneumonia, bronchiectasis
• Impaired respiratory mucus clearance (e.g., cystic fibrosis)

• Aspiration (most common etiology) of food or oropharyngeal secretions → pneumonitis and/or obstruction of the smaller airways → localized suppurative inflammation and necrosis → lung abscess formation (∼ after 1–2 weeks) ^[2][3][4]
• Other mechanisms by which lung abscesses can develop include: ^[5][6]
  • Impaired mucus clearance: e.g., bronchial neoplasms, cystic fibrosis, impacted lower airway foreign body
  • Hematogenous spread of bacteria: e.g., tricuspid valve endocarditis or Lemierre syndrome
  • Contiguous spread (direct extension): e.g., liver abscess, pleural empyema, mediastinitis
  • Direct inoculation: e.g., penetrating chest injury
  • Complication of necrotizing pneumonia

Onset

Typically indolent; symptoms can evolve over weeks to months, but may also be acute in onset.

• Acute: symptoms present for < 4–6 weeks
• Chronic: symptoms present for > 6 weeks

Characteristic symptoms

• Fever
• Cough with production of foul-smelling purulent sputum
• Night sweats
• Hemoptysis
• Anorexia, weight loss, fatigue
• Pleuritic chest pain

Pulmonary examination findings

• Digital clubbing in chronic abscess
• Dullness to percussion over the affected area
• Amphoric breath sounds may be present over the lung abscess

Reference: ^[2][3]

General principles ^[4]

• All patients should undergo chest imaging, either CT (gold standard) or chest x-ray.
• Laboratory findings are typically nonspecific, showing signs of infection.
• Cultures of blood, sputum (or bronchoalveolar lavage), and any pleural fluid should be ordered to determine the causative pathogen.
• Further diagnostics (e.g., PCR testing, bronchoscopic-guided biopsy for suspected underlying neoplasm) may be required if there is an inadequate response to antibiotic therapy or for suspected secondary lung abscess.

Imaging ^[2][3]

Modalities

• Chest x-ray: typically the initial investigation in any suspected lung pathology
• CT chest with IV contrast: to confirm the diagnosis and rule out differential diagnoses of lung abscesses ^[3]

Findings

• Common findings on x-ray and CT
  • Spherical intraparenchymal cavity with thick irregular walls
  • Air-fluid level within the cavity that is dependent on body position
• Additional findings on CT
  • Features of the underlying etiology (e.g., malignancy, lower airway foreign body)
  • Small abscesses, which may not be apparent on chest x-ray
• Location of abscess
  • Due to aspiration: typically unilateral ^[7]
    • Right middle lobe (typically caused by aspiration in the prone or upright position)
    • Posterior segments of the upper lobes or the superior segments of the lower lobes (typically caused by aspiration in the recumbent position).
  • Due to hematogenous dissemination: typically bilateral and multiple ^[8]

Laboratory studies ^[2][4]

Routine

• CBC
  • Typically shows elevated WBC
  • May show anemia of chronic disease in chronic lung abscess
• Blood culture
  • Frequently negative in anaerobic infections
  • More likely to be positive in patients with secondary lung abscess
• Gram stain and culture
  • Of sputum (or bronchoalveolar lavage), blood, and any pleural fluid (i.e., if a pleural empyema or pleural effusion is present)
  • Include cultures for aerobic and anaerobic bacteria, mycobacteria, and fungi.
  • Results depend on the causative pathogen (see “Etiology”).

Additional testing (as needed)

• HIV screening: Consider in the following situations. ^[3]
  • Atypical pathogens are cultured
  • No obvious risk factors for lung abscess identified
• Gram stain and culture of aspirate of abscess
  • Consider if initial sputum cultures are negative or equivocal and the patient has not improved on empiric antibiotic therapy.

Do not wait for culture results to start antibiotics. Start empirical treatment immediately after obtaining samples for culture, and tailor therapy as needed once culture results are available.

Diagnostics for the underlying cause

• Bronchoscopy for suspected malignancy or lower airway foreign body
• Echocardiography for suspected septic endocarditis
• Clinical swallow assessment for suspected dysphagia

• Differential diagnoses of cavitary lung lesions, such as:
  • Pleural empyema with air-fluid level
  • Bullae or cysts with air-fluid level
  • Pulmonary infarction
  • Tuberculosis
  • Aspergillosis
  • Cavitating pulmonary metastasis
• See also “Differential diagnosis of pulmonary nodules.”

As lung abscesses due to aspiration appear in characteristic locations, imaging can help to differentiate them from other cavitary lesions. For example, tuberculosis more commonly affects the lung apices or apical segments of the lower lobes; embolic pulmonary infarcts typically appear as multiple, diffuse lesions. ^[9]

References: ^[2][4]

The differential diagnoses listed here are not exhaustive.

Approach

• Admit the patient and start immediate empiric antibiotic therapy (after obtaining samples for culture).
• Consider the following consults as appropriate:
  • Pulmonary
  • Infectious disease
  • Cardiothoracic surgery or interventional radiology for patients with large abscesses
  • Speech and language pathology for patients with dysphagia
• Identify and treat the underlying cause.

Antimicrobial therapy ^[4][10][11]

Antimicrobial therapy should be tailored to local sensitivities. Follow local guidelines and infectious diseases advice, if available.

Suspected bacterial infection

Bacterial infection is the most common cause of lung abscess and empiric antibiotic therapy should be initiated in all patients with typical clinical features (e.g., fever, purulent sputum) and a cavitary lesion on imaging.

• Broad-spectrum antibiotics with anaerobic coverage are recommended; (e.g., ampicillin-sulbactam, carbapenems, or clindamycin).
• Ensure antibiotic coverage of gram-positive cocci in lung abscesses that are likely due to IV drug use. ^[8]
• Parenteral antibiotic therapy is preferred for seriously ill patients.
• Tailor antibiotic therapy as needed based on culture results and clinical response.
• Antibiotics are typically continued for 3–6 weeks or until clinical and radiological improvement is seen ^[4][8]

Lung abscesses due to aspiration are typically caused by anaerobic bacteria. Lung abscesses due to hematogenous spread from IV drug use are typically caused by Staphylococcus aureus and streptococci. ^[8]

Lung abscesses following a recent influenza infection are likely caused by Staphylococcus aureus in children and adults, but can also be caused by Streptococcus pneumoniae and Hemophilus influenzae.

Parasitic or fungal infection ^[13][14][15]

Suspect parasitic or fungal infection in patients with atypical clinical features (e.g., nonpurulent sputum), an inadequate response to empiric antibiotic therapy, and risk factors (e.g., immunocompromise)

• Management of immunocompromised status, which includes: ^[13]
  • Decrease or stop immunosuppressive medications.
  • Optimize management of the underlying condition (e.g., diabetes mellitus, HIV infection).
  • Consider administration of immunostimulants.
• Treatment depends on the underlying organism and whether disseminated disease is present.
  • Consult the infectious disease service for recommended regimens.
  • Antifungal and antiparasitic agents are typically required for a prolonged duration.
  • Surgery is often required to debride necrotic and devitalized parenchyma. ^[13]

Interventional therapy ^[2][3][4]

• Indications
  • Large abscess
  • Significant hemoptysis
  • Inadequate response despite appropriate antibiotic therapy, characterized by:
    • Persisting fever and/or purulent sputum
    • Inadequate signs of resolution on imaging
• Options ^[4]
  • First line: bronchoscopic drainage or image-guided percutaneous drainage
  • Rarely : surgical resection (segmentectomy, lobectomy, pneumonectomy)

Management of the underlying cause

Examples include:

• Bronchoscopy-guided removal of a lower airway foreign body
• Treatment of bacterial tonsillitis or gingitivitis
• Management of bronchial carcinoma
• Management of dysphagia
• Management of immunosuppression

• Extension or rupture into the pleural cavity, causing:
  • Pleural empyema
  • Pleural effusion
  • Bronchopulmonary fistula
  • Pneumothorax
• Recurrence of abscess
• Development of chronic reactive changes: e.g., bronchiectasis or pneumatoceles
• Massive hemoptysis

References: ^[2][4][16]

We list the most important complications. The selection is not exhaustive.

• Confirm diagnosis on imaging.
• Send cultures of sputum (or bronchoalveolar lavage), blood, and any pleural fluid.
• Start empiric antibiotic therapy for pyogenic lung abscess.
• Consider if there are any indications for invasive treatment (e.g., large abscess, underlying malignancy, or significant hemoptysis) and refer to appropriate services as needed.
• Identify and treat the underlying cause.