AUTHOR AND EDITOR INFORMATION

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INTRODUCTION

Section 2 of 11  

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Background

Lung abscess is defined as necrosis of the pulmonary tissue and formation of cavities containing necrotic debris or fluid caused by microbial infection. The formation of multiple small (<2 cm) abscesses is occasionally referred to as necrotizing pneumonia or lung gangrene. Both lung abscess and necrotizing pneumonia are manifestations of a similar pathologic process. Failure to recognize and treat lung abscess is associated with poor clinical outcome.

In 1920s, approximately one third of patients with lung abscess died; Dr Smith postulated that aspiration of oral bacteria was the mechanism of infection. He observed that the bacteria found in the walls of the lung abscesses at autopsy resembled the bacteria noted in the gingival crevice. A typical lung abscess could be reproduced not by one but with an inoculum containing 4 microbes, known as anaerobic spirochete: Fusobacterium nucleatum, Peptostreptococcus species, a fastidious gram-negative anaerobe, and possibly Prevotella melaninogenicus.

Lung abscess was a devastating disease in the preantibiotic era, when one third of the patients died, another one third recovered, and the remainder developed debilitating illnesses such as recurrent abscesses, chronic empyema, bronchiectasis, or other consequences of chronic pyogenic infections. In the early postantibiotic period, sulfonamides did not improve the outcome of patients with lung abscess until the penicillins and tetracyclines were available. Although resectional surgery was often considered a treatment option in the past, the role of surgery has greatly diminished over time because most patients with uncomplicated lung abscess eventually respond to prolonged antibiotic therapy.

Lung abscesses can be classified based on the duration and the likely etiology. Acute abscesses are less than 4-6 weeks old, whereas chronic abscesses are of longer duration. Primary abscess is infectious in origin, caused by aspiration or pneumonia in the healthy host; secondary abscess is caused by a preexisting condition (eg, obstruction), spread from an extrapulmonary site, bronchiectasis, and/or an immunocompromised state. Lung abscesses can be further characterized by the responsible pathogen, such as Staphylococcus lung abscess and anaerobic or Aspergillus lung abscess.

Pathophysiology

Most frequently, the lung abscess arises as a complication of aspiration pneumonia caused by mouth anaerobes. The patients who develop lung abscess are predisposed to aspiration and commonly have periodontal disease. A bacterial inoculum from the gingival crevice reaches the lower airways, and infection is initiated because the bacteria are not cleared by the patient's host defense mechanism. This results in aspiration pneumonitis and progression to tissue necrosis 7-14 days later, resulting in formation of lung abscess. Other mechanisms for lung abscess formation include bacteremia or tricuspid valve endocarditis, caused by septic emboli to the lung.

Microbiology

In lung abscesses, anaerobes are recovered in up to 89% of the patients. In a study by Bartlett et al in 1974, 46% of patients with lung abscesses had only anaerobes isolated from sputum cultures while 43% of patients had a mixture of anaerobes and aerobes. The most common anaerobes are Peptostreptococcus, Bacteroides, Fusobacterium species, and microaerophilic streptococcus.

Other organisms that may infrequently cause lung abscess include Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae (rarely), Klebsiella pneumoniae, Haemophilus influenzae, Actinomyces species, Nocardia species, and gram-negative bacilli.

Nonbacterial pathogens may also cause lung abscesses. These microorganisms include parasites (eg, Paragonimus, Entamoeba), fungi (eg, Aspergillus, Cryptococcus, Histoplasma, Blastomyces, Coccidioides), and Mycobacterium.

Frequency

United States

The frequency of lung abscess in the general population is not known.

Mortality/Morbidity

Most patients with primary lung abscess improve with antibiotics, with cure rates documented at 90-95% (Bartlett, 1992). The mortality rate for patients with underlying immunocompromised status or bronchial obstruction who develop lung abscess may be as high as 75% (Pohlson, 1985). A retrospective study (Hirshberg, 1995) reported the overall mortality rate of lung abscesses caused by mixed gram-positive and gram-negative bacteria at approximately 20%.

Age

These conditions occur more commonly in elderly patients because of the increased incidence of periodontal disease and the increased prevalence of microaspiration.

CLINICAL

Section 3 of 11  

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History

Symptoms depend on whether the abscess is caused by anaerobic or other bacterial infection.

• Anaerobic infection
• • Patients often present with indolent symptoms that evolve over a period of weeks to months.
  • The usual symptoms are fever, cough with sputum production, night sweats, anorexia, and weight loss.
  • The expectorated sputum characteristically is foul smelling and bad tasting.
  • Patients may develop hemoptysis or pleurisy.
• Other bacterial pathogens
• • These patients generally present with conditions that are more emergent in nature and are usually treated while they have bacterial pneumonia.
  • Cavitation occurs subsequently as parenchymal necrosis ensues.
  • Abscesses from fungi, Nocardia, and mycobacteria tend to have an indolent course and gradually progressive symptoms.

Physical

The findings on physical examination of a patient with lung abscess are variable. Physical findings may be secondary to associated conditions such as underlying pneumonia or pleural effusion. The physical examination findings may also vary depending on the organisms involved, the severity and extent of the disease, and the patient's health status and comorbidities.

• Patients may have low-grade fever in anaerobic infections and temperatures higher than 38.5°C in other infections.
• Generally, evidence of gingival disease is present.
• Clinical findings of concomitant consolidation may be present (eg, decreased breath sounds, dullness to percussion, bronchial breath sounds, course inspiratory crackles).
• The amphoric or cavernous breath sounds are only rarely elicited in modern practice.
• Evidence of pleural friction rub and signs of associated pleural effusion, empyema, and pyopneumothorax may be present. Signs include dullness to percussion, contralateral shift of the mediastinum, and absent breath sounds over the effusion.
• Digital clubbing may develop rapidly.

Causes

The bacterial infection may reach the lungs in several ways. The most common is aspiration of oropharyngeal contents.

• Patients at the highest risk for developing lung abscess have the following risk factors:
• • Poor dentition
  • Seizure disorder
  • Alcohol abuse
• Other patients at high risk for developing lung abscess include the following:
• • Individuals with an inability to protect their airways because of an absent gag reflex, such as during coma, loss of consciousness, or general anesthesia and sedation
  • Patients with primary lung disorders, such as septic emboli from tricuspid endocarditis, vasculitic disorders, cavitating lung malignancies, and pulmonary cystic diseases
• Infrequently, the following infectious etiologies of pneumonia may progress to parenchymal necrosis and lung abscess formation:
• • Pseudomonas aeruginosa
  • Klebsiella pneumoniae
  • Staphylococcus aureus (may result in multiple abscesses)
  • Streptococcal pneumonia
  • Nocardia species
  • Fungal species
• An abscess may develop as an infectious complication of a preexisting bulla or lung cyst.
• An abscess may occur secondary to carcinoma of the bronchus; the bronchial obstruction causes postobstructive pneumonia, which may lead to abscess formation.
• A recent study by Wang et al from Taiwan suggested that the bacteriologic characteristics of lung abscess have changed. By performing transthoracic lung aspiration, they showed that anaerobes were recovered from just 28 patients (31%); the predominant bacterium was Klebsiella pneumoniae, in 30 patients (33%). This study concluded that K pneumoniae has become more common as a cause of lung abscess, and the antibiotic coverage should include this organism, anaerobes, and S milleri (Wang, 2005).

DIFFERENTIALS

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Other Problems to be Considered

Cavitating lung cancer
Localized empyema
Infected bulla containing a fluid level
Infected congenital pulmonary lesion, such as bronchogenic cyst or sequestration
Pulmonary hematoma
Cavitating pneumoconiosis
Hiatus hernia
Lung parasites (eg, hydatid cyst, Paragonimus infection)
Actinomycosis
Wegener granulomatosis and other vasculitides
Cavitating lung infarcts
Cavitating sarcoidosis

WORKUP

Section 5 of 11  

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Lab Studies

• A complete white blood cell count with differential may reveal leukocytosis and a left shift.
• Obtain sputum for Gram stain, culture, and sensitivity.
• If tuberculosis is suspected, acid-fast bacilli stain and mycobacterial culture is requested.
• Blood culture may be helpful in establishing the etiology.
• Obtain sputum for ova and parasite whenever a parasitic cause for lung abscess is suspected.

Imaging Studies

• Chest radiograph
• • A typical chest radiographic appearance of a lung abscess is an irregularly shaped cavity with an air-fluid level inside. Lung abscesses as a result of aspiration most frequently occur in the posterior segments of the upper lobes or the superior segments of the lower lobes.
  • The wall thickness of a lung abscess progresses from thick to thin and from ill-defined to well-circumscribed as the surrounding lung infection resolves. The cavity wall can be smooth or ragged but is less commonly nodular, which raises the possibility of cavitating carcinoma.
  • The extent of the air-fluid level within a lung abscess is often the same in posteroanterior or lateral views. The abscess may extend to the pleural surface, in which case it forms acute angles with the pleural surface.
  • Anaerobic infection may be suggested by cavitation within a dense segmental consolidation in the dependent lung zones.
  • Lung infection with a virulent organism results in more widespread tissue necrosis, which facilitates progression of underlying infection to pulmonary gangrene.
  • Up to one third of lung abscesses may be accompanied by an empyema.
• CT scan
• • CT scan of the lung may help visualize the anatomy better than the chest radiograph. It is very useful in identification of concomitant empyema or lung infarction.
  • On CT scan, an abscess often is a rounded radiolucent lesion with a thick wall and ill-defined irregular margins.
  • The vessels and bronchi are not displaced by the lesion as they are by an empyema.
  • The lung abscess is located within the parenchyma compared to loculated empyema, which may be difficult to distinguish on chest radiograph.
  • The lesion forms acute angles with the pleural surface chest wall.

Procedures

• To obtain an uncontaminated specimen, transtracheal aspirate or transthoracic needle aspirate are the techniques employed in lung abscess.
• Although transtracheal aspirate and transthoracic needle aspiration may provide microbiologic diagnosis, obtaining pleural fluid and blood cultures in patients with lung abscess is easier.
• Flexible fiberoptic bronchoscopy is performed to exclude bronchogenic carcinoma whenever bronchial obstruction is suspected. Bronchoscopy using a protected brush to obtain a specimen uncontaminated by the upper airway or quantitative culture of organisms from the bronchoalveolar lavage fluid has been advocated to establish bacteriologic diagnosis in lung abscess. However, the experience with this technique in diagnosis of anaerobic lung infections is limited and the diagnostic yield is uncertain. Furthermore, risk of spillage of infected material into the uninvolved areas of lung exists.

Histologic Findings

Lung abscesses begin as small zones of necrosis developing within the consolidated segments in pneumonia. These areas may coalesce to form single or multiple areas of suppuration, which are referred to as lung abscesses. If antibiotics interrupt the natural history at an early stage, the healing results in no residual changes. When the progressive inflammation erodes into the adjacent bronchi, the contents of the abscess are expectorated as malodorous sputum. Subsequently, fibrosis occurs, which causes a dense scar and separates the abscess. The abscess may still occur, and spillage of pus into the bronchial tree may disseminate the infection.

TREATMENT

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Medical Care

Treatment of lung abscess is guided by the available microbiology and knowledge of the underlying or associated conditions.

• Antibiotic therapy
• • Standard treatment of an anaerobic lung infection is clindamycin (600 mg IV q8h followed by 150-300 mg PO qid).
  • This regimen has been shown to be superior over parenteral penicillin in published trials. Several anaerobes may produce beta-lactamase (eg, various species of Bacteroides and Fusobacterium) and therefore develop resistance to penicillin.
  • Although metronidazole is an effective drug against anaerobic bacteria, the experience with metronidazole in treating lung abscess has been rather disappointing because these infections are generally polymicrobial. A failure rate of 50% has been reported.
  • In hospitalized patients who have aspirated and developed a lung abscess, antibiotic therapy should include coverage against S aureus and Enterobacter and Pseudomonas species.
  • Cefoxitin is a second-generation cephalosporin that has gram-positive, gram-negative, and anaerobic coverage. This agent may be used when a polymicrobial infection is suspected as cause of lung abscess.
  • In a prospective open, randomized, trial to compare the safety and clinical and bacteriologic efficacy of ampicillin plus sulbactam versus clindamycin with or without cephalosporin, 95 patients with pulmonary infection following aspiration were included. In patients treated with ampicillin plus sulbactam, the clinical response was 73.0% at the end of therapy and 67.5% 7-14 days after therapy. For clindamycin, the rates were 66.7% and 63.5%, respectively. Both therapies were well tolerated and proved equally effective in the treatment of aspiration pneumonia and lung abscess (Allewelt, 2004).
• Duration of therapy
  • Although the duration of therapy is not well established, most clinicians prescribe antibiotic therapy generally for 4-6 weeks.
  • Current recommendations are that antibiotic treatment should be continued until the chest radiograph has shown either the resolution of lung abscess or the presence of a small stable lesion.
  • The rationale for extended treatment maintains that risk of relapse exists with a shorter antibiotic regimen.
• Response to therapy
  • Patients with lung abscesses usually show clinical improvement, with improvement of fever, within 3-4 days after initiating the antibiotic therapy. Defervescence is expected in 7-10 days. Persistent fever beyond this time indicates therapeutic failure, and these patients should undergo further diagnostic studies to determine the cause of failure.
  • Considerations in patients with poor response to antibiotic therapy include bronchial obstruction with a foreign body or neoplasm or infection with a resistant bacteria, mycobacteria, or fungi.
  • Large cavity size (ie, > 6 cm in diameter) usually requires prolonged therapy. Because empyema with an air-fluid level could be mistaken for parenchymal abscess, a CT scan may be used to differentiate this process from lung abscess.
  • A nonbacterial cause of cavitary lung disease may be present, such as lung infarction, cavitating neoplasm, and vasculitis. The infection of a preexisting sequestration, cyst, or bulla may be the cause of delayed response to antibiotics.

Surgical Care

Surgery is very rarely required for patients with uncomplicated lung abscesses. The usual indications for surgery are failure to respond to medical management, suspected neoplasm, or congenital lung malformation. The surgical procedure performed is either lobectomy or pneumonectomy.

When conventional therapy fails, either percutaneous catheter drainage or surgical resection is usually considered. Endoscopic lung abscess drainage was considered if an airway connection to the cavity could be demonstrated. Success of this treatment represents an additional option other than percutaneous catheter drainage or surgical resection (Herth, 2005).

Consultations

Consulting a pulmonary medicine or infectious diseases specialist is often helpful in workup and follow-up of patients with lung abscess.

MEDICATION

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Most lung abscesses occur secondary to aspiration and are caused by anaerobes. A history suggestive of community acquired pneumonia or a history of development of abscess in a hospitalized patient is important in deciding the appropriate antibiotic coverage.

Drug Category: Antibiotics

Empiric antimicrobial therapy must be comprehensive and should cover all likely pathogens suspected in this clinical setting.

FOLLOW-UP

Section 8 of 11  

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Further Inpatient Care

• For the following reasons, inpatient care is advisable in patients with lung abscess:
• • Evaluation and management of patient's respiratory status
  • Administration of intravenous antibiotics
  • Drainage of the abscess or empyema as needed

Further Outpatient Care

• In patients who have small lung abscess, who are not clinically ill, and who are reliable, outpatient care may be considered after obtaining appropriate diagnostic studies such as sputum culture, blood culture, and blood work.
• Following initial intravenous antibiotic therapy, the patient may be treated on an outpatient basis for completion of prolonged therapy, which is often required for cure.

Deterrence/Prevention

• Prevention of aspiration is important to minimize the risk of lung abscess. Early intubation and protection of airway in patients who do not have a gag reflex is essential.
• Positioning the supine patient at a 30-degree reclined angle minimizes the risk of aspiration. Vomiting patients should be placed on their sides.
• Improving oral hygiene and dental care in elderly and debilitated patients may decrease the risk of anaerobic lung abscess.

Complications

• Complications of pulmonary abscess
• • Rupture into pleural space causing empyema
  • Pleural fibrosis
  • Trapped lung
  • Respiratory failure
  • Bronchopleural fistula
  • Pleural cutaneous fistula
• In a patient with coexisting empyema and lung abscess, draining the empyema while continuing prolonged antibiotic therapy is often necessary.

Prognosis

• The prognosis for lung abscess following antibiotic treatment is generally favorable. Over 90% of lung abscesses are cured with medical management alone, unless caused by bronchial obstruction secondary to carcinoma.

Patient Education

• For excellent patient education resources, visit eMedicine's Infections Center, Lung and Airway Center, Pneumonia Center , and Procedures Center. Also, see eMedicine's patient education articles Bacterial Pneumonia, Antibiotics, and Bronchoscopy.

MISCELLANEOUS

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Medical/Legal Pitfalls

• A lung abscess may be asymptomatic in a small proportion of patients in the early stages; a chest radiograph may be helpful.
• In any patient who is producing foul-smelling or bad-tasting sputum, suspect a lung abscess.
• A shorter course of antibiotics may increase risk of a relapse. Therefore, antibiotic therapy for anaerobic lung abscess is prolonged, often extending up to 6-8 weeks.
• A lack of response to antibiotic therapy should lead to consideration of a cavitating lung neoplasm, lung infarction, or Wegener granulomatosis.

MULTIMEDIA

Section 10 of 11  

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Media file 1:  Histology of a lung abscess shows dense inflammatory reaction (low power).
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Media file 2:  A thick-walled lung abscess.
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Media file 3:  Pneumococcal pneumonia complicated by lung necrosis and abscess formation.
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Media file 4:  A lateral chest radiograph shows air-fluid level characteristic of lung abscess.
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Media file 5:  A 54-year-old patient developed cough with foul-smelling sputum production. A chest radiograph shows lung abscess in the left lower lobe, superior segment.
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Media file 6:  A 42-year-old man developed fever and production of foul-smelling sputum. He had a history of heavy alcohol use, and poor dentition was obvious on physical examination. Chest radiograph shows lung abscess in the posterior segment of the right upper lobe.
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Media type:  X-RAY

Media file 7:  A 42-year-old man developed fever and production of foul-smelling sputum. He had a history of heavy alcohol use, and poor dentition was obvious on physical examination. Lung abscess in the posterior segment of the right upper lobe was demonstrated on chest radiograph (see Image 6). CT scan shows a thin-walled cavity with surrounding consolidation.
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Media type:  CT

Media file 8:  Chest radiograph of a patient who had foul-smelling and bad-tasting sputum, an almost diagnostic feature of anaerobic lung abscess.
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Media type:  X-RAY

Media file 9:  Histology of a lung abscess shows dense inflammatory reaction (high power).
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Media type:  Photo

REFERENCES

Section 11 of 11

• Authors and Editors
• Introduction
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• Differentials
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Article Last Updated: May 8, 2006