Lung Abscess

A lung abscess is a necrotic lesion of lung tissue with cavity formation caused by infection with various pathogens. It is clinically characterized by high fever, cough, and the expectoration of large amounts of foul-smelling purulent sputum. Chest X-rays or CT scans typically show a single or multiple cavities with air-fluid levels. When multiple cavities smaller than 2 cm in diameter are present, the condition is referred to as necrotizing pneumonia. Lung abscesses can occur at any age but are more common in young and middle-aged adults, with a higher prevalence in males than females. The primary pathogens are anaerobic and facultatively anaerobic bacteria, though the proportion of aerobic bacterial infections has increased in recent years.

Etiology and Pathogenesis

The pathogens causing lung abscesses are closely related to the route of infection. Based on the route of infection, lung abscesses can be classified into the following types:

Primary Lung Abscess

Also known as aspiration lung abscess, this type is mainly caused by aspiration of pathogens (primarily anaerobes) from the oral and pharyngeal regions. Aspiration and impaired airway defense mechanisms are key contributing factors. Common anaerobes include species of Peptostreptococcus, Prevotella, Fusobacterium, and Bacteroides fragilis. Aerobic and facultatively anaerobic bacteria include Streptococcus, Staphylococcus aureus, Pseudomonas aeruginosa, and Klebsiella pneumoniae. Abscesses are often solitary and their location is influenced by bronchial anatomy and body position. For example:

• In the supine position, lesions commonly occur in the posterior segment of the upper lobe or the dorsal segment of the lower lobe.
• In the sitting position, the posterior basal segment of the lower lobe is more frequently involved.
• In the right lateral decubitus position, the anterior or posterior segment of the right upper lobe is often affected due to the steeper angle and larger diameter of the right main bronchus.

Secondary Lung Abscess

This type may occur as a complication of bacterial pneumonias caused by Staphylococcus aureus, Pseudomonas aeruginosa, or Klebsiella pneumoniae. Secondary lung abscesses can also result from infections superimposed on underlying pulmonary conditions such as bronchial obstruction (e.g., foreign bodies, bronchiectasis, bronchial cysts, bronchogenic carcinoma, or cavitary tuberculosis). Additionally, abscesses from adjacent organs, such as subphrenic abscesses, perinephric abscesses, spinal abscesses, or esophageal perforations, can extend into the lungs and cause abscess formation. An amebic liver abscess, which typically occurs in the upper right lobe of the liver, can rupture through the diaphragm into the right lower lobe of the lung, forming an amebic lung abscess.

Hematogenous Lung Abscess

This type occurs when bacteria spread to the lungs via the bloodstream, as in cases of skin and soft tissue infections, intravenous drug use, peritonsillar infections, or disseminated liver abscesses. The infection leads to septic emboli, small vessel thrombosis, inflammation, and necrosis, ultimately forming hematogenous lung abscesses. Common pathogens include:

• Staphylococcus aureus, which is often associated with skin and soft tissue infections or infections following intravenous drug use.
• Hypervirulent strains of Klebsiella pneumoniae, which frequently cause multiple abscesses in the liver and lungs, especially in patients with diabetes or impaired glucose tolerance.
• Streptococcal species, such as Streptococcus intermedius, Streptococcus anginosus, and Streptococcus constellatus, which are known to cause pyogenic infections, including brain abscesses, intra-abdominal abscesses, and lung abscesses associated with oral or respiratory tract infections.

Imaging of hematogenous lung abscesses typically reveals multiple subpleural wedgy consolidations in both lungs, often with abscess formation.

Pathology

The infection begins with pneumonia and obstruction of the small bronchi, leading to bacterial proliferation and inflammatory thrombosis of small blood vessels. This results in necrosis of lung tissue and the formation of a lung abscess. As necrotic tissue liquefies and ruptures into the bronchi, some of the purulent material is expelled, leaving a cavity with an air-fluid level. Necrotic tissue often remains on the cavity walls. The infection may spread to surrounding areas, potentially crossing interlobar fissures and visceral pleura. If the abscess is near the pleura, localized fibrinous pleuritis and pleural adhesions may occur. Tension abscesses that rupture into the pleural cavity can lead to empyema, pyopneumothorax, or bronchopleural fistulas.

Lung abscesses may completely resolve or leave behind small fibrous scars. Acute lung abscesses generally resolve within 4-6 weeks. However, if treatment is inadequate or bronchial drainage is poor, extensive necrotic tissue may persist, leading to chronic lung abscesses (lasting more than three months). In chronic cases, fibroblast proliferation and granulation tissue cause thickening of the cavity wall, which may involve surrounding small bronchi, resulting in deformation or dilation.

Clinical Manifestations

Symptoms

Early symptoms resemble those of pneumonia, including chills, high fever (up to 39-40°C), cough, and the production of mucopurulent or purulent sputum. Pleuritic chest pain may occur when the inflammation involves the pleura. Systemic symptoms such as fatigue, malaise, and anorexia are also common. Around 90% of patients have risk factors for aspiration, such as poor oral hygiene, gingival disease, or a history of surgery, intoxication, fatigue, cold exposure, or cerebrovascular disease. Lung abscesses caused by anaerobic bacteria may have an insidious onset.

If the infection is not controlled, symptoms worsen in 1-2 weeks, with increased coughing and the expectoration of large amounts of foul-smelling purulent sputum and necrotic material (300-500 mL/day), which separates into three layers when left to stand. About 1/3 of patients experience hemoptysis, ranging from minor to massive hemoptysis. After expelling large amounts of purulent sputum, fever typically subsides, systemic symptoms subside, and the patient gradually recovers over several weeks.

In cases of rapid disease progression or weakened immunity, the abscess may rupture into the pleural cavity, causing sudden chest pain and dyspnea, indicative of pyopneumothorax. Hematogenous lung abscesses often present with a history of extrapulmonary infections, such as skin or soft tissue infections, peritonsillar abscesses, or liver abscesses. Initial symptoms include local manifestations of the primary infection, along with chills and high fever. Pulmonary symptoms, such as cough and expectoration, appear in several days to two weeks, with minimal sputum production and rare hemoptysis.

Signs

Physical findings depend on the size and location of the abscess. In early stages or when the abscess is deep within the lung, there may be no abnormal findings on physical examination. Larger abscesses surrounded by inflammation may produce signs of lung consolidation, such as bronchial breath sounds. Large cavities may produce tympanic percussion sounds and amphoric breath sounds. Pleural involvement may result in pleural friction rubs or signs of pleural effusion. Chronic lung abscesses may cause weight loss, anemia, and clubbing of the fingers or toes. Hematogenous lung abscesses often show no specific physical findings.

Laboratory and Other Examinations

Biochemical Tests

In acute lung abscesses, the total white blood cell (WBC) count can reach (20-30)×10⁹/L, with neutrophils accounting for over 90% and a significant left shift in the nucleus, often accompanied by toxic granules. In chronic cases, the WBC count may be slightly elevated or normal, with reductions in red blood cells and hemoglobin.

Microbiological Tests

Gram staining of sputum, as well as aerobic and anaerobic cultures of sputum, pleural effusion, and blood, along with antimicrobial susceptibility testing, can help identify the causative pathogens and guide antibiotic selection.

The ideal sampling techniques include protected specimen brushing via bronchoscopy or percutaneous lung aspiration.

In patients with pleural effusion, a positive culture of the effusion provides greater diagnostic value for identifying the pathogen.

In hematogenous lung abscesses, blood cultures may reveal the causative organism.

Chest Imaging

A posteroanterior and lateral chest X-ray is the most commonly used diagnostic tool for lung abscesses. Radiographic findings vary depending on the type, stage, bronchial drainage, and presence of pleural complications.

Aspiration lung abscesses initially appear as large, dense, and poorly defined infiltrative opacities or patchy dense opacities involving one or more lung segments.

As the abscess forms and pus drains through the bronchi, a round radiolucent area with an air-fluid level surrounded by dense inflammatory infiltrates becomes visible. The cavity walls are smooth or slightly irregular.

Following pus drainage and antibiotic therapy, the surrounding inflammation resolves, the cavity shrinks, and it may eventually disappear, leaving behind fibrous streaks on imaging.

Chronic lung abscesses present as thick-walled cavities with irregular inner walls, sometimes multiloculated, accompanied by fibrotic changes, pleural thickening, varying degrees of lobar atelectasis, and mediastinal shift toward the affected side.

Empyema appears as extensive dense opacities on the affected side, while pneumothorax may show air-fluid levels.

Hematogenous lung abscesses manifest as multiple patchy or well-defined spherical or oval dense opacities of varying sizes in the peripheral regions of one or both lungs. Small cavities and air-fluid levels may be visible, and post-inflammation resolution may result in localized fibrosis or small air cysts.

Chest CT provides more precise localization of abscesses, especially small ones, and is invaluable for diagnosis, differential diagnosis, and determining treatment strategies.

Bronchoscopy

Bronchoscopy is useful for identifying the etiology, performing microbiological diagnostics, and aiding treatment. Protected bronchial needle aspiration and protected specimen brushing during bronchoscopy can be used for aerobic and anaerobic cultures to identify pathogens. Tissue biopsy can help differentiate between abscesses and tumors. Additionally, bronchoscopy can assist in draining pus and administering antibiotics directly to the lesion, promoting bronchial drainage and cavity healing.

Diagnosis

The diagnosis of a lung abscess can be established based on the following:

• A history of aspiration risk factors or aspiration events.
• Acute onset of chills, high fever, cough, and expectoration of large amounts of foul-smelling purulent sputum.
• Laboratory findings showing significantly elevated total WBC and neutrophil counts.
• Chest imaging revealing dense inflammatory opacities in the lungs with cavities and/or air-fluid levels.
• Microbiological studies, including sputum and blood cultures (both aerobic and anaerobic), and antimicrobial susceptibility testing, which are critical for identifying pathogens and selecting appropriate antibiotics.

For patients with risk factors such as skin or soft tissue infections, intravenous drug use, peritonsillar infections, or disseminated hypervirulent Klebsiella pneumoniae infections, presenting with persistent fever, cough, and sputum production, and chest imaging showing multiple subpleural wedgy consolidations with abscess formation in both lungs, hematogenous lung abscess can be diagnosed.

Differential Diagnosis

Bacterial Pneumonia

Early-stage lung abscesses may resemble bacterial pneumonia in symptoms and chest X-ray findings. However, typical pneumococcal pneumonia is often associated with herpes labialis and rust-colored sputum, rather than large amounts of foul-smelling purulent sputum. Chest X-rays show lobar or segmental consolidation or patchy inflammatory changes with poorly defined edges, without cavity formation.

Cavitary Pulmonary Tuberculosis

Pulmonary tuberculosis has an insidious onset and a prolonged course, often accompanied by low-grade afternoon fever, fatigue, diaphoresis, chronic cough, anorexia, and recurrent hemoptysis. Chest imaging reveals thick-walled cavities, usually without air-fluid levels, surrounded by minimal inflammatory changes. Irregular linear, nodular lesions, and calcifications may be present, and associated findings of ipsilateral or contralateral disseminated tuberculosis lesions may be seen. Tubercle bacilli can be identified in sputum samples.

Bronchogenic Carcinoma

Tumor-induced bronchial obstruction can lead to distal obstructive pneumonia, and secondary infection with pyogenic pathogens may result in lung abscesses. The course of the disease is relatively long, with less purulent sputum. Due to impaired bronchial drainage, antibiotic therapy is often ineffective. Squamous cell carcinoma may undergo necrosis and liquefaction, forming a cancerous cavity, but this is generally not accompanied by acute infection symptoms. Chest imaging shows thick-walled, eccentric cavities with irregular inner walls due to residual tumor tissue, minimal surrounding inflammatory infiltration, and possible hilar lymphadenopathy. Bronchoscopy or sputum cytology identifying cancer cells confirms the diagnosis.

Pulmonary Bullae or Cysts

Pulmonary bullae or cysts may show air-fluid levels within the bullae or cysts, with minimal surrounding inflammatory response, no significant systemic symptoms, and no production of purulent sputum. Comparison with previous imaging studies can aid in differentiation.

Treatment

The principles of treatment for acute lung abscess include effective anti-infective therapy and drainage of purulent secretions.

General Care

Patients with lung abscesses often exhibit signs of systemic depletion, particularly those with poor physical condition. Nutritional support should be emphasized. Oxygen therapy may be provided if hypoxia is present.

Antimicrobial Therapy

Aspiration lung abscess is primarily caused by mixed infections dominated by anaerobic bacteria. Most anaerobes are sensitive to penicillin, which can be empirically administered at a daily dose of 1.2-10 million units, divided into 3-4 intravenous infusions. If penicillin is ineffective, clindamycin (0.6-1.8 g/day) or metronidazole (1.0-1.5 g/day) can be administered in 2-3 intravenous doses. If treatment remains ineffective, antibiotics should be selected based on results from bacterial cultures and susceptibility testing. Other options include carbapenems or β-lactam/β-lactamase inhibitor combinations.

In cases of hematogenous lung abscess, common pathogens include Staphylococcus aureus, Klebsiella pneumoniae, and Streptococcus anginosus group.

For Staphylococcus aureus infections, penicillinase-resistant semi-synthetic penicillins such as oxacillin (6-12 g/day) can be administered via intravenous infusion in divided doses.

For methicillin-resistant Staphylococcus aureus (MRSA), vancomycin, teicoplanin, or linezolid should be the first choice.

For infections caused by Klebsiella pneumoniae or other Gram-negative bacilli, second- or third-generation cephalosporins or carbapenems are commonly used, often in combination with aminoglycosides or ciprofloxacin.

For Streptococcus anginosus group infections, penicillin is the first choice, with vancomycin as an alternative for penicillin-resistant strains.

For hematogenous lung abscesses, antibiotics with high local drug concentrations should be chosen. Additionally, primary infection foci, such as skin and soft tissue abscesses, peritonsillar abscesses, or liver abscesses, should be treated promptly with incision and drainage or catheter drainage.

The typical duration of antibiotic therapy is 6-8 weeks, continuing until clinical symptoms completely resolve and chest imaging shows that the abscess cavity and inflammatory lesions have disappeared, leaving only residual fibrotic streaks. With effective antibiotic treatment, body temperature typically begins to drop within 3-7 days and normalizes within 7-14 days. The foul odor of sputum disappears within 3-10 days. After clinical improvement, intravenous antibiotics can be switched to intramuscular injections or oral administration.

Drainage

Effective drainage can shorten the disease course and improve treatment outcomes. For patients with thick, viscous sputum that is difficult to expectorate, mucolytics such as ambroxol or acetylcysteine can be administered, and nebulized saline or bronchodilators can aid sputum drainage. For patients in good physical condition, postural drainage can be employed to place the abscess in the highest position, combined with gentle percussion of the affected area 2-3 times daily for 10-15 minutes each time. Bronchoscopic lavage and suction are also highly effective.

Surgical Treatment

Indications for surgery include:

• Chronic lung abscesses where the cavity fails to shrink in more than 3 months of medical treatment, infection remains uncontrolled or recurs, or the cavity is too large (>5 cm) and unlikely to close.
• Complications such as bronchopleural fistula or empyema that do not respond to aspiration or lavage.
• Massive hemoptysis that is unresponsive to medical treatment or life-threatening.
• Lung abscesses caused by bronchial obstruction, suspected to be due to bronchogenic carcinoma, leading to poor drainage.

Prevention

Emphasis should be placed on treating chronic infections of the oral cavity and upper respiratory tract. Before oral or thoracoabdominal surgery, oral hygiene should be maintained. During surgery, blood clots and secretions in the oral cavity and upper respiratory tract should be cleared. Patients should be encouraged to cough, and foreign bodies in the airway should be promptly removed to ensure unobstructed airway drainage. Special attention should be given to maintaining oral hygiene in comatose patients.