Lung Abscess

Etiology

Primary lung abscesses are generally solitary and develop in otherwise healthy patients without preexisting medical conditions. In contrast, secondary lung abscesses arise in individuals with underlying or predisposing conditions; often, secondary lung abscesses comprise multiple abscess collections. The abscess can be described as nonspecific if no likely pathogen is recognized in the expectorated sputum or as a putrid abscess if the cause is presumed to be an anaerobic bacteria. The classification depends on the microorganism causing the abscess. In the majority of cases, the causative pathogen is polymicrobial, which includes anaerobic bacteria (eg, Bacteroides, Prevotella, Peptostreptococcus, Fusobacterium, or streptococci).[3] A monomicrobial lung abscess is typically caused by streptococci, Staphylococcus aureus, Klebsiella pneumoniae, Streptococcus pyogenes, Burkholderia pseudomallei, Hemophilus influenzae type b, Nocardia, or Actinomyces.[4][5]

In patients with alcohol use disorder, the most common underlying organisms of a lung abscess include Staphylococcus aureus, Klebsiella pneumoniae, Streptococcus pyogenes, and Actinomyces. Poor dental hygiene is an independent risk factor for the development of lung abscesses.[6] The incidence of Mycobacterium abscessus complex infection is greater in lung transplant recipients than in other solid organ transplant recipients. This increased incidence is primarily because lung transplant recipients need higher levels of immunosuppression posttransplant than other solid organ transplant recipients. Additionally, lung transplant recipients possess numerous donor-derived dendritic cells capable of activating T-cells and may encounter environmental antigens that could lead to graft rejection.[7]

Epidemiology

Some of the most common factors that predispose a patient to develop a lung abscess include:

• Immunocompromised patients (eg, HIV-AIDS, posttransplantation, or those receiving prolonged immune suppressive therapy) [7][8]
• Patients at high risk for aspiration (eg, seizures, bulbar dysfunction, alcohol intoxication, and cognitive impairment) [9]

Pathophysiology

In most cases, patients have primary lung abscesses due to aspiration of oropharyngeal contents with anaerobes. These primary lung abscesses typically initially start as aspiration pneumonia and are later complicated by pneumonitis, progressing to tissue necrosis in 1 to 2 weeks if left untreated.[9] Bronchogenic causes include bronchial obstruction by a tumor, foreign body, enlarged lymph nodes, aspiration of oropharyngeal secretions, and congenital malformation. In remaining cases, a lung abscess results from the hematogenous spread of an extrapulmonary pathology, including abdominal sepsis, infective endocarditis, and septic thromboembolism.

Histopathology

Acute lung abscesses are typically characterized by poorly defined borders within the lung parenchyma and are filled with thick necrotic debris. Histologically, the central region of an acute abscess contains necrotic tissue intermingled with necrotic granulocytes and bacteria. Surrounding this central area are intact neutrophilic granulocytes, dilated blood vessels, and inflammatory edema (Image. Acute Lung Abscess). In contrast, chronic lung abscesses often have an irregular, star-like shape with well-defined borders in the lung parenchyma. These abscesses contain a grayish lining or thick debris. The center of a chronic abscess is filled with pus, with or without bacteria. Surrounding the abscess is a pyogenic membrane through which white blood cells migrate into the abscess cavity. The area around the pyogenic membrane includes lymphocytes, plasma cells, and histiocytes within the connective tissue.[2] (Image. Chronic Lung Abscess)

History and Physical

Early signs and symptoms of a lung abscess are similar to pneumonia and include fever, chills, cough, night sweats, dyspnea, weight loss, fatigue, chest pain, and occasionally anemia. Initially, the cough is nonproductive, but it becomes productive when bronchial communication develops, a hallmark sign of lung abscess. This productive cough may sometimes be accompanied by hemoptysis. Differential diagnoses for lung abscesses include conditions like cavitary tuberculosis and mycoses, although these rarely show the radiological sign of a gas-liquid level. Cavitating bronchial carcinomas (eg, squamous cell or small cell carcinomas) typically present with thicker, more irregular walls than infectious lung abscesses. The absence of fever, purulent sputum, and leukocytosis can suggest a carcinoma rather than an infectious disease. 

Evaluation

Chest radiographs and CT aid in the diagnosis of a lung abscess. Differentiating between lung abscesses and pulmonary cystic lesions, including intrapulmonary bronchial cysts, sequestration, or secondary infected emphysematous bullae, can be challenging. However, the lesion's location and clinical signs often guide the diagnosis. Localized pleural empyema can be distinguished using a CT scan (Image. Computed Tomography of a Lung Abscess) or ultrasound. The radiological sign of an air-fluid level can also be seen in hydatid cysts of the lung.[10] 

Sputum examination helps identify microbiological agents or confirm bronchial carcinoma. Microbiologic analysis of sputum can potentially assist in management. If the patient presents with risk factors for fungi or mycobacteria in their history, particular cultures should be requested.[11] Diagnostic bronchoscopy is crucial for obtaining material for microbiological examination and confirming an intrabronchial cause of the abscess, such as a tumor or foreign body. Pleural fluid analysis and bronchoscopy with bronchoalveolar lavage also have been utilized in rare cases.[12] Lung abscesses are more prone to develop over the posterior segment of the right upper lobe and middle lobe followed by the superior segment of the right lower lobe and sometimes of the left lung in case of aspiration of oropharyngeal contents.[13] Complicated lung abscess can cause pyopneumothorax or pleural empyema. In such cases, pleural fluid analysis can aid in diagnosis. Lung abscesses in patients who are immunocompromised have an increased risk of complications. However, immunocompetent patients with adequate treatment have a decreased risk of complications and usually resolve in 3 weeks. Furthermore, in cases of lung abscesses secondary to hematologic spread, blood cultures and echocardiography are essential in determining the appropriate management.

Treatment / Management

Clinicians should consider empiric antibiotic therapy in patients suspected of having a lung abscess. Empiric coverage should target colonized organisms in the upper airway and oropharynx, including gram-positive cocci, gram-negative cocci, and aerobic and anaerobic gram-negative bacilli. If the patient is exposed to healthcare set 3 months before the presentation, coverage for methicillin-resistant Staphylococcus aureus (MRSA) should be considered. Atypical organisms can be found in the setting of an abscess that is not improving with broad-spectrum antibiotic therapy. Poor prognostic factors include individuals who are older or with severe comorbidities, immunosuppression, bronchial obstruction, and neoplasms. Beta-lactamase inhibitors (eg, ticarcillin-clavulanate, ampicillin-sulbactam, amoxicillin-clavulanate, and piperacillin-tazobactam) are the preferred initial empiric antibiotic therapy followed by imipenem or meropenem.[14]

Clindamycin, as an empiric treatment for lung abscess, is no longer recommended given the risk of Clostridium difficile infection, but it remains an alternative for penicillin-allergic patients. For MRSA, vancomycin or linezolid are preferred. Daptomycin has no activity against pulmonary infections.[15] For methicillin-sensitive Staphylococcus aureus, preferred pharmacologic treatments are cefazolin 2 g intravenously (IV) every 8 hours, nafcillin 2 gm IV every 4 hours, or oxacillin 2 gm IV every 4 hours. Dosing adjustment is considered in patients with compromised renal function. Antibiotic duration is usually around 3 weeks but varies depending on clinical response. Clinicians should consider switching to oral antibiotics once patients are afebrile, stable, and able to tolerate an oral diet. Amoxicillin-clavulanate is the drug of choice for oral antibiotics for lung abscesses.[16] Metronidazole, as a single therapy, does not appear particularly useful due to polymicrobial flora.[17](A1)

Abscesses larger than 6 cm are unlikely to resolve with antibiotic therapy alone and might require surgical or percutaneous intervention.[2] In patients who fail to respond to medical therapy, surgical intervention is considered with lobectomy or pneumonectomy.[18] In patients who are poor surgical candidates, percutaneous and endoscopic drainage is considered.[19] When the clinical response is inadequate, associated conditions or alternative diagnoses are considered, especially for certain fungi and mycobacteria. Empyema sometimes might be misdiagnosed as a parenchymal abscess. Antibiotic administration has proven beneficial in the case of chronic lung abscesses.[20] Additionally, in the case of hematologic causes of lung abscess, any underlying conditions (eg, infective endocarditis, septic thromboembolism) should be thoroughly treated.(B3)

Differential Diagnosis

Differential diagnoses that should be considered when evaluating patients with a lung abscess include:

• Excavating bronchial carcinoma (squamocellular or microcellular)
• Excavating tuberculosis
• Localized pleural empyema 
• Infected emphysematous bullae
• Cavitary pneumoconiosis
• Hiatal hernia
• Pulmonary hematoma
• Hydatid cyst of the lung
• Cavitary infarcts of lung
• Polyangiitis with granulomatosis (Wegener granulomatosis)
• Foreign body aspiration
• Septic pulmonary emboli 

Prognosis

In most cases, empiric antibiotic therapy successfully treats primary lung abscesses, followed by targeted therapy depending on gram stain and culture results, with an approximately 90% cure rate. Secondary abscesses require treatment of the underlying cause to improve the outcome. The prognosis of patients with secondary lung abscesses is poor, especially in immunocompromised patients and patients with bronchial neoplasm, with a mortality rate of approximately 75%.[21]

Complications

Complications primarily develop secondary to the underrecognition or inappropriate treatment of a lung abscess's underlying etiology. Complications include abscess rupture into the pleural space, pleural fibrosis, trapped lung, respiratory failure, bronchopleural fistula, and pleurocutaneous fistula.[22]