A lung abscess is a collection of pus that is contained within a cavity formed by the the surrounding tissues.
The pus consists of leukocytes and a thin fluid referred to as “liquor puris.” Arbitrarily, abscesses are termed acute if the duration is less than 6 weeks and chronic if more than 6 weeks. Although the incidence of lung abscesses fell dramatically following the introduction of effective antibiotics in the 1940s and 1950s, a recent increase of immunocompromised individuals secondary to organ transplantation, chemotherapy, and AIDS has resulted in a resurgence in the numbers of lung abscesses requiring treatment.
Lung abscesses may be divided into two major categories based on etiology: primary and secondary. Primary lung abscesses occur because of aspiration of oropharyngeal contents (most common), acute necrotizing pneumonia (due to S aureus, K pneumoniae), chronic pneumonia (due to fungi, tubercle bacilli) and opportunistic infection in an immunodeficient host. Conditions that predispose to aspiration include anesthesia (both general and monitored), neurologic disorders (cerebrovascular accidents, seizures, diabetic coma, head trauma, etc), drug ingestion (alcohol, narcotics, etc), normal sleep, poor oral hygiene (increases bacterial load), and esophageal disease (gastroesophageal reflux, achalasia, cancer, tracheoesophageal fistula). Secondary causes of lung abscesses include bronchial obstruction (foreign body, hilar lymphadenopathy, cancer) cavitating lesions, direct extension (amebiasis), and hematogenous dissemination (S aureus, E coli, etc). It should be noted that secondary infections of congenital or acquired cystic lesions, such as bronchogenic cysts, bullae, tuberculous cavities, and hydatid cysts, are not true pulmonary abscesses because they occur in a preformed spaced. The bacteriologic findings of lung abscesses depend somewhat on the underlying cause and the thoroughness of the laboratory. Classically, aerobic gram-positive cocci and facilitative gram-negative bacilli (K pneumoniae, E coli, pseudomonas species) have been incriminated: however, with more fastidious culture techniques, anaerobic bacteria (bacteroides species, Clostridium ramosum, peptostreptococci, peptococci) are now isolated in over 85% of cultures. In immunocompromised patients, more unusual organisms predominate, eg, Candida albicans, Legionella micdadei and L pneumophila, and Pneumocystis carinii.
Clinical Findings & Diagnosis of lung abscesses
Patients typically complain of cough, fever, dyspnea, and occasionally pleuritic chest pain. The symptoms are insidious in onset and associated with malaise and weight loss. Complications include rupture into a bronchus, with initial hemoptysis followed by the production of foul-smelling, purulent sputum (and the potential for life-threatening pneumonia from aspiration of pus into normal lung); rupture into the pleural space with resulting pyopneumothorax, sepsis, and possibly empyema necessitatis; and, rarely, massive hemoptysis requiring emergent pulmonary resection. On physical examination, signs of lobar consolidation predominate. Laboratory studies should include a differential white blood cell count and sputum culture. Chest radiography may demonstrate an area of consolidation or a rounded density. In unusual cases, a CT may be required for better radiographic visualization, and in cases of bronchial obstruction or in all patients with unexplained lung abscesses, bronchoscopy is indicated. Fine-needle aspiration of the abscess cavity for diagnostic culture has been shown to isolate the offending pathogens in 94,4% of patients compared with only 11% and 3% from sputum culture and bronchoalveolar lavage, respectively. In addition, early fine-needle aspiration also has been reported to change the antibiotic regimen in 43% of cases and can be life-saving in immunocompromised patients with unusual organisms.
Treatment of lung abscesses
Antibiotic administration has been the mainstay of therapy following general resuscitation measures. The selection of antibiotics varies and depends on the underlying cause, but penicillin and clindamycin are commonly used. In immunocompromised individuals, trimethoprim-sulfamethoxazole, pentamidine, erythromycin, and amphotericin B are often indicated. Once the acute sepsis subsides (after up to 2 weeks), therapy can frequently be changed to an oral outpatient regimen and continued until complete resolution of the abscess occurs (3–5 months). Important adjuncts to antibiotic administration include chest physiotherapy, bronchoscopy (may require repeated examinations to maintain bronchial drainage), and health maintenance measures (general nutrition, dental hygiene, etc). In patient who do not respond to this initial regimen and who do not have surgical indications (see below), early percutaneous drainage has been shown to be an effective procedure (mortality rate 1.5%; morbidity rate 10%). Specific proposed indications for percutaneous drainage include an abscess under tension as evidenced by mediastinal shift, displacement of fissures or downward movement of the diaphragm, radiographic verification of contralateral lung, unremitting signs of sepsis, abscess size > 4 cm or increasing abscess size, rising fluid level, and persistent ventilatory dependency. Thoracotomy today is rarely indicated in the management of lung abscess but continues to be indicated in patient with massive hemoptysis, empyema, bronchial obstruction (particularly if secondary to resectable cancer), and failure of medical therapy. Furthermore, acute rupture into the pleural space (pyopneumothorax) is still a surgical emergency. When surgery is indicated, lobectomy generally is the preferred procedure.
Since the appearance of effective antibiotics, the mortality rate from lung abscesses has declined from 30–50% down to 5–20%. Medical therapy alone is successful in 75–88% of patients, and those requiring operation are cured 90% of the time with a mortality rate of only 1%. In the growing population of ICU and immunocompromised patients, however, the mortality rate remains high (approximately 28%).