A lung abscess is a subacute infection in which an area of necrosis forms in the lung parenchyma. It usually is in a dependent section of the lung, more often involves the right lung than the left, and is most commonly seen after aspiration of oropharyngeal secretions. Lung abscesses have a slow, insidious presentation and usually develop 1-2 weeks after the initial aspiration event. A lung abscess is shown in the image below.
Empyema is defined as pus in the pleural space. It typically is a complication of pneumonia. However, it can also arise from penetrating chest trauma, esophageal rupture, complication from lung surgery, or inoculation of the pleural cavity after thoracentesis or chest tube placement. An empyema can also occur from extension of a subdiaphragmatic or paravertebral abscess.
A lung abscess involves the lung parenchyma, whereas an empyema involves the pleural space.  In many patients with pneumonia, a sterile simple parapneumonic effusion develops in the pleural space. If this pleural effusion becomes infected, it is labeled a complicated parapneumonic effusion, whereas the presence of frank pus in the pleural space defines an empyema.
The development stages of an effusion can be divided into 3 phases: exudative, fibropurulent, and organizational. The initial effusion develops from increased pulmonary interstitial fluid along with progressive capillary vascular permeability. A simple effusion is frequently sterile and resolves with antibiotic treatment of the underlying pulmonary infection.
In 5-10% of the patients with a pleural effusion, the effusion becomes infected and neutrophils buildup. This inflammatory response, shown in the images below, also causes the production of chemokines, cytokines, oxidants, and protease mediators.
This more complicated parapneumonic effusion requires both antibiotics and some form of surgical drainage or alternative treatment modality to remove the purulent effusion. In these more complicated effusions, decreased fibrinolysis and activation of the coagulation cascade leads to the production of fibrin with subsequent adhesions and loculated fluid collections. This process ultimately can cause pleural fibrosis and impairment of lung expansion.
The rates of lung abscess have dropped significantly since the development of antibiotics. The frequency of lung abscess varies depending on underlying comorbidities and occurs most frequently in individuals with risk factors for aspiration (stroke, dementia, alcoholism, gastric reflux, and poorly controlled epilepsy), immunocompromise, malignancy, intravenous drug use, poor dental hygiene, and diabetes mellitus. 
Current rates of parapneumonic empyema in the United States are estimated at 6 cases per 100,000. 
In the 1940s, the rates of empyema had dropped precipitously with the advent of antibiotics, but, from 1996 to 2008, the rate of empyema nearly doubled across all age groups. This increase is partly explained by the rapid increase of antibiotic resistance but, interestingly, is theorized to have occurred secondary to a replacement phenomenon. The heptavalent pneumococcal vaccine had dramatically decreased the overall incidence of pneumococcal infections but had increased the incidence of nonvaccine pneumococcal serotypes 1, 3, and 19A and nonpneumococcal infections, especially Staphylococcus aureus, that are thought to confer a higher conversion from pneumonia to complicated pneumonia and empyema. Since 2010, a 13-valent pneumococcal conjugate vaccine has been available, which includes the 1, 3, and 19A serotypes. [3, 4]
Most empyemas are complications of pneumonia, but up to 20% are secondary to iatrogenic causes, including but not limited to, thoracic surgeries, chest tube insertion, or thoracentesis. Three percent of empyemas are estimated to occur as complications of chest trauma. 
The mortality rate for lung abscesses in adults can reach as high as 15-20%,  whereas children have mortality rates typically below 5%. Mortality is largely dependent on comorbid conditions. Healthy individuals generally have a very good prognosis while immunosuppression, malignancy, nosocomial infections, reduced level of consciousness, anemia, and low albumin portend a poorer prognosis. Lung abscesses secondary to Pseudomonas aeruginosa, Staphylococcus aureus, and Klebsiella pneumoniae infections have high mortality rates. 
The case fatality rate in the United States for parapneumonic empyema is 7.2%, with rates increasing with age. The fatality rate in children is 0.4%, and is 16.1% in adults older than 64 years.  Mortality is increased for hospital acquired, Saureus, gram-negative, and mixed aerobic infections.
A population study revealed that nearly 65% of individuals with parapneumonic empyema are male.  No studies have revealed male gender as an independent risk factor for empyema. [7, 8] This disparity may be attributed to gender-related risk factors, including alcohol abuse, drug use, and delay to treatment.
Lung abscesses are more common in individuals with comorbid conditions often found with increasing age, including dementia and malignancy.
There is an increased risk of parapneumonic empyema with extremes of age with rates of 7.6 and 9.9 cases per 100,000 for ages younger than 5 years and older than 64 years, respectively. Rates for empyema between ages of 5 and 64 years range from 1.9-5.4 cases per 100,000. [2, 4]
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