Lung abscess overview
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Lung abscess is necrosis of the pulmonary tissue and formation of cavities (more than 2 cm) containing necrotic debris or fluid caused by microbial infection. This pus-filled cavity is often caused by aspiration, which may occur during altered consciousness. Alcoholism is the most common condition predisposing to lung abscesses. Lung abscess is considered primary (60%) when it results from existing lung parenchymal process and is termed secondary when it is complicated by another process e.g. vascular emboli or follows rupture of extrapulmonary abscess into the lung.
Lung abscess has been a disease entity since the days of Hippocrates. Postural physiotherapy had been the mainstay of treatment until Harold Neuhof described his concept of new one-stage open drainage for the acute abscess in 1930's. With the invention of antibiotics in late 1940's and their promising results led the change in management of lung abscess.
Aspiration of bacteria is the inciting event for the development of lung abscess. Once the aspirate is localized it results in pneumonitis. Inflammatory mediators are released, resulting in the formation of colliquative necrosis. The right side lung is more commonly affected than the left. On gross morphology, the lesions are well circumscribed filled with necrotic debris and do not demonstrate well-defined borders with the surrounding lung parenchyma. Microscopic examination demonstrates neutrophils with dilated blood vessels and inflammatory edema. 
Lung abscess is commonly caused by bacterial infections and these include bacteroides, peptostreptococcus and prevotella.Lung abscess is commonly caused by bacterial infections and these include bacteroides, peptostreptococcus and prevotella. Most of the lung abscess is caused by polymicrobial organisms of which anaerobes play a crucial role . Less common include nocardia, actinomyces and fungi. If left untreated, life-threatening conditions may develop which may result in death or permanent disability within 24 hours .
Differentiating Lung Abscess from other Diseases
Lung abscess must be differentiated from other lesions that present with similar symptoms such as cough, fever with chills and rigor and chest includes malignancy, tuberculosis, Wegener's granulomatosis, rheumatoid nodules.
Epidemiology and Demographics
The incidence from lung abscess have greatly declined during the past several decades due to the widespread use of antibiotics and the availability of other treatment options. Lung abscess accounts for up to 4.0 to 5.5 per 10,000 hospital admissions each year in the US.
Factors resulting in the altered level of consciousness and decreased immune response play a key role in the development of lung abscess includes alcoholism, diabetes mellitus, neurological disorder and bronchial obstruction.
There is no screening recommendations for lung abscess.
Natural history, Complications and Prognosis
Symptoms of lung abscess begin approximately 10 days after aspiration of infected material. Acute symptoms include a prodrome accompanied by fever, productive cough, pleuritic chest pain and occasional episodes of hemoptysis. Patients with chronic lung abscess develop weight loss and malaise.Without treatment based on the immune status of the patient, lung abscess can either resolve by forming a granulation tissue scar, or progressively worsens and can result in septicemia, hemorrhage, and death.The prognosis of lung abscess is good with appropriate antibiotic treatment with a high success rate. The outcomes depend on the other associated conditions underlying lung abscess such as size of the cavity, age of the patient and other underlying lung diseases. Complications include hemorrhage pyopneumothorax, pleural empyema and sepsis.
History and Symptoms
Patient history is important to establish a diagnosis of lung abscess. Common history findings include conditions associated with a risk of gastric content aspiration, a recent history of hospitalization or surgery. Common symptoms include high fever (>101°F [>38.5°C]), productive cough with purulent sputum, and pleuritic chest pain. Anaerobic bacteria is responsible for purulent sputum in acute lung abscess.
Patients with acute lung abscess may present with fever, cough with purulent sputum, gag reflex may be absent in patients with an underlying neurological disorder such as stroke. Digital clubbing is present in patients with a chronic abscess.On examination of the chest, there will be features of consolidation. Abscess typically localizes in the upper lobes or apical segments of the lower lobes.
Diagnosis of lung abscess is made based on clinical symptoms, physical examination, radiographic studies and bacterial culture. Laboratory findings include increased acute phase reactants (ESR and CRP) levels and leukocytosis with consolidation being evident in a segmental or lobar distribution with central cavitation an air-fluid level on lung x-ray. Blood cultures should be performed in all suspected cases.
The mainstay of management for lung abscess is hospital admission for chest drain and systemic antibiotics. Antimicrobial therapy is based on predisposing host factors and local resistance patterns.Empiric treatment should be commenced after culture samples are obtained. The regimens should penetrate the lung parenchyma and target both anaerobes, aerobic and microaerophilic streptococci. Any combination of a beta-lactam–beta-lactamase inhibitor or a carbapenem are reasonable to use. The standard duration of the treatment of lung abscess is ≥ 4–6 weeks of parenteral antibiotics. 
Surgery is considered as the last resort of therapy when both medical and chest drain failed to resolve symptoms. Options for surgery includes chest tube drainage and surgical resection of the lung abscess with the surrounding lung tissue.Percutaneous and endoscopic drainage techniques are considered as a first-line management, especially for patients who are not candidates for surgery. Percutaneous drainage of lung abscesses is characterized by high therapeutic effectiveness and preservation of functional lung tissue, it is a minimally invasive method with fewer complications and lower mortality rates (approximately 4%) in comparison to surgical management. Surgical resection is considered in about 10% of the patients when the chest drain has failed to improve symptoms and patients presenting with one of the following conditions. The surgical approach is thoracotomy and the extent of surgical resection depends on the size of the underlying lesion.
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