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.^[1]

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.^[2]

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.^{[3] [4]}

Lung abscess is commonly caused by bacterial infections and these include bacteroides, peptostreptococcus and prevotella.^[5]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 .

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.^[6]

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.^[7]

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.^[8]

There is no screening recommendations for lung abscess.

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.^[9][10]

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.^[10]

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. ^[11]

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.^[12]

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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.

• In 1904, Guillemot first put weight to the theory that aspirated oropharyngeal organisms are responsible for cause of lung abscess^[1]
• In 1920-1930, Harold Neuhof was the first thoracic surgeon to develop a valid and reliable therapeutic concept of drainage operation for acute pulmonary abscess.^[2]
• In 1938, the first percutaneous drain of lung abscess was performed.^[3]
• In 1942, Brock added evidence in stating that aspirated contents gravitated to the dependent part of the lungs.
• In 1946, owing to the advent of antibiotics, and treatment of acute putrid lung abscess with penicillin showed good results and henceforth patients received antibiotics rather than surgical procedures.^[2]

Lung abscess is defined as an infection of lung resulting in necrosis of lung parenchyma. It can be classified into three categories based on the duration of symptoms, etiology, and mode of spread as acute or chronic, primary or secondary and bronchogenic or hematogenic.

• Acute: If the duration of symptoms is less than 4-6 weeks before presenting to medical care.^[1]
• Chronic: If the symptoms persist for more than 6 weeks.

• Primary: When the abscess develops after lung infection in previously healthy persons or in patients prone to aspiration. ^[2]
• Secondary: Abscess formation in patients due to complications of a co-existing lung disease such as post obstructive process (bronchial obstruction due to tumor, foreign body or enlarged lymphnodes) and systemic process resulting in decreased immune response like HIV, and patients on immunosuppressants and corticosteroids.

Bronchiogenic: Abscess formation is due to aspiration and inhalation.^[1]

• Aspiration of oropharyngeal secretions.
• Bronchial obstruction by a tumor.
• Foreign body, congenital malformations, and enlarged lymph nodes.

Hematogenic: Abscess formation due to dissemination of causative agents from other infected sites

• Infective endocarditis.
• Abdominal sepsis.
• Septic thromboembolism.

Aspiration of anaerobic 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 neutrophilic granulocytes with dilated blood vessels and inflammatory edema. .

• Aspiration of anaerobic bacteria from the oropharynx, due to altered level of consciousness, absent gag reflex or inability to swallow is the inciting event for the development of primary lung abscess.^[1]
• In healthy individuals, defense mechanisms cope up with the small amounts of aspirates with no effects, however, in conditions like alcoholism, diabetes mellitus, and immunocompromised state these defense mechanisms can be compromised leading to decreased activity of alveolar macrophages and mobility of leukocytes. ^[2].
• In secondary lung abscess, abscess formation depends on the underlying lung disease and predisposing factors, for example, bronchial obstruction from benign or malignant intrabronchial lesions or extrinsic compression of bronchus as in middle lobe syndrome results in distal abscess formation due to decreased oropharyngeal clearance and favoring abscess formation.
• Once the aspirate is localized it results in pneumonitis. ^[3]
• Inflammatory mediators along with various bacterial toxins and proteolytic enzymes from the neutrophils are released, leading to the rupture of small blood vessels resulting in the formation of colliquative necrosis. ^[4]
  

• The right side lung is more commonly affected than the left because the right bronchus is more acutely angulated when compared to left bronchi.
• The most common location is the posterior segment of the right apical lobe or apical segments of lower lobes of both the lungs.^[5]
• In alcoholics and hospitalized patients are in the supine position, right lateral part of the posterior segment of the upper lobe is more commonly involved.

• Congenital diseases such as cystic fibrosis, vasculitis, pulmonary sequestration and bronchial cysts are associated with increased the risk of lung abscess in children.^[6]

• In acute lung abscess, the lesions are well circumscribed filled with necrotic debris and do not demonstrate well-defined borders with the surrounding lung parenchyma.^[7]
• In chronic long standing abscess, the lesions are irregular and filled with grayish thick debris.

• 
  Gross photograph of the lung abscess. Note the abscesses (arrows) especially in the lower lobes.
• 
  In this section of upper lobe there are multiple areas of early abscess formation (arrows). Note the circumscribed whitish-tan lesions. These lesions are filled with white blood cells.

• In acute lung abscess, neutrophilic granulocytes are demonstrated with dilated blood vessels and inflammatory edema.^[7]
• In chronic lung abscess, biopsy specimen demonstrates lymphocytes, plasma cells, and histiocytes around a layer of pyogenic membrane surrounding the abscess cavity which is filled with pus.

• 
  Acute lung abscess abscess full of inflammatory cells (primarily neutrophils) (arrows). There is a bacterial colony in the center of this abscess (1)..
• 
  Chronic lung abscess containing necrotic debris (1) and there is a rim of viable inflammatory cells (arrows) surrounding this abscess..

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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.

• Include conditions which may result in death or permanent disability within 24 hours if left untreated.
• Aerobic bacteria are most commonly associated with rapid and flumiant course with high grade fever and worsening symptoms. These bacteria Include Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Streptococcus pneumonia.

• About 90% of the lung abscess is caused by polymicrobial infection.^{[1] [2]}
• Anaerobes are the predominant pathogens involved in primary lung abscess, followed by Streptococcus pneumoniae.

• Klebsiella pneumoniae is the more commonly seen in alcoholics.^[3]

• Staphylococcus aureus is the most common pathogen responsible for lung abscess in children with cystic fibrosis.^[4]

The following table elaborates the most common etiological pathogens responsible for lung abscess ^[5]

• Bacteroides
• Fusobacterium
• Peptostreptococcus
• Prevotella
• Streptococcus milleri

• Legionella
• Lemierre’s syndrome
• Listeriosis
• Melioidosis

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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, pulmonary tuberculosis, Wegener’s granulomatosis, rheumatoid nodules.

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.

• Incidence has declined in the late 1940s and 1950s, after the practice of performing oral surgery and tonsillectomy in the sitting position was abandoned, as it became clear that this could result in lung abscess formation.^[1]
• Lung abscess accounts for up to 40 to 55 per 100,000 hospital admissions each year in the US.
• It occurs at any age, but most frequently from the sixth to eighth decades, and is predominantly seen in men. ^[2]

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Factors resulting in 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.^[1]

• Alcoholism^[2]
• Seizure disorder
• Mechanical ventilation
• Decrease level of consciousness
• Neuromuscular disorders with bulbar dysfunction^[3]
• Nocturnal aspiration ^[4]
• Bronchial obstruction
• Gingivo–dental sepsis
• Diabetes mellitus
• Immunosuppression^[4]

• Drug abuse
• Malnutrition
• Mental retardation
• Gastroesophageal reflux disease

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There is no screening recommendations for lung abscess.

Symptoms of lung abscess begin approximately 10 days after aspiration of infected material. Symptoms lasting less than 14 days is considered acute lung abscess, while longer lasting symptoms constitute chronic lung abscess. 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. The prognosis of lung abscess is good with appropriate antibiotic treatment with a high success rate. Complications include hemorrhage, pyopneumothorax, pleural empyema and sepsis.

• The symptoms of lung abscess usually develop during the fifth decade of life with risk factors, and start with symptoms such as fever, productive cough, pleuritic chest pain and occasional episodes of hemoptysis, typically developing 8-14 days after aspiration ^[1]
• 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. The mortality rate of lung abscess is as high as 75% in patients with underlying immunocompromised state and bronchial obstruction favoring poor prognosis. The following factors are considered to be associated with poor prognosis among patients.^[2]

• Large size cavities (> 6 cm)
• Old age
• Necrotizing pneumonia
• Prolonged symptoms
• Abscess due to aerobic bacteria and hospital acquired
• Bronchial obstruction due to tumors or foreign body (secondary abscess)
• Immunocompromised individuals

Without treatment, lung abscess can result in the following complications:

• Hemorrhage ^[3]
• Pyopneumothorax
• Pleural empyema^[4]
• Fibrosis and calcification of lung tissue
• Mediastinal, pleural and cutaneous fistulas
• Sepsis

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