Lung abscess is a purulent inflammation of lung tissue caused by infection with a variety of pathogenic bacteria, resulting in tissue necrosis, destruction, and liquefaction to form an abscess. The main clinical features are high fever, cough, and coughing up large amounts of pus-flavored sputum. Common pathogens include Staphylococcus aureus, Streptococcus pyogenes, Klebsiella pneumoniae, and Pseudomonas aeruginosa.
Etiology and pathogenesis
The pathogens are often colonizing bacteria of the upper respiratory tract and oral cavity, including aerobic, anaerobic, and parthenogenic anaerobes. 90% of patients with lung abscess have combined anaerobic infections, and more virulent anaerobes can cause disease alone in some patients. Other common pathogens include Staphylococcus aureus, Streptococcus pyogenes, Klebsiella pneumoniae, and Pseudomonas aeruginosa. Escherichia coli and Haemophilus influenzae can also cause necrotizing pneumonia. Depending on the route of infection, lung abscesses can be divided into the following types.
Aspiration lung abscess
Pathogens cause disease by aspiration through the mouth, nose, and pharyngeal cavity. Under normal circumstances, inhalants can be rapidly cleared by the airway mucus and cilia transport system, cough reflex and lung macrophages. However, when there is impaired consciousness such as in anesthesia, intoxication, drug overdose, epilepsy, cerebrovascular accident, or due to cold, extreme fatigue and other triggers, the systemic immunity and airway defense clearance function is reduced, the inhaled pathogenic bacteria can cause disease. In addition, purulent secretions such as sinusitis and alveolar abscesses can also be inhaled and cause disease. Abscesses are often solitary, and their location is related to bronchial anatomy and body position. Since the right main bronchus is steeper and straighter, and the diameter of the tube is thicker, the aspirator is prone to enter the right lung. In the supine position, it occurs in the posterior segment of the upper lobe or the dorsal segment of the lower lobe; in the sitting position, it occurs in the posterior basal segment of the lower lobe, and in the right lateral position, it occurs in the anterior or posterior segment of the right upper lobe. The pathogens are mostly anaerobic bacteria.
Secondary lung abscess
Certain bacterial pneumonias, such as Staphylococcus aureus, Pseudomonas aeruginosa and Klebsiella pneumoniae pneumonia, as well as secondary infections such as bronchiectasis, bronchial cysts, bronchopulmonary carcinoma and tuberculosis cavity can lead to secondary lung abscess. Bronchial foreign body obstruction is also an important factor leading to lung abscess, especially in pediatric lung abscess. Purulent lesions in adjacent organs of the lung, such as subphrenic abscess, perinephric abscess, spinal abscess, or esophageal perforation, can also lead to lung abscess. Amoebic liver abscesses tend to develop on the top of the right liver and easily penetrate the diaphragm to the lower lobe of the right lung, forming an amoebic lung abscess.
Hematogenous lung abscess
Bacteremia due to traumatic skin infections, boils, carbuncles, otitis media or osteomyelitis, etc. Bacterial emboli spread to the lung via the bloodstream, causing embolism, inflammation and necrosis of small vessels and forming a lung abscess. In cases of bacterial endocarditis of the right heart in intravenous drug users, tricuspid valve redundancy is dislodged and obstructs small pulmonary vessels to form lung abscesses, often multiple abscesses in the outer fields of both lungs. Staphylococcus aureus, Staphylococcus epidermidis and Streptococcus are the common causative organisms.
Pathological changes
Infected material obstructs the fine bronchi, inflammatory embolism of small blood vessels, and multiplication of pathogenic bacteria causes purulent inflammation and necrosis of lung tissue, forming a lung abscess, followed by liquefaction and rupture of necrotic tissue to the bronchi, partial discharge of pus, and formation of a pus cavity with air-fluid level, and residual necrotic tissue is common on the surface of the cavity wall. The lesion has a tendency to expand peripherally, even beyond the interlobular fissure to adjacent lung segments. If the abscess is close to the pleura, limited fibrinous pleurisy and pleural adhesions may occur; if it is a tension abscess and breaks into the pleural cavity, an abscess chest, pneumothorax or bronchopleural fistula may be formed. Lung abscesses may be completely absorbed or only a small amount of fibrous scarring may remain.
Clinical manifestations
Symptoms
Rapid onset, chills, high fever, temperature of 39-40℃, accompanied by cough, coughing mucus sputum or mucopurulent sputum. Inflammation involving the mural pleura may cause chest pain and is associated with breathing. Shortness of breath may occur when the lesion is extensive. In addition, there are symptoms of systemic toxicity such as mental weakness, general weakness and loss of appetite. If the infection is not controlled in time, the patient coughs up a lot of purulent sputum and some patients have different degrees of hemoptysis. Hematogenous lung abscesses mostly start with the manifestations of infection toxicity such as chills and high fever caused by the primary lesion. Cough and sputum appear only after several days or weeks, and the amount of sputum is not much, and rarely hemoptysis. Patients with chronic lung abscess often have symptoms such as irregular fever, cough, coughing up purulent sputum, emaciation and anemia.
Physical signs
Pulmonary signs are related to the size and location of lung abscess. It is accompanied by cough, coughing mucus sputum or mucopurulent sputum. Inflammation spreading to local pleura may cause chest pain. If the lesion is more extensive, shortness of breath may occur. In addition, there is mental weakness, fatigue, and poor appetite. After about 10-14 days, the cough intensifies, the abscess breaks down in the bronchi, and a large amount of purulent sputum is coughed up, up to 300-500 ml per day, and the body temperature drops immediately. Since the pathogenic bacteria are mostly anaerobic bacteria, the sputum has a fishy odor. Sometimes there is blood in the sputum or a moderate amount of hemoptysis. Patients with chronic lung abscess have chronic cough, coughing up pus, recurrent hemoptysis, secondary infection and irregular fever, and often present with anemia, wasting and chronic wasting disease. Hematogenous lung abscesses are mostly preceded by symptoms of systemic sepsis such as chills and hyperthermia caused by the primary lesion. Pulmonary symptoms such as cough and sputum appear only after a few days to two weeks. Usually the sputum volume is small, and hemoptysis is rare. Signs: related to the size and location of the lung abscess. Smaller lesions or those located deep in the lungs may have no abnormal signs. In larger lesions with massive inflammation around the abscess, there are turbid or solid sounds on percussion, decreased breath sounds on auscultation, and sometimes wet rales can be heard. Signs of hematogenous lung abscess are mostly negative. In patients with chronic lung abscess, the affected side of the chest is slightly collapsed, with turbid percussion and decreased breath sounds. Pestle-like fingers (toes) may be present.
Laboratory and other tests
Blood count
Acute lung abscess blood leukocyte count can reach (20-30) × 109/L, and neutrophils are above 90%. The nuclei are markedly left-shifted and often have toxic granules. In chronic patients, blood leukocytes may be slightly elevated or normal, and red blood cells and hemoglobin are reduced.
Sputum bacteriological examination
Sputum smear Gram stain, sputum, pleural fluid and blood cultures and antimicrobial drug sensitivity tests help to identify the pathogen and select effective antimicrobial drugs. In particular, pleural effusions and blood cultures are of greater diagnostic value for pathogens when they are positive.
Chest X-ray examination
Early inflammation appears as a large, dense, faint infiltrative shadow with indistinct margins, or as a mass of dense shadows distributed over one or several lung segments. After the formation of lung abscess, a large amount of pus sputum is discharged through the bronchus, and a circular cavity with a plane containing gas fluid is visible on the chest X-ray, with smooth or slightly irregular inner wall. In chronic lung abscess, the cavity wall is thick, the pus cavity is irregular, the size varies, and it may be honeycomb-shaped, surrounded by fibrous tissue hyperplasia and thickening of the adjacent pleura.
CT examination of the chest: it can clearly show what is seen on the chest film, which can be more accurately localized and help to make postural drainage and surgical treatment.CT can be used to distinguish lung abscess from limited abscess chest with air-fluid flat, and to detect smaller abscesses and pulmonary air sac cavities caused by staphylococcal pneumonia. This test should be done further in patients whose diagnosis is not easily clarified clinically.
Bronchial iodine oil angiography: used in patients with chronic lung abscess suspected of being complicated by bronchiectasis. In elderly patients, cardiopulmonary insufficiency is often present, so this test should be done with caution.
Fiberoptic bronchoscopy: It helps to clarify the etiology and pathogenetic diagnosis. If there is a foreign body in the airway, the foreign body can be removed to make the airway drainage smooth. If tumor obstruction is suspected, pathological specimens can be obtained. A catheter can also be inserted through fiberoptic bronchoscopy to get as close as possible to or into the abscess cavity to attract pus, flush the bronchi and inject antibiotics to improve the efficacy and shorten the course of the disease.
Differential diagnosis
Bacterial pneumonia
Early lung abscess is very similar to bacterial pneumonia in terms of symptoms and radiographic chest manifestations, but common Streptococcus pneumonia is mostly accompanied by herpes of the mouth and lips, rust-colored sputum without a large amount of purulent sputum, and the radiographic chest shows solid lobe or segmental lung lesions or lamellar thin inflammatory lesions with blurred margins and no cavity formation. Lung abscess should be considered when high fever does not subside even after treatment with antibacterial drugs, and when cough and sputum increase and a large amount of pus sputum is coughed up.
Secondary infection of cavitary pulmonary tuberculosis
Cavitary tuberculosis is a chronic disease with slow onset and long duration, which may include prolonged cough, afternoon fever, malaise, night sweats, loss of appetite or recurrent hemoptysis. When combined with pulmonary infection, acute infection symptoms and coughing up large amounts of purulent sputum can occur, and it is difficult to find Mycobacterium tuberculosis in the sputum due to the proliferation of purulent bacteria. If it cannot be distinguished for a while, it can be treated as acute lung abscess. After controlling the acute infection, chest X-ray can show fibrous cavity and surrounding polymorphic tuberculosis lesions, and sputum Mycobacterium tuberculosis can turn positive.
Bronchopulmonary carcinoma
Obstruction of bronchus by bronchopulmonary carcinoma often causes purulent infection in the distal lung, but the course of lung abscess is relatively long, because there is a gradual obstruction process, toxic symptoms are not obvious, and the amount of pus sputum is also less. Obstructive infections are not effective with antibacterial drugs due to poor bronchial drainage. Therefore, for patients over 4O years old with recurrent infections in the same part of the lung and poor efficacy of antibacterial drugs, the possibility of obstructive pneumonia caused by bronchial lung cancer should be considered. The X-ray chest film shows that the wall of cavity is thicker, mostly eccentric cavity, and the residual tumor tissue makes the inner wall uneven, and there is a little inflammatory infiltration around the cavity, and the lymph nodes in the lung gate may be enlarged, so it is not difficult to distinguish from lung abscess.
Pulmonary cyst secondary infection
In the case of secondary infection of pulmonary cyst, gas-fluid flat is seen in the cyst, and the surrounding inflammatory reaction is mild, without obvious toxic symptoms and pus sputum. It is easier to distinguish if there is a previous X-ray chest film for comparison.
Treatment principles
Antibacterial drug treatment and pus drainage are the main treatment principles.
Antimicrobial drug treatment
Aspiration lung abscess is mostly anaerobic bacterial infection, which is generally sensitive to penicillin, only Bacteroides fragilis is not sensitive to penicillin, but sensitive to lincomycin, clindamycin and metronidazole. The dose of penicillin can be decided according to the severity of the disease, 1.2-2.4 million U/d for mild cases, and 10 million U/d can be used in divided intravenous doses for severe cases to increase the concentration of the drug in the necrotic tissue. The body temperature usually drops to normal within 3-10 days of treatment, and then can be changed to intramuscular injection. If penicillin is not effective, lincomycin 1.8-3.0g/d divided intravenous drip, or clindamycin 0.6-1.8g/d, or metronidazole 0.4g, 3 times daily orally or intravenously.
Hematogenous lung abscesses are mostly staphylococcal and streptococcal infections, and β-lactamase-resistant penicillin or cephalosporin can be used. In the case of methicillin-resistant staphylococci, vancomycin, teicoplanin or linhazolamide should be used.
In case of amoebic protozoal infection, treat with metronidazole. In the case of gram-negative bacilli, second- or third-generation cephalosporins, fluoroquinolones (e.g., moxifloxacin), and aminoglycoside antibacterial drugs may be used in combination. The course of antibacterial drugs is 8-12 weeks until the pus cavity and inflammation disappears on X-ray chest, or only a small amount of residual fibrosis is present.
Pus drainage
An effective measure to improve the efficacy of treatment. Expectorants or nebulized aspirated saline, expectorants or bronchodilators can be used for those whose sputum is thick and not easy to cough up to facilitate sputum drainage. Those who are in good health can adopt postural drainage to drain sputum, and the position of drainage should be such that the abscess is in the highest position, 2-3 times a day for 1-15 minutes each time. Trans-fiber bronchoscopic irrigation and suction are also effective methods of drainage [2].
Surgical treatment
Indications are.
(i) lung abscess of more than 3 months’ duration, where the abscess cavity is not reduced by medical treatment, or where the abscess cavity is too large (more than 5 cm) and is estimated not to close easily.
②Large hemoptysis which is ineffective or life-threatening by medical treatment.
(iii) those with bronchopleural fistula or abscess thorax treated with suction, drainage and flushing with poor results.
④Bronchial obstruction restricting airway drainage, such as lung cancer. For those who are too ill to tolerate surgery, a catheter can be inserted through the chest wall into the abscess cavity for drainage. The patient’s general condition and lung function should be evaluated preoperatively.