LUNG ABSCESS

LUNG ABSCESS
Definition

Lung abscess is a localized area of liquefactive necrosis of the lung. This would then include necrotizing gram negative and gram positive pneumonias eg. Klebsiella, Staph, Pseudomonas etc. However, by convention we reserve the term lung abscess for necrotizing anaerobic pneumonia.

Prerequisites and Predisposing Conditions

• Aspiration of a Large Bacterial Inoculum:
  The aspiration of oropharyngeal contents with a bacterial bolus inoculum is the prerequisite for development of lung abscess.
• Loss of Cough Reflex:
  If the cough reflex is intact, significant aspiration is not possible unless it is overwhelming. Altered sensorium is the most common state when cough reflex is suppressed, thus CVA, drug overdose, alcoholism, post-op state or coma from any cause is the most common predisposing factor for lung abscess.
• Trouble with Deglutition:
  This occurs with neurological disorders and esophageal diseases. Aspiration to lungs is frequent in this situation even if the cough reflex is intact. In many of the Esophageal diseases the mode of presentation is Lung abscess.
• Post Obstructive Pneumonia:
  Lung abscess can occur as a complication of post obstructive pneumonia as seen in some patients with lung cancer or foreign body aspiration.

·         Common Segments

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The superior segments of RLL, LLL and axillary subsegments of anterior and posterior segments of RUL are common sites for aspiration and will account for 85% of all Lung abscesses.

·         Gravitational forces determine the site of aspiration. Position of the patient at the time of aspiration determines the segment the aspiration is most likely to occur.

·         Basal segments of RLL used to be the most common site for aspiration during 1940 to 1960. During this period ENT surgery and Dental work was done in sitting position with Ether as the anaesthetic. The Right main bronchus is in straight line with Trachea while left main takes of at an angle. In this position gravity facilitates lodging of the aspirate to basal segments of RLL.

·         In supine position and with the patient on back superior segment of RLL is the most dependent segment.

·         In right lateral decubitus position the axillary subsegments of anterior and posterior segments of RUL is the dependant site for aspiration. Abscess is located in the middle of lateral CXR corresponding to RUL bronchus take off.

·         When the patient is on abdomen, aspiration does not occur, thus it is extremely unlikely for any anterior segments, middle lobe and lingula to be the site for aspiration lung abscess. When lung abscess is encountered in these sites on should suspect partial airway obstruction or trouble with deglutition as the predisposing factor for lung abscess.

Clinical Picture

• Most of the patients present with subacute onset of illness and do not seek medical attention for three to four weeks since the onset of illness.
• Patients complain of cough, low grade fever, anorexia and weight loss of few weeks duration .
• Patients often have cough with large amounts of foul smelling sputum.
• Lack of foul smell does not exclude lung abscess, as 50% of anaerobic infections do not produce a foul smell.
• The superior segment of RLL , LLL and axillary sub-segments of anterior and posterior segments of RUL usually account for 85% of all aspiration lung abscesses. The lesions will correspond to these sites in CXR.
• In the early stages one sees consolidation.
• The inflammatory mass eventually necroses and the necrotic material is expectorated through bronchus.
• The cavity that results has thick wall with irregular lumen. You may note stalactites and stalagmites in the lumen.
• Air fluid level is the hall mark of Lung abscess.
• The appearance of the cavity is similar to necrotizing squamous cell cancer of lung and has to be differentiated from it.

·         Bacteriology

Common pathogens are gram positive anaerobes such as peptococci and peptostreptococci, the micro-aerophilic streptococci which are part of normal oropharyngeal flora gram negative anaerobes such as the prevotella (P. Melaninogenicus), Fusobacteria (necrophorum and nucleatum)

Method of Obtaining Specimen
The options are as follows:

• Sputum Gram Stain:
  May occasionally be helpful if there is a large number of white blood cells and bacteria consistent with oropharyngeal flora.
• Bronchoscopy:

1.     Triple lumen catheter: Routine aspirates during bronchoscopy is useless for anaerobic cultures. The bronchoscope passes through oropharynx and will be contaminated by the oropharyngeal flora. You need to use triple lumen catheter to avoid contamination and obtain material selectively from the involved segment.

2.     Bronchial lavage: The second option is to obtain a bronchial lavage from the involved segment and perform quantitative bacterial cultures.

• Fine Needle Aspiration:
  In the pre-antibiotic era needle aspirations of lung abscess were fraught with fear of development of bronchopleural fistula and empyema. With the current option of FNAB under CT guidance, it is being done with increasing frequency and safety. Uncontaminated aspirate can be obtained by this method for cultures. This procedure is often the method of choice for obtaining the specimen in children as other options are not easily feasible in this population.
• No Need for Cultures:
  When the patient has foul smelling sputum the anaerobic infection is obvious and there may not be a need for confirmation, as many of these procedures are expensive and attendant with some risks. Most of the lung abscess respond to empiric therapy. The primary purpose for culture is to obtain antibiotic sensitivity and can be reserved to cases not responding to empiric therapy.

·         Antibiotic of Choice

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Traditionally, penicillin alone was used and produced satisfactory results. Of late, there has been increasing incidence of penicillin resistance in oropharyngeal anaerobes. Hence, penicillin alone is no longer recommended. Metronidazole alone has failed despite its superb anaerobic spectrum due to lack of activity against microaerophilic streptococci which are significant pathogens in lung abscesses. Penicillin added to metronidazole is an acceptable alternative.

·         Clindamycin is the most popular antimicrobial for treatment of lung abscesses and has produced excellent results. The intracellular uptake of clindamycin and its stability in abscess which have low pH and poor vascularity may offer an advantage.

·         Other beta lactams such as ampicillin and sulbactam, ticarcillin or amoxicillin with clavulanate, piperacillin with tazobactam, cefoxitin and cefotetan also have excellent activity against anaerobes and offer expensive alternatives. Imipenem also has excellent activity against anaerobes. Presently, available quinolonoses such as ciprofloxacin, norfloxacin, oflaxacin, etc. have very poor activity against anaerobes and streptococci. Prolonged treatment over several weeks is typically required.

Methods of Drainage

• Postural Drainage:
  Postural drainage should be instituted in all cases if possible to drain the abscess.This will facilitate drainage of pus. Patient should have empty stomach and prepared with bronchodilator and humidification prior to postural drainage.

In most patients antibiotics and postural drainage is sufficient to cure lung abscess. However in some patients additional drainage options have to be entertained.

• Bronchoscopy:
  Fiberoptic bronchoscopy is very useful to drain the lung abscess, however cannot be used as a method of drainage daily. This method has to be reserved for situations when the postural drainage and antibiotics are failing to control infection and the cavity is enlarging.
• Percutaneous Chest Tube:

1.     CT Guided Percutaneous Drainage: One can select the site where the parietal and visceral pleura are adherent and the drain can be placed into the cavity.

2.     In past chest tube was placed into the cavity in two surgical steps. First pleural space was marsupialized by sutures between visceral and parietal pleura. Few days later chest tube was placed into the cavity through this site. This precaution was taken to avoid spilling of pus into pleural cavity .

Role of Bronchoscopy

• Diagnosis:
  In the past bronchoscopy was done routinely in all patients with lung abscess with the intent of detecting foreign body or cancer. In most the bronchoscopy was non-contributory. Lung abscess is due to aspiration of bacterial bolus and not due aspiration of large foreign body. Cancer and foreign body aspiration account only for a small number of cases. Nowadays bronchoscopy is reserved only when the lung abscess is located in atypical segments or is refractory to therapy.
• Specimen Collection:
  As discussed under specimen collection methods, bronchoscopy is one method by which we can collect specimen for cultures and sensitivity studies.
• Drainage of Pus:
  Bronchoscopy is also useful to drain lung abscess trans-bronchially in selected cases.

Role of Surgery

In the pre-antibiotic era surgery was the only method of therapy for lung abscess. In the modern era there is very limited role for surgery in patients with lung abscess. Most of the lung abscesses are curable with antibiotics and postural drainage. Massive hemoptysis is often the most common indication for surgery in the modern days. In patients with partial airway obstruction, lung abscess may increase in size even with antibiotics and one may have to resort to drainage procedure or surgical resection.