Lung Abscess
PKGhatak, MD
In any infection, the body tries to limit the spread of infection by pouring in inflammatory cells at the site of entry of the infective organisms. A dense layer of inflammatory cells, white cells and their products are formed around the infection is known as a pyogenic membrane. In the center of it, the dead and liquefied cells accumulate. In the lungs, a few smaller branches of bronchioles remain open and air enters the liquefied center giving a distinct picture of air-fluid level on the x-rays. These bronchioles, however, are too narrow to drain the thick accumulated secretion. Unless the cellular derbies are cleared from the site, the repair and healing cannot begin. This may lead to one or more complications - enlargement of abscess, breaking open into a larger bronchiole forming a pulmonary fistula, opening in the pleural space forming empyema, eroding into a blood vessel producing hemoptysis, the distant heterogeneous spread of infection and abscess formation in other organs.
The lung abscess is called primary when the lung is directly infected. An abscess is called a secondary abscess when the lung infection comes from an infected blood clot from below the diaphragm via the inferior vena cava and infected cardiac vegetation. From direct contact with the lung as it happens in amoebic liver abscess, and subphrenic abscess.
The usual source of lung abscess is the aspiration of anaerobic bacteria of the mouth from choking, general anesthesia, dental procedures, alcoholism, seizures, reflux esophagitis, hiatal hernia, esophageal-pulmonary fistula, head and neck cancers, strokes with dysphagia, bulbar palsy, Parkinson's disease, ALS, etc.
Abscess formation can also develop from preexisting cavities in the lungs as cystic fibrosis, emphysema bullae, bronchiectasis, tuberculosis, carcinoma, foreign body- specially partial dentures, loose tooth, small toys in children, and intra- pulmonary sequestration and bronchial cysts - are congenital conditions. Narcotizing vasculitis and infarction of lung from Pulmonary emboli.
Common Bacteria.
Anaerobic gram negative and anaerobic gram positive bacteria predominate in adults. Staphylococcus aureus, including MRSA, is a common bacterium in children. Other rare infections are fungus, nocardia, mycobacteria, actinomyces, lung parasites, and amoeba.
Bacteroides fragilis, Fusobacterium capsulatum and F.nacrophorum are the most frequent causes of anaerobic lung abscess. Occasionally gram positive Pepto streptococcus and microaerophilic streptococcus infections are seen. Mixed bacterial infection is also frequent. In rare instances gram negative rods like E. coli, Hemophilus influenza, Pseudonymous aeruginosa are seen specially in hospitalized patients with ventilator associated lung abscesses.
Pathology.
In the first 48 hrs. following aspiration inflammatory exudate takes place. Toxins released by the bacteria initiate vasculitis and venous thrombosis and necrosis of lung tissues.
Common areas of the lung are prone to abscess formation.
Lobes involved in abscesses are the result of aspiration to the most dependent part of the lung. In a bedridden patient, the posterior segment of the right middle lobe generally develops an abscess. In alcoholic and unconscious patients' apical segments of the lower lobes are involved.
Signs and symptoms.
Patients are sick and a distinct foul odor due to anaerobic bacteria is often detected. Chills, fever, night sweats, loss of significant weight are common. Chest pain and shortness of breath are usual. Cough may start as non-productive but soon becomes productive thick pus, at times blood mixed or frank hemoptysis is seen.
Clubbing of fingers is often seen. Fixed rales and other auscultation features may be present based on how close the abscess is to the chest wall.
Diagnosis.
Chest x-ray shows air-fluid level in a thick-walled cavity, rarely, more than one abscess is present. CT scans often are not required for diagnosis but are needed in case surgical treatment is deemed necessary.
Bronchoscopy is mandatory to detect bronchial obstruction by a tumor or foreign body. And to obtain bronchial secretion/pus for both anaerobic and aerobic cultures, gram stain, fungal and mycobacteria cultures, Bronchoscopic biopsy, brushing and cytology are routinely performed.
Cultures.
Aerobic and anaerobic cultures and sensitivity tests are most valuable in selecting proper antibiotics.
Treatment.
As soon as a lung abscess is suspected or detected on x-rays, intravenous antibiotics are initiated. The initial choice of antibiotic is Clindamycin 600 mg given every 8 hrs.; some physicians also add another agent like metronidazole and beta lactam cephalosporin, Fluoroquinolone. Macrolide antibiotic is used in mixed bacterial infections. Antibiotic coverage depends very much on the prevalence of common lung abscess bacteria of the local community and reports of culture and drug sensitivity tests.
It is to be noted that if proper antibiotics are chosen, a clinical improvement is expected in 3 to 5 days – lowering of temperature, less sputum production and a general sense of wellbeing.
The antibiotic coverage should be continued till the patient is afebrile and nearly symptoms free. That may take 3 to 6 weeks. At that stage, antibiotics are usually switched to orally.
In addition to IV antibiotics, chest physiotherapy, postural drainage, chest percussion, bronchodilator therapy, etc. are prescribed including the attention paid to improve nutrition.
Switching mode of therapy.
If the expected improvement is not forthcoming at the end of 2nd week of therapy, then further assessment is necessary with repeat CT scans and cultures.
If the size of the abscess is seen to have enlarged to 6 cm or more, then some forms of surgical interventions are required. The less invasive procedure was tried first. CT guided or Ultrasound guided transcutaneous drainage of the abscess is performed under local anesthesia.
If more than 12 weeks have passed and still the abscess remains then lobectomy is generally required.
Those patients respond well to the initial therapy, and a complete resolution of abscess takes place in 3 to 6 weeks. Complete resolution of abscess by chest x-ray usually takes 12 weeks, even though the patient is recovered clinically and symptoms free from having an abscess.
****************************************
