Introduction
This topic covers three lung conditions: lung abscess (a localised pus-filled cavity of necrotic lung), pulmonary gangrene (putrid necrosis of lung tissue that spreads without a wall) and atelectasis (collapse or incomplete expansion of lung).
Lung abscess
A lung abscess is a localised area of suppurative inflammation with liquefactive necrosis of lung tissue, forming a cavity filled with pus and sharply walled off from the neighbouring lung.
- Aspiration of gastric or oropharyngeal contents (alcoholism, coma, anaesthesia, seizures, swallowing disorders). The commonest route; it favours the right lower lobe and the posterior segment of the upper lobe, because of the course of the right main bronchus. Mixed anaerobes are usual.
- Complication of pneumonia: especially Staphylococcus aureus, Klebsiella and Streptococcus pneumoniae type 3.
- Bronchial obstruction (often by a carcinoma) with infection behind it.
- Septic embolism (right-sided endocarditis, septic thrombophlebitis), producing multiple peripheral abscesses; direct spread from neighbouring infection; trauma; immunosuppression.
Morphology. Macroscopically there is a cavity, from a few millimetres to several centimetres, with a ragged inner surface containing yellow-green pus and often with an air-fluid level. Aspiration abscesses are usually single; embolic ones are multiple. Microscopically there is liquefactive necrosis, a mass of neutrophils, a surrounding zone of congestion and oedema, and with time a pyogenic membrane of granulation tissue followed by fibrous capsule (chronic abscess).
Complications and outcome.
- rupture into a bronchus: expectoration of foul pus, which can lead to healing with a scar, or to a chronic abscess;
- rupture into the pleura: empyema, pyopneumothorax;
- haemoptysis, bronchiectasis, amyloidosis (secondary) in chronicity;
- septicaemia, brain abscess by haematogenous spread, metastatic infections.