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Describe lung abscess with its causes, morphology and complications.

Pathology · 2nd Professional — Para-Clinical

Definition

A lung abscess is a localised area of suppurative necrosis of pulmonary parenchyma resulting in one or more cavities containing pus.

Causes and Routes of Infection

Route	Details
Aspiration — the commonest	Of oropharyngeal contents; in unconsciousness, alcoholism, anaesthesia, stroke, epilepsy, poor dentition, oesophageal disease. Usually mixed anaerobes from the mouth
Complication of pneumonia	Staphylococcus aureus, klebsiella, Streptococcus pyogenes, type 3 pneumococcus
Bronchial obstruction	Carcinoma (always to be excluded in an adult), foreign body, mucus plug — infection develops distal to the obstruction
Haematogenous (septic emboli)	Right-sided infective endocarditis, septic thrombophlebitis, intravenous drug use; gives multiple and bilateral abscesses
Direct extension	From empyema, subphrenic or hepatic abscess (amoebic liver abscess rupturing through the diaphragm)
Trauma	Penetrating chest injury

Aspiration abscesses are commoner on the right, because the right main bronchus is wider, shorter and more vertical
Site depends on posture at the time of aspiration — the posterior segment of the upper lobe and the apical segment of the lower lobe when supine

Morphology

A cavity containing pus, surrounded by a zone of suppurative inflammation, then organising pneumonia, then fibrosis in chronic cases
If it ruptures into a bronchus the pus is expectorated, leaving an air-fluid level visible on an erect radiograph
Chronic abscesses develop a fibrous wall and may be lined by squamous metaplastic epithelium
Rupture into the pleura causes empyema or a bronchopleural fistula

CLINICAL PEARL

A lung abscess in an adult over 40 is a bronchial carcinoma until proved otherwise. Tumour obstructs a bronchus, secretions stagnate behind it, and infection follows. Treating the infection alone lets the abscess recur in exactly the same place. Bronchoscopy is mandatory in that setting, and in any abscess that fails to resolve.

Clinical Features and Complications

Swinging fever with rigors, malaise and weight loss
Cough with copious, foul-smelling, purulent sputum, often expectorated suddenly in large amounts when the abscess ruptures into a bronchus
Haemoptysis, pleuritic pain, clubbing in chronic cases
Complications — empyema, bronchopleural fistula, massive haemoptysis, metastatic brain abscess from septic emboli, secondary AA amyloidosis, and bronchiectasis

Applied Aspects

Chest radiograph shows a cavity with an air-fluid level; CT defines it better and may show an underlying tumour
Differential diagnosis of a cavity — tuberculosis (especially in India), cavitating carcinoma, infected bulla, hydatid cyst, fungal infection, granulomatosis with polyangiitis, and pulmonary infarct
Prolonged antibiotics for 4 to 6 weeks with anaerobic cover, plus postural drainage and physiotherapy; most resolve without surgery
Drainage or resection for failure to respond, very large abscesses, or suspected malignancy
Prevention — dental hygiene, care with the unconscious patient, and attention to swallowing after a stroke

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