Write a short note on lung abscess.

Definition

A localised area of suppuration and necrosis within the lung parenchyma producing a cavity containing pus, usually with an air-fluid level. PRIMARY (normal lung, usually aspiration or immunosuppression) or SECONDARY (bronchial obstruction, septic embolism, adjacent spread); acute (< 6 weeks) or chronic.

Aetiology

1. ASPIRATION — by far the commonest mechanism. Risk factors are those causing impaired consciousness or swallowing: alcohol, general anaesthesia, seizures, stroke, head injury, drug overdose, dysphagia, reflux, nasogastric tubes, and POOR DENTAL HYGIENE AND PERIODONTAL DISEASE, which supplies the inoculum. Edentulous patients rarely develop anaerobic lung abscess — a useful pointer.

The SITE follows GRAVITY: supine — POSTERIOR SEGMENT OF THE RIGHT UPPER LOBE and SUPERIOR SEGMENT OF THE RIGHT LOWER LOBE; upright — basal segments. The RIGHT lung is affected more often because the right main bronchus is wider, shorter and more vertical.

2. Other mechanisms:

• bronchial obstruction by carcinoma (always consider malignancy in an older smoker), foreign body or nodes
• haematogenous — septic emboli in right-sided endocarditis and intravenous drug use (multiple bilateral peripheral cavities), Lemierre's syndrome
• direct extension — amoebic liver abscess rupturing into the chest is an important cause in India
• infection of a pre-existing cavity or bulla.

Organisms: ANAEROBES (commonest in aspiration) — usually polymicrobial oral flora; aerobes — *S. aureus* (including MRSA), *Klebsiella* (classically alcoholics and diabetics, with upper lobe cavitation and a "bulging fissure"), *Pseudomonas*, *Nocardia*. In India always consider *M. TUBERCULOSIS*, melioidosis, amoebic abscess and fungi.

Clinical Features

• INSIDIOUS onset over weeks in anaerobic abscess (acute in *Staphylococcus* and *Klebsiella*)
• High swinging fever with rigors, night sweats and marked weight loss
• Cough with COPIOUS FOUL-SMELLING (PUTRID) PURULENT SPUTUM — the foul odour is virtually diagnostic of ANAEROBIC infection
• A sudden large expectoration ("vomica") as the abscess ruptures into a bronchus
• HAEMOPTYSIS; pleuritic pain
• Signs: CLUBBING (may develop within weeks — unusually rapidly), anaemia, poor dentition; localised dullness, bronchial or amphoric breathing, coarse crackles

Investigations

• CHEST X-RAY — a cavity with a thick irregular wall and an AIR-FLUID LEVEL (visible only on an erect film), in the characteristic aspiration segments
• CT CHEST — superior; distinguishes abscess from empyema (abscess: thick irregular wall, acute angle with the pleura, no lung compression; empyema: smooth thin walls, obtuse angle, "split pleura", compressed lung), and identifies underlying obstruction or malignancy
• Sputum — Gram stain and culture, AFB, CBNAAT and mycobacterial culture (essential in India), fungal studies
• Blood cultures; CBC (neutrophil leucocytosis and anaemia), inflammatory markers, blood glucose, HIV testing
• BRONCHOSCOPY — indicated where there is suspected obstruction, atypical location, no response, or in any older smoker — to exclude carcinoma and foreign body
• Echocardiography for suspected septic embolism; amoebic serology and ultrasound where the right lower zone is involved

Differential of a Cavitating Lesion

Infective: lung abscess, TUBERCULOSIS (the commonest cause of a cavity in India), necrotising pneumonia, fungal (aspergilloma), melioidosis, hydatid. Malignant: cavitating squamous cell carcinoma, metastases. Vasculitic: granulomatosis with polyangiitis, rheumatoid nodules. Vascular: infarcted embolus.

Management

1. Prolonged antibiotics — the mainstay; most abscesses resolve medically

• Cover ANAEROBES and streptococci — a beta-lactam with beta-lactamase inhibitor, or clindamycin. Metronidazole must NOT be used alone, as it misses microaerophilic streptococci
• Duration is PROLONGED — typically 4–6 weeks or until radiological resolution; premature discontinuation is the commonest cause of relapse
• Antitubercular therapy if TB is confirmed or strongly suspected

2. Drainage — postural drainage and physiotherapy; percutaneous CT-guided catheter drainage for large abscesses (> 6–8 cm) or failure to respond after 5–7 days; bronchoscopic drainage in selected cases.

3. Surgery — rarely required; for failure of medical therapy, massive haemoptysis, suspected carcinoma, bronchopleural fistula or empyema requiring decortication.

4. Supportive and preventive — nutrition, glycaemic control, treatment of dental and periodontal disease (essential to prevent recurrence), management of dysphagia and reflux, alcohol cessation.