Lung Abscess — SAQ

Questions

a) What is the diagnosis, and what are the two key risk factors that produced it? (2 marks) [1]

Diagnosis: aspiration (pyogenic) lung abscess — a localised collection of pus in a cavitating area of lung parenchyma, with an air-fluid level on imaging.[1]

The two key risk factors are chronic alcoholism (depressed consciousness, impaired airway protection, vomiting) and poor dentition (which raises the bacterial load of the aspirated oropharyngeal secretions with anaerobes). [1]

b) Outline the pathogenesis of this abscess. (2 marks) [1]

Aspiration of oropharyngeal contents carrying mixed anaerobic flora (Bacteroides, Prevotella, Fusobacterium, Peptostreptococcus, Actinomyces) into the posterior segment of the upper lobe — a dependent segment in a recumbent patient.[1] Alveolar macrophages are overwhelmed and release IL-1 and TNF-alpha, recruiting neutrophils whose proteases and reactive oxygen species cause liquefactive necrosis of the parenchyma. The necrotic core drains into a bronchus (producing fetid sputum) and air enters, forming the air-fluid level. The right lung is commoner because the right main bronchus is shorter, wider and more vertical.

c) Give the empiric antibiotic regimen — drug, dose, route, and duration. (3 marks) [1]

• Clindamycin 600 mg IV three times daily OR amoxicillin-clavulanate 1.2 g IV three times daily.[1]
• Rationale: cover anaerobes and microaerophilic streptococci; high lung penetration.
• Duration 4 to 6 weeks, switching to oral when the patient is afebrile, improving and with a falling CRP.
• Metronidazole must NOT be used alone — it lacks reliable activity against microaerophilic streptococci and Actinomyces.

d) When would you escalate to drainage, and by what method? (2 marks) [1]

Drainage is indicated for a large abscess (over 6 cm), failure to improve after 1 to 2 weeks of appropriate antibiotics, imminent rupture into the pleura, the immunocompromised, or severe sepsis. The method of choice is percutaneous CT-guided catheter drainage.[2] Surgery (lobectomy/pneumonectomy) is reserved for failure of antibiotics and drainage, massive haemoptysis, suspected malignancy, or a bronchopleural fistula.

e) What is the single most important investigation if the abscess fails to resolve, and why? (1 mark) [1]

Bronchoscopy — to exclude an obstructing bronchogenic carcinoma or foreign body (post-obstructive abscess), which cannot be cured by antibiotics until the obstruction is relieved.[1] Also send sputum for AFB and GeneXpert to exclude tuberculosis.