Lung Abscess imaging

Characteristics

  1. Cavitating infective consolidation.
  2. Single or multiple lesions.
  3. Bacterial (Staphylococcus aureus, Klebsiella, Proteus, Pseudomonas, TB and anaerobes) or fungal pathogens are the most common causative organisms.
  4. ‘Primary’ lung abscess – large solitary abscess without underlying lung disease is usually due to anaerobic bacteria.
  5. Associated with aspiration and/or impaired local or systemic immune response (elderly, epileptics, diabetics, alcoholics and the immunosuppressed).

Clinical features

  1. There is often a predisposing risk factor, e.g. antecedent history of aspiration or symptoms developing in an immunocompromised patient.
  2. Cough with purulent sputum.
  3. Swinging pyrexia.
  4. Consider in chest infections that fail to respond to antibiotics.
  5. It can run an indolent course with persistent and sometimes mild symptoms. These are associated with weight loss and anorexia mimicking pulmonary neoplastic disease or TB infection.

Radiological features

  1. Most commonly occur in the apicoposterior aspect of the upper lobes or the apical segment of the lower lobe.
  2. CXR may be normal in the first 72 h.
  3. CXR – a cavitating essentially spherical area of consolidation usually >2 cm in diameter, but can measure up to 12 cm. There is usually an air-fluid level present.
  4. Characteristically the dimensions of the abscess are approximately equal when measured in the frontal and lateral projections.
  5. CT is important in characterising the lesion and discriminating from other differential lesions. The abscess wall is thick and irregular and may contain locules of free gas. Abscesses abutting the pleura form acute angles. There is no compression of the surrounding lung. The abscess does not cross fissures. It is important to make sure no direct communication with the bronchial tree is present (bronchopleural fistula).

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Fig: Lung abscess – frontal and lateral views. Cavitating lung abscess in the left upper zone.

Differential diagnosis

  1. Bronchopleural fistula – direct communication with bronchial tree. Enhancing split pleural layers on CT.
  2. Empyema – enhancing split pleural layers, forming obtuse margins with the lung on CT.
  3. Primary or secondary lung neoplasms (e.g. squamous cell carcinomas) these lesions can run a slow indolent course. Failure to respond to antibiotic therapy should alert the clinician to the diagnosis.
  4. TB (usually reactivation) – again suspected following slow response to treatment. Other findings on the CT may support old tuberculous infection such as lymph node and/or lung calcification. Lymphadenopathy, although uncommon, may be present on the CT scans in patients with lung abscesses. It is therefore not a discriminating tool for differentiating neoplasms or TB infection.

Management

  1. Sputum – M, C & S.
  2. Protracted course of antibiotics is usually a sufficient treatment regime.
  3. Physiotherapy may be helpful.
  4. Occasionally percutaneous drainage may be required.
  5. Lastly, some lesions failing to respond to treatment and demonstrating soft tissue growth or associated with systemic upset (e.g. weight loss) may need biopsy. This is done to exclude underlying neoplasm (e.g. squamous cell carcinoma).

Reference:

  • Andrew P., Mangerira C. and Rakesh R. A-Z Chest Radiology, pp.22-24

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