Septic pulmonary emboli

Septic pulmonary emboli refer to the embolization of infectious particles (intravascular thrombus containing microorganisms) into the lungs via the pulmonary arterial system. 

Clinical presentation

Symptoms can be not specific but most manifest as a bacteremia ¹⁸ with, dyspnea, chest pain, cough and other respiratory symptoms. Often concurrent symptoms of the extrapulmonary primary infective focus are also present.

Pathology

Septic emboli can originate from different sources ⁵:

• right-sided infective endocarditis, particularly tricuspid valve (occasionally pulmonary valve ¹⁹)
• infection elsewhere in the body (e.g. soft tissue infection) with associated septal defects
• infected deep venous thrombosis
• immunological deficiencies
• infected catheters/lines
  • venous lines ²¹ (e.g. central venous catheters) 
  • pacemaker wires
• post-anginal septicemia ¹⁰
• periodontal disease ¹⁰

Common pathogens include ²²:

• Klebsiella pneumoniae
• Staphylococcus aureus
• Pseudomonas aeruginosa
• Escherichia coli
• Salmonella group B
• Fusobacterium necrophorum (in Lemierre syndrome)

Radiographic features

The clinical context is significant in image interpretation and differential considerations. Most patients will be clinically septic and have positive blood cultures at the time of imaging assessment.

Plain radiograph

Chest x-ray features are nonspecific but may show ³:

• peripheral, lower lobe predominant infiltrative densities: can be unilateral or bilateral
• diffuse bilateral nodular densities (often poorly marginated) in varying stages of cavitation
  • these nodules generally range between 1-3 cm
  • the nodules vary greatly in size, which is a reflection of repeated episodes of embolic shower ¹⁵
  • may increase in number or change in appearance (size or degree of cavitation) on subsequent short-term follow up radiographs ¹²
• accompanying small pleural effusions can be common
• features suggestive of complicating pleura empyemas may be seen

CT

Bilateral abnormalities may be present in as much as 80% of cases ¹⁸.

• feeding vessel sign ⁹: peripheral nodules with clearly identifiable feeding vessels associated with lung abscesses ^2,3
• subpleural nodular lesions or wedge-shaped densities with or without necrosis caused by septic infarcts (these can manifest as cavitary pulmonary infarcts)
  • these may have a dependent, lower zone predication ^13,15
  • the wedge-shaped lesions usually range between 10-20 mm ⁶
  • peripheral nodular densities usually range between 5-35 mm ⁷

Treatment and prognosis

Management focuses on treatment of the underlying infection and any associated complications. 

Complications

Recognized complications include:

• empyema formation ¹⁵
• pneumothorax formation ^16,17

Differential diagnosis

For small and/or different size cavitary lung lesions on HRCT or CT chest, consider:

• cavitary pulmonary metastases
• necrobiotic lung nodules
• granulomatosis with polyangiitis
• pulmonary embolism (thromboembolism)

See also

• pulmonary embolism
• septic-embolic encephalitis