Septic Emboli

Aqeel A. Chowdhry, MD

Tan-Lucien H. Mohammed, MD, FCCP

Key Facts

Terminology

Septic embolism occurs when infected purulent material is dislodged and embolizes to lung

Imaging Findings

Nodules or wedge-shaped opacities
- Usually small (< 3 cm diameter) and few in number
- Basilar and peripheral
Cavitation (50%)
- Cavitation more common in nodules than wedge-shaped opacities
- Rapid cavitation, typically over 24-48 hours
- Nodules often in various stages of cavitation: No wall to thin wall
Visible embolus extremely uncommon
Pleural effusions (70%)
CT may be abnormal before blood cultures positive

Top Differential Diagnoses

Pulmonary Emboli
Lung Abscess
Metastases

Pathology

Source of septic emboli
- Indwelling venous catheters
- Periodontal disease
- Skin infections

Clinical Issues

Median duration of symptoms before diagnosis: 18 days

Diagnostic Checklist

Septic embolism in patient with multiple pulmonary nodules and extrapulmonary focus of infection

Axial CECT shows multiple peripheral thick-walled cavitary nodules

from septic emboli.

Axial CECT coronal reconstruction shows distribution and number of cavitary nodules

TERMINOLOGY

Definitions

Septic embolism occurs when infected purulent material is dislodged and embolizes to lung

IMAGING FINDINGS

General Features

Best diagnostic clue: Multiple nodular opacities rapidly evolving into cavitary nodules
Patient position/location: Basilar and peripheral
Size: Usually small (< 3 cm diameter)
Morphology: Nodules more common than wedge-shaped opacities

CT Findings

Morphology
- Nodules
  - Majority < 3 cm diameter although they can grow much larger
  - Edges indistinct, may have halo sign
  - Air bronchograms seen in 25%
  - Average number 15 per patient
- Wedge-shaped opacities
  - Peripheral heterogeneous density
  - Edge may enhance with intravenous contrast
  - Average number 6 per patient
- Cavitation (50%)
  - More common in nodules than wedge-shaped opacities
  - Thickness of cavity wall varies from thick to thin (reflects stages of evolution)
  - Usually no air-fluid level
- Distribution
  - More common in bases (reflects gravity-dependent blood flow)
  - Bilateral, right lung more nodules than left
  - Peripheral (within 2 cm of pleura) (90%)
- Feeding vessel sign
  - Vessel leading directly to edge of nodule, found in up to 60-70% of patients
  - MDCT with reconstructions shows that vessel sign usually represents pulmonary veins coursing next to nodule
Evolution
- Rapid cavitation, typically over 24-48 hours
- Nodules often in various stages of cavitation: No wall to thin wall
- May change in number or appearance (size or degree of cavitation) from day to day
- Resolution: May resolve completely with antibiotic therapy, may also result in small residual linear or nodular scars
Pulmonary artery
- Visible embolus extremely uncommon
Adenopathy (20%)
Pleural effusions (70%)
- May be bilateral or unilateral
- Small common; if loculated consider empyema
Heart
- May see valve vegetations, especially with gated studies
Indwelling central catheters or pacemaker lead common source
- Examine closely for adherent clot (may be source of infection)
May have infection in other organs seen on chest CT (liver abscess, osteomyelitis, renal abscesses)

Radiographic Findings

Radiography: Poorly defined nodular opacities often small and few in number; may be easily missed

Imaging Recommendations

Best imaging tool: CT may be abnormal before blood cultures are positive
Protocol advice: ECG-gated MDCT may be useful to look for valve vegetations

Echocardiographic Findings

Transesophageal echocardiography is procedure of choice to examine valves for vegetations

DIFFERENTIAL DIAGNOSIS

Pulmonary Emboli

Visible emboli in pulmonary artery, uncommon with septic emboli
With infarcts
- Infarcts uncommon, usually require some underlying cardiopulmonary disease
- Pulmonary infarctions rarely cavitate, cavitation common with septic emboli
Pleural effusions nearly always present with infarction, also common with septic emboli

Lung Abscess

Typically have air-fluid level, less common in septic emboli
Evolves more slowly over days and weeks
Fewer in number
Etiology varies
- Periodontal disease
  - Gravity-dependent segments: Posterior segments of upper lobe or superior segments of lower lobes
  - Polymicrobial, especially anaerobic organisms like Peptostreptococcus or Fusobacterium

Metastases

Cavitation seen with
- Squamous cell carcinoma, sarcomas, most common cell types
Do not rapidly evolve and do not respond to antibiotic therapy
Usually more numerous in number
Usually variable in size
Also have feeding vessel sign

Wegener Granulomatosis

Nodules with varying degrees of cavitation
Do not rapidly evolve
May have subglottic tracheal stenosis
Respond to steroid therapy but not to antimicrobial treatment

Rheumatoid Nodules

Nodules uncommon manifestation of rheumatoid arthritis (< 5%)
Also few in number, typically subpleural
Cavitation (50%) thick- or thin-walled
Often associated with pneumothorax
Usually associated with subcutaneous nodules

Foreign Body Embolus

Source: Catheter fragments, vertebroplasty cement, radioactive prostatic seeds
Rarely causes nodules or infraction

Tumor Embolism

Seen with hepatocellular carcinoma, renal cell carcinoma, or any tumor with extension into venous system
Rarely causes infarcts
Smaller tumors may grow and expand vessel, mass maintains shape of vessel
Intraluminal clot similar to venous embolism

PATHOLOGY

General Features