What is a Phlegmon?
A phlegmon is an ill-defined, non-drainable inflammatory mass composed of mixed fat and soft tissue without a discrete fluid collection, representing localized inflammation that lacks the organized wall and fluid content characteristic of an abscess. 1
Definition and Imaging Characteristics
The term "phlegmon" has been actively discouraged in modern medical terminology due to its ambiguous definition, with major gastroenterology and radiology consensus guidelines recommending the more precise term "inflammatory mass" instead. 1 The 1998 British Society of Gastroenterology guidelines for acute pancreatitis similarly abandoned the term phlegmon as ambiguous and no longer recommended. 1
Key Distinguishing Features from Abscess
The critical distinction is that a phlegmon lacks a drainable fluid component:
- On CT/MRI: Appears as ill-defined perienteric mesenteric increased signal or attenuation without fluid content (not water attenuation/signal intensity). 1
- On ultrasound: Manifests as an ill-defined hypoechoic mass without a definable wall, typically with detectable internal color Doppler signal. 1
- Contrast enhancement: Shows central enhancement on contrast-enhanced ultrasound, unlike an abscess which demonstrates rim enhancement with a fluid center. 1
In contrast, an abscess contains hypoechoic fluid with a defined (often irregular) wall and peripheral Doppler signal, shows fluid attenuation/signal characteristics, and demonstrates rim enhancement after intravenous contrast. 1
Clinical Context
Phlegmons most commonly occur in:
- Crohn's disease: Dense mesenteric inflammation adjacent to severe mural inflammation or penetrating complications (fistulas, sinus tracts). 1
- Appendicitis: Diffuse inflammation in the right lower quadrant representing complicated appendicitis. 2, 3, 4
- Kawasaki disease: Retropharyngeal edema or non-suppurative phlegmon associated with cervical lymphadenopathy. 1
- Soft tissue infections: Unencapsulated collections of inflammation tracking along tissue planes. 5
Clinical Management Implications
The distinction between phlegmon and abscess is therapeutically critical:
Phlegmon (inflammatory mass): Managed with broad-spectrum antibiotics covering aerobic and anaerobic bacteria (ertapenem, meropenem, imipenem-cilastatin, or piperacillin-tazobactam). 2, 6 Percutaneous drainage is not indicated as there is no drainable fluid component. 1
Abscess: Requires source control through percutaneous drainage or surgical intervention in addition to antibiotics. 2, 6, 7
Antibiotic Duration
For phlegmons with adequate source control (when applicable), antibiotics should not be prolonged beyond 3-5 days postoperatively, with typical duration ranging 5-14 days depending on clinical response. 2
Common Pitfall
The most critical error is misidentifying a phlegmon as an abscess and attempting unnecessary drainage procedures, or conversely, missing an abscess that requires drainage by incorrectly labeling it as a phlegmon. 1 Advanced imaging with contrast-enhanced CT or MRI is essential for accurate differentiation, as clinical examination alone is unreliable. 1, 8