Phlegmon Definition
A phlegmon is an inflammatory mass composed of mixed fat and/or soft tissue without water attenuation or fluid signal intensity, characterized by dense mesenteric inflammation that lacks a well-defined fluid component and is therefore non-drainable. 1
Core Defining Characteristics
Phlegmon represents unencapsulated collections of inflammation that track along soft tissues, distinguishing it fundamentally from an abscess which has a defined fluid collection with rim enhancement 1, 2
The American Gastroenterological Association specifically describes phlegmon as developing adjacent to severe mural inflammation or penetrating complications, particularly in conditions like Crohn's disease 1
The European Crohn's and Colitis Organisation recommends using the term "inflammatory mass" instead of "phlegmon" to describe ill-defined perienteric mesenteric increased signal or attenuation without fluid content 1
Imaging Features That Define Phlegmon
On ultrasound, phlegmon demonstrates an ill-defined hypoechoic mass without a definable wall with detectable internal color Doppler signal 1
On contrast-enhanced ultrasound, phlegmon shows central enhancement, which is the key feature differentiating it from abscess 1
On CT and MRI, phlegmon appears as ill-defined perienteric mesenteric increased attenuation or signal without conforming to normal peritoneal reflections 1
Critical Clinical Distinction from Abscess
The American College of Radiology recommends using the term "abscess" when there is a defined fluid collection with rim enhancement and/or internal air, which phlegmon lacks 1
Phlegmons typically respond to antibiotics alone without requiring drainage, whereas abscesses require drainage (percutaneous or surgical) in addition to antibiotics 1
Important Terminology Caveat
The British Society of Gastroenterology guidelines discourage the use of the term "phlegmon" due to its ambiguous definition, recommending instead the use of specific anatomic descriptions 1
Despite this controversy, the term remains in clinical use across multiple specialties including gastroenterology, podiatry, and head and neck surgery 1, 2, 3
Common Clinical Contexts
In Crohn's disease, phlegmon terminology describes mesenteric inflammation adjacent to fistulas or strictures, often associated with penetrating complications 1, 4
In diabetic foot infections, phlegmon may describe soft tissue inflammation without discrete abscess formation 1, 2
In head and neck infections, phlegmons can derive from odontogenic sources and may be diffusely located in carotid and parapharyngeal spaces 3
In pancreatic disease, phlegmon represents inflammatory mass formation that can be difficult to differentiate clinically from pancreatic abscess without percutaneous aspiration 5