What is a phlegmon?

What is a Phlegmon?

A phlegmon is an ill-defined inflammatory mass of mixed fat and/or soft tissue density (not water density) usually associated with penetrating disease, such as complex fistulas. The term "phlegmon" is discouraged in modern medical practice in favor of more precise terminology like "inflammatory mass." 1

Definition and Characteristics

• A phlegmon traditionally refers to an unencapsulated collection of inflammation that tracks along soft tissues in various parts of the human body 2
• In gastrointestinal imaging, the term describes an inflammatory mass without a drainable component, as opposed to an abscess which contains fluid that can be drained 1
• Current guidelines discourage the use of the term "phlegmon" due to its ambiguous definition, recommending "inflammatory mass" instead 1
• An inflammatory mass appears as an ill-defined mass-like process of mixed fat and/or soft tissue attenuation/signal intensity, not water attenuation/signal intensity 1

Radiological Appearance

• On ultrasound, a mesenteric inflammatory mass typically appears as an ill-defined hypoechoic mass without a definable wall and usually has detectable internal color Doppler signal 1
• On MRI and CT, an inflammatory mass manifests as ill-defined perienteric mesenteric increased signal or attenuation, respectively, without fluid content 1
• In contrast, an abscess shows signal or attenuation characteristics of fluid and typically demonstrates rim enhancement after intravenous contrast 1
• Contrast-enhanced ultrasound may aid in differentiating between a mesenteric mass and abscess, with the inflammatory mass demonstrating central enhancement 1

Clinical Significance

• Inflammatory masses are commonly associated with penetrating complications of Crohn's disease 1, 3
• They can also develop in other contexts, such as in the head and neck region (often odontogenic in origin) 4, urinary tract 5, or lower extremities 2
• The presence of an inflammatory mass should be documented in all imaging reports, including dimensions, enteric location, and relationship to adjacent structures 1
• Inflammatory masses without drainable components are typically treated medically rather than with percutaneous drainage 1

Differential Diagnosis from Abscess

• Unlike an inflammatory mass, an abscess contains hypoechoic fluid, has a defined wall (often irregular), and peripheral Doppler signal in the wall 1
• An abscess shows rim enhancement after intravenous contrast and restriction on diffusion-weighted imaging 1
• The distinction is clinically important as abscesses generally require drainage while inflammatory masses are typically treated with antibiotics and other medical therapy 1

Management Considerations

• Management of inflammatory masses typically involves antibiotics and, in cases of Crohn's disease, may include anti-TNF therapy after infection has been controlled 3
• Surgical intervention may be necessary depending on the location and clinical context 6, 4
• In Crohn's disease, inflammatory masses may respond to medical therapy, though more slowly than acute inflammatory changes 7
• The presence of an inflammatory mass should be communicated to the clinical care team as it may influence treatment decisions 1

Importance of Precise Terminology

• The 2018 consensus recommendations from Gastroenterology specifically discourage the term "phlegmon" due to its ambiguity 1
• Using more precise terminology helps clarify whether there is a drainable component (abscess) or not (inflammatory mass) 1
• This distinction is crucial for appropriate clinical management decisions 1