Last revised by Daniel J Bell on 10 Jul 2022

Phlegmon (plural: phlegmons) refers to soft connective tissue inflammation, usually in the context of infectious disease. It is distinct from an abscess, which is a collection of pus walled-off by granulation tissue.

Historically and contemporaneously, phlegmon is used in an inconsistent fashion. The Oxford English Dictionary documents that it can mean an inflammatory mass or localized area of inflammation; diffuse, spreading inflammation; or cellulitis 1. The variant "phlegmonous change" may be used instead of "phlegmon" to suggest an ill-defined rather than mass-like process, but this distinction is not strict. 

Cellulitis commonly carries a narrower dermatologic connotation of a spreading skin infection, while phlegmon is more often applied toward non-cutaneous soft tissue sites and a more advanced stage of inflammation, though one which has not yet developed into an encapsulated or liquefied abscess. Phlegmon is variously used to describe sterile and infected entities 2. The usage may also differ by body part:

In acute pancreatitis, the revised Atlanta classification recommends against the use of phlegmon, in favor of specific terms for pancreatic/peripancreatic collections that specify the presence of a definable wall, necrosis, and infection 3.

In acute appendicitis, appendiceal phlegmon is a form of complicated appendicitis defined as an inflammatory mass (including the appendix, adjacent viscera, and greater omentum) without pus 4. However, different radiological studies have defined phlegmon differently and often simply referred to severe periappendiceal fat stranding and ill-defined periappendiceal fluid 5.

In acute colonic diverticulitis, phlegmonous diverticulitis is a form of uncomplicated/mild diverticulitis defined by pericolic inflammation without abscess 6.

In acute tonsillitis or pharyngitis, peritonsillar phlegmon or cellulitis is an intermediate stage between uncomplicated tonsillitis and peritonsillar abscess, defined as inflammation of the tissue between the tonsillar capsule and pharyngeal muscles without a discrete purulent collection and thus typically managed medically 7,8.

In musculoskeletal imaging, the use of phlegmon has been discouraged as, once it is clear that the pathology in question is not an abscess, it can be better described by specifying the soft tissue involved, whether skin (cellulitis), fascia (fasciitis) or muscle (myositis) 10.

Contrast administration is necessary to distinguish phlegmon from an abscess. Phlegmon is an area of soft tissue or fluid density effacing or displacing usual fat or other connective tissue and shows variable enhancement. However, an abscess typically has a well-defined, complete rim of enhancement around a non-enhancing core.

The word phlegmon derives from the French "flegmon/fleugmon", in turn from Latin phlegma, which was one of the four bodily humors of ancient medicine. These terms in turn come from the ancient Greek, ϕλέγμα (transliterated as phlegma), which translates as inflammation; this itself came from ϕλέγειν (transliterated as phlegein), which means "to burn" and is also the origin of the English word phlegm 1,9.

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