an acute, diffuse, purulent inflammation of adipose tissue. Unlike an abscess, a phlegmon lacks precise boundaries.

Phlegmons are classified by site. They may occur in many parts of the body, including the foot and the hand, and may be subcutaneous, subfascial, intermuscular, retroperitoneal, paranephric (paranephritis), pararectal (paraproctitis), or mediastinal. The causative agents are primarily staphilococci, other pyogenic microorganisms, and, less commonly, Escherichia coli and anaerobes. Phlegmons may also be classified according to the characteristics of the causative agents as suppurative, putrefactive, and anaerobic.

The causative agents penetrate the tissue through breaks in the skin or from nearby foci of infection, such as furuncles, dental caries, or suppurating lymph nodes. Sometimes they are brought to the site hematogenously—that is, through the bloodstream— from foci at some distance.

The symptoms of phlegmon include pain, edema, elevated body temperature, and chills; subcutaneous phlegmon has the added symptom of cutaneous hyperemia. The inflammatory process may spread to neighboring organs, and sepsis may develop.

Phlegmon is treated by exposing and draining the suppurative focus and by antibiotic therapy. It is prevented by keeping the skin clean, attending to minor injuries, and promptly treating pyodermas and other local foci of infection.