This patient has an epidermoid cyst which has recently become inflamed and “infected”. I put “infected” in quotes, because technically this is not an infection: When an epidermoid cyst ruptures, it creates a vigorous foreign body inflammatory response – this skin becomes warm, tender, painful, and swells, simulating an abscess.
Incision and Drainage (I & D), will confirm the diagnosis of inflamed cyst, when the cheesy, sometimes odiferous material is evacuated, and this process often leads to rapid resolution of symptoms. These episodes are often misdiagnosed as “infection” of the cyst, but cultures are usually negative and antibiotic treatments is not required. Intralesional steroids can hasten the resolution of symptoms as well.
All this being said, we often prescribe antibiotics- it’s hard not to when you see something so red and inflamed and painful.. it seems almost cruel not to! Also, we as dermatologists, will just call the cyst infected, or call it an abscess. It’s more difficult to explain to patients why we would not call an area that looks angry and red and is painful non-infected. However, I felt I needed to clarify all of this in detail here, for medical education purposes!