How to treat an inflamed and infected sebaceous cyst?

Sebaceous cysts refer to initially harmless lumps that slowly develop under the skin and contain fluid and a semi-solid substance. They typically appear on the scalp, face, trunk, and back, that is, on parts of the body that house numerous sebaceous glands. These lumps usually occur following the rupture of a hair-sebaceous follicle, due to an accumulation of sebum and keratin within the structure. It is also hypothesized that some sebaceous cysts have a genetic origin. The cysts may maintain their initial size or gradually grow. Sometimes, they become inflamed or suddenly rupture, thereby posing a risk of infection.

The interior of a sebaceous cyst, composed of keratin and sebum, is typically protected by a thin membrane, known as a capsule or cystic wall, made of epidermal and keratinized cells, separating the internal mass from the surrounding tissues and the exterior. However, when the surface of the sebaceous cyst is weakened, due to repeated friction or manipulations for example, it can crack and provide an opportunity for bacteria present on the skin surface, such as Cutibacterium acnes and Staphylococcus aureus, to penetrate into the cyst.

Once bacteria are inside, they multiply rapidly due to the sebum and cellular debris they find, which serve as nutrients. In response to bacterial colonization in the sebaceous cyst, the body triggers an immune response, leading to the formation of pus, a mixture of bacteria, cells, and neutrophils, which are white blood cells. However, the pus increases the pressure inside the sebaceous cyst, causing visible swelling. The cyst then becomes red and painful, indicating local inflammation.

It is also possible for a sebaceous cyst to become inflamed even when there is no bacterial colonization. This inflammation, referred to as aseptic, is often the result of the rupture of the cyst's wall within the skin. This event triggers the release of sebum and keratin from the sebaceous cyst into the surrounding tissues, which in turn initiates an inflammatory response. Macrophages are then mobilized to phagocytize these debris, but they create redness and a sensation of heat in the area. The wearing of tight clothing or rubbing at the site of the sebaceous cyst can exacerbate the inflammatory processes by inducing local irritation and facilitating the rupture of the cystic wall, and thus the penetration of bacteria.