A cyst is any fluid-filled closed cavity or sac that is lined by an epithelium. Cysts can be of normal, abnormal, non-neoplastic, or neoplastic tissues and come in several different varieties, including follicular (epidermoid), dermoid, apocrine, and sebaceous. Epidermoid cysts result from the proliferation of epidermal cells that are part of hair follicle in the dermis. Dermoid cysts are rare developmental abnormalities that may occur as single or multiple lesions resembling epidermoid cysts. They often develop along the back, but are also found on the extremities. Apocrine cysts are caused by obstruction of sweat gland ducts and usually have a blue tint. They develop most often on the head, neck and upper areas of the trunk. Sebaceous cysts are uncommon and usually involve the sebaceous glands leading to hair follicles. Epidermoid cysts are sac-like cavities filled with a foul-smelling cheesy or doughy material that consists of sebum and epithelial debris, which may dry in the centre to form a horn. Partial hair loss may be seen on the skin overlying the cyst. Epidermoid cysts grow slowly and usually do not cause symptoms, but they may become inflamed or secondarily infected, resulting in pain and tenderness. In rare cases, basal cell carcinoma develops in epidermoid cysts, which is marked by rapid growth and bleeding. Treatment is surgical excision (cutting out). If an epidermoid cyst is infected, it is first drained and tested for bacterial contamination. If drainage fails to resolve the infection, antibiotics are usually started. Manual rupture of the cyst can result in intense inflammation. Total removal of the cyst is only performed once infection and inflammation have settled.
- Mordecai Siegal. UC Davis Book of Dogs
- Kenneth A Becker, MD. Epidermal Inclusion Cyst
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