Epidermoid Cysts

Cysts come in several different varieties, including follicular (or 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 glands are uncommon lesions involving the sebaceous glands leading to hair follicles.

Epidermal cysts are the result of growth of epidermal cells within the dermis. Epidermal cysts are common non-cancerous (benign) sac-like cavities filled with a foul-smelling cheesy or doughy material composed with sebum (oily material secreted by sebaceous glands) and epithelial debris. This may dry in the centre to form a horn. The cyst may become secondarily infected. 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. Rarely, malignancies, including basal cell carcinoma developed in epidermoid cysts with rapid growth and bleeding.

Causes of Epidermoid Cysts

True epidermal inclusion cysts result from the implantation of epithelial elements in the dermis and are commonly caused by injuries or a surgical procedure. In humans, even seemingly minor procedures, such as needle biopsy of the breast, have reportedly induced epidermoid cysts.

Treatment of Epidermoid Cysts

Treatment is surgical excision (cutting out). Caution: Manual rupture of the cyst can result in intense inflammation. 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. An anti-Staphylococcal drug is used if cultures and sensitivities cannot be obtained immediately as this is the most common infecting organism. Total removal of the cyst is usually only performed once infection and inflammation have settled.

References:
1. UC Davis Book of Dogs : The Complete Medical Reference Guide for Dogs and Puppies Mordecai Siegal
2. Epidermal Inclusion Cyst Author: Kenneth A Becker, MD, Consulting Staff, Department of Dermatology, Rhode Island Hospital, Memorial Hospital