Excerpt from Epidermal Inclusion Cyst

Synonyms, Key Words, and Related Terms: epidermal cyst, epidermoid cyst, epithelial cyst, keratin cyst, sebaceous cyst, milia, basal cell carcinoma, Bowen disease, SCC, mycosis fungoides, Gardner syndrome, flesh-colored subcutaneous nodules, yellow subcutaneous nodules, white subcutaneous nodules, onycholysis, subungual hyperkeratosis, pincer nails, eccrine duct occlusion, HPV infection, comedonal plugging, hypercornification, molluscum contagiosum, seborrheic keratoses, nevi, subungual epidermoid cysts, terminal phalanx epidermoid cysts, female circumcision, rhinoplasty, reduction mammoplasty, dermal grafts, myocutaneous flaps, needle biopsy of the breast, plantar epidermoid cysts, acrosyringeal epithelium, basal cell nevus syndrome, pachyonychia congenita, idiopathic scrotal calcinosis, leukonychia totalis, renal calculi, multiple epidermoid cysts, scrotal epidermoid cysts

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Background

Several different terms have been used to describe epidermal cysts. Epidermal inclusion cyst refers to those cysts that are the result of the implantation of epidermal elements in the dermis. However, many cysts originate from the infundibular portion of the hair follicle, and the more general term, epidermoid cyst, is favored. Milia merely represent miniature epidermoid cysts. The term wen should be reserved for trichilemmal or pilar cysts. Sebaceous cyst is a misnomer, and the term should not be used at all because these cysts are not of sebaceous origin.

Pathophysiology

Epidermoid cysts result from the proliferation of epidermal cells within a circumscribed space of the dermis. They have been shown to not be of sebaceous origin based on the analysis of their lipid pattern, which demonstrates similarities to the epidermis. In addition, epidermoid cysts express cytokeratins 1 and 10, which are constituents of the suprabasilar layers of the epidermis. The source of this epidermis is often the infundibulum of the hair follicle, as evidenced by the observation that the lining of the 2 structures is identical.

Inflammation is in part mediated by the horny material contained in epidermoid cysts. Extracts of this material have been shown to be chemotactic for polymorphonucleocytes.

The manner in which carcinomas may rarely arise within epidermoid cysts is unknown. In a series of epidermoid cysts with carcinoma, immunohistochemical results for human papillomavirus (HPV) were negative, leading the authors to conclude that HPV is not likely to be the cause of squamous cell carcinomas (SCCs) in these cysts. Some have proposed that repetitive trauma and inflammation may play a role.

Mortality/Morbidity

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, Bowen disease, SCC, and even mycosis fungoides, have developed in epidermoid cysts.

Sex

In one study, epidermoid cysts were approximately twice as common in men as in women.

Age

Epidermoid cysts may occur at any time in life, but they are most common in the third and fourth decades of life. Gardner syndrome is an exception; the average patient age at onset is 13 years.

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