Epidermal Inclusion Cyst

Updated: Jun 30, 2017
  • Author: Linda J Fromm, MD, MA, FAAD; Chief Editor: Dirk M Elston, MD  more...
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Overview

Background

Epidermoid cysts represent the most common cutaneous cysts. While they may occur anywhere on the body, they occur most frequently on the face, scalp, neck, and trunk. [1]

Historically, epidermoid cysts have been referred to by various terms, including follicular infundibular cysts, epidermal cysts, and epidermal inclusion cysts. The term epidermal inclusion cyst refers specifically to an epidermoid cyst that is the result of the implantation of epidermal elements in the dermis. Because most lesions originate from the follicular infundibulum, the more general term epidermoid cyst is favored. The term sebaceous cyst should be avoided because it implies that the cyst is of sebaceous origin. Finally, the term milia refers to very small, superficial epidermoid cysts.

Epidermoid cysts are benign lesions; however, very rare cases of various associated malignancies have been reported. [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]

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Pathophysiology

Epidermoid cysts result from the proliferation of epidermal cells within a circumscribed space of the dermis. Analysis of their lipid pattern 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 nearly always the infundibulum of the hair follicle, as evidenced by the observation that the lining of the 2 structures is identical. [13]

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

Studies have suggested that human papillomavirus (HPV) and exposure to ultraviolet light (UV) may play a role in the formation of some epidermoid cysts, particularly verrucous cysts with coarse hypergranulosis. [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]

The manner in which carcinomas may arise within epidermoid cysts is unclear. In a series of epidermoid cysts with carcinoma, immunohistochemical results for HPV were negative, suggesting that HPV is not likely to play a role in the development in squamous cell carcinoma (SCC) in epidermoid cysts. Chronic irritation or repetitive trauma to the epithelial lining of the cyst has been suggested to play a role in malignant transformation; however, this relationship has not been established.

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Epidemiology

Race

No racial predilection has been identified. Pigmentation of epidermoid cysts is common in individuals with dark skin. In a study of Indian patients with epidermoid cysts, 63% of the cysts contained melanin pigment. [26]

Sex

Epidermoid cysts are approximately twice as common in men as in women.

Age

Epidermoid cysts may occur at any age; however, they most commonly arise in the third and fourth decades of life. Small epidermoid cysts known as milia are common in the neonatal period.

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Prognosis

Epidermoid cysts are usually asymptomatic; however, they may become inflamed or secondarily infected, resulting in swelling and tenderness. Rarely, malignancies, including basal cell carcinoma, Bowen disease, SCC (most common of these rarities), mycosis fungoides, and melanoma in situ, have developed in epidermoid cysts. [13]

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