Epidermal inclusion cyst

-          refers to those cysts that are the result of the implantation of epidermal elements in the dermis.

-          many cysts originate from the infundibular portion of the hair follicle, and the more general term, epidermoid cyst, is favored.

-          Sebaceous cyst is a misnomer, and the term should not be used at all because these cysts are not of sebaceous origin.

Pathophysiology

-          results 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.

-          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.

Causes

Epidermoid cysts likely form by several mechanisms.

-          Congenital epidermoid cysts of the anterior fontanelle or those that are orogenital in location presumably result from sequestration or trapping of epidermal rests along embryonic fusion planes during development.

-           Lip and lingual lesions may be related to aberrant fusion of the branchial arches, and genital lesions could result from improper closure of the genital folds.

-          Any benign or malignant process affecting or growing near the pilosebaceous unit may lead to occlusion or impingement of the follicular ostia and subsequent formation of a cyst.

-          Many cysts with an acneiform distribution are probably the result of follicular occlusion.

-          True epidermal inclusion cysts result from the implantation of epithelial elements in the dermis.

• Injuries, especially of the crushing type, such as the slamming of a car door on a finger, are frequently reported in association with subungual or terminal phalanx epidermoid cysts.
• Female circumcision is associated with the formation of epidermoid cysts, perhaps from instruments that are not sharp or from imprecise cutting.
• Theoretically, any surgical procedure may result in epidermoid cysts, and it is surprising that they are not a more common occurrence. Unusual examples of this mechanism include the formation of multiple epidermoid cysts after rhinoplasty and reduction mammoplasty. The use of dermal grafts, presumably because of the inclusion of epithelial elements, has also resulted in the formation of epidermoid cysts. A similar situation has been observed with the use of myocutaneous flaps where the cutaneous portion is buried. Even seemingly minor procedures, such as needle biopsy of the breast, have reportedly induced epidermoid cysts.