1. Distribution: trunk, face, ears, neck, and scalp

2.Palpable as an encapsulated dermal lesion

Clinical Presentation

Epidermoid cysts are slope-shouldered or dome-shaped nodules 1-4 cm in diameter. They have a smooth surface. Lesions that have not been traumatized are skin colored or, if close enough to the surface, slightly white in hue. Traumatized lessions may become inflamed and tender. A central pore is sometimes visible at the summit of the nodule. Palpation reveals a well-defined (encapsulated) spherical nodule that feels as though it lies deep within the skin even beneath the skin, in the subcutaneous tissue. The surface skin mayor may not be independently movable over the nodule. Small lesions have a very firm feel, whereas large ones may be slightly fluctuant. Epidermoid cysts may be confused with lipomas, but the latter feel softer and more lobular.

Epidermoid cysts are most commonly found on the trunk, neck, face, and scalp. Those that occur on the scalp (trichilemmal or pilar cysts) are colloquially known as "wens."

Milia represent a subtype of epidermoid cyst. They are very small (less than 2 mm in diameter) and are bright white in color.

Confirmation of a clinical diagnosis is possible by way of a stab incision into the nodule. Expression of semisolid white material through the incision documents the keratinous nature of the nodule.

Uncommon Clinical Presentations. True sebaceous cysts (those filled with sebum rather than keratin) are seen only in the rare familial syndrome of stealocystoma multiplex. Epidermoid cysts represent one of the cutaneous components in the nevoid basal cell carcinoma syndrome and in Gardner's syndrome. On very rare occasions, microscopic evidence of basal or squamous cell carcinoma may be found on histologic examination of the lining of otherwise-typical epidermoid cysts.

Course and Prognosis

Epidermoid cysts, when they first appear, enlarge over weeks or months to a given size, thereafter, unless rupture occurs, they remain unchanged and asymptomatic. Chronic irritation or direct trauma occasionally causes rupture and a resultant marked inflammatory response .

Pathogenesis

Epidermoid cysts probably arise from embryologic remnants of malformed hair follicles. At some point in adult life the epithelial cells of these remnants begin making keratin. Some cysts open to the surface through a thin follicular orifice, but in spite of a follicular origin, most have no continuity with the surface. A small minority of epidermoid cysts occur as the result of traumatic implantation of epithelial fragments. These buried bits of epithelium then "round up" and begin producing keratin. Such cysts are termed inclusion cysts.

As is probably evident at this point, the old terminology of "sebaceous" cysts is erroneous, since the cells that line epidermoid cysts produced semisolid white keratin rather than colorless, oily sebum.

Therapy

Asymptomatic cysts require no medical attention. Those that are cosmetically unacceptable and those that are repeatedly traumatized can be treated in a variety of ways. For small lesions, elliptical excision with suture closure is appropriate and definitive. Some cysts, particularly those on the scalp, can be "delivered" through a simple incision, but cyst rupture and resultant incomplete removal of the wall sometimes complicate the procedure. For large cysts, it is perhaps best to incise the cyst and extrude the contents in an initial procedure. Over the next several weeks the cyst shrinks considerably in size. A subsequent, much smaller, excisional removal can then be carried out.