These are cysts containing cornified cells and their breakdown products, surrounded by an epidermoid wall.

The cyst is normally situated in the dermis, raising the epidermis to produce a firm, elastic, dome-shaped swelling which is mobile over the deep structures. It is tethered to the epidermis and there may be a central keratin-filled punctum. Cysts near the surface, such as on the scrotum or ear lobe, are yellowish and white.

The size of the lesion varies from a few millimetres to 5 cm or more. The common sites are the areas favoured by acne and the lesions may be solitary, but are commonly multiple ..

They enlarge slowly and may, at times, become inflamed and tender; suppuration may follow and such lesions are often more difficult to remove completely. Of uncomplicated epidermoid cysts, 73 per cent have been shown to contain the common skin commensals Staphylococcus epidermidis and Propionibacterium acnes, type I.5 This confirms that these cysts, which have a punctum, may, under normal circumstances, become colonized.

Calcification of the contents of the cyst, although not confirmable clinically, may be so radiologically..

Epidermoid cysts may also arise following trauma which results in the implantation of epidermis within the dermis. These traumatic cysts usually occur at exposed sites such as on the palm, sole, buttocks and knees..

Histology of epidermoid cysts shows a unilocular spherical cyst in the dermis. The lining reproduces the layers of the epidermis, although these may be attenuated in large cysts. The keratin is lamellated and birefringent and cholesterol clefts may be seen. If rupture of the cyst has occurred, chronic inflammation of the foreign-body type of reaction may be detected and occasionally a few small hairs may be found coiled up within the cyst..

A rare complication is intrafollicular Bowen's disease. This was reported in an 81-year-old female with a rapidly growing epidermoid cyst.

The diagnosis of epidermoid cysts is rarely difficult and noninflamed cysts are normally easily treated by excisional biopsy.

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