Epidermoid Cyst (Infundibular Cyst)
Keratin-filled cysts of the skin are common lesions that arise from hair follicles. The most common example is the epidermoid cyst, which is derived from the follicular infundibulum. Such lesions may develop secondary to inflammation of the hair follicle, and they are found most frequently in sites prone to acne, such as the head, neck, and back. Although follicular cysts of the skin often are referred to as sebaceous cysts, this term is a misnomer because neither the epidermoid cyst nor the pilar cyst (see next topic) arises from sebaceous glands. Another type of keratin-filled cyst of the skin is unrelated to hair follicles but instead arises secondary to traumatic implantation of the surface epithelium. Such lesions also can develop on oral mucosa and are designated as epithelial (or epidermal) inclusion cysts.
Epidermoid cysts on the skin occur more frequently in males than in females. They present as nodular subcutaneous growths that are often fluctuant to palpation. If the cyst becomes inflamed or infected, then the epidermal surface will appear red. Rupture of the cyst may release keratinaceous material, which typically will elicit a pronounced foreign-body response, resulting in pain and swelling. Epidermoid cysts rarely develop during childhood unless the patient has Gardner syndrome. Younger adults are more likely to develop these cysts on the face, whereas older adults are more likely to have them on the back. Epithelial inclusion cysts related to implanted epithelium usually appear as small, yellowish-white papules.
Epidermoid and epithelial inclusion cysts usually are treated by conservative surgical excision, and recurrence is uncommon. Extremely rare examples of malignant transformation of epidermoid cysts have been reported.
Yellow keratin-filled cyst located on the earlobe. (from Color Atlas of Oral and Maxillofacial Diseases)
Yellowish-white papule on the lateral tongue secondary to traumatically implanted epithelium. (from Color Atlas of Oral and Maxillofacial Diseases)
In addition to the epidermoid cyst, the pilar cyst is the second type of follicular cyst that arises from the outer sheath of the hair follicle. Also known as a trichilemmal cyst or isthmus-catagen cyst, it comprises approximately 10% to 15% of skin cysts. Unlike the epidermoid cyst, the pilar cyst occurs most frequently on the scalp and exhibits a female predilection. A tendency to develop such cysts may run in families, and some patients will develop multiple lesions. Pilar cysts appear as movable nodules that typically shell out easily when surgically removed.
Nodular mass located at the edge of the scalp line. (from Color Atlas of Oral and Maxillofacial Diseases)