|Classification and external resources|
Trichilemmal Cyst - Note the thick durable white shell.
A trichilemmal cyst, also known as a wen, pilar cyst or isthmus-catagen cyst,:779:677 is a common cyst that forms from a hair follicle. They are most often found on the scalp. The cysts are smooth, mobile and filled with keratin, a protein component found in hair, nails, and skin. They may or may not be tender. Trichilemmal cysts may run in families. Rarely, these cysts may grow more extensively and form rapidly multiplying trichilemmal tumors, also called proliferating trichilemmal cysts, which are benign but may grow aggressively at the cyst site. Very rarely, trichilemmal cysts can become cancerous.
Trichilemmal cysts are derived from the outer root sheath of the hair follicle. Their origin is unknown, but it has been suggested that they are produced by budding from the external root sheath as a genetically determined structural aberration. They arise preferentially in areas of high hair follicle concentrations, therefore, 90% of cases occur on the scalp. They are solitary in 30% of cases and multiple in 70% of cases.
Histologically, they are lined by stratified squamous epithelium that lacks a granular cell layer and are filled with compact "wet" keratin. Areas consistent with proliferation can be found in some cysts. In rare cases, this leads to formation of a tumor, known as a proliferating trichilemmal cyst. The tumor is clinically benign, although it may display nuclear atypia, dyskeratotic cells, and mitotic figures. These features can be misleading, and a diagnosis of squamous cell carcinoma may be mistakenly rendered.
Surgical excision is required to treat a trichilemmal cyst. The method of treatment varies depending on the physician's training. Most physicians perform the procedure under local anesthetic. Others prefer a more conservative approach. This involves the use of a small punch biopsy about 1/4 the diameter of the cyst. The punch biopsy is used to enter the cyst cavity. The content of the cyst is emptied, leaving an empty sac. As the pilar cyst wall is the thickest and most durable of the many varieties of cysts, it can be grabbed with forceps and pulled out of the small incision. This method is best performed on cysts larger than a pea which have formed a thick enough wall to be easily identified after the sac is emptied. Small cysts have walls which are thin, and easily fragmented on traction. This increases the likelihood of cyst recurrence. This method often results in only a small scar, and very little if any bleeding.
- Freedberg, et al. (2003). Fitzpatrick's Dermatology in General Medicine. (6th ed.). McGraw-Hill. ISBN 0-07-138076-0.
- James, William; Berger, Timothy; Elston, Dirk (2005). Andrews' Diseases of the Skin: Clinical Dermatology. (10th ed.). Saunders. ISBN 0-7216-2921-0.