Pilar Cyst (Trichilemmal Cyst)
Pilar cysts are relatively common benign growths that occur most frequently on the scalp. They go by many names, including wen, trichilemmal cyst, and isthmuscatagen cyst. Most are solitary, but it is not uncommon to see multiple pilar cysts in a single individual. Their appearance is similar to that of epidermal inclusion cysts, but the pathogenesis is completely different. There is a malignant counterpart called a metastasizing proliferating trichilemmal cyst. The malignant trans- formation of a pilar cyst is exceedingly rare. Subsets of these growths are inherited.
Clinical Findings: Pilar cysts occur most frequently on the scalp. They can be mistaken for epidermal inclusion cysts. The main clinical differentiating points are that pilar cysts do not have an overlying central punctum, and they tend to be a bit firmer to touch. These cysts occur more commonly in adults, and they have a tendency to affect women more often than men. They typically manifest as slowly growing, firm dermal nodules with no overlying epidermal changes and no central punctum. These cysts do not drain, as epidermal cysts sometimes do. They also rarely get inflamed. Almost exclusively found in the scalp, they are for the most part asymptomatic. Patients present to the clinician because of an enlarging nodule. As opposed to the epidermal inclusion cyst, which essentially has no malignant potential, the pilar cyst does have a small proliferating and malignant potential. This risk is very low.
Some families show an autosomal dominant inheritance pattern. The exact gene defect has yet to be determined, but a possible gene has been mapped to chromosome 3. Most patients with the hereditary version of this condition have solitary lesions. Numerous lesions are infrequently encountered in the inherited form.
Pathogenesis: Pilar cysts are also called trichilemmal cysts, because they are derived from the outer root sheath of the hair follicle, which undergoes trichilemmal keratinization. This form of keratinization is unique in that there is no granular layer. The hereditary version of this disease was originally thought to be caused by a defect in the gene encoding β-catenin. This has been disproven, and the familial gene has been mapped to the short arm of chromosome 3, although the exact genetic defect has yet to be elucidated. These cysts are believed to be derived from the isthmus of anagen-type hairs. They are formed from deeper elements of the hair shaft apparatus than the epidermal inclusion cyst are.
Histology: Pilar cysts are composed of compact layers of stratified squamous epithelium without a granular cell layer. The cysts are found within the dermis, and the overlying epidermis is unaffected. These cysts show an absence of intercellular adhesion molecules. The cysts can become calcified or ossified. The cysts have a unique peripheral rim of keratinocyte nuclei, which is very helpful in classifying them. The central aspect of the cyst contains homogenous pale, eosinophilic, compressed keratin.
Treatment: Simple surgical excision is curative. The recurrence rate is minimal. These cysts typically are removed very easily after excision through the overlying skin into the cyst wall. The cyst almost always “pops” out with slight lateral pressure, and only a small incision is needed. After removal, care needs to be taken to decrease the amount of dead space left, to avoid seroma formation. This can be prevented by removing some of the redundant overlying epidermis and suturing the deeper tiss es together to close the space left by the removed cyst.