Trichilemmal or pilar cysts are common intradermal or subcutaneous cysts, occurring in 5-10% of the population. Greater than 90% occur on the scalp, where trichilemmal cysts are the most common cutaneous cyst.[2, 3] Trichilemmal cysts are almost always benign. They may be sporadic or they may be autosomal dominantly inherited.[4, 5] They contain keratin and its breakdown products and are lined by walls resembling the external (outer) root sheath of the hair.
In 2% of trichilemmal cysts, single or multiple foci of proliferating cells lead to tumors called proliferating trichilemmal cysts. Proliferating trichilemmal cysts are gradually enlarging (up to 25 cm in diameter), exophytic nodules that occasionally ulcerate. Although biologically benign tumors, they may be locally aggressive. Recurrences and metastases have been observed, with rare malignant transformation.[5, 6]
Erroneously but often called sebaceous cysts, trichilemmal cysts are lined by stratified squamous epithelium similar to that in the isthmus of the hair follicle. This is the segment between the insertion of the erector pili muscle and the sebaceous gland duct, where no inner root sheath exists. The keratinization is similar to that which occurs in the outer root sheath. The squamous epithelium undergoes “trichilemmal keratinization” or rapid keratin formation without a granular cell layer, resulting in a cyst wall without a granular cell layer.[2, 7]
Trichilemmal cysts are common, occurring in 5-10% of the population.
Trichilemmal cysts have no known racial predilection.
Trichilemmal cysts occur more commonly in women than in men.
Trichilemmal cysts are more common in persons of middle age than in younger persons.
Trichilemmal cysts occur preferentially in areas with dense hair follicle concentrations; therefore, 90% occur on the scalp. They may be seen infrequently on the face, neck, trunk, and extremities.[1, 2, 8]
Trichilemmal cysts are solitary in 30% and multiple in 70% of patients.
Trichilemmal cysts may be red, swollen, and tender if they have ruptured or become infected.
A family history may be present because the condition can have an autosomal dominant pattern of inheritance.[4, 5] Proposed clinical criteria for recognizing autosomal dominant hereditary cases include the diagnosis of trichilemmal cyst in 2 or more first- or second-degree relatives, age of diagnosis younger than 45 years, and diagnosis of multiple or large (>5 cm) cysts or rare histologic features such as proliferating and ossifying cysts.
A proliferating trichilemmal cysts presents as a slow-growing nodule. They are more common in women, and occur at a mean age of 65 years.
Rapid growth is unusual and may be a sign of infection or malignancy. Other suspicious features include nonscalp location, size larger than 5 cm, and an infiltrative growth pattern.
Trichilemmal cysts manifest as skin-colored, smooth, mobile, firm, and well-circumscribed nodules,[1, 5] as shown in the image below.
A firm, smooth swelling on the scalp.
Ninety percent of trichilemmal cysts occur on the head, and 70% occur as multiple lesions.[2, 7]
Unlike epidermoid cysts, no punctum is seen. If they are inflamed, they may be tender, erythematous, or both. The contents occasionally extrude to form a soft, cutaneous horn.
The wall of a marsupialized cyst may fuse with the overlying epidermis to form a crypt. This may occasionally discharge its contents and then self-resolve.
If the cysts are hereditary, the inheritance pattern is autosomal dominant.[4, 5] Hereditary trichilemmal cysts link to the short arm of chromosome 3.
Radiography of the head, CT scanning, and MRI may be needed to differentiate midline scalp lesions that may have a connection to the meninges or the central nervous system.
Excision and histopathologic evaluation can confirm the diagnosis. Trichilemmal cysts can usually be extracted more easily than epidermoid cysts.
If a trichilemmal cyst becomes inflamed and ruptures, the cyst should be excised and submitted for histopathology in order to exclude carcinoma, particularly nodular or nodulocystic basal cell carcinomas. Inflamed, ruptured cysts may have an infectious etiology. Wound culture can elucidate infection and guide therapy if necessary.
Trichilemmal cysts are surrounded by fibrous capsules against which rest layers of small, cuboidal, dark-staining basal epithelial cells in a palisade arrangement with no distinct intercellular bridging. These merge with characteristic squamous epithelium composed of swollen pale keratinocytes, which increase in height as they mature and transform abruptly into solid eosinophilic-staining keratin without the formation of a granular cell layer. They often have foci of calcification.
Trichilemmal cyst keratin stains with antikeratin antibodies derived from human hair. In contrast, epidermal cyst keratin stains with human callus–derived antikeratin antibodies.
In contrast, epidermoid cysts have a granular cell layer in the lining epithelium. Epidermoid cysts (nonimplantation variant) have laminated keratin, which is believed to be derived from the follicular infundibulum. Note the slides below.
The trichilemmal cyst is lined by a squamous epithelium without a granular layer and with swelling of the cells close to the cyst cavity, which is fil....
At higher magnification, the cyst is lined by a squamous epithelium without a granular layer and with swelling of the cells close to the cyst cavity, ....
Very rarely, sebaceous or apocrine differentiation occurs in a cyst wall. Calcification may be seen, and a few reports have described ossification.[13, 14]
If cyst wall rupture occurs, a foreign body giant cell reaction may surround the cyst.[5, 7] Note the slide below.
Numerous cholesterol clefts are identified within the homogenous keratin of the trichilemmal cyst.
Lobules of squamous epithelium in the cyst wall suggest a proliferating trichilemmal cyst. Other histologic characteristics of proliferating trichilemmal cysts may include mitoses, cell atypia, and necrosis. Penetration of tumor cells through the cyst lining suggests malignancy.
Painful, swollen, erythematous, and/or purulent cysts may indicate the presence of infection. A wound culture with directed therapy may be necessary.
Definitive treatment is complete excision of the cyst. Several methods can be used to surgically remove these cysts. A small linear incision, an elliptical excision (see image below), and a circular dermal punch incision all are effective ways to remove the cysts.
Surgical removal of an intact pilar cyst through an elliptical excision.
For all methods, the procedure involves the following steps:
If the cyst is ruptured or infected, deferring excision until the inflammation is reduced decreases the likelihood of spreading infection and wound healing problems.
Most proliferating trichilemmal cysts are cured with complete surgical removal. In the very occasional instances when multiple proliferating trichilemmal cysts require several local excisions, additional radiotherapy and/or chemotherapy may be considered.
Although most are asymptomatic, cyst infection and rupture may occur. Cysts may be traumatized when combing or brushing hair. Proliferating trichilemmal cysts can invade surrounding structures and ulcerate.
Malignant transformation is exceedingly rare but can occur.
Trichilemmal cysts are biologically benign, but they may be locally aggressive. Malignant transformation is very rare but may lead to distant metastases.[5, 6]
Case reports have described Merkel cell carcinoma arising from Merkel cells in trichilemmal cysts.[18, 19]