Trichilemmal Cyst (Pilar Cyst)

Background

Trichilemmal (pilar) cysts are common intradermal or subcutaneous cysts, occurring in 5-10% of the population.^{[1, 2]} More than 90% of trichilemma cysts occur on the scalp, where they are the most common type of cutaneous cyst.^{[3, 4]} Trichilemmal cysts are almost always benign. They may be sporadic, or they may be inherited in an autosomal dominant manner.^[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.^[6] Proliferating trichilemmal cysts are gradually enlarging (up to 25 cm in diameter) exophytic nodules that occasionally ulcerate.^[3] Although these tumors are biologically benign, they may be locally aggressive. Recurrences and metastases have been observed, with rare malignant transformation.^{[7, 5, 6, 8, 9]}

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Pathophysiology

Frequently but erroneously called sebaceous cysts, trichilemmal cysts are lined by stratified squamous epithelium similar to that in the isthmus of the hair follicle.^[5] 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.^[10] The squamous epithelium undergoes so-called trichilemmal keratinization or rapid keratin formation without a granular cell layer, resulting in a cyst wall that lacks a granular cell layer.^{[3, 10]}

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Etiology

If the cysts are hereditary, the inheritance pattern is usually autosomal dominant.^{[5, 11]} Hereditary trichilemmal cysts link to the short arm of chromosome 3.^[12]

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Epidemiology

Trichilemmal cysts are common in the United States, occurring in 5-10% of the population.^[1]  They are more common in middle-aged persons than in younger persons, and they occur more frequently in women than in men. Trichilemmal cysts have no known racial predilection.

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Prognosis

Trichilemmal cysts are biologically benign but may be locally aggressive in some cases.^[13] 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.^{[14, 15]}

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History

Trichilemmal (pilar) cysts occur preferentially in areas with dense hair follicle concentrations; consequently, 90% occur on the scalp.^[3] They may be seen infrequently on the face, neck, trunk, and extremities.^{[1, 3, 16, 17, 18]}

Trichilemmal cysts are solitary in 30% of patients and multiple in 70%.^[10]

Trichilemmal cysts may be red, swollen, and tender if they have ruptured or become infected.

A family history may be present, given that the condition may have an autosomal dominant pattern of inheritance.^[5]  Proposed clinical criteria for recognizing autosomal dominant hereditary cases include the following^[19] :

• Diagnosis of trichilemmal cyst in two or more first- or second-degree relatives
• Age younger than 45 years at diagnosis
• Diagnosis of multiple or large (>5 cm) cysts or rare histologic features such as proliferating and ossifying cysts

A proliferating trichilemmal cyst presents as a slow-growing nodule.^[5] They are more common in women and occur at a mean age of 65 years.^[3]

Rapid growth is unusual and may be a sign of infection or malignancy.^[1] Other suspicious features include the following^[3] :

• Nonscalp location
• Size larger than 5 cm
• Infiltrative growth pattern

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Physical Examination

Trichilemmal cysts manifest as skin-colored, smooth, mobile, firm, and well-circumscribed nodules (see the image below).^{[1, 5]}

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Firm, smooth swelling on scalp.

As noted, 90% of trichilemmal cysts occur on the head, and 70% occur as multiple lesions.^{[3, 10]}

Unlike epidermoid cysts, trichilemmal cysts do not have a visible punctum.^[3] 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.

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Complications

Although most trichilemmal cysts are asymptomatic, cyst infection and rupture may occur.^[1] Cysts may be traumatized by combing or brushing the hair. Proliferating trichilemmal cysts can invade surrounding structures and ulcerate.

Malignant transformation is exceedingly rare but can occur.^{[1, 20]}

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Imaging Studies

Radiography of the head, computed tomography (CT), and magnetic resonance imaging (MRI) may be needed to differentiate midline scalp lesions that may have a connection to the meninges or the central nervous system (CNS). Ultrasonography (US) may be a cheaper, less invasive, and equally diagnostic option.^[21]

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Procedures

Excision and histopathologic evaluation can confirm the diagnosis. Trichilemmal (pilar) cysts can usually be extracted more easily than epidermoid cysts can.^[3]

If a trichilemmal cyst becomes inflamed and ruptures, it should be excised and submitted for histopathologic examination in order to exclude carcinoma, particularly nodular or nodulocystic basal cell carcinomas.^[22] Inflamed, ruptured cysts may have an infectious etiology. Wound culture may identify infection and guide therapy if necessary.^[1]

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Histologic Findings

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. (See the images below.) They often have foci of calcification.^[10] 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.

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Trichilemmal (pilar) cyst, low power. Trichilemmal cyst is lined by squamous epithelium without granular layer (trichilemmal keratinization) and with ....

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Trichilemmal (pilar) cyst, medium magnification. Higher magnification shows stratified squamous epithelium without granular layer and shows swelling o....

Tricholemmal cyst keratin stains with antikeratin antibodies derived from human hair. In contrast, epidermal cyst keratin stains with human callus–derived antikeratin antibodies.

Very rarely, sebaceous or apocrine differentiation occurs in a cyst wall. Calcification may be seen, and a few reports have described ossification.^{[23, 24]}

If cyst-wall rupture occurs, a foreign body giant cell reaction may surround the cyst.^{[5, 10]}  (See the images below.)

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Ruptured trichilemmal (pilar) cyst. Replacement of squamous lining by histiocytes and rare multinucleated giant cells.

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Ruptured trichilemmal (pilar) cyst, low power. Replacement of epithelial lining by granulomatous reaction with numerous cholesterol clefts and calcifi....

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Ruptured trichilemmal (pilar) cyst, high power. Cholesterol clefts and calcifications are highlighted.

Lobules of squamous epithelium in the cyst wall suggest a proliferating trichilemmal cyst.^[1] Other histologic characteristics of proliferating trichilemmal cysts may include mitoses, cell atypia, and necrosis. Penetration of tumor cells through the cyst lining suggests malignancy.^[3]

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Medical Care

Painful, swollen, erythematous, or purulent trichilemmal (pilar) cysts may indicate the presence of infection. A wound culture with directed therapy may be necessary.^[1]

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Surgical Care

Definitive treatment of a trichilemmal cyst consists of complete excision. Several methods can be employed for surgical removal. A small linear incision, an elliptical excision (see the image below), and a circular dermal punch incision all are effective ways of excising these cysts.^[25]

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Surgical removal of intact trichilemmal (pilar) cyst through elliptical excision.

For all methods, the procedure involves the following steps:

• Anesthetize the area with local anesthetic
• Over the center of the cyst, make a small linear incision, an elliptical excision, or a punch incision using a dermal punch biopsy tool
• If,as is often the case, the fibrous capsule of the cyst is thick enough, remove the cyst intact via blunt dissection without expressing its contents; alternatively, express the contents of the cyst, then use a curette to press against the inner wall, moving it back and forth to dislodge the outer side of the cyst from the surrounding tissue
• Grasp the edge of the cyst with forceps, and separate the cyst wall from the surrounding connective tissue via blunt dissection
• Control the bleeding, and suture (see the image below) or dress the wound as necessary

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Closure of defect after surgical removal of trichilemmal (pilar) cyst.

If the cyst is ruptured or infected, deferring excision until the inflammation is reduced decreases the likelihood of spreading infection and wound healing problems.^[1]

Most proliferating trichilemmal cysts are cured with complete surgical removal.^[3] In those very occasional instances when multiple proliferating trichilemmal cysts necessitate several local excisions,^[8] additional radiotherapy, chemotherapy, or both may be considered.^[9]

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