Epidermoid Cyst

Background

Epidermoid cysts represent the most common cutaneous cysts. They may occur anywhere on the body but are most frequently seen on the face, scalp, neck, and trunk.^[1]

Historically, epidermoid cysts have been referred to by various terms, including follicular infundibular cysts, epidermal cysts, and epidermal inclusion cysts. The term epidermal inclusion cyst refers specifically to an epidermoid cyst that is the result of the implantation of epidermal elements in the dermis. Because most lesions originate from the follicular infundibulum, the more general term epidermoid cyst is favored. The term sebaceous cyst should be avoided because it implies that the cyst is of sebaceous origin. Finally, the term milia refers to very small, superficial epidermoid cysts.

Epidermoid cysts are benign lesions; however, very rare cases of various associated malignancies have been reported.^{[2, 3, 4, 5, 6, 7]}

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Pathophysiology

Epidermoid cysts result from the proliferation of epidermal cells within a circumscribed space of the dermis. Analysis of their lipid pattern demonstrates similarities to the epidermis. In addition, epidermoid cysts express cytokeratins 1 and 10, which are constituents of the suprabasilar layers of the epidermis. The source of this epidermis is nearly always the infundibulum of the hair follicle, as evidenced by the observation that the lining of the two structures is identical.^[7]

Inflammation is mediated in part by the horny material contained in epidermoid cysts. Extracts of this material have been shown to be chemotactic for polymorphonucleocytes.

Studies have suggested that human papillomavirus (HPV) and exposure to ultraviolet (UV) light may play a role in the formation of some epidermoid cysts, particularly verrucous cysts with coarse hypergranulosis.^{[8, 9, 10, 11, 12, 13, 14]}

The manner in which carcinomas may arise within epidermoid cysts is unclear. In a series of epidermoid cysts with carcinoma, immunohistochemical results for HPV were negative, suggesting that HPV is not likely to play a role in the development in squamous cell carcinoma (SCC) in epidermoid cysts. Chronic irritation or repetitive trauma to the epithelial lining of the cyst has been suggested to play a role in malignant transformation; however, this relation has not been established.^[15]

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Etiology

Epidermoid cysts likely form via several mechanisms. They may result from the sequestration of epidermal rests during embryonic life, occlusion of the pilosebaceous unit, or traumatic or surgical implantation of epithelial elements. HPV infection, UV light exposure, and eccrine duct occlusion may be additional factors in the development of palmoplantar epidermoid cysts.^[16] HPV has also been identified in nonpalmoplantar epidermoid cysts.

Congenital epidermoid cysts of the anterior fontanelle or those that are orogenital in location presumably result from sequestration or trapping of epidermal rests along embryonic fusion planes during development. Lip and lingual lesions may be related to aberrant fusion of the branchial arches, whereas genital lesions may result from improper closure of the genital folds.

Any benign or malignant process affecting or growing near the pilosebaceous unit may lead to occlusion or impingement of the follicular ostia with subsequent formation of a cyst. Cysts with an acneiform distribution are likely the result of follicular occlusion. In elderly persons, accumulated sun damage can injure the pilosebaceous unit, causing abnormalities such as comedonal plugging and hypercornification, both of which can eventuate in cyst formation.^[17] This condition is referred to as Favre-Racouchot syndrome.

True epidermal inclusion cysts result from the implantation of epithelial elements in the dermis. Certain injuries, especially of the crushing type, have been associated with subungual or terminal phalanx epidermoid cysts. A crush injury sustained from slamming a car door on a digit is frequently reported. Any surgical procedure can theoretically result in epidermoid cysts. The formation of multiple epidermoid cysts after rhinoplasty, breast augmentation, and liposuction has been described.^[18] The use of dermal grafts, myocutaneous grafts, and needle biopsies has also been associated with the formation of epidermoid cysts.

Certain hereditary syndromes are associated with epidermoid cysts. Such syndromes include Gardner syndrome (see the image below), basal cell nevus syndrome,^[5]  and pachyonychia congenita.^[19] In addition, idiopathic scrotal calcinosis may actually represent an end stage of dystrophic calcification of epidermoid cysts.

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Multiple epidermoid cysts on forehead of patient with Gardner syndrome.

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Epidemiology

United States statistics

In a study using data from the National Ambulatory Medical Care Survey for the period from 2007 and 2016, epidermoid cysts were found to be the fifth most common skin diagnosis across all medical specialties in the United States.^[20]

Age-, sex-, and race-related demographics

Epidermoid cysts may occur at any age; however, they most commonly arise in the third and fourth decades of life. Small epidermoid cysts known as milia are common in the neonatal period.

Epidermoid cysts are approximately twice as common in men as in women.

No racial predilection has been identified. Pigmentation of epidermoid cysts is common in individuals with dark skin. In a study of Indian patients with epidermoid cysts, 63% of the cysts contained melanin pigment.^[21]

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Prognosis

Epidermoid cysts are usually asymptomatic; however, they may become inflamed or secondarily infected, resulting in swelling and tenderness. Rarely, malignancies, including basal cell carcinoma, Bowen disease, SCC (the most common of these rarities), mycosis fungoides, and melanoma in situ, have developed in epidermoid cysts.^[7]

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History

Epidermoid cysts are usually asymptomatic. Discharge of a foul-smelling cheeselike material may be described. Less frequently, the cysts can become inflamed or infected, resulting in pain and tenderness. In the uncommon event of malignancy, rapid growth, friability, and bleeding may be reported.

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Physical Examination

Epidermoid cysts appear as flesh-colored–to-yellowish, firm, round nodules of variable size. A central pore or punctum may be present (see the image below).

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Unusually large epidermoid cyst with prominent punctum on back of patient. (Ruler is in centimeters.)

Epidermoid cysts have been reported to be most common (in descending order of frequency) on the face, trunk, neck, extremities, and scalp. Rare cases of epidermoid cysts occurring in bone, breast, and various intracranial locations have been reported.^{[22, 23]} Epidermoid cysts in the breast and genital area are not uncommon in the general population.^[24] The ocular and oral mucosae can also be affected, and cysts have been reported on the palpebral conjunctivae, lips, buccal mucosa, tongue, and uvula.

Epidermoid cysts can manifest in various ways on the extremities. Epidermoid cysts on the distal portions of the digits may extend into the terminal phalanx. These lesions may produce changes in the nails (eg, pincer nails), erythema, edema, tenderness, and pain.

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Laboratory Studies

Laboratory studies are typically unnecessary; however, with recurrent infection or lack of response to antibiotics, culture and sensitivity testing may be indicated.

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Imaging Studies

If an epidermoid cyst is suspected in an unusual location (eg, breast, bone, or an intracranial site), imaging with ultrasonography (US), radiography, computed tomography (CT), or magnetic resonance imaging (MRI) is appropriate. Different types of skin lesions exhibit distinct characteristics on high-frequency US (HFUS).^[25] HFUS has also been used to assess treatment results after therapeutic dermatologic procedures.

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Other Tests

Fine-needle aspiration (FNA) has been used to help diagnose epidermoid cysts in unusual locations (eg, the breast). Smears of aspirated material stained with Wright-Giemsa stain demonstrate nucleated keratinocytes and wavy keratin material.^[1]

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Histologic Findings

Epidermoid cysts are lined with stratified squamous epithelium that contains a granular layer. Laminated keratin contents are noted inside the cyst (see the images below). An inflammatory response may be present in cysts that have ruptured. Older cysts may exhibit calcification. Pilomatrical differentiation may be noted, especially in patients with Gardner syndrome.^[26]

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Cyst containing keratinous material (hematoxylin and eosin, original magnification X1.6).

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Higher-magnification view of wall of cyst in preceding image demonstrates true epidermis with granular layer and adjacent laminated keratinous materia....

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Medical Care

For asymptomatic epidermoid cysts, no treatment is required. If inflammation is present, intralesional injection of triamcinolone may hasten its resolution. Oral antibiotics may occasionally be indicated for infection.^[27]  Intralesional injection of hydrolytic enzymes (eg, hyaluronidase, collagenase, and lipase) has reportedly been successful, but further data from controlled studies are required to determine its utility.^[28]

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Surgical Care

Epidermoid cysts may be removed via simple excision or incision with removal of the cyst and the cyst wall though the surgical defect.^[29] If the entire cyst wall is not removed, the lesion may recur. Excision with punch biopsy technique may be used if the size of the lesion permits.^[30] Minimal-incision surgery, with reduced scarring, has been reported.^{[31, 32]}  An intraoral approach has been used to minimize facial scarring.^[33]

Incision and drainage may be performed if a cyst is inflamed. Injection of triamcinolone into the tissue surrounding the inflamed cyst results in faster improvement in symptoms. This may facilitate the clearing of infection; however, it does not eradicate the cyst.

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Complications

Complications are rare but can include infection, scarring from removal, and recurrence. Malignancies in epidermoid cysts are very rare.

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Prevention

When the cutaneous portion of a myocutaneous flap is to be buried, a dermatome should be used to remove the epidermis.

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Consultations

Lesions located in atypical locations warrant appropriate consultation.

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Medication Summary

The goals of pharmacotherapy are to reduce morbidity and to prevent complications.

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Triamcinolone (Amcort, Aristocort)

• Dosing, Interactions, etc.

Clinical Context:  Triamcinolone decreases inflammation by suppressing the migration of PMN leukocytes and reversing capillary permeability. Intralesional injections may be used for localized skin disorders.

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Class Summary

These agents have anti-inflammatory properties and cause varied metabolic effects.

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