Epidermal Inclusion Cyst

Background

Epidermoid cysts represent the most common cutaneous cysts. While they may occur anywhere on the body, they occur most frequently 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, 8, 9, 10, 11, 12, 13]}

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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 2 structures is identical.^[13]

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 light (UV) may play a role in the formation of some epidermoid cysts, particularly verrucous cysts with coarse hypergranulosis.^{[14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]}

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 relationship has not been established.^[44]

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Epidemiology

Race

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.^[26]

Sex

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

Age

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.

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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 (most common of these rarities), mycosis fungoides, and melanoma in situ, have developed in epidermoid cysts.^[13]

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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. Note the image below.

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

Reportedly, epidermoid cysts are 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.^[33] Epidermoid cysts in the breast and genital area are not uncommon in the general population. 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 such as pincer nails, erythema, edema, tenderness, and pain.^[34]

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Causes

Epidermoid cysts likely form by 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, ultraviolet exposure, and eccrine duct occlusion may be additional factors in the development of palmoplantar epidermoid cysts.^[27] 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, while 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.^[28] 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. Reports describe the formation of multiple epidermoid cysts after rhinoplasty, breast augmentation, and liposuction.^[29] 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,^[30] basal cell nevus syndrome,^{[3, 10, 25]} and pachyonychia congenita.^{[31, 32]} In addition, idiopathic scrotal calcinosis may actually represent an end stage of dystrophic calcification of epidermoid cysts. Note the image below.

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Multiple epidermoid cysts on the forehead of a patient with Gardner syndrome.

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Laboratory Studies

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

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Imaging Studies

If an epidermoid cyst is suspected in an unusual location, such as breast, bone, or intracranial locations, imaging with ultrasonography, radiography, CT scanning, or MRI is appropriate.

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Other Tests

Fine-needle aspiration has been used to help diagnose epidermoid cysts in unusual locations, such as 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. 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.^[35]

Note the images below.

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

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Higher-magnification view of the cyst wall of the cyst in Media File 3 demonstrates a true epidermis with a granular layer and adjacent laminated kera....

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Medical Care

Asymptomatic epidermoid cysts do not need to be treated. Intralesional injection with triamcinolone may hasten the resolution of inflammation. Oral antibiotics may occasionally be indicated.

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Surgical Care

Epidermoid cysts may be removed via simple excision or incision with removal of the cyst and cyst wall though the surgical defect.^[36] 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.^{[37, 38]} Minimal-incision surgery, with reduced scarring, has been reported.^{[39, 40]}  An intraoral approach has been used to minimize facial scarring.^[45]

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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Consultations

Lesions located in atypical locations warrant appropriate consultation.

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Complications

Complications are rare, but they 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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Guideline Summary

The Infectious Diseases Society of America recently updated their guidelines for the diagnosis and management of skin and soft tissue infections. For the full guidelines, see Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the Infectious Diseases Society of America.^{[41, 42]}

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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)

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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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