Answer: C

An epidermal inclusion cyst — also known as an epidermoid cyst, infundibular cyst, keratin cyst, or inclusion cyst — is the most common type of cutaneous cyst. Patients present with a nodule that is movable, firm, and generally asymptomatic unless infected. The cyst can range in size from millimeters to a few centimeters and is usually covered by shiny skin with a central punctum. Because they are filled with keratin and not sebum, epidermal inclusion cysts should not be classified as a sebaceous cyst.8,9

Men in their 20s to 40s are most commonly affected by this condition. Women are affected less commonly, with a male-to-female ratio of 2 to 1. When this condition is seen in neonates, it is called milia. Occurrence during puberty is uncommon and genetic conditions, such as Gardner syndrome, should be considered when epidermal inclusion cysts are present in patients in this age group.8,9

Epidermal inclusion cysts result from inflammation and irritation that cause blockage of the infundibulum. This blockage results in epidermal proliferation and the formation of a keratin-filled cyst. A central punctum is often seen when the cyst connects to the surface of the skin. Although they can occur anywhere on the body, they are usually found on the face, head, neck, back, and scrotum. This condition is extremely common, sporadic, and generally benign.8 Rarely, malignancy is diagnosed as an incidental finding upon histologic examination of the excised cyst; this occurs in approximately 1% of cysts. Of those that progress to malignancy, squamous cell carcinoma is the most common presentation (approximately 70% of cases), followed by basal cell carcinoma.9 Currently, the mechanism of neoplastic progression is unknown, but it is hypothesized that chronic irritation plays a central role. Higher clinical suspicion of neoplasm may be warranted in cases where the cyst is associated with pain, rapid growth, or ulceration.10

Known risk factors for the development of epidermal inclusion cysts are those associated with inflammation and irritation, including chronic sun damage and acne vulgaris. BRAF inhibitors have also been studied as a potential risk factor.8 This condition is not known to be contagious, but studies have suggested an association with human papillomavirus (HPV).11 Epidermal inclusion cysts are the most common complication seen in female circumcision, a still common practice worldwide.12

If a patient presents with an epidermal inclusion cyst outside of the normal epidemiologic distribution, it is important to consider genetic syndromes as a cause. The most common of these is Gardner syndrome, which is due to a mutation in the APC gene and is a subcategory of familial adenomatous polyposis (FAP). Gardner syndrome presents with numerous colonic polyps, as well as extracolonic growths, including osteomas, epidermal cysts, and fibromas. Other genetic syndromes associated with epidermal inclusion cysts include Favre-Racouchot syndrome (senile comedones, solar comedones, and nodular elastosis with cysts and comedones) and Gorlin syndrome (basal cell nevus syndrome).8

The diagnosis of epidermal inclusion cyst is made clinically with the history and physical examination findings of a small, discrete, single, freely movable cyst. If the cyst is surgically removed, histology can be used to confirm the diagnosis or confirm/rule out malignancy. Epidermal elements implanted into the dermis layer of the skin with a cystic cavity filled with laminated keratinous material should be visualized by histology. A granular layer filled with keratohyalin granules may be present.8 Generally, the cysts are not infected, but in infectious cases the agent is often normal skin flora, including but not limited to Staphylococcus aureus, S epidermidis, and Propionibacterium.13

The differential diagnosis of epidermal cysts includes other types of cysts (pilar cyst, dermoid cyst, ganglion cyst, brachial cleft cyst, and pilonidal cyst), lipoma, abscess, neuroma, benign growths, skin carcinomas (squamous cell carcinoma and basal cell carcinoma), metastatic cutaneous lesions, pilomatrixoma, neurofibroma, calcinosis cutis, pachyonychia congenita, steatocystoma simplex, and steatocystoma multiplex.1,6 The punctum is a feature of epidermal cysts that distinguishes it from others on the differential diagnosis, such as a pilar cyst or a lipoma. A history of an unchanging, asymptomatic mass differentiates epidermal cyst from an abscess.14

Surgery is first-line therapy. Complete removal of the cyst is essential to prevent recurrence. It is best that the cyst be removed during a period when it is not inflamed to ensure the wall of the cyst is intact. The 2 most common surgical techniques are minimal incision and punch biopsy; the minimal incision technique provides better cosmetic results.15 Local anesthetic should be injected around, not directly into, the cyst. If cellulitis is present, oral antibiotic therapy should be included in the treatment plan. An alternative, less-invasive therapy is injecting triamcinolone into the cyst; this reduces inflammation and infection risk, potentially to the point that surgery is no longer necessary.8

For the patient in the case described, the cyst was surgically removed using the minimal incision technique. Histologic examination of the tissue confirmed the diagnosis of epidermal inclusion cyst, and no evidence of malignancy was found. The patient required no further treatment.

                           Vellus Hair Cyst1-8Epidermal Inclusion Cyst9-16
Dermatologic Presentation·Collection of discrete follicular papules, each about 1-4 mm in size
·Range in color from brown to gray, blue,  yellow, or erythematous
Single, movable nodule less than a few centimeters in diameter with a visible central punctum that is compressible, asymptomatic
Associations·Can be familial
·May be pruritic or erythematous in some cases
·Acne vuglaris
·Men aged 20-40, chronic sun damage
EtiologyBlockage at infundibulum of hair follicle with subsequent cystic dilatation and bulb atrophyMost cases are sporadic and result from inflammation and irritation that causes blockage of the infundibulum and formation of keratin-filled cyst; generally benign, but rarely contain squamous cell carcinoma or basal cell carcinoma
Characteristic LocationChest or flexor and extensor surfaces of the arms and legsFace, neck, back, or genitals
Histology·Mid-dermal cyst lined by stratified squamous epithelium with laminated keratin and vellus hairs within cyst
·Possible granulomatous inflammation
Cyst wall derived from infundibulum and cavity filled with keratin; if infected, acute inflammation and neutrophils present
DiagnosisHistologic analysis of punch biopsy or potassium hydroxide wet mount of cyst contents, both depicting vellus hairsHistory and physical examination; confirm with histology
Treatment·Topical modalities such as retinoic acid or tazarotene cream
·Incision and drainage
·Dermabrasion, erbium:yttrium aluminum garnet laser, CO2
·Avoid oral isotretinoin
·Surgical removal of entire cyst with intact cyst wall; for better cosmetic results, extract via the minimal incision technique
·Direct injections of triamcinolone can be used as an alternative in cases of active inflammation or patient preference

Yelena Docik, BS, is a medical student; Eleanor Johnson, BS, is a medical student; and Christopher Rizk, MD, is a dermatology fellow at Baylor College of Medicine, Houston, Texas.


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