Cutaneous cysts appear as well-demarcated dermal or subdermal nodules. There are many different types, the most common of which is the epidermal inclusion cyst. Cysts can be categorized based on the histological appearance of the cyst wall.
Epidermal Inclusion Cyst (follicular cyst)
An “epidermal” inclusion cyst is something of a misnomer since it is located in the dermis and not the epidermis. Its name reflects the fact that it is derived from the epidermal cells that line the upper portion of the hair follicle. For this reason it is also known as a follicular cyst. The contents of the cyst are keratin (not sebum as is sometimes thought). It is flesh-colored and “dome-shaped” in appearance and firm but malleable (i.e. able to be deformed) in consistency. It is benign with no potential for malignant transformation. They are common and occur at all ages, often on the face or trunk. They range from several millimeters to several centimeters in diameter.
With what can an epidermal inclusion cyst be confused?
A pilar cyst (trichilemmal cyst), derived from the middle third of the follicle, occurs frequently on the scalp and less often in other locations. It, too, is benign. A lipoma is a subcutaneous nodule derived from fat cells. Its consistency is usually described as “rubbery” but not “malleable”. In other words, it is more firm than an epidermal inclusion cyst. A steatocystoma is a true sebaceous cyst and is filled with sebum. If there is any doubt about the identity of a firm lesion, a malignant tumor must be ruled out.
How is an epidermal inclusion cyst diagnosed?
Epidermal inclusion cysts are typically asymptomatic and are usually discovered incidentally. Occasionally a cyst can become secondarily infected or rupture, leading to inflammation and drainage. It is usually diagnosed clinically based on appearance and feel. The presence of a central pore within the cyst, indicative of the opening of the follicular unit, is a useful clue that the lesion originates from that structure. It is sometimes possible to express material from the central pore, which will be cheesy and foul smelling. Incision and drainage can also be performed; the drainage of cheesy keratinaceous material is diagnostic. Biopsy is not necessary unless the diagnosis is in doubt. Histologically, the cyst wall is composed of stratified squamous epithelium with a granular layer and the cavity is filled with laminated keratin.
How is an epidermal inclusion cyst treated?
Unless the cyst is troublesome, no treatment is required. If the cyst is to be removed, the entire cyst lining must be excised to prevent recurrence.
What is the prognosis for an epidermal inclusion cyst?
Epidermal inclusion cysts have no malignant potential. They enlarge then reach a stable size of up to several centimeters in diameter. They may become infected or rupture, necessitating treatment. Inflamed lesions may require incision and drainage and possibly treatment with antibiotics. Intralesional steroids may help resolve the inflammation. Excision is sometimes required.
Steatocystoma is a dermal cyst that is filled with sebum. It is the only true “sebaceous cyst”. It is most commonly located on the trunk, but lesions can also be found in the axillae and groin. Lesions range in size from several millimeters to about a centimeter. They usually have a whitish coloration.
Lesions can occur singly (steatocystoma simplex), or can be multiple, sometimes as part of a dominantly inherited disorder termed steatocystoma multiplex.
With what can steatocystoma be confused?
Steatocystoma can be confused with other dermal cysts, lipoma, or other dermal and subcutaneous tumors.
How is steatocystoma diagnosed?
Although it may be suspected clinically, confirmation requires histological examination. In steatocystoma, the cyst wall is lined with stratified squamous epithelium with an eosinophilic cuticle. Sebaceous lobules are locate in or adjacent to the cyst.
How is steatocystoma treated?
Individual lesions can be excised or incised, and the cyst wall removed.
What is the prognosis for steatocystoma?
These cysts do not spontaneously involute. They are typically only of cosmetic concern. If multiple, then a workup for associated syndromes may be warranted.