International Academy of Cosmetic Dermatology
A cyst is a closed, saclike structure that contains fluid, gas, semisolid, or solid material. A true cyst is lined by epithelium. A pseudocyst is a cyst-like structure without epithelial lining. Cysts are common and can occur anywhere in the body in people of any age. They vary in size from microscopic to grossly visible and are usually easily felt or seen when in the skin. The vast majority of cutaneous cysts are benign and do not require treatment in the absence of medical complications (rapid growth, infection, inflammation). Rarely, malignancies, including basal cell carcinoma, Bowen disease, squamous cell carcinoma, mycosis fungoides, and melanoma in situ, have developed in cutaneous cysts.
Cysts may be classified on the basis of their pathogenesis and histologic features. The primary categories are adnexal cysts, developmental cysts, pseudocysts, and neoplasms with cystic changes. The most common lining is stratified squamous epithelium. Some developmental cysts have a lining of columnar epithelium and are classified as cutaneous columnar cysts; these cysts include branchial, thyroglossal, thymic, bronchogenic, cutaneous ciliated, and median raphe cysts.
Cysts can arise through a variety of processes, including
• simple obstruction to the flow of fluid,
• chronic inflammatory conditions,
• genetic (inherited) conditions,
• defects in developing organs in the embryo.
Epidermoid cysts (syn. follicular infundibular cysts, epidermal cysts, epidermal inclusion 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. Their wall is made up of tissue resembling the follicular infundibulum (uppermost part of the hair follicle), while the cavity is filled with loosely packed soft keratin. A central plug is often easily visible. Epidermoid cysts are usually asymptomatic; however, they may become inflamed or secondarily infected, resulting in swelling and tenderness.
Milia are very small, superficial epidermoid cysts. Common on the face at any age, they can be especially numerous in infants, in whom they resolve spontaneously and require no treatment.
Pilar cysts (syn. trichilemmal cysts, isthmus-catagen cysts, wens) are the second most frequent type of cysts which contain keratin and its breakdown products and are lined by walls resembling the external (outer) root sheath of the hair. They may be sporadic or inherited in an autosomal dominantly way. Pilar cysts occur preferentially in areas of high hair follicle density; therefore, 90% arise on the scalp. Although biologically benign, they may be locally aggressive. On the scalp, they often result in permanent hair loss of the skin above them. Malignant transformation is very rare but may lead to distant metastases. Rarely, single or multiple foci of proliferating cells lead to proliferating tumors, often called proliferating trichilemmal cysts. Pilar cysts may become tender or painful if they rupture or become infected, but, in the absence of these complications, they are asymptomatic.
Eruptive vellus hair cysts appear as small asymptomatic papules on the chest or extremities. They have a lining similar to epidermoid cysts, but contain vellus hairs intermixed with keratin in the cavity. Either acquired or inherited in an autosomal dominant fashion, eruptive vellus hair cysts are, generally, an isolated finding.
Digital mucous cysts are benign pseudocysts (lacking a true epithelial wall) typically located at the distal interphalangeal (DIP) joints or in the proximal nail fold. They usually occur on the hands. Often, these cysts are asymptomatic and do not require treatment, although they may result in permanent nail dystrophy due to pressure on the nail matrix. When treatment is indicated, medical therapies and surgical interventions may be attempted. Recurrence is common.
Vesna Petronic-Rosic, MD, MSc
Chicago, IL, USA