One of the most commonly encountered of all benign skin growths is the seborrheic keratosis. These growths come in all sizes and shapes and invariably can be found on any human older than 40 years of age. They commonly begin in the fourth decade of life and tend to increase in number over one’s lifetime. They have no malignant potential but are often brought to the attention of physicians because they can mimic other skin growths, most importantly malignant melanoma.
Clinical Findings: Seborrheic keratoses are found equally in males and females, and they are seen in all races. They begin to manifest in the third to fifth decade of life and continue to increase in number there-after. They come in various sizes and shapes. Some are quite small, whereas others can be 5 to 6 cm in diameter. They occur almost exclusively in sun-exposed regions of the body. The classic description is that of a 1- to 2-cm plaque with a “stuck-on” appearance and small horn cysts. Most commonly flesh colored, they can also be tan, brown, or almost black. It is for this reason that they are occasionally mistaken for melanoma. Most individuals have a few scattered keratoses, but not infrequently a patient has thousands of these skin growths.
Many clinical variants of seborrheic keratosis can be seen. Stucco keratoses are small (1-5 mm), graytan papules with a stuck-on appearance or thin patches on the lower extremities. Dermatosis papulosis is a condition in which multiple seborrheic keratoses occur on the face and neck. This condition has a definite inheritance pattern.
Some seborrheic keratoses are smooth surfaced, but more commonly they have a pebbly or dry, rough surface. They have a characteristic stuck-on appearance, and in some instances they are easily removed by gently peeling from one side. These growths can easily become irritated or inflamed. The resulting pain, itching, or bleeding often brings the patient to medical treatment.
The sign of Leser-Trélat is the rapid onset of multiple seborrheic keratoses associated with an underlying internal malignancy. This sign has not been validated and is not a reliable indicator of an internal malignancy.
Histology: There is a well-circumscribed proliferation of keratinocytes. They have an exophytic growth pattern. The keratinocytes show acanthosis and hyperkeratosis. Marked papillomatosis is also commonly encountered. Two types of cysts are seen within the seborrheic keratosis. The horn cyst develops within the epidermis and is made of a keratin-filled cystic space with a surrounding granular cell layer. A pseudo-horn cyst is formed by an invagination of the stratum corneum into the underlying epidermis. Multiple histological subtypes have been described.
Pathogenesis: The formation of this benign epidermal tumor is not fully understood. It is caused by a proliferation of keratinocytes within the epidermis. The location in sun-exposed skin and the increasing number of lesions with increasing age has led some to believe that they are caused by a local suppression of the immune system that results in the epidermal proliferations. A definitive inheritance pattern has not been discovered, but these keratoses show some genetic predisposition. Chromosomal analysis of these tumors has not revealed any chromosomal defects. A link with the human papillomavirus has been proposed but has yet to be proven.
Treatment: These keratoses require no therapy. If they become inflamed or irritated, a simple shave biopsy removal is curative. Cryotherapy and curettage are often used to treat these benign skin growths, and both are extremely effective. After cryotherapy treatment, a blister usually forms at the base of the seborrheic keratosis, and within a day or two the keratosis falls off.
Another extremely effective method of removal that can be done in the office is cryotherapy followed by a light curettage; this also allows for histological evaluation. Occasionally, dark brown or black seborrheic keratoses can mimic melanoma, and in other cases a melanoma may arise adjacent to a seborrheic keratosis and mislead the clinician. If there is ever a doubt that the growth could be a melanoma, a biopsy is required. Thi allows for pathological confirmation of the diagnosis.