Excerpt from Seborrheic Keratosis

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Background

Seborrheic keratoses are the most common benign tumor in older individuals. Seborrheic keratoses have a variety of clinical appearances, and they develop from the proliferation of epidermal cells. Although no specific etiologic factors have been identified, they occur more frequently in sunlight-exposed areas.

Pathophysiology

The etiology of the development of a seborrheic keratosis is not known. Seborrheic keratoses exhibit histologic evidence of proliferation. Increased cell replication has been demonstrated in seborrheic keratoses with bromodeoxyuridine incorporation studies and immunohistochemistry for proliferation-associated antigens. A moderate increase is observed in the rates of apoptosis in all varieties of seborrheic keratoses compared to normal skin.

Reticulated seborrheic keratoses are usually found on sun-exposed skin, and the reticulated type of seborrheic keratoses may develop from solar lentigines.

Epidermal growth factors or their receptors have been implicated in the development of seborrheic keratoses. No difference was observed in the expression of immunoreactive growth hormone receptors in keratinocytes from normal epidermis and keratinocytes from seborrheic keratoses. The expression of BCL2, an apoptosis-suppressing oncogene, is low in seborrheic keratosis in contrast to the high values in basal cell and squamous cell carcinoma. No increase is observed in the sonic hedgehog signal transducers patched (ptc) and smoothened (smo) messenger RNA (mRNA) in seborrheic keratosis over normal skin.

Seborrheic keratoses have a varying degree of pigmentation. In pigmented seborrheic keratoses, the proliferating keratinocytes trigger the activation of neighboring melanocytes by secreting melanocyte-stimulating cytokines. Endothelin-1 has dual stimulatory effects on DNA synthesis and melanization of human melanocytes and has been implicated as playing a part in the hyperpigmentation observed in seborrheic keratoses. Immunohistochemically, the keratinocytes of seborrheic keratoses express low molecular weight keratin but often exhibit a partial lack of the high molecular weight forms of keratin.

Frequency

United States

Seborrheic keratoses are the most common benign tumor in older individuals. The frequency appears to increase with age. In 1963, Tindall and Smith examined a population of individuals older than 64 years in North Carolina and found that 88% of the people had at least one seborrheic keratosis. In this study, 38% of the white women, 54% of the white men, and 61% of the black men and women were found to have 10 or more seborrheic keratoses. In 1965, Young examined 222 residents of the Orthodox Jewish Home for the Aged in New York and found that 29.3% of the men and 37.9% of the women had seborrheic keratosis.

International

In 2000, Memon et al found in a British population younger than 40 years that 8.3% of the males and 16.7% of the females had at least one seborrheic keratosis. In an Australian population, 23.5% of individuals aged 15-30 years were found to have at least one seborrheic keratosis, with no significant differences between the sexes. In another Australian study of 100 people composed of hospital staff and nondermatologic day patients, 12% of people aged 15-25 years (n = 34), 79% of people aged 26-50 years (n = 24), 100% of people aged 51-75 years (n = 25), and 100% of people older than 75 years (n = 17) had seborrheic keratoses. The median number of seborrheic keratoses per person was 6 in the group aged 15-25 years, 5 in the group aged 26-50 years, 23 in the group aged 51-75 years, and 69 in those older than 75 years.

Mortality/Morbidity

Seborrheic keratoses are benign but secondary tumors, and Bowen disease (squamous cell carcinoma in situ) or malignant melanoma may occasionally arise within the lesion. Seborrheic keratoses can also catch on clothing and become irritated. They can itch, grow, and bleed.

• Scratching seborrheic keratoses or trying to pick them off the skin can result in a secondary infection.
• People sometimes have many seborrheic keratoses, and they may obscure the detection of a dysplastic nevus or malignant melanoma.

Race

Seborrheic keratoses are less common in populations with dark skin compared to those having white skin; however, black individuals develop a variant of seborrheic keratoses termed dermatosis papulosa nigra. These lesions affect the face, especially the upper cheeks and lateral orbital areas. They are small, pedunculated, and heavily pigmented with a minimal keratotic element. The onset of these lesions generally is earlier than that of ordinary seborrheic keratoses.

Sex

No sex difference is apparent in the frequency of occurrence of seborrheic keratoses.

Age

Seborrheic keratoses are the most common benign tumor in older individuals. They appear to increase with age. Seborrheic keratoses have also been found to occur in younger individuals.

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