A rare case of a sebaceous nevus in the external auditory canal - Original Article

Ear, Nose & Throat Journal,  Jan, 2003  by Capt. Nici Eddy Botliwell,  Col. Craig C. Willard,  Maj. Douglas M. Sorensen,  Maj. Timothy J. Downey

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Stages of development. Mehregan and Pinkus examined 150 cases of sebaceous nevi, and in 1965 they became the first to describe the three phases of development. (4) Their report was the first to document the natural course of sebaceous nevi and their potential to develop into secondary neoplasms.

In their first stage, sebaceous nevi are generally found in children (from birth to prepuberty). These lesions appear as a smooth, waxy, thick, yellow-orange papules or plaques accompanied by local alopecia. Histologically, first-stage sebaceous nevi are characterized by papillomatous epidermal hyperplasia, hypergranulosis, hyperkeratosis, and increased numbers of prickle cells. (4,5) Numerous small, underdeveloped lobules are associated with abnormally developed and immature hair follicles. Most specimens feature a thick dermis that is marked by increased vascularity and prolific fibrous connective tissue. (4)

The second phase usually begins around the time of puberty, and the progression to the new stage can become grossly apparent with the onset of changes in the color, size, and/or shape of the nevus. The color often changes from yellow-orange to purple or dark brown. Second-stage nevi can be verrucous or linear, and they can project an uneven or nodular appearance. Again, the nevus is either completely devoid of hair or nearly so. It will often feel firm or rubbery on palpation. Histologically, the papillomatous epidermal hyperplasia becomes more pronounced in the second phase. The well-developed sebaceous glands increase in size and number, and an abnormally high number of them arise in the dermis. (3) The number of apocrine glands also increases, and they can become cystic or hyperplastic. (4)

Not every sebaceous nevus advances to the third stage, which is characterized by the transformation of skin appendages into secondary neoplasms--namely, papilliferous syringadenomas and basal cell carcinomas. Of the 52 tumors found in sebaceous nevi in the Mehregan and Pinkus study, 21 were basal cell epitheliomas, eight were papilliferous syringadenomas, and six were hidradenomas. (4)

These findings prompted a change in the standard management of sebaceous nevi to include early and complete excision to preclude the development of neoplastic transformation.

Secondary neoplasms. Between 10 and 30% of sebaceous nevi become secondary neoplasms. (3, 4) Jones and Heyl studied 140 cases of sebaceous nevi and found that nearly 20% transformed into papilliferous syringadenomas. They also found that the incidence of basal cell carcinoma was less than half the rate that had been reported by Mehregan and Pinkus (6 and 14%, respectively). (3, 4) This difference might be attributable to Jones and Heyl's exclusion of specimens that exhibited "balaloid proliferation" but that had not yet demonstrated any malignant characteristics. In a recent article, Misago et al wrote that many basal cell carcinomas that have been reported to have arisen from sebaceous nevi were actually trichoblastomas. (6) They suggested that the incidence of basal cell carcinoma secondary to sebaceous nevi is actually lower than what has been reported in previous studies. (6) But regardless of the actual incidence of basal cell carcinomas, it has no effect on the management of sebaceous nevi because thei r neoplastic potential necessitates prompt, complete excision and long-term follow up.