Hypermelanotic nevus: clinical, histopathologic, and ultrastructural features in 316 cases.

We report on a series of benign melanocytic nevi that have unique clinical, histopathologic, and ultrastructural features. Between March 1993 and February 1994, 316 examples of hypermelanotic nevi were received by the dermatopathology laboratory at Denver General Hospital. Our study identified the clinical characteristics, histopathologic criteria, and ultrastructure of this lesion. Clinically, the lesions were dark brown to black macules or papules. The most common location was the back. There was a slight female predominance, and the mean age of our patients was 40 years. Histopathologically, the nevus showed the following characteristics: (a) melanin within a compact stratum corneum, (b) small nests of nevus cells at the dermal-epidermal junction and (in 52% of the cases), nests within the papillary dermis, (c) heavy melanin within keratinocytes in the lower epidermis, (d) a sparse to moderate lymphocytic infiltrate and melanophages in the superficial dermis, and (e) an absence of cytologic atypia. Electron microscopy revealed that abundant melanin was packaged in melanosome complexes within keratinocytes. Less pigmented melanocytes and nevus cells contained well-developed dendritic processes and golgi, indicative of efficient melanin transfer. According to our retrospective case control analysis, patients with hypermelanotic nevi were older and more likely to be male than those with ordinary nevi. Hypermelanotic nevi were more likely than controls to be junctional nevi; they were smaller, dark brown or black in color, and clinically suspicious for melanoma. We propose the name "hypermelanotic nevus" to describe this benign lesion, which is often biopsied to exclude melanoma.