Benign melanocytic proliferative lesions form a spectrum of disorders ranging from a massive accumulation of cells in multiple tissue elements to epidermal foci with a simple increase in epidermal melanocyte number. Here these lesions are separated into melanocytic neoplasias and melanocytic hyperplasias. The term melanocytic neoplasia is used to describe the presence of melanocytic cells in epidermal nests [defined as three or more melanocytic cells in direct contact (also known as thèque)], within the dermis, or in other tissues. The melanocytic neoplasms are referred to as nevi (singular: nevus) and the melanocytic cells forming these nevi are referred to as nevomelanocytes. The term melanocytic hyperplasia is used to indicate increased melanocytes confined to the basal layer of the epidermis.
The specific molecular events causing melanocytic neoplasias and hyperplasias are beginning to be defined. Given the presence of immature (less melanized) cells in many melanocytic neoplasias, it seems likely that specific underlying mutations (such as N-RAS, GNAQ, and B-RAF) disrupt normal melanocytic development and result in accumulation of the nevomelanocytic cells that do not complete typical migration and differentiation. The melanocytic hyperplasias are comprised of epidermal melanocytes at an increased concentration and thus alterations of normal melanocytic homeostatic mechanisms appear to be operative. These alterations could be due to a primary melanocytic defect [such as an ultraviolet (UV)-induced mutation] or homeostatic signaling changes in the local environment (possibly driven by mutations within the local keratinocytes, fibroblasts, or other resident cells).
This heterogeneous group of disorders is currently loosely subcategorized based on clinical features and microscopic characteristics. The neoplasias described include congenital nevomelanocytic nevi (CNNs), nevus spilus, common acquired nevomelanocytic nevi (excluding atypical/dysplastic nevi covered in Chapter 123), blue nevi, pigmented spindle cell nevi (PSCN), Spitz nevi, and nodal nevi. Cutaneous melanoma is covered separately in Chapter 124. The benign melanocytic hyperplasias described in this chapter include lentigo simplex (including agminated lentigines) and solar lentigines.
Congenital Nevomelanocytic Nevus at a Glance
- Melanocytic neoplasm often exhibiting extensive nevomelanocytic infiltration of the dermis with potential involvement of the underlying adipose and muscular tissue.
- Lesions may be small or cover a substantial portion of the body surface. Nevi are generally darker than the surrounding skin, have a rugose or smooth elevated surface, and may demonstrate long, darker, thicker hairs.
- Present at birth (or shortly thereafter—tardive congenital nevomelanocytic nevus).
- Large lesions have a significant risk of melanoma development. Cranial or midline lesions and large congenital nevomelanocytic nevus lesions with satellite lesions have an increased risk of leptomeningeal involvement.
- Synonyms: garment nevus, nevus pigmentosus et pilosus, giant nevus, verrucous nevus, and giant pigmented nevus.
The vast majority of CNNs noted at birth are small and singular, and no gender predilection has been demonstrated. The most reliable prevalence rate for CNN in a homogeneous ethnic group was obtained by biopsying all pigmented lesions in 841 white infants examined ...