This chapter discusses melanocytic nevi, defined as benign neoplasms of melanocytes, most of which manifest themselves as cutaneous pigmented lesions. They are characterized by the presence of banal melanocytic cells in nests (defined as three or more melanocytes in direct contact [also known as thèque]), within the epidermis, dermis, or in other tissues. The melanocytic cells forming these nevi are referred to as nevomelanocytes. The term melanocytic hyperplasia is a descriptive term used to indicate increased melanocytes confined to the basal layer of the epidermis in the absence of nest formation.
Simplistically, melanocytic nevi can be divided into those that arise from junctional melanocytes (often termed acquired nevi) and those that arise from neural-crest derived melanocytic precursors that migrate along neurovascular bundles, stop before they reach the epidermis, and proliferate at those sites. The latter are termed congenital melanocytic nevi (CMNs).
Dysplastic nevi are defined as those that display atypical architectural and cytologic features. Historically, the nomenclature associated with these lesions includes B-K moles (recognizing the first two families described with such lesions whose surnames began with the letters B and K),1 familial atypical multiple mole and melanoma syndrome,2 atypical mole syndrome,3 Clark nevus,4 atypical moles, and nevus with architectural disorder and (varying degrees of) cytologic atypia.5 Although the nomenclature for these lesions remains contentious, the term dysplastic nevus (DN) is most frequently utilized5,6 and is the term that will be used in this chapter. It is also important to note that in practice, the terms dysplastic and atypical are sometimes used as a modifier for other melanocytic neoplasias (eg, Spitz nevi) or hyperplasias to indicate a variation from a typical pattern or to recognize an increased concern for malignant biologic potential.
The nevi described in this chapter include CMNs, nevus spilus, common acquired melanocytic nevi, blue nevi, Spitz nevi, pigmented spindle cell nevi (PSCN), dysplastic nevi, and nodal nevi. Benign melanocytic hyperplasias include lentigo simplex and solar lentigo.
CONGENITAL MELANOCYTIC NEVI
Pigmented neoplasms of melanocytes that are evident at birth or shortly thereafter.
Lesions may be small, medium-sized, or large and cover substantial body surface area.
Large or giant lesions have a significant risk for melanoma development. Cranial or midline congenital melanocytic nevi and those with satellite lesions have an increased risk of leptomeningeal involvement.
Histology shows extensive nevomelanocytic infiltration of the dermis, accentuation along the adventitia of skin adnexa and nerves, single-filing between collagen bundles, often with involvement of subcutaneous tissues.
CMNs are found in approximately 1% to 3% of neonates across ethnicities.7 The majority of lesions are small to medium in size and are present at birth; however, there are also congenital nevi (>1.5 cm) that appear for the first time between 1 month and 2 years of life (termed tardive congenital ...