Congenital Melanocytic Nevus

The congenital melanocytic nevus is a type of melanocytic nevus (or mole) found in infants at birth. This type of birthmark occurs in about 1% of infants in the United States; it is located in the area of the head and neck 15% of the time.

The congenital melanocytic nevus (plural, nevi) presents as a circumscribed, light brown to black patch or plaque, potentially very heterogeneous in consistency, covering any size surface area and any part of the body. The appearance of the congenital melanocytic nevus is similar to those of nevi acquired after birth. However, congenital ones are usually larger in diameter and may have excess terminal hair, a condition called hypertrichosis. If over 40 cm projected adult diameter with hypertrichosis, it is sometimes called giant hairy nevus; more usually these largest forms are known as large or giant congenital melanocytic nevus. The estimated prevalence for the largest forms is 0.002% of births.

Microscopically, congenital melanocytic nevi appear similar to acquired nevi with two notable exceptions. For the congenital nevus, the neval cells are found deeper into the dermis. Also, the deeper nevus cells can be found along with neurovascular bundles, with both surrounding hair follicles, sebaceous glands, and subcutaneous fat. Such annexes and the hypodermis can also be hypoplasic or, conversely, present aspects of hamartoma.

Many are surgically removed for aesthetics and relief of psychosocial burden, but larger ones are also excised for prevention of cancer, although the benefit is impossible to assess for any individual patient. Proliferative nodules are usually biopsied and are regularly but not systematically found to be benign. Estimates of transformation into melanoma vary from 2-42% in the literature, but are most commonly considered to be at the low end of that spectrum due to early observer bias.