3.1.1 Melanocytic Nevus

Acquired benign neoplasms of melanocytic lineage are termed melanocytic nevi (plural of nevus). Melanocytic nevi are collections of melanocytes (nevus cells) in the epidermis (junctional nevus), dermis (dermal nevus) or both (compound nevus).

Melanocytes are derived from the neural crest during embryogenesis. Two developmental pathways give rise to melanocyte progenitor cells. The dorsolateral pathway generates melanoblasts that travel through the mesoderm to colonize the epidermis, mucosa, and hair follicles. A second population of neural crest cells gives rise to bipotent precursors of Schwann cells and melanocytes that migrate ventromedially to reach the skin.

Acquired melanocytic nevi express a high diversity of clinical appearance and histomorphology. Age of onset mostly during first two decades of life. Common acquired nevi have the highest BRAF mutation frequency. Increased exposure to UV light in childhood seems to be a promoting factor. Melanocytic nevi tend to disappear with aging.

Lesions present as mostly pigmented or less commonly skin-coloured macular, papular or nodular lesions. Atypical (by dermpathology dysplastic) nevi are often asymmetrical with a red-brown border and non-homogenous pigmentation. They fulfil the ABCD(E) rules, different melanoma types are a differential diagnosis and must be ruled out (see Melanoma and atypical nevi 9.7.).

Melanocytic nevi may occur anywhere on the body, the scalp or plantar surfaces, interdigital folds, the umbilicus or sometimes on the oral and ano-genital mucosa.

There are several variants of melanocytic nevi which include: common acquired nevus, congenital nevus, blue nevus, Mongolian spot, congenital (hairy and non-hairy) nevus (including small, medium, large and giant), halo nevus, Spit/Reed nevus, Wiesner nevus, combined nevus and nevi associated with syndromes. A halo nevus is a mole surrounded by a white ring. These halo naevi are also called Sutton nevi. They are more common in children and young adults.

The histological examination shows increased melanocytic cells, single or in nests in the basal layer of the epidermis or dropping down in the dermis. This helps distinguish between epidermal proliferation of melanocytes (junctional nevus), dermal proliferation (dermal nevus), or proliferation at both levels (compound nevus). Often macrophages storing melanin pigment can be found. Melanocytic nevi with histological signs of atypia are referred to as dysplastic nevi.

ABCDE rules/’ugly duck’/’mole out of context’ concept may help distinguish benign from malignant lesions. The diagnosis is generally made on clinical features alone. Dermatoscopy (where images are magnified x10) is a useful tool to increase diagnostic accuracy. Histological examination is required if the lesion is clinically unusual to rule out melanoma.

ABCDE rules/’ugly duck’/’mole out of context’ concept may help distinguish benign from malignant lesions. The diagnosis is generally made on clinical features alone. Dermatoscopy (where images are magnified x10) is a useful tool to increase diagnostic accuracy. Histological examination is required if the lesion is clinically unusual to rule out melanoma.

The differential diagnosis includes other pigmented, usually acquired skin lesions such as

• early melanoma,

• pigmented seborrheic keratosis (melanoacanthoma),

• pigmented basal cell carcinoma;

• less often histiocytoma, haemangioma, fibroma.

Observation (using the ABCDE rules to guide decision by the patient or clinician) is encouraged, but sometimes excision is required if there is diagnostic doubt or functional problems. Digital images in the clinical record may be helpful to facilitate accurate long-term follow-up. Visible change in the digital record within three months of follow-up is an absolute indication for excision. Abstention is preferred if there is no suspicion of atypical nevi. Excision may be performed if there is aesthetic discomfort, irritated nevi by trauma or localization or in case of a suspicious melanoma. Patients with giant congenital nevi or high numbers of dysplastic nevi must be regularly screened for melanoma.