3.1.2 Congenital Melanocytic Nevus

Congenital melanocytic nevi (CN) are collection of benign pigmented melanocytes in epidermis and dermis, present at birth, often associated with increased numbers of hair follicles.

Circumscribed brown to black nodules or plaques, often with hypertrichosis. Small and medium- sized congenital melanocytic nevi are usually round or oval and fairly symmetric.

Projected adult size (PAS) is the estimated size of nevus at birth expected to reach in adulthood:

PAS 1-3 cm (Small CN): common and risk of melanoma similar to common nevi. Incidence = 1:100.

PAS 3-20 cm (medium CN). Incidence=uncommon, 1:1000-1:20000, rarely malignant change.

PAS >20 cm (large CN)

PAS >40 cm (giant CN)

CN >20 cm usually are on the trunk (i.e. bathing trunk naevus): Incidence = 1:500000; risk of melanoma 3-5%.

In large or giant CN loci of melanin-producing cells within the brain parenchyma, found on magnetic resonance imaging in ∼20% of affected children. Other neurological associations comprise communicating hydrocephalus, arachnoid cysts, syringomyelia, tumors (including astrocytoma, choroid plexus papilloma, ependymoma, and pineal germinoma), and malformations such as Dandy–Walker or Arnold–Chiari. Congenital nevus syndrome is the term that has been introduced for the combination of cutaneous CN and neurological abnormalities.

Accumulation of melanocytes at the epidermal-dermal junction and in the dermis, often extending deeply, into the lower reticular dermis, subcutaneous fat and fascia, and even deeper. Helpful is the presence of nevus cells surrounding (cuffing) and within the walls of blood vessels, within adnexal structures such as hair follicles and sweat glands, and within cutaneous nerves, particularly when in the lower half of the reticular dermis.

Permanent, no tendency to regression. Mean age at diagnosis of melanoma: 15.5 years.

Melanoma. In giant naevi, risk of leptomeningeal melanosis. Lumbosacral: spina bifida, myelomeningocele. Increased risk of neurological disorders in large (>20 cm) and giant nevi in >40 cm or those is expected to reach this size in adulthood.

Histiocytoma, haemangioma, fibroma, melanoma, plexiform neurofibromas. The differential diagnosis of small congenital nevi includes congenital smooth muscle hamartoma, atypical nevus, melanoma, Becker melanosis.

Preventive excision has not shown clinical benefits. The excision is indicated if the congenital nevus exhibits atypical features and melanoma has to be ruled out. For cosmetic reasons the surgical excision of large or congenital nevi is controversial. Excision can be recommended of lesions that are itching, traumatised or in case of verrucous tumours. In large lesions or tumour in visible areas dermabrasion shortly after birth or laser ablation has been suggested as an alternative to surgical excision.

The decision to remove a lesion is linked to the risk of melanoma and the cosmetically disfiguring appearance. However, it is very difficult to remove large or giant congenital nevi completely, and thus the risk of melanoma is reduced but not abolished.