Large cogenital melanocytic nevus.

cause any symptoms
Congenital melanocytic naevi are usually asymptomatic, however, some may be itchy, particularly larger lesions. It is thought there may be reduced function of sebaceous (oil) and eccrine (sweat) glands, which may result in skin dryness and a heightened sensation of itch.

The overlying skin may become fragile and erode or ulcerate. Deep nests of melanocytes in the dermis may weaken the bonds between the epidermis and the dermis and account for skin fragility.

Congenital melanocytic naevi are often unsightly, especially when extensive, ie large or giant congenital melanocytic naevi. They may therefore result in anxiety and impaired self image, especially when the lesions are in visible areas.

Giant melanocytic naevi, and to a lesser degree small lesions, are associated with increased risk of developing cutaneous melanoma, neurocutaneous melanoma and rarely other tumours (see below).
causes congenital melanocytic naevi.
Congenital melanocytic naevi are caused by localised genetic abnormalities resulting in the proliferation of melanocytes; these are cells in the skin responsible for normal skin colour. This abnormal proliferation is thought to occur between the 5th and 24th weeks of gestation. If proliferation starts early in development, giant and medium sized congenital melanocytic naevi are formed . Smaller congenital melanocytic naevi are formed later in development, after the melanoblasts (immature melanocytes) have migrated from the neural crest to the skin .

In some cases, there are also overgrowth of hair-forming cells and epidermis, forming an organoid naevus.

Very early onset of congenital naevus before separation of the upper and lower eyelids results in kissing naevi, ie one part of the naevus is on the upper lid and the other part is on the lower eyelid.