A large, or giant, congenital melanocytic nevus (LCMN or GCMN) is a pigmented skin lesion of more than 20 cm - or 40 cm- respectively, projected adult diameter, composed of melanocytes, and presenting with an elevated risk of malignant transformation.
LCMN has a prevalence of about 1/20,000, while GCMN is estimated to occur in 1/50,000 to 1/500,000 births, becoming increasingly rare as more body surface is implicated. While present in all ethnic groups examined to date and in both genders, there is a slight female predominance.
CMN develop during the first trimester of pregnancy. An LCMN is a darkly colored, circumscribed area of the skin sometimes covered with dense hair or proliferative nodules, and/or accompanied by multiple small satellite nevi that develop at birth or during early childhood (tardive satellites). Certain forms of LCMN entail only congenital "satellites" (also known as "multiple medium CMN") with a total surface, when summed, similar to that of other LCMN; these forms seem to be associated more frequently with complications. Neurological involvement, such as leptomeningeal melanocytosis (see this term), epilepsy, hydrocephalus and tethered spinal cord has been observed. Patients with LCMN also present an elevated risk of malignant pediatric melanoma, particularly intracranial, and other neuroectodermal tumors of varying severity (rhabdomyosarcoma, schwannoma (see these terms), lipoma, neurofibroma).
LCMN is a neurocristopathy (a disorder of the development of the embryonic neural crest) but its etiology is unknown.
The diagnosis is clinical. Magnetic resonance imaging (MRI) and neurological evaluations are also performed in order to screen for complications. Skin biopsy shows an abnormal histological accumulation of melanocytes at the epidermal/dermal junction and below, even to the subfascia, as well as disorganization of the dermis. When malignant transformation is suspected, biopsy is necessary.
Differential diagnoses include atypical mole, Becker's nevus syndrome, malignant melanoma, nevus of Ota, nevus of Ito, Spitz nevus, blue nevus, and congenital smooth muscle hamartome (see these terms).
Familial cases have been observed, but the vast majority of LCMN cases are sporadic. A paradominant mode of transmission (postzygotic mutation occurring at an early developmental stage and resulting in somatic loss of heterozygosity) has been proposed to explain this observation.
Because of possible malignant transformation, dermatological follow-up of the nevus should be regular. In some cases excision may be advisable by an experienced plastic surgeon. Dermabrasion, curettage and laser treatments are performed but are currently considered inferior options to full-thickness removal, sometimes in stages, or simple surveillance.
The majority of patients have normal adult lives without any complications.
Last update: January 2011