Neurocutaneous melanosis

Neurocutaneous melanosis, or neurocutaneous melanomatosis, is a rare sporadic phakomatosis characterised by dermalmultiple congenital cutaneous nevi and meningeal melanocytosis/meningeal melanomatosis. 

Epidemiology

Neurocutaneous melanosis tends to be diagnosed in the first few years of life, and no gender predilection is reported ^5,7. The condition is most frequently reported in Caucasians ⁷. 

Clinical presentation

The disease is characterised by multiple melanotic naevinevi in the skin. Approximately two-thirds of patients have a giant nevus covering the back, known as a "bathing trunk" congenital nevus ⁷.

The diagnosis is often made in the first few years of life when the child presents with hydrocephalus due to meningeal melanocytosis/meningeal melanomatosis (aka diffuse melanosis) ⁵. Occasionally a delayed presentation occurs towards the end of the second decade, in which case neuropsychiatric presentation is more common ⁷.

Diagnostic features have been proposed ²:

1. unduly large or unusually numerous pigmented(pigmented nevi in association with leptomeningeal melanosis or melanoma
2. no evidence of malignant change in any of the cutaneous lesions
3. no evidence of malignant melanoma in any organ apart from the meninges

Pathology

It is believed that neurocutaneous melanosis is the result of congenital dysplasia of melanoblasts (melanocyte precursors) which are of neural crest cells origin^1,4,7. These are located in the leptomeninges, globes, inner ear, sinonasal cavity and skin ^1,4. 

Radiographic features

For leptomeningeal imaging features, please refer to meningeal melanocytosis.

In addition to, or instead of leptomeningeal involvement, parenchymal changes are seen in some individuals characterised by T1 signal hyperintensity involving the mesial temporal lobes (particularly the amygdala), ventral pons and medulla⁶.

Dandy-Walker malformation is also not infrequently identified, cerebellum and inferior frontal lobes ^6,7. It is believed this is due to melanocytes tracking along perivascular spaces ⁷. 

Treatment and prognosis

Prognosis in symptomatic cases is extremely poor, even in the absence of malignant transformations, particularly in when associated with a Dandy-Walker malformation⁷. Hydrocephalus is the most common complication. Involvement of the cord also may result in myelopathy, syringomyelia and arachnoiditis. 

Malignant transformation of cutaneous naevi is variably reported from 2-13% ⁴. 

Malignant transformation of CNS melanosis occurs very frequently, in up to 50% ⁵.