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Meningeal melanomatosis is an aggressive version of meningeal melanocytosis and one of the recognized primary melanocytic tumors of the CNS. It represents proliferation of melanocytic cells within the subarachnoid space 1. There is a strong association with cutaneous melanocytic lesions, in which case it is known as neurocutaneous melanosis 3.
Meningeal melanomatosis is the aggressive form of meningeal melanocytosis. Together they have frequently been referred to as diffuse melanosis although nomenclature seems somewhat variable.
Usually, the disease is discovered in childhood due to hydrocephalus from blocked subarachnoid and perivascular spaces.
The melanocytic cells found in the subarachnoid space are believed to arise from leptomeningeal melanocytes of neural crest origin 1. They range in thickness from bulky black cellular material entirely filling the subarachnoid space, to subtle clouding of the meninges 1.
These cells frequently extend to involve the perivascular spaces, often with direct invasion into the brain, a feature that is helpful in distinguishing melanomatosis from meningeal melanocytosis 1. Other features also reflect the more aggressive nature of melanomatosis: cellular atypia, high mitotic rate and necrosis.
hyperdensity (due to melanin) outlining sulci
melanin may be seen on unenhanced T1 weighted MRI as hyperintensity
diffuse enhancement of meninges in brain and spine (20%)
On CT hyperdensity within sulci on CT can mimic subarachnoid hemorrhage.
In addition to leptomeningeal dissemination or more aggressive primary or secondary melanocytic tumors of the CNS, leptomeningeal carcinomatosis, meningitis and even diffuse leptomeningeal glioneuronal tumor should be considered.