= rare sporadic congenital syndrome characterized by large multiple melanocytic nevi (in 5–15%) + melanotic lesions of CNS (in 40–60%)

Age: first 2 years of life (most); 2^nd / 3^rd decade (less common); M÷F = 1÷1

Cause: abnormal migration of melanocyte precursors, abnormal expression of melanin-producing genes within leptomeningeal cells, rapid proliferation of melanin-producing leptomeningeal cells

Histo: abnormal abundance of melanotic cells (which are normally found in basilar leptomeninges) with concomitant infiltration of perivascular spaces

• increased intracranial pressure
• seizures, ataxia, cranial nerve VI + VII palsies
• high attenuation of melanin pigments on CT scan
• hyperintense on T1WI, hypointense on T2WI (← paramagnetic effect of oxygen-free radicals in melanin)
• leptomeningeal melanosis = foci of abnormally thickened leptomeninges
  Location: inferior surface of cerebellum; inferior surface of frontal, temporal, occipital lobes; ventral aspect of pons; cerebral peduncles; upper cervical spinal cord
• parenchymal melanosis (less common)
  Location: cerebellum, anterior temporal lobes (esp. amygdala)
• frankly hemorrhagic necrotic invasive mass with transformation into malignant melanoma
• hydrocephalus
• posterior fossa cyst
• cerebellar hypoplasia
• Dandy-Walker malformation
• syringomyelia
• intraspinal arachnoid cyst
• intraspinal lipoma

Prognosis: rapid deterioration + death within 3 years of diagnosis ← development of malignant melanoma / complication of hydrocephalus