The hallmark of neurocutaneous melanosis in the neonatal period is the presence of a large bilateral hairy dark nevus with satellite nevi over the trunk and neck (Figure 303.1). The diagnosis should be considered in neonates with large pigmented nevi and in those with more than three hairy dark nevi regardless of their size. Neonates with neurocutaneous melanosis are at risk of developing neurological problems. The neurological complications of neurocutaneous melanosis are hydrocephalus, seizures, cranial nerve dysfunction, and signs of spinal cord and root involvement.
Hydrocephalus may have different causes. The most frequent cause of hydrocephalus is obstruction of the cerebrospinal fluid flow at the base of the skull due to thickening of the meninges (communicating hydrocephalus). This phenomenon occurs in neonates with a pigmented nevus in the distribution of the neck. Thickening of the meninges results from melanocytic infiltration of the arachnoid. Noncommunicating hydrocephalus due to aqueductal stenosis may also occur.
Magnetic resonance imaging of the area underlying the nevus or guided by the neurological findings is the study of choice in neonates with neurocutaneous melanosis. The most frequent MRI finding is the presence of areas of increased signal on T1. These areas of increased signal represent accumulation of melanocytic cells (Figure 303.1 B). The most frequent areas of melanocytic cell infiltration are the anterior temporal region, close to the amygdala, and the cerebellum. Malignant transformation of these areas is suggested by the presence of necrosis, hemorrhages, edema, growth, or contrast enhancement by CT.
The cerebrospinal fluid of neonates with neurocutaneous melanosis may reveal increased protein, decreased sugar, and a normal cell count. Melanin-filled cells are sometimes found in the cerebrospinal fluid. Neurocutaneous melanosis is a sporadic condition.

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Figure 303.1.— Neurocutaneous melanosis. [A] Hyperpigmented nevus; [B] MRI findings showing bilateral increased signal from deep brain regions and the left mesial temporal lobe.

MRI of the spine may demostrate the presence of an arachnoid cyst (Figure 303.2).

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Figure 303.2.— Neurocutaneous melanosis. [A and B] Large dorsal arachnoid cyst with mass effect.