Neurocutaneous Melanosis: Prenatal Presentation as a Posterior Fossa Cyst

A 35-year-old mother with fetal magnetic resonance imaging (MRI) at fetal age of 34^4/7 weeks was presented to characterize findings of macrocephaly, ventriculomegaly, and a posterior fossa fluid collection seen on prenatal ultrasound. A large posterior fossa cyst was seen ([Fig. 1]) as the cause of ventriculomegaly due to mass effect. The postpartum course was uneventful. The neonate was noted to have melanocytic nevi. Postnatal brain MRI was also obtained on the third day of life, showing extensive melanin deposits in the cerebellum and amygdalae ([Figs. 2] and [3]).

Fig. 1 Fetal MRI. T2-weighted axial (A), coronal (B), and sagittal (C) images of the fetal brain demonstrate a large arachnoid cyst in the posterior fossa causing mass effect on the cerebellum and moderate supratentorial ventriculomegaly. Note the lack of severe vermian hypoplasia or upward vermian rotation, which would characterize a Dandy–Walker malformation. MRI, magnetic resonance imaging.

Fig. 2 Postnatal brain MRI. T2-weighted axial (A) and sagittal (B) images of the postnatal brain at day of life 3. Once again, there is a large posterior fossa arachnoid cyst causing mass effect on the cerebellum but no severe vermian hypoplasia or upward vermian rotation. MRI, magnetic resonance imaging.

Fig. 3 Postnatal brain MRI. T1-weighted unenhanced axial (A) and coronal (B) images of the brain at day of life 3 demonstrate multiple leptomeningeal T1-hyperintense linear deposits in the cerebellum. Melanin deposits are also present in the amygdalae, more conspicuous on the right. MRI, magnetic resonance imaging.

Neurocutaneous melanosis (NCM) is characterized by the presence of congenital melanocytic cutaneous nevi, an uncommon birthmark occurring in approximately 1 in 20,000 to 50,000 live births, and melanocytic deposits in the meninges or the brain parenchyma. NCM is caused by a mosaicism for heterozygous activating mutations in codon 61 of NRAS (neuroblastoma RAS viral oncogene homolog), a developmental gene involved in the control of key cell signaling pathways.[1] Melanin can be demonstrated with T1-weighted imaging and is manifested by parenchymal nodular or leptomeningeal linear T1-hyperintense lesions, particularly conspicuous in the unmyelinated brain.[2]

Most children with NCM are asymptomatic but have increased risk of developing melanoma. However, some children develop central nervous system (CNS) manifestations before 2 years of age (or even prenatally as in this case), mainly progressive hydrocephalus from leptomeningeal melanocytic infiltration and associated signs of intracranial hypertension, seizures, or cranial nerve dysfunction. Symptomatic children have poor prognosis with the majority dead within 3 years of symptom onset.

Dandy–Walker malformation has been described in association with NCM.[1] Pathogenesis of posterior fossa cystic anomalies involves impaired cerebrospinal fluid (CSF) absorption by melanocytic deposits along the leptomeninges of the cerebellum. Our case with formation of a posterior fossa arachnoid cyst expands the spectrum of prenatal presentations of NCM. As there are no specific imaging criteria for prenatal diagnosis, NCM should be included in the differential diagnosis of posterior fossa cystic abnormalities on fetal imaging, and T1-weighted images should be acquired on fetal MRI for potential detection of CNS melanin deposits.

Location of Work

The study was conducted at the University of Vermont Medical Center.

Publication History

Received: 11 September 2020

Accepted: 20 November 2020

Article published online:
14 January 2021

• References

• 1 Gocmen R, Guler E, Arslan EA. A case of neurocutaneous melanosis and neuroimaging findings. J Radiol Case Rep 2015; 9 (03) 1-6
• 2 Barros FS, Marussi VHR, Amaral LLF. et al. The rare neurocutaneous disorders: update on clinical, molecular, and neuroimaging features. Top Magn Reson Imaging 2018; 27 (06) 433-462