Patients present with large or multiple congenital melanocytic nevi with benign or malignant proliferation of melanocytes in the leptomeninges. The classic appearance is a large melanocytic nevus in a posterior axial location with satellite melanocytic nevi. Leptomeningeal involvement can result in hydrocephalus, spinal cord compression, or other mass effect, usually in the first 2 years of life. Neurological symptoms are usually progressive and rapidly fatal.
MR reveals thickened and enhancing leptomeninges (predominantly over the cerebral convexity and quadrigeminal plate cistern), ventricular dilatation, and inferior vermian hypoplasia. Focal parenchymal melanocytomas can also be seen, sometimes in the absence of leptomeningeal involvement. Leptomeningeal T1-hyperintensity has been reported, but it is not unusual to have amelanotic melanocytoma or amelanotic melanocytosis, so contrast-enhanced images are necessary for full evaluation.
The post-contrast images of the brain and spine above reveal diffuse meningeal enhancement and mild residual dilatation of the ventricles post ventriculoperitoneal shunt (not shown). The enhancement is both leptomeningeal (predominantly on the left) and pachymeningeal. The imaging findings are not pathognomonic. Differential considerations for leptomeningeal enhancement include chronic meningitis (tuberculosis or coccidioidomycosis), and leptomeningeal metastases (e.g., medulloblastoma, ependymoma, high-grade astrocytoma, pineoblastoma and choroid plexus carcinoma).
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