Spinal arachnoid cyst containing nevus cells in a patient with a large congenital melanocytic nevus: case report.

Objective and Importance: Spinal arachnoid cysts are rare, and their cause and pathogenesis remain controversial. We experienced a rare case with a large congenital melanocytic nevus in which a spinal arachnoid cyst contained nevus cells, suggesting the congenital nature of a spinal arachnoid cyst.
Clinical Presentation: A 37-year-old Japanese man had been born with a large melanocytic nevus on his back. He experienced intermittent pain radiating to both thighs and to the lower back and waist. A magnetic resonance imaging study disclosed the presence of a posterior intradural extramedullary arachnoid cyst extending from T10 through T12. His spinal cord was displaced anteriorly and flattened.
Intervention: An osteoplastic laminoplasty was performed, and the arachnoid cyst was totally removed. The cyst membrane exhibited many foci of brown deposits, and histological examination disclosed the presence of melanin-containing cells in the cyst membrane. Morphologically and immunohistochemically, the melanin-containing cells in the cyst membrane were similar to nevus cells in the dermis.
Conclusion: The histological findings of our case suggest that the patient's spinal arachnoid cyst was formed at the same stage of development as the melanocytic nevus.