A retrocerebellar arachnoid cyst causing syringomyelia is extremely rare without tonsillar herniation. The authors present a 44-year-old woman with symptoms of foramen magnum compression and syringomyelia. Magnetic resonance imaging demonstrated a large retrocerebellar arachnoid cyst with a large cervicothoracic syrinx but no signs of tonsillar herniation or hydrocephalus. The patient underwent a foramen magnum decompression with C1 laminectomy, microsurgical fenestration of the cyst, and duraplasty. After successful reconstruction of CSF flow, the patient experienced a relief of symptoms and a significant reduction of the syrinx. The intraoperative findings support the theory of a piston mechanism in the development of syringomyelia. Additional arachnoidal adhesions may also obstruct the CSF flow around the craniocervical junction. We recommend the surgical treatment should consist of an adequate decompression of the foramen magnum, wide microsurgical arachnoidal debridement, and duraplasty with autologous grafts sutured in a watertight way.
Keywords: Arachnoid cyst; Cerebrospinal fluid circulation; Foramen magnum; Microsurgical fenestration; Posterior fossa; Syringomyelia.