Case Report: Reversible acquired Chiari-like state and upper cervical syringomyelia caused by a giant posterior tentorial tumor confined to the supratentorial compartment.

Acquired tonsillar descent with syringomyelia secondary to intracranial space-occupying lesions is rare, and supratentorial causes are particularly uncommon. These cases are of particular pathophysiological interest because they suggest that tonsillar descent and syrinx formation may arise as downstream consequences of altered intracranial compliance, compartmental pressure transmission, and impaired craniocervical cerebrospinal fluid buffering, rather than from a primary hindbrain malformation. We report a previously healthy 39-year-old woman who presented with progressive headache, papilledema, and left homonymous hemianopia. Imaging demonstrated a giant posterior tentorial dural-based mass confined to the supratentorial compartment, with marked mass effect, inferior tentorial deformation, 7-mm caudal tonsillar descent below McRae's line, and a small C1 syrinx. The patient underwent preoperative embolization and near-total microsurgical resection, followed 6 weeks later by single-fraction LINAC-based stereotactic radiosurgery to a small residual remnant. Follow-up MRI showed reduction of the remnant, resolution of the midline shift, restoration of normal tonsillar morphology and position, and complete resolution of the syrinx, accompanied by substantial visual recovery. The observed reversal after treatment of the causative lesion supports a model in which the supratentorial mass secondarily disturbed craniocervical hydrodynamics and produced a reversible acquired Chiari-like state with upper cervical syringomyelia. This case highlights the need for careful cranial imaging before posterior fossa decompression is considered in adults with newly identified tonsillar descent and syringomyelia.