Quadrigeminal cistern arachnoid cyst as a probable cause of hemifacial spasm

Arachnoid cysts arising in the quadrigeminal cistern (ACQCs) are uncommon. A 68-year-old woman presented with an unsteady gait, facial spasm, and cerebellar ataxia. Non-contrast head computed tomography showed a cystic mass centered in the quadrigeminal cistern accompanying ventriculomegaly. On MRI, the cyst appeared hypointense on T1- and hyperintense on T2-weighted sequence. There was no restricted diffusion on diffusion-weighted imaging. The cerebral aqueduct was obstructed and the prepontine cistern was narrowed. The left vertebral artery (VA) coursed adjacent to the facial nerve at its origin. The patient underwent neuroendoscopic fenestration of the posterior wall of the third ventricle and ventral wall of the ACQC. Postoperatively, the patient's symptoms resolved. MRI showed a considerable reduction in the ACQC and expansion of the prepontine cistern, whereas the relationship between the left VA and the proximal segment of the facial nerve did not change. We assumed that the pre-existing close relationship between the VA and facial nerve might have been aggravated by the anterior displacement of the brainstem, thus causing the facial spasm.