Arachnoid Cyst in Oculomotor Cistern

Oculomotor cistern is normal anatomic structure that is like an arachnoid-lined cerebrospinal fluid-filled sleeve, containing oculomotor nerve. We report a case of arachnoid cyst in oculomotor cistern, manifesting as oculomotor nerve palsy. The oblique sagittal MRI, parallel to the oculomotor nerve, showed well-defined and enlarged subarachnoid spaces along the course of oculomotor nerve. Simple fenestration was done with immediate regression of symptom. When a disease develops in oculomotor cistern, precise evaluation with proper MRI sequence should be performed to rule out tumorous condition and prevent injury of the oculomotor nerve.