Reversible cognitive and gait impairment by a giant quadrigeminal cistern arachnoid cyst: A case report of marked reversal following neuroendoscopic fenestration

Background: Quadrigeminal cistern arachnoid cysts are rare in adults, but can cause reversible gait and cognitive impairment secondary to obstructive hydrocephalus. Differentiating these structural lesions from idiopathic normal-pressure hydrocephalus (iNPH) or neurodegenerative diseases is essential because they are surgically treatable. Case Description: A 71-year-old woman presented with progressive gait disturbance and mild cognitive impairment. Neuropsychological tests showed a Montreal Cognitive Assessment–Japanese version (MoCA-J) score of 21/30 and a frontal assessment battery (FAB) score of 13/18, indicating executive dysfunction. Standard cognitive screening such as mini-mental state examination did not detect the subtle deficits identified by MoCA. Magnetic resonance imaging revealed a giant arachnoid cyst in the quadrigeminal cistern compressing the midbrain and cerebral aqueduct, resulting in obstructive hydrocephalus. Endoscopic cyst fenestration was performed through a right frontal burr hole. The cyst was opened and communication with the third ventricle was achieved. Gait and cognitive functions markedly improved postoperatively (MoCA-J 26/30, FAB 17/18) and radiographic findings showed that the hydrocephalus had resolved. Ocular symptoms, including difficulty gazing upward, also resolved after surgery. Conclusion: Quadrigeminal cistern arachnoid cysts can mimic iNPH or neurodegenerative disorders, but represent a reversible cause of cognitive and gait impairments. Detailed neuroimaging and sensitive cognitive assessments are essential for diagnosis. Neuroendoscopic fenestration provides a treatment, i.e., minimally invasive and effective, even in elderly patients.