Trochlear Nerve Palsy Caused by a Quadrigeminal Cistern Arachnoid Cyst Successfully Treated with a Hybrid Exoscope–endoscope Keyhole Approach: A Case Report

Quadrigeminal cistern arachnoid cysts are rare intracranial lesions. While most intracranial arachnoid cysts in adults are incidentally detected and asymptomatic, quadrigeminal cistern arachnoid cysts, located adjacent to the dorsal midbrain and cerebral aqueduct, may become symptomatic, most often with obstructive hydrocephalus. Trochlear nerve palsy secondary to quadrigeminal cistern arachnoid cysts has been reported only rarely, and the pathophysiology and optimal management remain unclear. We report a 60-yr-old man who presented with progressive diplopia due to left trochlear nerve palsy. Magnetic resonance imaging revealed a 35-mm arachnoid cyst in the quadrigeminal cistern compressing the dorsal midbrain near the left trochlear nerve without hydrocephalus; no other causative lesion was identified. As the diplopia interfered with the patients daily activities, hybrid exoscope–endoscope–assisted keyhole cyst fenestration was performed 1 month after symptom onset. Communication between the cyst cavity and the ambient and superior cerebellar cisterns was successfully established to reduce the risk of cyst re-expansion. Postoperatively, the diplopia improved rapidly. Follow-up imaging demonstrated cyst shrinkage and decompression around the dorsal midbrain and left trochlear nerve. No perioperative complications were observed. This case supports a compressive mechanism for quadrigeminal cistern arachnoid cyst-related trochlear nerve palsy and suggests that timely surgical decompression may provide favorable outcomes when symptoms correlate with radiological compression. Hybrid exoscope–endoscope–assisted keyhole fenestration may be a useful surgical option for quadrigeminal cistern arachnoid cysts requiring surgical decompression.