Onodi cell mucocele causing isolated trochlear nerve palsy: A case report

RATIONALE: Acquired vertical diplopia is commonly observed in trochlear nerve palsy, often resulting from blunt head trauma or vascular problems. It is rarely caused by tumorous conditions or space occupying lesion. We report the first case of Onodi cell mucocele causing isolated trochlear nerve palsy. PATIENT CONCERNS: A 62-year-old male noticed a double vision which worsened when looking down. On ophthalmologic examinations, the patient showed no abnormalities in visual acuity, intraocular pressure, and no swelling in optic disc. In ocular motility test, he was notable for 10° left hypertropia in primary position, 6° in right head tilt position, 14° in left head tilt position and this was aggravated in right and down gaze. Ostiomeatal complex CT depicted an expansile soft tissue density completely filling the left Onodi cell. DIAGNOSES: He was diagnosed with a trochlear nerve palsy caused by an isolated mucocele in the left Onodi cell. INTERVENTIONS: Three days after presentation, he underwent endoscopic sinus surgery for marsupialization of the mucocele in the left Onodi cell. OUTCOMES: The mucocele was completely removed through the endoscopic endonasal approach. Within 4 months after surgery, his 4th nerve palsy had gradually and completely improved. LESSONS: Onodi cell mucoceles that cause trochlear nerve palsy are extremely rare. Timely surgical decompression is essential to achieve optimal recovery of the neural function. Combined trochlear nerve palsy should be evaluated when Onodi cell mucocele involves the orbital apex from above.