Trochlear Nerve Palsy Caused by a Superior Cerebellar Artery Aneurysm

Purpose: We report a case of trochlear nerve palsy caused by a superior cerebellar artery (SCA) aneurysm. Case summary: A 34-year-old woman visited our clinic complaining of distance diplopia and a temporal headache 7 days in duration. She had no previous relevant medical or trauma history. Her visual acuity; intraocular pressure; and pupil, anterior segment, and fundus evaluations were unremarkable. Extraocular muscle examination (EOM) revealed six prism diopters (PD) of right hypertropia in the primary gaze; this worsened when the head was tilted to the right. Brain magnetic resonance imaging revealed a round solid nodule between the temporal lobe and pons; we thus suspected a petrous ridge meningioma. She was referred to our neurosurgery department for meningioma treatment. However, 1 hour later, she visited our emergency room with severe headache and nausea. Brain computed tomography angiography revealed a subarachnoid hemorrhage and a ruptured SCA aneurysm. Emergency coil embolization was successfully performed. Four weeks after surgery, the right hypertropia was slightly decreased. Four months later, the diplopia had disappeared and EOM revealed orthotropia. Conclusions: SCA aneurysms are rare and can be misdiagnosed even after radiological examination. Our case emphasizes that an SCA aneurysm should be considered during the differential diagnosis of patients with diplopia and headache. J Korean Ophthalmol Soc 2022;63(2):216-220.