Pupil-sparing Isolated Inferior Rectus Paresis Due to Compression of the Oculomotor Nerve by the Posterior Cerebral Artery.

Direct compression of the oculomotor nerve (OcN) is usually accompanied by OcN palsy, with an abnormal pupillary function. We herein present the case of a 76-year-old man with pupil-sparing paresis of the right inferior rectus muscle. A radiological examination revealed an epidermoid cyst at the right cerebellopontine angle, which came in contact with the right OcN. However, surgical intervention revealed compression of the inferomedial aspect of the OcN by the right posterior cerebral artery. Decompression of the nerve gradually improved the ocular movement. This unique case offers insights into the potential topographical arrangement of the OcN fibers.