Anterolaterally Projecting Clinoid Segment Aneurysm Causing Oculomotor Palsy, with an Anatomical Review of the Clinoid Segment of the Internal Carotid Artery

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We describe a rare case of an anterolaterally projecting clinoid segment aneurysm of the internal carotid artery (ICA) causing oculomotor palsy. A 76-year-old woman was referred to our facility because of right oculomotor palsy that had been found just before surgery to remove bilateral cataracts. Neuroimaging revealed that the patient had an aneurysm at the clinoid segment that projected anterolaterally, eroding the anterior clinoid process. The aneurysm was thought to be compressing the oculomotor nerve, which runs at the upper part of the lateral wall of the cavernous sinus, thereby causing oculomotor palsy. Endovascular coiling of the aneurysm was successfully performed, and the oculomotor palsy was alleviated postoperatively. Anatomically, there exists the carotid collar between the arterial wall of the clinoid segment and the anterior clinoid process, containing the clinoid venous plexus in it. Hence, the anterolateral wall of the clinoid segment, although protected by a stiff bony structure, has an anatomical base that allows it to protrude centrifugally. Once protrusion occurs, the bone may be eroded by remodeling caused by the aneurysm’s pulsed beating.