Oculomotor nerve palsy (ONP) may be caused by compression of the oculomotor nerve by the internal carotid artery-posterior communicating artery aneurysm (IC-PC AN) and/or basilar artery-superior cerebellar artery aneurysm (BA-SCA AN). During clipping, the oculomotor nerve can be observed directly and decompressed. However, the positional relationship between the aneurysm and oculomotor nerve is difficult to evaluate during intra-aneurysmal embolization. Constructive interference in steady state (CISS) allows detailed study of blood vessels and nerves in subarachnoid cisterns with high resolution. We herein report two cases of cerebral aneurysms presenting with ONP that were anatomically evaluated using CISS. Case 1 involved an 81-year-old woman who presented with a severe headache. She had no other neurological deficits. Computed tomography (CT) showed subarachnoid hemorrhage, while cerebral angiography revealed a left BA-SCA AN. Intra-aneurysmal embolization was performed; the framing coil spread throughout the aneurysm, and the other coils were placed using the Russian doll technique. The volume embolization rate was 43.4%. The following day, she developed left ONP. CISS on the 10th postoperative day revealed that the aneurysm was in contact with the left oculomotor nerve. The patient's ONP gradually improved although the recovery was partial. Case 2 involved a 62-year-old woman who presented with a headache and subsequent right ONP. CT showed no subarachnoid hemorrhage, and cerebral angiography revealed the right IC-PC AN with a posterolateral projection. The aneurysm was constricted in the middle portion. CISS visualized that the right oculomotor nerve spanned along with the constriction. Intra-aneurysmal embolization was performed; the aneurysm was compartmentalized with the segment around the apex and the neck and embolized in each segment intentionally by the double-catheter technique while preventing damage to the oculomotor nerve. The volume embolization rate was 44.0%. The patient's ONP steadily improved, and she fully recovered 1 year later. In endovascular treatment, anatomical information regarding structures around blood vessels is challenging to comprehend. CISS may be helpful for the evaluation of the surrounding structures. Utilizing information from CISS for endovascular treatment remains a challenge for the future.
