The cisternal segments of the oculomotor nerve: a magnetic resonance imaging study.

PURPOSE: The cisternal segments of the oculomotor nerve (OMN), which courses through the interpeduncular and oculomotor cisterns (OMC) have not been well delineated on neuroimages. The present study aimed to explore the cisternal segments of the OMN using magnetic resonance (MR) imaging.

METHODS: A total of 92 patients were enrolled in this study. A constructive interference in steady-state sequence was performed in coronal and axial sections.

RESULTS: On coronal images, cisternal portions of the OMN were entirely delineated in 97 % on the right and in 98.5 % on the left. Most of the OMCs were of a round shape, with a centrally located OMN, while 9 % were ectatic with the OMN located eccentrically. In 5.3 % of cases, fetal-type posterior communicating arteries (PCoAs), which coursed adjacent to the superior surfaces of the OMNs at the oculomotor triangle (OMT), were observed. On axial images, cisternal portions of the OMN were identified in all cases. The OMN segment passing through the OMT showed medial, central, and lateral courses. The PCoAs and P2 segments of the posterior cerebral artery (PCA) were adjacent to the OMNs in 17 and 19 % of cases, respectively.

CONCLUSIONS: The OMN most frequently courses in the medial part of the OMT and enters into the OMC. These findings indicate that OMN paresis can be caused by vascular compression at any site of the interpeduncular cistern and OMT.