Objective: To define the characteristics of the arachnoidal sleeve and cistern that accompany the oculomotor nerve through the cavernous sinus roof.
Methods: Forty cavernous sinuses were examined using 3 to 40x magnification. Information was obtained about the size of the oculomotor cistern and its relationship to the roof of the cavernous sinus and anterior clinoid process.
Results: An arachnoidal sleeve and cistern, referred to as the oculomotor cistern, accompanied the oculomotor nerve into the roof of all the cavernous sinuses examined. The oculomotor cistern extends from the oculomotor porus, where the nerve enters the roof of the cavernous sinus, to the area below the tip or the adjacent part of the lower margin of the anterior clinoid process. From the porus, the nerve passes forward and downward to the depth of the cistern where it becomes incorporated into the fibrous lateral wall of the cavernous sinus. The width of the cistern was maximal at the oculomotor porus averaged 5.5 mm (range, 3.0-9.2 mm), and tapered slightly towards the midpoint and deep end of the cistern. The cistern's average length was 6.5 mm (range, 3.0-11.0 mm). The oculomotor nerve usually coursed closer to the anterior than the posterior wall of the cistern at the level of the oculomotor porus.
Conclusion: The oculomotor cistern, an arachnoidal and dural cuff, accompanies the oculomotor nerve through the cavernous sinus roof to the area just below or anterior to the lower edge of the tip of the anterior clinoid process. The segment of the nerve inside the oculomotor cistern is interposed between its free portion in the interpeduncular cistern and the part of its course where it is incorporated into the fibrous lateral wall of the cavernous sinus. The cistern can be opened to aid in the exposure and mobilization of the nerve in dealing with pathology in the area.