The tentorial and intradural course of the trochlear nerve

Objective: To determine the tentorial and intradural course of the trochlear nerve from its tentorial penetration to its entrance into the superior orbital fissure and to appreciate its relationship to surrounding anatomic structures. Methods: six formalin, fixed human cadaveric heads were dissected with the operative microscope. Results: We found that the relationships were constant but the distances varied. We noted the initial penetration of the trochlear nerve at the tentorium and measured the distance to the oculomotor nerve, anterior clinoid, posterior clinoid, and the junction of the lateral edge of the optic nerve and falciform ligament. In addition, the distance from the superior edge of the tentorium and cavernous sinus, from original penetration to the entrance of the superior orbital fissure, was appreciated. The trochlear nerve is closest to the oculomotor nerve in the posterior cavernous sinus dura and furthest anteriorly. The trochlear nerve was never superiorly located to the oculomotor nerve in the cavernous sinus. At the junction of the cavernous sinus and superior orbital fissure, the trochlear nerve diverges slightly laterally and superiorly becoming separated from the oculomotor nerve by a thin membrane. The trochlear nerve at this point becomes closely related, and in four of five cases, shares a dura) attachment with the lacrimal and frontal branches of the trigeminal nerve. The trochlear nerve enters the superior orbital fissure above the annulus of Zinn and beneath and between both the lacrimal and frontal nerves. Conclusion: The trochlear nerve has a fairly constant intradural and tentorial course. The knowledge of its relationships and anatomic course enables mobilization and safe removal of appropriate skull base lesions.