The microsurgical anatomy of the cisternal segment of the trochlear nerve, as seen through different neurosurgical operative windows

Objective. To describe the anatomy of the cisternal segment of the trochlear nerve as seen through different neurosurgical approaches. Methods. The cisternal course of ten trochlear nerves was observed in five cadaveric embalmed heads, through the view afforded by the median infratentorial-supracerebellar, the extreme-lateral infratentorial-supracerebellar, and the combined presigmoid-subtemporal transtentorial approaches. The relationships of the trochlear nerve with the surrounding neuro-vascular structures were analyzed. Results. We identified 3 segments of the cisternal trochlear nerve: quadrigeminal, ambient and tentorial. The median infratentorial-supracerebellar approach allowed exposure of the quadrigeminal segment, including the origin of the nerve. The extreme-lateral supracerebellar and the combined presigmoid-subtemporal transtentorial approaches provided visualization of the ambient and tentorial segments of the nerve. The tentorial segment runs in a dural canal contained in the free edge of the tentorium, surrounded by its own arachnoidal sleeve. Conclusion. The trochlear nerve is a very delicate structure that can be easily injured during approaches to the tentorial incisura. Accurate knowledge of its anatomy as seen through different operative windows is helpful in maintaining its integrity during surgery.