Trochlear Nerve (Fourth Cranial Nerve)
The trochlear nerve (CN IV) contains the fewest nerve fibers (approximately 3400) of any CN but has the longest intracranial course (75 mm). The nerve nucleus is located in the caudal midbrain at the level of the inferior colliculus near the periaqueductal gray matter, ventral to the aqueduct of Sylvius. It is continuous with the caudal end of the CN III nucleus and differs histologically from that nucleus only in the smaller size of its cells. Like the CN III nucleus, it is bounded ventrolaterally by the medial longitudinal fasciculus.
Figure 3-19 Schematic of the lateral orbit demonstrating the ciliary ganglion and CNs II–VI.
(Illustration by Dave Peace.)
The fascicles of CN IV curve dorsocaudally around the periaqueductal gray matter and completely decussate in the superior medullary velum. The nerves exit the brainstem just beneath the inferior colliculus (see Figs 3-1, 3-2). CN IV is the only CN that is completely decussated (the superior rectus subnuclei of CN III project contralaterally; however, the CN III fascicles themselves do not decussate once they leave the nuclear complex) and the only CN to exit the dorsal surface of the brainstem (see Figs 3-2, 3-22). As it curves around the brainstem in the ambient cistern, CN IV runs beneath the free edge of the tentorium, passes between the posterior cerebral and superior cerebellar arteries (like CN III, but more laterally), and then pierces the dura mater to enter the cavernous sinus (see Fig 3-24).
CN IV travels beneath CN III and above the ophthalmic division of CN V in the lateral wall of the cavernous sinus (see Fig 3-25). It enters the orbit through the superior orbital fissure outside the annulus of Zinn and runs superiorly to innervate the superior oblique muscle. Because of its location outside the muscle cone, CN IV is usually not affected by injection of retrobulbar anesthetics (see Fig 3-17).
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.