The trochlear nerve, along with the abducens nerve, are perhaps the simplest of the cranial nerves in their function. Yet, the trochlear has a couple of unique properties and certainly some clinical correlates for the neurosurgeon.
The nerve carries somatic efferent fibers which do but a single job, innervate the superior oblique muscle. The nucleus trochlear nerve is located near the midline, just inferior to the motor nucleus of the occluomotor nerve and at the level of the inferior colliculus. The axons of the fourth cranial nerve leave the nucleus and actually decussates in the superior medullary velum. This is the only cranial nerve to decussate prior to exiting the brainstem. The significance is obvious that the right superior oblique muscle is controlled by neuronal cell bodies in the left midbrain.
The trochlear nerve exits dorsally from the midbrain. Again, this is unique amongst cranial nerves.
It travels along the medial, free edge of the tentorium cerebelli through which the brainstem is passing. It pierces the dura just posterior to the posterior clinoid process and enters the cavernous sinus where runs just medial and posterior to the occulomotor nerve before entering the orbit through the superior orbital fissure.
Since the superior oblique muscle acts to rotate the eye down and laterally a palsy of the trochlear nerve leaves the affected eye rotated up and in.
Attempts to compensate lead to the patient tilting their head to the contralateral side.
Amongst cranial nerve palsies a trochlear injury is the second most common manifestation of raised intracranial pressure, only the abducens more likely. It is the longest nerve and the smallest in diameter and particularly prone to the effects of raised ICP due to its lengthy free course starting dorsally and then extending through the anterior cisterns.
Compressive injuries can also occur uniquely with posterior cerebral artery as early in its course the trochlear passes between the superior cerebellar and posterior cerebral arteries. Obviously pathology in the cavernous sinus can affect the trochlear, as it can the occulomotor or abducens or the first branch of the trigeminal.
A more unique consideration for the neurosurgeon is the trochlears path along the medial edge of the tentorium. Transtentorial surgical approaches to the lower brainstem or the proximal cranial nerves located there, must be careful when incising the tentorium not to incur sharp damage to the trochlear or to overstretch it with retraction.
Thanks to Lauren for pointing out errors on the page.