The trochlear nerve is the fourth paired cranial nerve, and is the smallest of the cranial nerves (by number of axons) and has the longest intracranial course. It has a purely somatic motor function in the human body, and in this article, the anatomical course, motor functions, and clinical relevance of the nerve will be examined.
The trochlear nerve originates from the trochlear nucleus of the brain, emerging from the posterior aspect of the midbrain. It is the only cranial nerve to exit from the posterior midbrain. From there, the nerve then runs anteriorly and inferiorly within the subarachnoid space before piercing the dura mater adjacent to the posterior clinoid process of the sphenoid bone. It then moves along the lateral wall of the cavernous sinus, which is in the company of the oculomotor nerve, the abducens nerve, the ophthalmic and maxillary branches of the trigeminal nerve, and the internal carotid artery, before entering the orbit of the eye via the superior orbital fissure.
The trochlear nerve innervates a single muscle, the superior oblique, that is responsible for oculomotor functions. As the fibres from the trochlear nucleus cross in the midbrain prior to exiting, the trochlear neurons innervate the contralateral superior oblique. The tendon of the superior oblique is tethered by a fibrous structure known as the trochlea, giving the nerve its name. While the mechanism of action of the superior oblique is rather complex, it can be regarded that it overall gives the eyeball the capability to depress and intort.
The trochlear nerve is commonly examined in conjunction with the oculomotor and abducens nerves by testing the movements of the eye. The patient is asked to follow a point of reference, such as the tip of a pen, with their eyes without moving their head. The target is moved in an 'H-shape' and the patient is asked to report any blurring of vision or diplopia (double vision).
Trochlear nerve palsy is a rather common occurrence, often presenting itself with vertical diplopia that is more apparent when looking downwards and inwards (when reading or walking down the stairs, for example). Patients can also develop a head tilt away from the affected side. This is commonly caused by microvascular damage from diabetes mellitus or hypertensive disease, however congenital malformation, thrombophlebitis of the cavernous sinus, and raised intracranial pressure can also be factors in this condition.
For many medical professionals, an understanding of the trochlear nerve can be of tremendous value for diagnosing and treating various oculomotor system conditions. Further research into the trochlear nerve is needed to develop a greater insight into the way it functions, as well as develop new and improved treatment options for those suffering from trochlear nerve palsy and other related conditions.
