The trochlear nerve is the fourth paired cranial nerve. It is the smallest cranial nerve (by number of axons), yet has the longest intracranial course. It has a purely somatic motor function. In this article, we shall look at the anatomy of the trochear nerve – its course, motor functions and clinical relevance. Pro Feature - 3D Model You've Discovered a Pro Feature Access our 3D Model Library Explore, cut, dissect, annotate and manipulate our 3D models to visualise anatomy in a dynamic, interactive way. Learn More Anatomical Course The trochlear nerve arises 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). It runs anteriorly and inferiorly within the subarachnoid space before piercing the dura mater adjacent to the posterior clinoid process of the sphenoid bone. The nerve then moves along the lateral wall of the cavernous sinus (along with 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. By TeachMeSeries Ltd (2025) Fig 1The trochlear nerve and superior oblique muscle Motor Function The trochlear nerve innervates a single muscle – the superior oblique, which is a muscle of oculomotion. As the fibres from the trochlear nucleus cross in the midbrain before they exit, the trochlear neurones 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. Although the mechanism of action of the superior oblique is complex, in clinical practice it is sufficient to understand that the overall action of the superior oblique is to depress and intort the eyeball. By TeachMeSeries Ltd (2025) Fig 2Lateral view of the extraocular muscles. Clinical Relevance Examination of the Trochlear Nerve The trochlear nerve is examined in conjunction with the oculomotor and abducens nerves by testing the movements of the eye. The patient is asked to follow a point (commonly 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). Palsy of the Trochlear Nerve Trochlear nerve palsy commonly presents with vertical diplopia, exacerbated when looking downwards and inwards (such as when reading or walking down the stairs). Patients can also develop a head tilt away from the affected side. They are commonly caused by microvascular damage from diabetes mellitus or hypertensive disease. Other causes include congenital malformation, thrombophlebitis of the cavernous sinus, and raised intracranial pressure. By TeachMeSeries Ltd (2025) Fig 3Left trochlear nerve palsy, characterised by vertical diplopia and head tilt away from the affected side. Do you think you’re ready? Take the quiz below Pro Feature - Quiz The Trochlear Nerve (CN IV) Question 1 of 3 Submitting... Skip Next Rate question: You scored 0% Skipped: 0/3 1800 More Questions Available Upgrade to TeachMeAnatomy Pro Challenge yourself with over 1800 multiple-choice questions to reinforce learning Learn More Print Article Rate This Article