The fourth cranial nerve is the trochlear nerve. This tiny nerve has one job to do, and that is to innervate the superior oblique m., one of the extraocular muscles responsible for changing the direction of gaze.
Though it only has this one job, it is pretty important! The formation and course of the trochlear nerve is rather unique, too, as we’ll see. Before we get to the nitty gritty of the superior oblique muscle, let’s review some facts about the trochlear nerve itself.
The “trochlear” nerve
Let’s start by understanding where this nerve gets its name. The superior oblique m., the only muscle innervated by cranial nerve 4, uses a connective tissue pulley called a trochlea. The trochlea sits in the superior medial corner of the orbit. The superior oblique m. uses the trochlea to change direction along its course.
Course of cranial nerve four
The trochlear nerve arises from the trochlear nucleus in the midbrain. However, it does so on the opposite side of it’s target – meaning fibers that innervate the right superior oblique m. originate in the left trochlear nucleus. So, the trochlear n. arises from the contralateral trochlear nucleus, decussates (crosses) in the midbrain, and emerges on the dorsal surface of the brain stem, caudal to the inferior colliculus. The trochlear nerve is unique in doing this; it is the only nerve to emerge from the brain stem on the dorsal side – all other cranial nerves emerge on the ventral surface.
From the brain stem the trochlear nerves passes through subarachnoid space until it finds the folded edge of the tentorium cerebelli, the tentorial notch. Cranial nerve four then rides the folded edge of the tentorium towards the cavernous sinus. It passes through the cavernous sinus along it’s lateral wall.
From the cavernous sinus the trochlear nerve moves through the superior orbital fissure – a bony space between the lesser and greater wings of the sphenoid bone through which all cranial nerves in the orbit must pass.
To reach the superior oblique m. the trochlear nerve passes superior to the common tendinous ring (while other nerves that innervate extraocular eye muscles move through the ring). Since it passes superior to the ring rather than through it, the trochlear nerve is not impinged by swellings or lesions at the common tendinous ring.
Trochlear nerve and relationship to the common tendinous ring
The course of the trochlear nerve outside of the common tendinous ring means that the target of CN 4, the superior oblique m., remains active if the other extraocular muscles are paralyzed because their nerves course through the ring. We’ll come back to this as we review the action of the superior oblique m. below.
Fiber types and targets in the trochlear nerve
The only type of fibers in the trochlear n. are GSE – or general somatic efferent fibers. These are somatic fibers, like the rest of the extraocular muscles, because they originate from paraxial mesoderm rather than neural crest cells like the connective tissues in the orbit. They do not originate from branchial arches.
The one target of the trochlear nerve is the superior oblique m. This muscle originates from the lateral side of the body of the sphenoid bone, then runs anteriorly towards the trochlea. As it moves through the trochlea the muscle belly bends to move obliquely towards the posterior-inferior-lateral portion of the eye to insert onto the sclera there. The angle made by the two pre- and post-trochlear portions of this muscle is ~53 degrees. As an oblique muscle, superior oblique inserts onto the sclera of the eye posterior to a vertical equator of the eye. This fact is crucial for understanding the action of this muscle.
The superior oblique m. is known as the “down and out” muscle – but it isn’t in Beverly Hills, it’s in your orbit! Recall that the functional portion of the muscle in terms of fiber orientation is the part from the trochlea to the insertion on the sclera. Also recall that the course of those fibers is oblique from the trochlea in the superior-medial corner of the orbit to the posterior-lateral-inferior portion of the eyeball. This point of insertion is important for understanding the action of this muscle.
Muscle insertions move towards their origins so that means as the superior oblique m. pulls, it draws the posterior-lateral-inferior portion of the eyeball towards the trochlea in the superior medial corner of the orbit. This moves the direction of gaze down and out, or depression and abduction. It also rotates the eye towards the nose, or intorsion. THIS is where the “down and out” moniker comes from for the superior oblique.
Be solid on the action of superior oblique m. – it depresses, abducts, and medially rotates (intorts) the eye. Go over that again if you need to, because we’re about to discuss how to TEST the superior oblique m., and therefore, cranial nerve four.
You’re probably familiar with the test in an eye doctor’s office in which you follow the finger of the examiner as they move it in an H pattern in front of you then towards your face. The tricky thing about testing extraocular muscles is that they all must work together so while we learn their actions we must also understand that they work collectively to produce those actions. This means that to test one of them, we need to isolate an action they do, and negate the other eye muscles from doing that same action. After all, the superior oblique m. depresses the eye, but so does inferior rectus m., and it can abduct the eye, but so can inferior oblique m.
So think again of the position and fiber direction for the superior oblique m…it points towards the nose where the trochlea sits. Asking a patient to follow a finger that is in front of their nose orients the gaze in line with the functional portion of the superior oblique m., the 53 degree angled portion that matters. When the eye is directed towards the nose the line of pull of the superior oblique m. crosses that of the other depressor, the inferior rectus m. The inferior rectus cannot pull when the eye is in this position, leaving superior oblique as the only depressor. Isolating the superior oblique in this way allows for independent testing of the trochlear nerve.
What would happen if the trochlear nerve were impinged or lesioned? Strabismus is the technical term for “cross-eyed.” Again, all extraocular muscles must work together to produce coordinated gaze and movements. When one muscle isn’t working properly the visual signals from left and right eyes do not match.
In the case of superior oblique m. paralysis or trochlear nerve impingement, the eye will be left in hypertropia – upward deviation of the eye due to the unopposed action of the superior rectus m. – and it is most pronounced when looking towards the opposite side of lesion. For example, someone whose left trochlear nerve is damaged (meaning the left superior oblique is paralyzed) will experience hypertrophia when looking towards their right side. Their left eye will be deviated upward due to the inaction of the superior oblique to pull it down (inferior rectus m. on the left could not pull down either if the gaze is directed across the nose to the right side). The difference in gaze between left and right eyes produces strabismus, or cross-eye, and results in double vision, or diplopia.
The trochlear nerve may experience a lesion anywhere along its course. In fact, it’s intimate association with the tentorial notch means that problems may arise if the patient is experiencing increased intracranial pressure which may push or pull on the tentorium and its notch, and therefore the trochlear nerve. Tumors, trauma, or vascular issues may also impinge on CN 4. As we learned when describing testing of CN 4 and superior oblique m., this patient would not be able to easily look downwards. Patients with this issue tend to compensate by moving their whole head down, or tilting it to the opposite side (towards the same side makes the double vision worse).