The Trochlear nerve

This episode deals with the anatomy of the trochlear nerve and the clinical features of patients presenting with oculomotor palsy at various locations through its course in the brain.

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In this session we will discuss the trochlear nerve. Before starting the pre test multiple choice questions, let me thank all of you for the wonderful support you have been giving for the season 2 of the podcast. The podcast reached the number three position in the Apple Podcast India science charts. It is a great motivation and thank you all from the bottom of my heart. Now let us start with the pre-test multiple choice questions.

1. The trochlear nerve nucleus supplies which side?

A) Ipsilateral superior oblique

B) Contralateral superior oblique

C) Both superior obliques

D) Contralateral superior rectus

2. In congenital fourth nerve palsy, patients typically:

A) Complain of severe torsional diplopia

B) Have large vertical fusional amplitudes

C) Present with skew deviation

D) Develop bilateral ptosis

3. During right head tilt, which muscles should normally be activated to intort the right eye?

A) Superior rectus and inferior rectus

B) Superior oblique and superior rectus

C) Inferior oblique and superior rectus

D) Superior rectus and inferior oblique

4. In bilateral superior oblique palsy, what is the classic head posture?

A) Chin up position

B) Chin down position

C) Head turned to one side

D) No abnormal head posture

5. How does skew deviation differ from trochlear palsy on supine-sitting testing?

A) Skew deviation worsens on lying down

B) Vertical strabismus reduces >50% in the supine position in skew deviation

C) No change occurs with position in skew deviation

D) Trochlear palsy reduces diplopia when lying flat

Anatomy of the Trochlear nerve

The trochlear nerve is the smallest cranial nerve. The trochlear nerve nucleus lies caudal to the oculomotor nucleus in the midbrain at the level of the inferior colliculus. It lies dorsal to the medial longitudinal fasciculus and ventrolateral to the cerebral aqueduct. The trochlear nucleus contains somatic motor neurons. The nerve fascicles travel posteriorly and inferiorly around the aqueduct, crossing over to the opposite side in the dorsal midbrain at the anterior medullary velum. As a result, the right trochlear nerve nucleus supplies the left side and vice versa. 

The nerve fascicles exit the brainstem close to the dorsal midline, just beneath the inferior colliculi. Remember that the trochlear nerve is the only cranial nerve that crosses within the brainstem and emerges dorsally. The nerve has the longest intracranial course because of this. The cisternal segment proceeds anteriorly along the lateral side of the brainstem, passing through the quadrigeminal, ambient, crural, and pontomesencephalic cisterns in succession. Here, the trochlear nerve is in close proximity to the tentorium cerebelli. The nerve travels through the undersurface of the tentorium. Then, it pierces the dura behind and lateral to the posterior clinoid process just below the oculomotor nerve to enter the cavernous sinus. In the cavernous sinus, the trochlear nerve lies below the oculomotor nerve and above the ophthalmic division of the trigeminal nerve. The nerve enters the orbit through the superior orbital fissure and supplies the superior oblique muscle. The trochlear nerve will not pass through the annulus of Zinn. The trochlear nerve nucleus supplies the opposite eye superior oblique muscle. The opposite eye's superior oblique is weak in a nuclear trochlear nerve lesion. When the lesion is extramedullary outside the midbrain, along the course of the nerve, the ipsilateral superior oblique is involved.

Unlike other extraocular muscles, the superior oblique does not arise from the common tendinous ring. The superior oblique arises from the sphenoid bone near the optic canal and passes through a fibrocartilaginous trochlea just inside the superior medial orbital rim. It then inserts on the superior lateral aspect of the globe, posterior to the equator. This pulley mechanism alters the muscle's line of pull, enabling its distinct actions. Its main action depends upon the position of the eye. When the eye is abducted and in a neutral position, the superior oblique is a strong intorter, and when the eye is adducted, it is a depressor. Its tertiary action is the abduction of the globe. So, the superior oblique helps in the intorsion of the eye and moves it downwards in the adducted position. Intorsion is a movement where the top of the eye moves inwards or medially towards the nose. The movement is like a wheel rotating clockwise in the right eye and anticlockwise in the left eye. It is examined by focusing on a capillary on the limbus at around 12 o'clock position and asking the patient to look at the tip of the nose. The capillary is supposed to move clockwise towards a 3 o'clock position in the right eye and anticlockwise towards a 9 o'clock position in the left eye if the intorsion is normal. 

Clinical features

Patients with trochlear nerve palsy usually will have a primary position strabismus. The affected eye will be elevated or hypertropic, as well as be externally rotated or excyclotropic. A cover-uncover test may be needed to unmask the hypertropia in mild cases. The patients usually complain of binocular diplopia with vertically separated images, more when looking down and to the opposite side, which is the action of the ipsilateral superior oblique. A small caveat here is that in long-standing cases, the diplopia can sometimes be maximum in the direction of the gaze of the overacting inferior oblique or may be relatively equal in the various gaze positions due to the spread of comitance. Some patients won't have diplopia and will complain of blurring of vision when looking down and to the opposite side. To avoid diplopia, patients tilt their heads to the opposite side. Some patients will have only head tilt and no diplopia. 3% of patients can have a paradoxical head tilt to the same side. The wide separation of images in these cases will help to ignore one of the images. 

On duction testing, there will be impaired action of the ipsilateral superior oblique, and there can be overaction of the ipsilateral inferior oblique due to lack of counterbalance and contralateral superior oblique due to Hering's law. There is impaired intorsion and an inability to move the adducted eye downward. The affected eye has an incomitant hypertropia, meaning one eye is positioned higher than the other, and the misalignment varies with the direction of gaze. With the patient looking down and in and fixating with the normal eye, an alternate cover test will show corrective downward refixation on uncovering, suggesting upward drift of the eye under cover. If the patient fixes with the abnormal eye, the normal eye will have hypotropia. The Bielschowsky head tilt test helps to localize the fourth nerve palsy and the side affected in a case of hypertropia.

The Park Bielschowsky test is a three-step test used to isolate and identify paretic extraocular muscle in cases of acquired vertical diplopia. This systematic approach narrows down the potential culprit from eight possible muscles to a single muscle through three sequential examination steps.

Step 1: Determine which eye is hypertropic 

The first step involves determining which eye is hypertropic or elevated in the primary position of gaze. The evaluation uses the cover-uncover and alternate-cover tests while the patient looks straight ahead if the primary gaze does not show hypertropia. This initial step narrows the potential affected muscles to four from eight possibilities.

For example, if right hypertropia is present, either the depressors of the right eye, i.e., right inferior rectus or right superior oblique, or the elevators of the left eye, i.e., left superior rectus or left inferior oblique, is weak.

Step 2: Does the hypertropia increase in left gaze or right gaze?

The second step determines whether the hypertropia increases in the right or left gaze. This assessment is based on the principle that the rectus muscles show their vertical action when the eye is abducted, while the oblique muscles display their vertical action when the eye is adducted. 

For example, in the previous case, if diplopia is worse in the left gaze, the superior or inferior oblique muscle in the right eye or the superior or inferior rectus in the left eye is affected.

After completing step 2, the number of potentially affected muscles is reduced from four to two. The weak muscle iseither the right superior oblique or the left superior rectus which are affected in both steps.

Step 3: Is the hypertropia worse on the right head tilt or the left head tilt?

The superiors are intorters, and the inferiors are extorters. This evaluation is based on the principle that during head tilt, the intorting muscles (superior oblique and superior rectus) of the eye toward the tilted shoulder are stimulated, as are the extorting muscles (inferior oblique and inferior rectus) of the opposite eye.

In the previous example, if the hypertropia increases with right head tilt, the affected muscle is the right superior oblique and right superior rectus or the left inferior oblique and left inferior rectus.

After completing all three steps, only one muscle remains weak in all the steps, the right superior oblique. Thus, with the Bielschowsky test, we can come to a reasonable conclusion regarding the paretic muscle in a heterotopia case in three steps.

During right head tilt, the right eye should usually undergo intorsion to avoid diplopia. Since the right superior oblique is weak, the intorsion will not occur, and hypertropia and diplopia will increase. Hypertropia worsens with ipsilateral head tilt because the ocular counter-roll reflex stimulates ipsilateral intorters (superior oblique and superior rectus) and contralateral extorters (inferior oblique and inferior rectus). When the superior oblique is weak, this reflex causes a compensatory increase in ipsilateral superior rectus action, resulting in additional hypertropia (since the superior rectus is an elevator). 

Head tilt to the opposite side will correct the hypertropia but fails to correct excyclotropia due to the loss of intorsion. Excyclotropia can be evaluated using a double Maddox rod test, often considered the fourth step of the Bielschowsky head tilt test. Maddox rods of different colors are used over each eye. When viewing a horizontal bar, the two images appear slanted with respect to each other, with the apparent intersection of the lines directed toward the side of the affected, excyclodeviated eye. The severity of exocyclotropia correlates with the severity of superior oblique palsy acutely but not in congenital fourth cranial nerve palsy. Cyclotropia is symptomatic only in acquired cases. In congenital fourth nerve palsy, cyclotropia will be present, but patients are usually asymptomatic. Exocyclotropia can be evaluated during fundus examination as well. The fovea will be seen more inferior and lateral to the disc than normal.

Skew deviation

Skew deviation is a vertical strabismus caused by lesions affecting the otolithic pathways (utricle and its central connections) in the brainstem or cerebellum, leading to an imbalance in the vestibular-ocular reflex. It can mimic fourth cranial nerve palsy and is challenging to differentiate. Doing the Bielscowsky 3-step test in supine and sitting will help to distinguish between the two. If the vertical strabismus decreases by more than 50% from sitting to supine position, it is suggestive of skew deviation. This test has a high sensitivity and specificity to diagnose skew deviation. It is described by Wong et al. and is sometimes referred to as the Fifth step. Imaging will help to detect the brainstem and cerebellar lesions responsible for sqew deviation. 

Combined third and fourth cranial nerve palsy.

In patients with combined third and fourth cranial nerve palsy, the ability to test the depression of the eye in the adducted position is compromised because the eye cannot be adducted due to the third cranial nerve palsy. Instead, in these patients, the function of the superior oblique muscle can be evaluated by testing intorsion. If intorsion is normal, it is a pure third nerve palsy. If intorsion is absent, it is a combined third and fourth cranial nerve palsy.

Congenital fourth cranial nerve palsy

Decompensation of a congenital fourth cranial palsy can present acutely. Congenital fourth cranial nerve palsy patients can have facial asymmetry on the side of the head tilt. Old photographs can show head tilt. Exocyclodeviation will be present in clinical examinations, but patients are usually asymptomatic. Large vertical fusional diameters greater than 6-8 prism diopters are characteristic of congenital fourth cranial nerve palsy. The most important feature suggesting congenital fourth cranial nerve palsy is hypertropia, greater in upgaze than down gaze due to overaction of the inferior oblique muscle.

Bilateral fourth cranial nerve palsy

The clinical features of bilateral fourth cranial nerve palsy include

1. The patient will have right hypertropia in the left gaze and left hypertropia in the right gaze.
2. Bielschowsky test on tilt to either shoulder will be positive. (“double Bielschowsky test”)
3. The patient will have a large excyclotropia of more than 10 degrees.
4. V pattern esotropia with esotropia more looking down than up, producing the pattern of V. There is a 15 prism dioptre difference in ET between downward and upward gaze. It results from a decrease in the abducting effect of the superior oblique and the overactivity of the inferior oblique.
5. There is underaction of both superior obliques and overaction of both inferior obliques.
6. Bilateral superior oblique palsy will have lesser hypertropia in the primary position than unilateral superior oblique palsy.
7. The patient will have a head-down position, having difficulty looking down with both eyes. Post motor cycle accident bilateral fourth nerve palsy causing head down position is called Sogg sign.

Localization of fourth cranial nerve palsy

Fourth cranial nerve palsy is one of the commonest causes of vertical strabismus. However, it is less common than a third or sixth cranial nerve palsy. When a patient develops a fourth nerve palsy, the localization can be in the nucleus or fasciculus in the brainstem, the subarachnoid space, the cavernous sinus, the superior orbital fissure, or inside the orbital cavity. 

Nuclear lesion

A nuclear fourth cranial nerve palsy causes opposite-side superior oblique palsy as the nerves decussate at the superior medullary velum. It is the only cranial nerve nucleus that supplies the opposite side. The patient will often have other long tract signs that help localization. The involvement of MLF will result in ipsilateral internuclear ophthalmoplegia and contralateral superior oblique palsy. Involvement of the superior cerebellar peduncle will result in ipsilateral cerebellar signs. If the descending sympathetic tract is involved, the patient can have Horner syndrome and contralateral superior oblique palsy. A lesion affecting the brachium of the superior colliculus and the adjacent trochlear nucleus or fascicle may cause a contralateral relative afferent pupillary defect without visual impairment, along with contralateral superior oblique palsy. 

Fascicular lesion

A fascicular lesion before the decussation in the anterior medullary velum will result in superior oblique palsy on the opposite side. It is impossible to clinically differentiate between a nuclear fourth nerve and a fascicular lesion of this part of the trochlear nerve. A fascicular lesion distal to decussation will cause ipsilateral superior oblique palsy. Thus, a fascicular lesion can cause ipsilateral or contralateral superior oblique palsy. Usually, lesions posterior to the cerebral aqueduct cause ipsilateral superior oblique palsy. A lesion at the superior medullary velum can cause bilateral superior oblique palsy. The involvement of the pretectal nucleus can cause vertical gaze palsy. Nuclear and fascicular lesions may be due to infarction, hemorrhage, tumor, infection, inflammation, or trauma. An imaging, ideally an MRI of the brain, will confirm the lesion.

Lesions of subarachnoid space

Isolated fourth cranial palsy in the subarachnoid space is much less common than third cranial nerve palsy. Patients often have associated symptoms like headache, neck stiffness, and other cranial nerve palsy. Imaging followed by lumbar puncture is indicated if etiology is not evident. Isolated fourth cranial nerve palsy can occur due to superior cerebellar artery and posterior communicating artery aneurysm and schwannomas of the fourth cranial nerve. Fourth, cranial palsy can be rarely associated with idiopathic intracranial hypertension and superficial siderosis. 

Cavernous sinus and superior orbital fissure lesions

The fourth cranial nerve will be affected along with the third, sixth, and ophthalmic divisions of the trigeminal nerve in the cavernous sinus. If present, the sympathetic involvement causing Horner's syndrome will differentiate a cavernous sinus lesion from a superior orbital fissure lesion, which also involves the same combination of cranial nerves. If Horner's syndrome is absent, it is difficult to differentiate a cavernous sinus lesion from the superior orbital fissure. Isolated involvement of the trochlear nerve can occur rarely in these sites.

Orbital lesion

The fourth cranial nerve can be damaged inside the orbital cavity to produce superior oblique palsy. Proptosis, chemosis, and optic nerve involvement may also be present, along with other ocular motor nerve involvement. Direct involvement of the superior oblique muscle and trochlear pulley is a more common cause of vertical diplopia than the trochlear nerve injury inside the orbital cavity. Tenosinovitis can restrict the motion of the superior oblique tendon through the trochlear pulley. A forced duction test can be used to unmask the mechanical restriction of depression on the adduction of the eye. Myasthenia gravis and thyroid ophthalmopathy may sometimes simulate fourth nerve palsy. In thyroid ophthalmopathy, the opposite eye inferior rectus restriction can mimic a fourth nerve palsy due to vertical diplopia and head tilt. A forced duction test will be able to differentiate between the two.

Super oblique myokymia

It is a uniocular rotatory microtremor that causes paroxysmal vertical oscillopsia and diplopia. It is usually benign and can follow superior oblique palsy. The myokymia can occur in the primary position and can occur within the direction of action of the superior oblique muscle or away from it. It is usually idiopathic but can be the manifestation of a tectal lesion. It is associated with lead poisoning, adrenoleukodystrophy, and neurovascular compromise at the nerve exit zone. Spontaneous discharges of the trochlear motor neurons result in myokymia. Inferior oblique myokymic characterized by monocular, high-frequency, low-amplitude contractions causing excyclotorsion, not incyclotorsion, induced by looking up and out, is rarely described. Alternating superior and inferior oblique myokymia is also described.

Brown's superior oblique tendon sheath syndrome

Mechanical restriction of the superior oblique tendon in the trochlea leads to limited upward movement of the eye in the adducted position, mimicking inferior oblique palsy. The superior oblique is trapped intermittently, producing intermittent vertical diplopia. The eye may release suddenly, producing an audible click. The etiology may be congenital or acquired, like trauma. A forced duction test and MRI of the orbit will confirm the diagnosis.

MCQ

1. The trochlear nerve nucleus supplies which side?

A) Ipsilateral superior oblique

B) Contralateral superior oblique

C) Both superior obliques

D) Contralateral superior rectus

Answer: B) Contralateral superior oblique

The trochlear nucleus controls the opposite (contralateral) superior oblique muscle because the nerve crosses at the superior medullary velum in the midbrain.

2. In congenital fourth nerve palsy, patients typically:

A) Complain of severe torsional diplopia

B) Have large vertical fusional amplitudes

C) Present with skew deviation

D) Develop bilateral ptosis

Answer: B) Have large vertical fusional amplitudes

Vertical fusional amplitude (or vertical fusional vergence amplitude) refers to the maximum vertical disparity (difference in vertical position of images seen by each eye) that a person can fuse into a single image. This large vertical fusional amplitude allows patients to fuse images even when the eyes are vertically misaligned beyond normal limits, reducing symptoms like diplopia (double vision) in the primary gaze and many gaze positions.

3. During right head tilt, which muscles should normally be activated to intort the right eye?

A) Superior rectus and inferior rectus

B) Superior oblique and superior rectus

C) Inferior oblique and superior rectus

D) Superior rectus and inferior oblique

Answer: B) Superior oblique and superior rectus

Superiors are intorters. When the head tilts to the right, the ocular counter-roll reflex stimulates ipsilateral intorters (superior oblique and superior rectus) and contralateral extorters (inferior oblique and inferior rectus).

4. In bilateral superior oblique palsy, what is the classic head posture?

A) Chin up position

B) Chin down position

C) Head turned to one side

D) No abnormal head posture

Answer: B) Chin down position

The patient will have a head-down position, having difficulty looking down with both eyes. Post motor cycle accident bilateral fourth nerve palsy causing head down position is called Sogg sign.

5. How does skew deviation differ from trochlear palsy on supine-sitting testing?

A) Skew deviation worsens on lying down

B) Vertical strabismus reduces >50% in the supine position in skew deviation

C) No change occurs with position in skew deviation

D) Trochlear palsy reduces diplopia when lying flat

Answer: B) Vertical strabismus reduces >50% in the supine position in skew deviation

This test has a high sensitivity and specificity to diagnose skew deviation, as described by Wong et al. It is sometimes referred to as the Fifth step of Bielschowsky.