Anatomy of Fourth and Sixth Cranial Nerve

Dr.Yajuvendra Singh Rathore

4th Nerve-Trochlear NerveThe Trochlear nerve is entirely

motor in function and supplies only the Superior Oblique muscle of the eyeball.

Trochlear Nerve differs from other cranial nerves in being:

The only cranial nerve to arise from the dorsal aspect of the midbrain.

The only cranial nerve to cross completely on the other side i.e the trochlear nerve arises from the contralateral nucleus.

The longest and the thinnest of all cranial nerves.

Functional components-

Somatic efferent component of the nerve is concerned with the movement of the eyeball through the superior oblique muscle.

General somatic afferent component carries propioceptive impulses from the superior oblique muscle. These impulses are relayed to the mesencephalic nucleus of the trigeminal nerve.

Nucleus

The trochlear nerve is situated in the ventromedial part of the central grey matter of the midbrain at the level of inferior colliculus.

It is caudal to and continuous with the third nerve nucleus complex.

It belongs to somatic efferent column of nuclei and is closely related to the medial longitudinal bundle.

Connections of the Nucleus-

1. Cerebral Cortexa) Motor Cortex of both sides through the

corticonuclear tracts.b) Visual Cortex through the superior

colliculus and the tactobulbar tract.c) Frontal eye fiels

2. Nuclei of 3rd ,6th and 8th cranial nerves through the medial longitudinal bundle.

3. Superior colliculi through the descending predorsal bundle.

4. Vertical and torsional gaze centres.5. Cerebellum through the vestibular nuclei.

Course of Trochlear Nerve

Fascicular PartFrom each nucleus, the nerve fibers

run laterally and emerge from the dorsal aspect of the midbrain at the level of the inferior colliculus.

They pass medially and decussate completely (thus each superior oblique is supplied from the contralateral trochlear nucleus).

Pre-cavernous partTrochlear nerve exits the brain from the

dorsal side It turns towards the ventral side and

passes between the PCA and SCA.

It then pierces the dura on the posterior corner of the roof of the cavernous sinus to

enter into it.

Cavernous SinusThe nerve runs forwards in the lateral wall

lying below the oculomotor nerve and above the first division of the fifth cranial nerve.

In the anterior part of the cavernous sinus it rises, crosses over the third nerve and leaves the sinus to pass through the lateral part of the superior orbital fissure.

Superior Orbital FissureThe trochlear nerve enters the orbit through

the lateral portion of the superior orbital fissure. The nerve passes medially above the origin of the LPS and ends by supplying the superior oblique muscle through its orbital surface.

Orbital CourseIn the orbit, the trochlear nerve divides up in

a fan-shaped manner into 3 or 4 branches, which supply the superior oblique on its upper surface near the lateral border. The most anterior branch enters the muscle at the junction of the posterior and middle thirds and the most posterior around 8 mm beyond its origin.

Course of Trochlear Nerve

Lesions of the Trochlear Nerve

Nuclear Fascicular Syndrome

• It is difficult to distinguish between nuclear and fascicular lesions due to the short course of the fascicles within the midbrain.

• It could be due to hemorrhage, trauma or demyelination. It is seen with contralateral Horner’s syndrome, since the sympathetic pathways descend through the dorsolateral tegmentum of the midbrain adjacent to the trochlear fascicles.

Subarachnoid Space SyndromeAs the fourth nerve emerges from

the dorsal surface of the brainstem, it can get injured easily.

When bilateral fourth nerve palsies occur, the site of injury is likely in the anterior medullary velum.

Contracoup forces transmitted to the brainstem by the free tentorial edge may injure the nerves at this site.

Other causes could be tumors like tentorial meningiomas.

Cavernous Sinus SyndromeIf the lesion is in the cavernous sinus,

other cranial nerves in close association with the fourth cranial nerve also get involved.

Orbital SyndromeThis could be due to trauma,

inflammation or tumors. In this other cranial nerves close to the fourth cranial nerve are also involved.

Other orbital signs like proptosis, chemosis are also seen.

Isolated Fourth Nerve Palsy Isolated fourth nerve palsy could be due to a congenital cause or it could be acquired. The features of a fourth nerve palsy are:

Hyperdeviation The involved eye is higher as a result of the weakness of the superior oblique muscle. Bielschowsky’s head tilting is done to determine hyperdeviation.

Ocular Movements Depression is limited in adduction. Intorsion is also limited.

Diplopia Homonymous vertical diplopia occurs on looking downwards. Usually the vision is single as long as the eyes look above the horizontal

plane. The patient especially notices diplopia when coming down the stairs.

Abnormal Head Posture To avoid diplopia, the head takes an abnormal head posture towards the action of the superior oblique muscle, i.e. the face is slightly

turned to the opposite side, chin is depressed and the head is tilted towards the opposite shoulder.

CHECKING FOURTH NERVE FUNCTION IN THE SETTING OF A THIRD NERVE PARESISAs the eye cannot be adducted, one cannot test the vertical action (depression)of the superior oblique muscle.

Note a limbal or conjunctival landmark like a blood vessel or pterygium.

Ask the patient to look down.

The eye will intort as the superior oblique works.

If the conjunctival landmark is moving, the eye is intortingand that means the fourth nerve is intact.

The Bielschowsky’s head tilting test can diagnose which muscle is paralyzed. Let us look at a case of R/L hypertropia in which the right eye is at a higher position than the left eye.R/L HypertropiaStep 1-

Step.2

Step 3- Tilt the patient’s head to the right and then to the left. If we tilt the head to the right, the right eye will intort and the left eye will extort. This is because nervous impulses will be sent from the semicircular canals to keep the eyes in a straight position.

Now, remember the superiors are intorters. So, if the right eye intorts, it means the superiors in that eye (RSR and RSO) work and if the left eye extorts it means the inferiors of that eye (LIO and LIR) work.

BIELSCHOWSKY’S HEAD TILTING TEST

Bielschowsky’s head tilting test for R/L hypertropia

When this happens in the right eye the RIR will not be used at all as it is an extorter and in the right eye extortion is not taking place.

But, in the left eye, extortion will take place and the LIO and LIR will work. Now, the LIO is paralyzed and so cannot work.

This will make the LIR only work in that eye and as a balance will not be maintained between these two muscles the left eye will move down as the LIR is also a depressor. Thus, one can diagnose the case of LIO.

6th Nerve-Abducent NerveThe abducent nerve is a small,

entirely motor nerve that supplies the lateral rectus muscle of the eyeball.

Functional components-Somatic Efferent- for lateral

movement of the eyeGeneral Somatic afferent- for

propioceptive impulses from the lateral rectus muscle.

NucleusThe abducent nucleus is situated in the lower

part of the pons, close to the midline. The facial nerve lies close to it and crosses it

and turns around the nucleus to emerge from the brain just adjacent to the abducent nerve.

Medial to the abducent nerve nucleus lies the medial longitudinal fasciculus and the pontine paramedian reticular formation (PPRF).

Lateral to it lies the fifth cranial nerve and the sympathetic neuron. Just ventral to it lies the pyramidal tract.

Connections of the Nucleus-1. Cerebral cortexa) Motor Cortex through the afferent corticonuclear

fibres from both cerebral hemispheres.b) Visual Cortex through the superior colliculus and

tactobulbar tract.c) Frontal Cortex

2. Nuclei of 3rd , 4th and 8th cranial nerves through the medial longitudinal bundle.

3. Pretectal nucleus of both sides through the tectobulbar tract.

4. Horizontal gaze centre through the medial longitudinal bundle.

5. Cerebellum through vestibular nuclei.

Course of Abducent NerveFascicular part• Fasciculus consists of efferent fibres

which start from the nucleus,pass forward traversing the medial leminiscus and pyramidal tract.

• It then emerges by 7 to 8 rootlets from the junction of pons and medulla.

• The rootlets then join to form one nerve.

Basilar part• The nerve runs forwards, upwards

and laterally between the pons and the occipital bone.

• At the upper border of petrous bone the nerve bends forward under petrospenoidal ligament and enters the Cavernous sinus by piercing its posterior wall.

Cavernous SinusThe sixth nerve runs almost horizontally

forwards. In the posterior part of the sinus, the

nerve winds around the lateral aspect of the ascending portion of the ICA.

Further forwards the sixth nerve lies below and lateral to the horizontal portion of the ICA.

The nerve then leaves the cavernous sinus to enter the orbit through the middle part of superior orbital fissure.

OrbitIn the orbit, the abducent nerve

runs forwards and enters the ocular surface of the lateral rectus muscle just behind its middle portion before dividing into 3 to 4 branches.

Course of Abducent Nerve

Lesions of Abducent Nerve

The Brainstem SyndromeA brainstem lesion of the sixth nerve may

also affect the fifth, seventh and eight cranial nerves and also the cerebellum. The sixth nerve nucleus has also connections via the medial longitudinal fasciculus with the III nerve nucleus and so a lesion here produces a gaze palsy.Three syndromes can occur in the brainstem.

1. Millard-Gubler SyndromeIn this the lesion is ventral and involves the

facial nerve and the pyramidal tract. Thus, there is a sixth nerve paresis, ipsilateral VII nerve paresis and contralateral hemiparesis.

2. Raymond’s SyndromeIn this syndrome the lesion involves only the

sixth cranial nerve and the pyramidal tract. Thus, the patient has a sixth nerve paresis and contralateral hemiparesis.

3. Foville’s SyndromeAs the lesion is dorsal the areas affected are the

medial longitudinal fasciculus, the pontine paramedian reticular formation, the fifth nerve and the sympathetic neurons. Thus, the patient has horizontal conjugate gaze palsy, ipsilateral V, VI, VII and VIII nerve palsies with ipsilateral Horner’s syndrome.

Subarachnoid Space SyndromeElevated intracranial pressure may

result in downward displacement of the brainstem, with stretching of the sixth nerve, which is tethered at its exit from the pons and in Dorello’s canal. This gives rise to nonlocalizing sixth nerve palsies of raised intracranial pressure. 30% of patients with pseudotumor cerebri have sixth nerve paresis,besides papilledema and its visual field changes.

Petrous Apex SyndromeThe sixth nerve passes under the Gruber’s

ligament in the Dorello’s canal. This makes it liable to a lesion.

1. Gradenigo’s SyndromeThis is due to a localized inflammation or

extradural abscess of the petrous apex following complicated otitis media. This leads to:

• Sixth nerve palsy • Ipsilateral decreased hearing (eighth nerve involvement) • Ipsilateral facial pain in the distribution of the fifth nerve • Ipsilateral facial paralysis.

Pseudo-Gradenigo’s SyndromeThis is seen in two conditions: 1. Nasopharyngeal carcinoma-

This may cause serous otitis media due to obstruction of the eustachian tube and the carcinoma may subsequently invade the cavernous sinus causing sixth nerve paresis.

2. Cerebellopontine angle tumor- This may cause sixth nerve paresis with decreased hearing (VIII nerve), VII nerve palsy, V nerve palsy, ataxia and papilledema.

Cavernous Sinus SyndromeIn this other nerves in the cavernous

sinus also are involved like the third, fourth and fifth nerves.

Orbital SyndromeIn this proptosis is an early sign and the

optic nerve may appear normal or demonstrate atrophy or edema. The ophthalmic division of the fifth nerve is involved. The third, fourth and sixth nerves are also involved. It occurs due to trauma, tumors or inflammations

Clinical features of Isolated 6th Nerve Palsy1. Deviation- In primary position,eyeball is

converged due to unopposed action of medial rectus muscle.

2. Ocular movements- Abduction is limited due to weakness of the lateral rectus muscle.

3. Diplopia- Uncrossed diplopia occurs, which becomes worse toward the action of paralysed muscle.

4. Head Posture- The face is turned towards the action of paralysed muscle to minimise diplopia.

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