Anatomy of fourth and sixth cranial nerv

1. Anatomy of Fourth and Sixth
Cranial Nerve
Dr.Yajuvendra Singh Rathore

2. 4th Nerve-Trochlear Nerve
 The Trochlear nerve is entirely motor
in function and supplies only the
Superior Oblique muscle of the
eyeball.

3. 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.

4. 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.

5. 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.

6. Connections of the Nucleus-
1. Cerebral Cortex
a) 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.

7. Course of Trochlear Nerve
Fascicular Part
 From 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).

8. Pre-cavernous part
 Trochlear 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.

9.  Cavernous Sinus
 The 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.

10.  Superior Orbital Fissure
 The 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 Course
 In 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.

11. Course of Trochlear Nerve

12. Lesions of the Trochlear Nerve

13. 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.

14. Subarachnoid Space Syndrome
 As 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.

15. Cavernous Sinus Syndrome
 If the lesion is in the cavernous sinus,
other cranial nerves in close
association with the fourth cranial nerve
also get involved.
Orbital Syndrome
 This 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.

16.  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

17. CHECKING FOURTH NERVE FUNCTION IN THE
SETTING OF A THIRD NERVE PARESIS
As 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
intorting

18. 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 Hypertropia
Step 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

19. 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

20. 6th Nerve-Abducent Nerve
 The abducent nerve is a small, entirely
motor nerve that supplies the lateral
rectus muscle of the eyeball.

21. Functional components-
Somatic Efferent- for lateral
movement of the eye
General Somatic afferent- for
propioceptive impulses from the lateral
rectus muscle.

22. Nucleus
 The 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.

23. Connections of the Nucleus-
1. Cerebral cortex
a) 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.

24. Course of Abducent Nerve
Fascicular 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.

25. 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.

26. Cavernous Sinus
 The 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

27. Orbit
 In 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.

28. Course of Abducent Nerve

29. Lesions of Abducent Nerve

30. The Brainstem Syndrome
 A 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 Syndrome
 In 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.

31. 2. Raymond’s Syndrome
 In 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 Syndrome
 As 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.

32. Subarachnoid Space Syndrome
 Elevated 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.

33.  Petrous Apex Syndrome
 The sixth nerve passes under the Gruber’s
ligament in the Dorello’s canal. This makes it
liable to a lesion.
1. Gradenigo’s Syndrome
 This 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.

34. Pseudo-Gradenigo’s Syndrome
 This 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.

35. Cavernous Sinus Syndrome
 In this other nerves in the cavernous sinus
also are involved like the third, fourth and
fifth nerves.
Orbital Syndrome
 In 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

36. Clinical features of Isolated 6th
Nerve Palsy
1. 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.