Trochlear nerve

Question 1

What is special about the way the trochlear nerve (CNIV) emerges from the CNS?

Answer 1

only somatic efferent nerve to emerge from the posterior aspect of the central nerve system

Question 2

Why is the course of the trochlear nerve in the brainstem unusual?

Answer 2

decussates before leaving the brainstem

Question 3

What 1 muscle is supplied by the trohlear nerve?

Answer 3

superior oblique

Question 4

Where is the trochlear nerve nucleus located?

Answer 4

anterior part of the periaqueductal grey matter at the level fo the inferior colliculus (caudal to III nerve nucleus) in line with the other oculomotor nuclei

Question 5

What are 5 steps to the course of CNIV from its origin between the intracranial portion emerges from the brainstem?

Answer 5

1. fibres pass from nucleus anterior and laterally towards tegmentum
2. turn and pass posteriorly around periaqueductal grey matter
3. pass into superior medullary velum (part of roof of fourth ventricle)
4. decussate in SMV
5. emerge from posterior surface of brainstem in posterior cranial fossa

Question 6

How many cranial nerves decussate completely and which are they?

Answer 6

CNIV is the only one

Question 7

Where on the posterior surface of the brainstem do CNIV slender nerves emerge?

Answer 7

caudal to the inferior colliculi on dorsal surface of the brainstem

Question 8

What are 4 stages of the course of CNIV from emerging on the posterior aspect of the brainstem to entering the cavernous sinus?

Answer 8

1. emerge below inferior colliculi
2. each nerve winds around crura of the midbrain (cerebral peduncles), above superior cerebellar artery and pons, and below posterior cerebral artery
3. continue anteriorly immediately under free edge of tentorium cerebelli
4. pierces this dura and enters lateral wall of cavernous sinus, beneath oculomotor nerve

Question 9

What 3 structures do the trochlear nerves run between after emerging on the posterior aspect of the brainstem?

Answer 9

above superior cerebellar artery and pons

below posterior cerebral artery

Question 10

How do the trochlear nerves enter the cavernous sinus?

Answer 10

after piercing dura of tentorium cerebelli, enters lateral wall of cavernous sinus beneath CNIII

Question 11

What is the course of the CNIV inside the cavernous sinus to reach the orbit?

Answer 11

• enters lateral wall of cavernous sinus under CNIII
• passes upwards, coming to lie above oculomotor nerve
• enters orbit outside tendinous ring in lateral part of superior orbital fissure

Question 12

How does CNIV enter the orbit?

Answer 12

from cavernous sinus, enters orbit outside the tendinous ring in the lateral part of the superior orbital fissure

Question 13

What is the intraorbital course of CNIV?

Answer 13

passes forwards above origin of levator palpebrae superioris close to the bone, to enter upper free edge of superior oblique

Question 14

How frequently is the trochlear nerve paralysed alone?

Answer 14

rarely

Question 15

At what 2 points is the trochlear nerve particularly vulnerable?

Answer 15

1. at posterior exit from brainstem
2. as it winds round midbrain

Question 16

Lesions in which 2 regions causing compression may affect the trochlear nerve?

Answer 16

1. undersurface of the tentorium
2. cavernous sinus

Question 17

What are 3 key symptoms of superior oblique paralysis due to trochlear nerve lesions?

Answer 17

1. diplopia when looking down
2. difficulty looking down when eye is adducted, because superior oblique is only depressor in the adducted state
3. patients carry head tilted to non-affected side with chin lowered

Question 18

Why do trochlear nerve lesions cause difficulty looking down in the adducted state?

Answer 18

in the adducted state, superior oblique is the only depressor

Question 19

Why do patients characteristically carry the head tilted to non-affected side, chin down in CNIV lesions?

Answer 19

to compensate for overaction of inferior oblique producing unopposed torsion on eye

Question 20

In which structure in the brainstem is the trochlear nucleus found?

Answer 20

midbrain (level of inferior nucleus)