4.2.3 Cranial Nerves III,IV & VI

Question 1

What do cranials nerves III,IV and VI do?

Answer 1

Involved in innervating muscles & structures relating to the orbit/eye

Question 2

After exiting the brainstem what commonality in route do CNIII, IV and VI have?

Answer 2

1. Cavernous Sinus
2. Superior orbital fissure
3. Into orbital fissure

Question 3

What nerves are associated with the lateral wall of the cavernous sinus?

Answer 3

Oculomotor
Trochlear
Opthalmic (CNVa)
Maxillary (CNVb)

Abducens is in the middle

Question 4

How do you test CN III,IV,VI?

Answer 4

• Observation of the resting position of patients gaze
• Ask patients to perform a series of eye movements gear stick sign, H with line in the middle (more detail in 8.2.3)

Question 5

What two fibres does the oculomotor CNIII have?

Answer 5

Motor- Somatic efferent
Parasympathetic- Visceral efferent

Question 6

What do the somatic efferent fibres supply?

Answer 6

All extra ocular muslces (muscles that move eyeball) except for lateral rectus and superior oblique & muscle in the eye lid- leavtor palpebrae superiosis

Question 7

What do visceral efferent fibres of CNIII do? (parasympathetic)

Answer 7

Muscles inside the eye
Ciliary muscle, alters thickness of the lens
Sphinter pupillae, alters the size of the pupil in response to light

AUTONOMIC PARASYMPATHETIC

Question 8

How do you test CNIII?

Answer 8

• Inspection of resting gaze
• Eyelid position, supplies LPS, eyelid kept up
• Eye movements (responsible for most muscles)
• Pupils and pupillary light reflexes

Question 9

When do signs arise due to oculomotor nerve lesions?

Answer 9

Arise when there is involvement of somatic fibres (motor) +/- visceral efferent fibres (parasympathetic)

Question 10

What are the symptoms of oculomotor nerve lesions?

Answer 10

Dipoplia- double vision

Question 11

What signs of oculomotor nerve lesion would be found on clinical examination?

Answer 11

• Ptosis- eyelid drooping
• Abnormal position of eye- down and out
• Pupil may or may not be dilated (depending on cause, if compression of parasympathetic fibres will have blown pupil)

Question 12

Where does CNIII arise from?

Answer 12

Midbrain (remember 2244)

Question 13

What does CNIII have a close relationship to?

Answer 13

Tentorium cerebelli

Question 14

Outline the path of oculomotor nerve

Answer 14

Question 15

What are some causes of oculomotor nerve lesions sparing the pupil?

Answer 15

Microvascular ischaemia
Risk factors:
* Age >50
* Diabetes/hypertension

Question 16

What are some causes of oculomotor nerve lesions involving the pupil?

Answer 16

Compressive lesion
compresses outside of CNIII
Parasympathetic (visceral efferents) run on the periphery of the oculomotor nerve
e.g.
* Aneurysmal posterior communicating artery (associated with headache/retroorbital pain)
* Head injury
* Tentorial (uncul) herniation secondary to raised ICP

More concerning as means raised ICP, which causes damage to other structures

Question 17

What is the function of the trochlear nerve (CNIV)?

Answer 17

Motor only, therefore only motor fibres

Question 18

What muscle does the trochlear nerve innervate?

Answer 18

Superior oblique, patient will complain of diplopia

On examination there will be:
* Abnormal eye position
* Difficulting moving eye downwards when eye is adducted

Question 19

Where does CNIV arise from?

Answer 19

Dorsal midbrain

Question 20

Outline the path of the trochlear nerve

Answer 20

Question 21

What are the acquired causes of trochlear nerve lesions?

Answer 21

• Microvascular ischaemia (>50 years, diabetes, hypertension)
• Trauma (head injury, even minor)
• Intracranial tumour (compresses stretches the nerve)

Can have congenital trochlear nerve lesions, NOT acquired

Question 22

What muscle does the Abducens nerve innervate? (CNVI)

Answer 22

Lateral rectus- abducts the eye, only has motor fibres

Question 23

What are the symptoms of a CNVI lesion?

Answer 23

Dipoplia- worse in lateral gaze

Question 24

What signs on examination would an abducens nerve lesion present with?

Answer 24

• Abnormal eye position at rest
• Difficulty/unable to move affected eye laterally

Question 25

What are the causes of CNVI lesions?

Answer 25

• Microvascular ischaemia (diabetes/hypertension)
• Head injury, tumour
• Raised ICP of any cause, nerve gets stretched

Question 26

Where does CNVI arise from?

Answer 26

Caudal pons

Question 27

Outline the path of CNVI

Answer 27

Question 28

Why is CNVI so susceptible to being damaged in raised ICP?

Answer 28

Vertical route & between brainstem and cavernous sinus,

CNVI is fixed at these points

Raised ICP causes downward displacement of brain which can stretch CNVI

Question 29

What is false localising sign?

Answer 29

Local symptoms due to general raise in ICP as the abducens becomes stretched