4.2.3 Cranial Nerves III,IV & VI Flashcards
What do cranials nerves III,IV and VI do?
Involved in innervating muscles & structures relating to the orbit/eye
After exiting the brainstem what commonality in route do CNIII, IV and VI have?
- Cavernous Sinus
- Superior orbital fissure
- Into orbital fissure
What nerves are associated with the lateral wall of the cavernous sinus?
Oculomotor
Trochlear
Opthalmic (CNVa)
Maxillary (CNVb)
Abducens is in the middle
How do you test CN III,IV,VI?
- 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)
What two fibres does the oculomotor CNIII have?
Motor- Somatic efferent
Parasympathetic- Visceral efferent
What do the somatic efferent fibres supply?
All extra ocular muslces (muscles that move eyeball) except for lateral rectus and superior oblique & muscle in the eye lid- leavtor palpebrae superiosis
What do visceral efferent fibres of CNIII do? (parasympathetic)
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
How do you test CNIII?
- Inspection of resting gaze
- Eyelid position, supplies LPS, eyelid kept up
- Eye movements (responsible for most muscles)
- Pupils and pupillary light reflexes
When do signs arise due to oculomotor nerve lesions?
Arise when there is involvement of somatic fibres (motor) +/- visceral efferent fibres (parasympathetic)
What are the symptoms of oculomotor nerve lesions?
Dipoplia- double vision
What signs of oculomotor nerve lesion would be found on clinical examination?
- 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)
Where does CNIII arise from?
Midbrain (remember 2244)
What does CNIII have a close relationship to?
Tentorium cerebelli
Outline the path of oculomotor nerve
What are some causes of oculomotor nerve lesions sparing the pupil?
Microvascular ischaemia
Risk factors:
* Age >50
* Diabetes/hypertension
What are some causes of oculomotor nerve lesions involving the pupil?
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
What is the function of the trochlear nerve (CNIV)?
Motor only, therefore only motor fibres
What muscle does the trochlear nerve innervate?
Superior oblique, patient will complain of diplopia
On examination there will be:
* Abnormal eye position
* Difficulting moving eye downwards when eye is adducted
Where does CNIV arise from?
Dorsal midbrain
Outline the path of the trochlear nerve
What are the acquired causes of trochlear nerve lesions?
- 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
What muscle does the Abducens nerve innervate? (CNVI)
Lateral rectus- abducts the eye, only has motor fibres
What are the symptoms of a CNVI lesion?
Dipoplia- worse in lateral gaze
What signs on examination would an abducens nerve lesion present with?
- Abnormal eye position at rest
- Difficulty/unable to move affected eye laterally
What are the causes of CNVI lesions?
- Microvascular ischaemia (diabetes/hypertension)
- Head injury, tumour
- Raised ICP of any cause, nerve gets stretched
Where does CNVI arise from?
Caudal pons
Outline the path of CNVI
Why is CNVI so susceptible to being damaged in raised ICP?
Vertical route & between brainstem and cavernous sinus,
CNVI is fixed at these points
Raised ICP causes downward displacement of brain which can stretch CNVI
What is false localising sign?
Local symptoms due to general raise in ICP as the abducens becomes stretched
