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Optic Nerve Palsies Notes

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This is an extract of our Optic Nerve Palsies document, which we sell as part of our Medical Finals & OSCEs Notes collection written by the top tier of Oxford University students.

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Optic Nerve Palsies Cranial Nerve III: Oculomotor nerve Functions of oculomotor:

*

The efferent for the pupillary reflex (i.e. constricts) since it carries parasympathetic fibres

*

Innervates all extraocular muscles except lateral rectus and superior oblique: o

Medial rectus ?adduction (move medially)

o

Superior rectus ?up

o

Inferior rectus ?down

o

Inferior oblique?up and out

o

Levator palpebrae?lift eyelid

A lesion therefore produces:

*

Levator palpebrae damage?ptosis

*

Parasympathetic efferent damage?fixed dilated pupil

*

Unopposed lateral rectus?abducted eye

*

Unopposed superior oblique?down

*

Muscle damage?eye fixed down and out

Pathway:

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Originates in midbrain

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Travels near the posterior communicating artery near the Circle of Willis

*

Travels through the cavernous sinus (below the pituitary gland)

*

Travels through the superior orbital fissure

Anatomy:

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Parasympathetic fibres carried on the outside

*

The muscle nerves are in the middle

*

There are small arteries that supply the middle nerve

Mechanisms of damage:

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Iatrogenic CNIII palsy?complete palsy d/t severing during an operation (e.g. to fix a posterior communicating artery aneurysm)

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Surgical CNIII palsy:

*

o

Aneurysm of posterior communicating artery or internal carotid artery in the cavernous sinus?compress CNIII?outer parasympathetic fibres first?pupil dilated first

o

Increased ICP compressing CNIII?outer parasympathetic fibres first?pupil dilated first

o

Uncal herniation down into the midbrain compresses CNIII nucleus almost before anything else?evolving CNIII palsy Uncal herniation may be unilateral but give bilateral CNIII palsies as it pushes acrossUnilateral pressure (e.g. haematoma)?unilateral uncal herniationBilateral pressure (e.g. hydrocephalus)?bilateral uncal herniation

Medical CNIII palsy: o

o

*Oculomotor middle nerve blood supply damaged d/t:Small ischaemic episodesVasculitides

Oculomotor nerve damaged d/t:Mononeuritis multiplexMSDiabetic polyneuropathy

Cavernous Sinus Syndrome

Patterns of damage:

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A surgical CNIII palsy will theoretically compress the outer parasympathetic fibres first and cause a dilated pupil without motor involvement.

*

A medical CNIII palsy will theoretically damage the inner motor fibres first and cause a ptosis and ophthalmoplegia without a dilated pupil.

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Practically, all features (lid ptosis, pupil dilation, eye down-and-out) occur simultaneously.

Cranial Nerve IV: Trochlear nerve Functions of trochlear:

*

Innervation of superior oblique muscle o

pulls the eye nasally

o

pulls the eye down when it is facing inwards

o

makes the eye look at the nose

o

the gaze for going downstairs or reading the newspaper

o

cannot turn nasally when the oculomotor muscles are paralysed

o

when the oculomotor muscles are paralysed it can only turn downwards

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