ICL 6.4: Anatomical & Clinical Pearls Of CN III, IV, VI, Eye Movements And Pupillary Control Flashcards
what is the origin of all the rectus muscles of the eye?
common tendinous ring aka annulus of zinn
it’s a ring of fibrous tissue that the ophthalmic artery and nerve pass through
what are the 6 extra-ocular eye muscles?
- medial rectus
- lateral rectus
- superior rectus
- inferior rectus
- superior oblique
- inferior oblique
which nerves innervate the extra-ocular muscles?
CN 6 = lateral rectus
CN 4 = superior oblique
CN 3 = medial, superior and inferior rectus and the inferior oblique
which muscles help you look up vs. down
up gaze = inferior oblique, superior rectus
down gaze = inferior rectus, superior oblique
what is the function of torsional eye movements and which muscles help us do them?
intorsion = counter-clockwise rotation –> superior oblique with a bit of the superior rectus
extorsion = clockwise rotation –> inferior oblique with a bit of the inferior rectus
torsional eye movement help compensate for head tilt; they’re what maintains a visual horizon when your head/neck tied –> your eyes rotate in the direction opposite form the head/neck tied
what is the function of the GSE fibers of CN 3 and where do they originate from?
GSE fibers of CN 3 originate from the oculomotor nucleus in the midbrain and innervate:
- superior rectus
- inferior rectus
- medial rectus
- inferior oblique
- levator palpebrae
these are all innervated by the ipsilateral subnucleus with the exception of the superior rectus which is innervated by the contralateral sub nucleus
what is the function of the GVE fibers of CN 3 and where do they originate from?
the GVE of CN 3 synapse in the Edinger-Westphal nucleus in the midbrain and have parasympathetic innervation to:
- ciliary muscles for accommodation of the lens
- iris sphincter for pupillary constriction
what is the path of the oculomotor nerve?
the nuclei are in the midbrain and then it emerges from the brainstem and runs with the dural wall of the cavernous sinus and enters the orbit through the common tendinous ring to innervate the eye muscles
what are the divisions of the oculomotor nerve and which muscles do they innervate?
- superior division which innervates the superior rectus and levator palpebrae
- inferior division which innervates the inferior oblique, medial rectus, inferior rectus and contains the parasympathetic fibers for pupillary constriction
what will you find during a PE of a patient with CN 3 palsy?
CN 3 = superior rectus, medial rectus, inferior rectus, inferior oblique, pupil sphincter and levator palpebrae
- the superior oblique and lateral rectus will be unopposed so the eye will be down and out
- superior eyelid droops (ptosis) and cannot be raised voluntarily because of the unopposed action of the orbicularis oculi (facial nerve)
- pupil is dilated and non-reactive because of the unopposed dilator pupillae
how do you classify a CN 3 palsy?
- pupil involvement or no pupil involvement?
- complete vs partial?
- inferior vs superior division, or both?
- aberrant regeneration? (clue to timing)
- neighborhood signs?
- if age > 50, are there signs of giant cell arteritis?
- pain? (Not really helpful)
all of these are trying to help you figure out if it’s an aneurism or not
which fibers are found in the peripheral vs central part of CN 3?
the parasympathetic fibers controlling pupil constriction are on the peripheral part of the nerve
the motor fibers controlling the eye muscles and the arteries are on the inside of the nerve
what will happen if there’s compression on CN 3? what about ischemia?
the parasympathetic fibers controlling pupil constriction are on the peripheral part of the nerve while the motor fibers controlling the eye muscles are on the inside of the nerve so with a compression you will probably just have mydriasis alone (pupil dilation) –> this is bad because ti means there’s a growing aneurism in the vertebrobasilar artery system that’s growing
with ischemia, it will more likely effect the motor fibers and spare the pupils and this isn’t as big of a deal because it’s not life threatening
what could cause a CN 3 palsy?
- stroke
- tumor
- wernicke-korsakoff syndrome = thiamin deficiency that can cause hemorrhagic necrosis of CN 3 nuclei
- leigh’s syndrome = disease in kids that can cause necrotizing encephalopathy that also effects CN 3
- stroke
- compression = tumor, hemorrhage, aneurism
- distention (Ex. cerebral displacement)
- inflammation in the cavernous sinus
- trauma to the orbit
what is the presentation of a nuclear third nerve palsy?
when we say 3rd nerve palsy, usually we mean the nerve fibers or some lower motor neuron problem; a nuclear third nerve palsy involves the nuclei of CN 3
- bilateral gaze limitation (superior rectus nuclei sends fibers to the contralateral superior rectus so a nuclear third nerve palsy will cause bilateral deficits; usually worst on the contralateral side)
- bilateral ptosis (central caudal nucleus sends fibers to the ipsilateral levator palpebrae)
so with the nuclear CN 3 palsy it’s bilateral symptoms instead of just one side with a normal CN 3 palsy
what is Weber’s syndrome?
a form of stroke characterized by the presence of:
- ipsilateral oculomotor nerve palsy
- contralateral hemiparesis
- Parkinsonism
- facial expression, speech problems
so this is a brainstem problem which is why it’s causing ipsilateral CN 3 problems since that’s where the CN 3 nuclei are and it’s causing contralateral hemiparesis because it’s messing with the corticospinal tract which has already decussated in the medulla and Parkinsonism because it’s effecting the substantia nigra
what is Claude’s syndrome?
same thing as Weber’s but also includes red nucleus involvement so patient will have incoordination
- ipsilateral oculomotor nerve palsy
- contralateral hemiparesis
- Parkinsonism
- facial expression, speech problems
- incoordination
what is an uncal herniation?
it’s where the medial temporal lobe is forced inferiorly over the tentorium which causes downward displacement of the brainstem
it’s caused by expanding supratentorial mass like a hematoma or tumor
patient will present with ipsilateral pupillary dilation due to CN 3 compression
what is aberrant regeneration?
years after a CN 3 palsy, injured fibers meant for the inferior or medial rectus may regrow incorrectly and innervate the levator palpebrae
what you end up having instead of adduction or depression, you end up with lifting of the lid –> so every time you try to adduct the injured eye, you lift the eyelid instead
what is the function of CN 6?
the abducens has GSE motor fibers to the lateral rectus to abduct the eye
where is the nucleus of the abducens nerve located?
the nucleus is located in the dorsal caudal pons
it’s separated anteriorly from 4th ventricle by the genu of facial nerve (facial colliculus)
what is the function of the medial longitudinal fasiculus?
adduction of the contralateral eye and abduction of the ipsilateral eye
what is the pathway of the abducens nerve?
Ventral brainstem (ponto-medullary jxn) –> enters subarachnoid space –> runs upward (rostral) between pons and clivus –> pierces dura and travels via epidural space –> travels anteriorly over the petrous part of temporal bone
then it passes via (medial) cavernous sinus
then it passes via superior orbital fissure and enters the orbit via common tendinous ring to innervate the lateral rectus
what does an abducens nerve palsy look like?
the person can’t abduct their eye, they get stuck in the middle
what could cause an abducens nerve palsy?
usually due to increase intracranial pressure to cloves tumors because it runs near the clivus
usually it’s idiopathic though
there’s other stuff too but elevated ICP is what’s important
what is idiopathic cranial hypertension?
condition that effects young women of child bearing age that are over weight and this usually causes CN 6 palsy!
how do you initiate a saccade to the left?
RIGHT frontal eye field tells the LEFT Paramedian Pontine Reticular Fibers (PPRF) toactivate two types of neurons in the LEFT abducens nucleus ( –> CN VI nerve and medial longitudinal fasiculus)
this will allow the LEFT eye to turn laterally and the RIGHT eye will turn medially = this is a voluntary LEFT horizontal saccade
what is the function of the trochlear nerve?
GSE fibers innervate the contralateral superior oblique muscle
where is the nucleus of the trochlear nerve located?
it’s located near the midline of the midbrain – ventral to the periaqueductal grey at the level of inferior colliculus
what is the pathway of the trochlear nerve?
it’s the only CN to exit from the dorsal surface of the brainstem!!
it passes between posterior cerebral and superior cerebellar arteries then travels via lateral wall of cavernous sinus via superior orbital fissure then enters orbit passing superior to common tendinous ring
it’s the longest pathway so it’s prone to trauma
what is the clinical presentation of a trochlear palsy?
- extortion (outward rotation) of the affected eye that is due to the unopposed action of the inferior oblique muscle
- this gives rise to vertical diplopia (double vision) and weakness in downward gaze and is most pronounced in medial gaze
- chief complaints: visual difficulty when going down stairs.
- patients compensate by tilting the head
what is the function of the tarsal muscle?
aka mueller’s muscle
assists in elevating upper lid
what happens when there’s damage to the superior or inferior tarsal muscle?
ptosis
what is the difference between acquired vs. congenital ptosis?
Acquired Ptosis (upper lid)
- “drooping” eyelid
- paralysis or paresis of muscles that elevate the upper eyelid
- muscles involved = levator palpebrae superioris and superior tarsal muscle
Congenital ptosis is poor development of the levator palpebralis muscle
what is Horner’s syndrome?
- ptosis
- miosis
- anhydrosis
what could cause Horner’s syndrome?
carotid dissection of the carotid artery that could cause Horner’s syndrome
most common cause of acute isolated Horner Syndrome is an extra-cranial dissection of the internal carotid artery at the skull base
this also gives you a higher risk of stroke….
