Oculomotor system lesion

Question 1

Oculomotor nuclei

Answer 1

Located in ro strab midbrain, with projections shown in red

Question 2

Trochlear nuclie

Answer 2

Located in caudal midbrain, with projections shown in red

Question 3

Abducens nuclie

Answer 3

Located in caudal pons with projections shown in red

Question 4

3 fibers of the midbrain

Answer 4

Projections of CNIII

Question 5

CN4 as they exit the midbrain

Answer 5

They Criss cross each other as they exit the midbrain on the dorsal side

Question 6

Left CN4 nucleus innervate

Answer 6

Right SO

Question 7

Where are the UMN for voluntary eye movements

Answer 7

FEF
Parietal-occipitotemproal area
visual cortex

Question 8

Lower motor neurons

Answer 8

These are the neurons that actually innervate the muscle

Question 9

Projections to CN 3 nuclei are

Answer 9

Bilateral

Question 10

Projections to the CN4 are

Answer 10

Bilateral

Question 11

Projections to the CN6 are

Answer 11

To the contralateral side

Question 12

Circuitry for horizontal gaze

Answer 12

• The CN6 nucleus drives activation in the contralateral 3 nucleus
• function:L when one 6nucleus is going to abduct the ipsilateral eye (lateral rectus), it activates the contralateral 3 nucleus and medial rectus
• VI nucleus projects to contralateral III nucleus by sending axons across the midline, then ascending up the MLF
• PPRF=paramedian pontine reticular formation a. Projects to ipsilateral 6 nucleus
• this circuit is recruited during both voluntary horizontal gaze and by reflex horixtonal gaze drive by the vestibular system responds to head movement
• left motor cortex projects down ipsilateral corticonuclear (corticobulbar) tract, through cerebral peduncle and basilar pons, then turns horizontally and crosses midline in pons to activate right PPRF, which activates right 6 nucleus
• right motor cortex projects down ipsilateral corticonuclear tract, through cerebral peduncle to basilar pons, and then turns horizontally and crosses midline in pons to left PPRF, activates 6 nucleus
• analogous to how motor cortex targets contralteral anterior (ventral ) horn in spinal cord

Question 13

Clinical significance of the vestibulo-ocular reflex

Answer 13

• can be used to evaluate the brainstem oculomotor system in an unconscious or unresponsive patient
• in a conscious cooperative pateitns with conjugate gaze palsy, VOR testing can be used to test the integrity of the brainstem conjugate gaze circuitry . If the VOR is intact, suggests the conjugate gaze plays is the result of an UMN lesion, e.g. lesion in a cerebral hemisphere, rather than a brainstem lesion
• can be performed by the examiner moving the patient’s head: cools head maneauver, or oculocephalic testing. Also by caloric testing

Question 14

Review of how horizontal canal detects lateral head turn

Answer 14

Decrease in firing of the right CNIII firing from the hair cells

Increase in firing from the left CNIII to the right CN 6

Question 15

Damage to the FEF and internal capsule

Answer 15

Only voluntary eye movements affected

• impairment/loss of voluntary gaze to left or right, but intact VOR
• pure CNIII functions (adduction) or CN3 and CN4 functions (vertical gaze) likely spared due to bilateral projections from intact hemisphere (FEF)

Question 16

What could cause damage to the FEF

Answer 16

MCA superior divions infarct, hematoma, tumor

Question 17

CNIII nucleus lesion

Answer 17

Likely to hit the cerebral peduncle

• descending axon projections of the corticospinal tract
• R damage causes left sided weakness

Question 18

Parinauds syndrome

Answer 18

• dorsal midbrain compression
• pineal tumor expands downward to compress dorsal midbrain
• hydrocephalus causes the suprapineal recess to displace downward, compressing dorsal midbrain
• ischemic damage
• MS lesion

Question 19

Parinaud’s syndrome and the eye

Answer 19

First disruption of pupillary light reflex, then CN3

• bialteral compression of midbrain rectus, the dorsal region continuing the superior colliculus
• disruption of the pretectal nuclei and potentially CN3 nuelcues and its neighbor the rostrum interstitial nucleus of the MLF
• disruption of the pretectal nuclei: loss of the light reflex
• disruption of the CN3 nucleus and riMLF, disruption of vergence and accommodation, adduction, and conjugate gaze, vertical gaze

Question 20

If there is a further downward lesion expansion or compression in parinauds suydrome

Answer 20

Potential invovlemtn of CN4

Question 21

CN6 damage results in

Answer 21

Only LR palsy

Question 22

6 nucleus damage not only paralyzes LR by

Answer 22

Also damages projection via the MLF to contralteral 3 nucleus. Lateral gaze deficit. VO reflex wont work either-LMN lesion

Question 23

Nystagmus

Answer 23

Type of reflex eye movement, which can be a normal event or an abnormal event caused by a pathology that stimulates the oculomotor year system.

Question 24

How is nystagmus always named

Answer 24

By the fast eye movement that re-fixates the eyes in forward gaze after a slow movement in some direction, usually a smooth pursuit

Question 25

Neural circuitry for nystagmus

Answer 25

It is a reflex (automatic) eye movement triggered by the visual system seeking re-fixation of gaze. The neural circuitry is not well defined, but likely invokes descending projections from visual cortex and possible the parietal eye fields to the superior colliculus (optic tectum), which communicates with the brainstem motor nuclei for CN 3/4/6

Question 26

OKR

Answer 26

Example of normal occurrence of nystagmus. Another normal form of nystagmus occurs after each repeated slo lateral conjugate gaze movement (after spinning in a chair)

Question 27

Caloric testing

Answer 27

Test the VOR

-COWS

Question 28

Abnormal nystagmus

Answer 28

-nystagmus can follow abnormally spontaneous slow eye movements that are caused by some pathology that stimulates the VOR circuitry. When a pathology activates the ocular reflex circuitry, an inappropriate conjugate eye movement occurs, which in turn activates the normal nystagmus reflex circuit and induces nystagmus

Question 29

Examples or sources of an inappropriate conjugate eye movements followed by nystagmus

Answer 29

Pathology stimulating CN8 nerve on one side (Schwannoma)

Damage to the vestibular nucleus on one side: abolishes the baseline amount neural activation from the vestibular nucleus to the contralateral abducens nucleus. As a result, there is unequal activity in the two abducens nuclie, which activates a reflex slow conjugate lateral eye movement and a nystagmus

Damage to cerebellar systems, especially cerebellum itself. Cerebellar systems receive vestibular input, proprioceptive input, and motor cortex input and sends output to coordinate eye movements