Control of Eye Movements

Question 1

Purpose of eye movements:

Answer 1

to keep object of interest on fovea (the center of macula), where visual acuity is highest due to high density of cones

Question 2

Definition: Nystagmus

Answer 2

fast oscillatory eye movements

• mediated by cortex (either FEF or parietal eye fields)
• fast component & slow component
• horizontal, vertical, or rotational
• physiologic or pathologic

Question 3

What causes nystagmus?

Answer 3

• A defect of the slow eye movement system
  
  • usually due to impaired VOR
• Vertical nystagmus (ie. eyes are ‘beating’ up & down) is ALWAYS a central lesion
  
  • ​cerebellum, vestibular nuclei or pathways connecting the two
• Horizontal or rotational nystagmus could be due to a peripheral or central lesion

Question 4

Diplopia:

Answer 4

double vision

• a neurological issue (unlike blurred vision)
• usually caused by the image not resting on the fovea of each eye due to muscle, NMJ, nerve, or CNS disorder

Question 5

Blurred vision:

Answer 5

• decreased visual acuity
• could be neurological or non-neurological cause
  
  • eg. corneal abrasion, optic neuritis

Question 6

Mechanisms of eye movement:
2 major divisions

Answer 6

1. Fast, voluntary system:
   
   • Saccades = voluntarily brings object in peripheral vision onto fovea
2. Slow, involuntary system:
   
   • ​​Smooth pursuits = allows eyes to follow a moving object
   • Vestibuloocular reflex (VOR) = coordinates eye & head movement to stabilize object on fovea
   • Optokinetic system = stabilizes object on fovea during sustained head rotations

Question 7

Final common pathway for all extraocular movements:

Answer 7

• All the different systems involved in eye movements use:
  
  • trochlear nerve/nuclei
  • abducens nerve & nuclei
  • oculomotor nerve & nuclei
  • paramedian pontine reticular formation (PPRF)
  • medial longitudinal fasciculus (MLF)
• execute yoked eye movements so that image is on each fovea

Question 8

Abducens nucleus:

Answer 8

horizontal gaze center

• equivalent in function to the ipsilateral PPRF
  
  • helps both eyes move horizontally using MLF

1. right abducens nucleus/right PPRF ⇒ helps both eyes move horizontally to the RIGHT
2. lesion of R abducens nucleus/right PPRF ⇒ difficulty moving eyes to the RIGHT
   
   • ipsilateral

Question 9

MLF:

Answer 9

helps yoke the eyes

• helps the eyes move as a pair
  
  • R MLF helps the R eye move medially
• Lesion of the MLF ⇒ internuclear ophthalmoplegia (INO)
• R MLF lesion causes R INO:
  
  • R eye does not adduct (move towards nose) & L eye has nystagmus on leftward gaze
  • nystagmus occurs in left eye b/c of double vision (secondary to R eye not adducting)
  • VOR moves L eye back towards nose but b/c of instruction to look to the left ⇒ cortex overrides (suppresses) VOR ⇒ moves eye back to the left

Question 10

Saccades:

Answer 10

• Move eyes quickly to object of interest to improve visual acuity
  
  • voluntary control
• Eye movements are very fast

Question 11

What mediates saccades?

Answer 11

• Mediated by frontal eye fields (FEF)
  
  • ​located in frontal cortex
  • moves eyes contralaterally
• Final common pathway:
  
  • includes the PPRF/abducens nuclei
• If patient wants to move eyes to right:
  
  • activate L FEF ⇒ + R PPRF/abducens nucleus ⇒ R CN6 to lateral rectus & L MLF ⇒ activates L oculomotor nucleus & L CN3 to medial rectus ⇒ both eyes move to right (R lateral rectus & L medial rectus)

Question 12

Lesions of saccadic system:
Right-way eyes

Answer 12

contralateral hemiparesis & ipsilateral gaze preference

• R body/face hemiparesis & L gaze preference
  
  • due to lesion of L frontal cortex
• L primary motor cortex lesion ⇒ causes R body/face paresis
• L frontal eye fields lesion ⇒ does not allow saccading to R
  
  • R FEFs are still functional & allows looking to L

Question 13

Lesions of the saccadic system:

Wrong-way eyes

Answer 13

contralateral hemiparesis & contralateral gaze preference

• R body/face hemiparesis & R gaze preference
  
  • ​due to lesion of L pons
    
    • common final pathway
• L pontine corticobulbar/corticospinal tracts lesion ⇒ R body/face paresis
• L PPRF/abducens nuclei lesion ⇒ loss of L horizontal gaze center ⇒ cannot look to the left
  
  • eyes are deviated to the right

Question 14

What is the function of smooth pursuits?

Answer 14

Allows smooth tracking of a moving object

• if object is traveling faster than ability of smooth pursuit system to follow ⇒ saccadic system takes over

Question 15

What mediates smooth pursuits?

Answer 15

• Mediated by parietal-occipital-temporal eye fields
  
  • stimulate ipsilateral PPRF/abducens nucleus ⇒ activate the final common pathway
• Smooth pursuits are modulated by cerebellum
  
  • flocculonodulus

Question 16

Lesion of smooth pursuit system:

Answer 16

• results in “jerky” eye movements
  
  • saccades are being used instead
• R parietal-occipital-temporal lesion ⇒ ’jerky’ pursuits to the right
  
  • Lesion of cerebellum ⇒ can also cause ‘jerky’ pursuits or very slow pursuits

Question 17

Vestibuloocular Reflex (VOR):

Answer 17

• Maintains visual fixation on stationary object while head is moving
• Mediated by vestibular nuclei/nerve/semicircular canals (SCC)
• Influenced by cerebellum (flocculonodular lobe)

Question 18

Each VOR moves both eyes ____________.

Describe how VOR works:

Answer 18

contralaterally

• if one maintains fixation of an object in front & head is moving to the left ⇒ **VOR activated **⇒ eyes to move to the right
• to be more accurate:
  
  • L head turn ⇒ L ear horizontal SCC to be activated ⇒ moves eyes to the right
  • part of slow eye movement system

Question 19

What happens if the VOR is lesioned?

Answer 19

• pt will not be able to fixate any object while moving
• everything will appear ‘blurred’ due to decreased visual acuity
• pt c/o of feeling ‘dizzy’

Question 20

VOR can be _________ if you want to track an object using both EYES & HEAD motion

Answer 20

suppressed

• Eg: tracking object with head & eye movement & head is moving to the left & you want eyes to move L, then VOR needs to be suppressed
• If VOR was not suppressed:
  
  • head movement will cause eyes to go in opposite direction
• VOR is suppressed by cortex (such as FEF) to track a fast moving object

Question 21

Where is the lesion?

Answer 21

Lesion: either R CN6 or R lateral rectus

• Right eye is esotropic (inward) on primary gaze
• Right eye does not abduct fully

Question 22

Where is the lesion?

Answer 22

Lesion:

R abducens nucleus & R MLF = 1 ½ syndrome

• Impaired R eye adduction ⇒ R INO ⇒ R MLF
• Impaired R gaze ⇒ R horizontal gaze center ⇒ R abducens nucleus (&/or PPRF)

Question 23

Where is the lesion?

Answer 23

Lesion: impaired R eye adduction ⇒ R INO ⇒ R MLF

Question 24

Where is the lesion?

Answer 24

Lesion: bilateral abducens nucleus/PPRF lesions

• Cannot look L ⇒ L horizontal gaze center lesion ⇒ L abducens nucleus/PPRF
• Cannot look R ⇒ R horizontal gaze center
• Intact up, down, convergence ⇒ intact CN 3 & 4