VOR Flashcards
Why do we need reflex eye movements that stabilize images on the retina during head movement?
decreases potential retinal image motion– prevents blurred images and can prevent oscillopsia
How do we stabilize images on the retina during head movements?
by producing an eye movement in the direction opposite head movement
What is the point of adaptive head and eye movements?
preserves the image on the center of the visual field
What are the two component parts of Vestibular-Optokinetic movments?
vestibular system and optokinetic system
What is the general idea of the vestibular system?
compensates for brief, transient head movements, produces VOR
What is the general idea of the optokinetic system?
compensates for prolonged, sustained movements, low frequency, aided by pursuit system, result is OKN
How to vestibular and optokinetic movements interact?
over time VOR declines, responds to acceleration/deceleration, when that becomes constant VOR breaks down and optokinetic takes over to hold image stable until rotation is finished
When rotation is complete, we should get a…
nystagmus in the opposite direction
T/F the optokinetic system does not account for nystagmus
false
What is OKAN?
optokinetic after-nystagmus
What does OKAN do?
helps cancel out the post-rotational nystagmus and minimizes vertigo
Describe VOR movements
head moved quickly to 1 side while eyes stay on target, eye position in orbit is opposite head motion in space, result: keep eye position in space constant
What does OKAN result from?
velocity storage phenomenon
What is the velocity storage phenomenon?
indirect central neuro integrating circuit activated by initial input of rotational acceleration and velocity, stores velocity info and discharges in after the input has ceased
What is the stimulus for VOR?
angular or rotational acceleration, acceleration info integrated into head velocity signal and integrated again to get head displacement
Describe what happens during VOR with brain injury?
when head rotated to the right, eyes moved to the right and then made a compensatory movement back to the target
What three additional thing does VOR serve to maintain?
body posture, equilibrium, and muscle tone
How many semicircular canals are each side and what do they sense?
3, sense angular acceleration when fluid hits the cupula and deflects it
What is the cupula?
gel like substance containing crista ampullaris that sends a neurological/electrical signal to the CNS
What do the maculae of the saccule and utricle and the otoliths sense?
transient linear accelerations and static head tilt
What contains endolymph?
the endolymphatic space of the semicircular canal
What are otoliths?
calcium particles that sit on top of the gelatin in the macula of the saccule or utricle
What are otoliths affected by?
gravity and inertia
When do the otoliths stay in place?
when the head is level
What forces take place when walking forward?
acceleration forward, inertial force opposite of acceleration, gravity, and resultant from inertia and gravity
What are the otolith forces with a head tilt?
gravity and gravitational component in the plane of the otoliths creates resultant force
What is the result of head tilt?
translational VOR with total latency a bit longer, 35 msec
What do the best runners do?
keep head relatively still and suppress VOR to an extent
Why do runners suppress VOR to some extent?
prevents disruptive, conflicting info between internal and external inputs
VOR displacement?
10 to 20 degrees
VOR peak head velocity?
100 to 250 deg/sec (constant velocity means OKN)
VOR peak head acceleration?
1000 to 2500 deg/sec^2 (trigger for VOR)
VOR latency?
15 msec (shortest)
Is VOR gain better for horizontal or vertical?
horizontal, barely
T/F fluid movement is in the same direction as head movement
false
What makes a gain greater than 1.0?
eye movement > head movement
Is VOR gain present in congenitally blind people?
no, need early normal visual experience for development
What happens to VOR with acquired blindness?
reduced VOR, visual experience is needed for maintenance
T/F VOR is less accurate in infants
true
T/F VOR can compensate for optical magnification changes
true
What happens to VOR gain with minus lenses?
patient needs less gain for a given head rotation
What happens to VOR gain with plus lenses?
hyperope needs more gain, around 2.5% per diopter
T/F VOR gain effects take place with contact lenses and spectacle lenses
false, does NOT happen w/ CLs
What happens to patients with lesions of the cerebellum?
patients lose visual inhibition of the VOR reflex, adaptation response
T/F each canal has connections to more than one EOM
true, not 1:1 canal:EOM ratio
T/F each canal has either excitatory or inhibitory connections
false, each canal has both
What structure are VOR EOM connections passing through?
medial longitudinal fasciculus
With head translation, result is…
opposite eye movement and otolith movement
With head tilt, result is… counter-roll of the eye
counter-roll of the eye
T/F VOR translation and tilt have different neuro pathways
true
T/F injury to the side/back of the head impacts VOR
true
What is ICS impulse?
new tech for testing eye movements
What does ICS impulse test?
VOR, BPPV, other vestibular disorders, nystagmus, gaze position, skew deviation
In the VOR data of a healthy subject gain is ___ and one side should ___ the other
1.0, balance
Describe VOR data with bilateral loss
lots of catch up saccades to stay on target, gain is symmetric but bad 0.3-0.4, patient has some symptoms
Describe VOR data with unilateral loss
asymmetric gain (one bad one normal) and overt saccades, patient has severe symptoms
T/F after time passes, gain recovers after acute loss
true
What is benign paroxysmal postural vertigo?
disorder caused by problem in inner ear, symptom=positional vertigo, spinning sensation after change in head position
Why does BPPV happen and what is the cure?
happens when an otolith gets stuck in a semicircular canal and hit cupula, fixed with epley maneuver to get crystal back in place in the saccule/utricle
What is optokinetic nystagmus?
involuntary jerk nystagmus induced by motion of the visual field, not the fovea
What triggers OKN?
repetitive stimuli, sustained, low frequency
What is the neural integrator for OKN?
nucleus of the ophthalmic tract
What is the pathway?
semicircular canals to primary vestibular nerve to vestibular nucleus to sec vestibular neurons to medial longitudinal fasciculus to oculomotor neurons
What is the OKN specific pathway?
retina to visual cortex to dorsal terminal nucleus of the ophthalmic tract to nucleus of the ophthalmic tract to inferior olive to cerebellum to vestibular nuclei oculomotor nuclei
In the OKN system, ___ retina dominates
peripheral
How does a deep central scotoma affect OKN?
reduction of gain by only 10-30%
T/F OKN is heavily affected by blur
false, not affected much by blur
What is the latency for OKN?
140 msec
Is the image clear in OKN?
no image is stable but not clear
How do horizontal and vertical gain values compare?
similar but vertical degrades faster as target moves faster
What is gain with torsion?
.03 low
T/F OKN declines with age
true
T/F all speeds of optokinetic drum trigger the reflex
false, only certain ranges trigger the reflex, too fast=blur
In children how are temporal to nasal and nasal to temporal OKN different?
nasal to temporal OKN is not intact as infant, temporal to nasal is intact
When do the two directions of OKN equalize in babies?
directions equalize between 3 and 6 months
What happens in children whose OKN on each eye does not equalize?
high tendency for strabismus, usually esotropia
Describe functional amblyopia
asymmetric, reduced response, similar to neurological disease, especially found in congenital ET/strabismic amblyopia
What are the three vestibular disorders?
unilateral peripheral (acute), bilateral peripheral (acute), and central (lots of problems)
What are the two subtypes of unilateral peripheral vestibular disorder?
transient imbalance, spontaneous nystagmus
What is the subtype of bilateral peripheral vestibular disorder?
oscillopsia (world tilt), decreased vision
Describe nystagmus in unilateral
slow phase toward lesion
Describe nystagmus in bilateral
decreased vision, inadequate compensation from VOR
Describe congenital nystagmus?
asymmetric, reduced gain
Describe neurological disease/developmental delay or VOR/OKN
newborns have decreased nasal to temporal, anterior/cortical lesions of the visual pathway
