The Eye Flashcards
Rods
scotopic vision- black & white, shapes & size, use at night most
Cones
photopic vision- color, use during the day
parts of Retina
optic disk
maclua
optic disk
blind spot, where vessels leave
maclua
higher concentration of cones than rods
contains fovea- has even greater concentration of cones
fovea
high concentration of cones
greater clarity when image is projected on the fovea
Extraocular muscles
lateral rectus medial rectus superior rectus inferior rectus superior oblique inferior oblique
vestibular pathways for the control of gaze, balance, and posture
visions
vestibular
proprioception
transmitters in Vestibule
glycine
acetylcholine
glutamate
2 important characteristics of human eye and their importance
- both eyes are frontally positioned
- the retina has a region called the fovea
allow for:
binocular vision
depth perception
fovea allow for sharp detail
gaze definition
the direction or position in which the eyes are directions
how is eye movement
conjugate (move as a pair)
2 main types of conjugate eye movements
those that shift gaze
those that stabilize gaze
gaze shifting
saccades (rapid movement of eye to acquire target)
gaze stabilizing
pursuit vestibulo-ocular reflex optokinetics fixation vergence
pursuit
eye movements that are trying to stabilize an image in front of them without moving head
vestibulo-ocular reflex
following a target while moving head
optokinetics
focus on something as the target is moving in a sustained fashion, look at multiple images
fixation
staring at a target without moving eyes
vergence
only eye movement that is not conjugate- eyes are not moving the same way: convergence- eyes move toward each other, divergence- eyes move away from each other
neural structures responsible for eye movements due to vestibular input
paramedian pontine reticular formation nucleus prepositus hypoglossi vestibular cerebellum interstitial nucleus of caja interstitial nucleus of the medial longitudinal fasciculus vestibular nucleus
vestibular input travels:
primary afferents–> vestibular nucleus–> nucleus prepositus hypoglossi (bilaterally)–> vestibular cerebellum–> back to nucleus prepositus hypoglossi–> paramedian pontine reticular formation–> superior colliculi–> interstitial nucleus of the medial longitudinal fasciculus–> interstitial nucleus of caja–> OMN
secondary pathways
vestibular cerebellum on the PMT back to vestibular cebellum
role of the neural integrator
convert vestibular info about acceleration and velocity into info of step commands
important factors about eye movements
all of eye muscle fibers participate in contraction
the firing rate of neurons is proportional to contraction of EOM
position of the eyes is a sum of agoinist/antagonist muscles
several signals that are required for quick eye movements
require a surge of activity of agonist and quick inhibition of antagonists
several signals that are required for smooth eye movements
step to hold eye in position
ramp to give integral of stop so the eye moves smoothly
laws of eye movement
Hering’s Law of Equal Innervation
Listing’s Law
Hering’s Law of equal innervation
in order to have conjugate movement, the eyes must move in the same directions and the same amount
Listing’s Law
when eye moves from the primary axes of movement, a torsion of the eye is generated
Ways to record eye movement
EOG (electro-oculography) VOG (video oculography)/IROG (infrared oculography) Scleral Coil ENG (electro-nystagmography) VNG (video nystagmography)
EOG (electro-oculography)
use electrodes
cheap but has a lot of noise
can only record horizontal and vertical not torsion
VOG (video oculography)/IROG (Infrared oculography)
common methods for recording eye movement much more reliable results than EOG doesn't matter if lights are on or off less noise can do horizontal, vertical, and torsion
Scleral Coil
use contact lens
series of magnets (electrical field) around the person
not commonly used
expensive & invasive
ENG (electro-nystagmography)
use electrodes to record nystagmus/eyemovement, a type of EOG
VOG (video nystagmography)
use video to record nystagmus/eyemovement, a type of VOG
Calibration
must calibrate, the system doesn’t know anything about the patient or their eye movement so we have to calibrate the system to know their eye movements
spontaneous nystagmus
test of nystagmus with eyes focused in primary position (straight ahead)
performed in 2 phases: fixation (eyes open), no fixation (closed)
4 types of nystagmus
peripheral
central
congenital
periodic alternating nystagmus
what is the problem for peripheral nystagmus
combine, but primarily horizontal nystagmus
fast phase usually away from the lesion (right beat nystagmus= left lesion)
always maintains same direction
always stops during fixation task
present with lights out
what is the problem with central nystagmus
not combined, only 1 type (horizontal or vertical) will often change direction changes in characteristics or if present during fixation task present with lights on
problems with congenital nystagmus
system cannot maintain foveate vision
usually horizontal in one direction but can change by gaze opposite fast phase (right beat nystagmus and look to left can briefly get a left beat nystagmus)
other ocular motility tests normal
Congenital nystagmus is characterized by:
long history
horizontal nature
can be slowed by convergence
velocity increases with time (age)
problems with periodic alternating nystagmus
changes directions without change in eye/head position
cycles last 1-6 minutes in time
nystagmus properties on test results
has both a slow and fast component
fast component gives us
direction
slow component gives us
degrees (velocity)
Alexander’s Law
- 1st degree nystagmus- present when looking in the direction of fast phase
- 2nd degree nystagmus- present when looking in direction of fast phase and in primary position
- 3rd degree nystagmus- present in all directions of gaze
What is direction fixed
if it is a right beat nystagmus, it is always right beat no matter what direction the eyes are looking
Characteristics of Peripheral Gaze Evoked Nystagmus
- Follows Alexander’s Law
- Direction Fixed
- Enhanced in absence of fixation
- Usually beats away from affected ear
Characteristics of Central Gaze Evoked Nystagmus
- pure vertical or torsional nystagmus in primary position is aways central (unless proven otherwise)
- Downbeat: Arnold-Chiari Malformation (cerebellum hanging out of spinal hole thing?)
- Upbeat: problem in brainstem or medullary regions
- horizontal gaze (left or right)
- possibly ipsilateral NPH (nucleause p…. hypoglossi) or medial vestibular nucleus problems
- possible vestibulocerebellar problems
- vertical
- possible involvement of interstitial nucleus of Cajal or cerebellar/brainstem lesion
- torsion in primary position
- never normal with central position unless moving head
- pons or pontomedullary region
- rebound
- nystagmus beats in direction of last direction the eye moved as it returns to primary position (vary rare)
- typically horizontal
- Dissociated
- eyes are not moving in same directions
- most likely will never see
What is Saccades?
rapid eye movement that shifts gaze between successive fixation points
Paradigms of Saccades testing
- fixed- fixed time and location
- pseudorandom-fixed time, random location & vice versa
- random- random time & location
Neural involvement of Saccades
- brainstem
- superior colliculus
- frontal eye fields
- supplementary eye fields
- posterior parietal cortex
- thalamus
- basal ganglia
- cerebellum
What do you look at when testing Saccades?
- latency
- accuracy
- velocity
What is Saccadic Latency? Norms? Abnormal results?
- time between movement of target & eye
- Norm- around 200 msec
- Prolonged latency- more than 400 msec (causes: basal ganglion disorders, parkinson’s disorder, improper instruction)
- Shortened Latency- pt anticipating movement
- Asymmetric latency- seen in occipital or parietal lesions
What is Saccadic Accuracy? Abnormal results?
- ability to move eye to exact placement of the target
- Overshoots (hypermetric)-cerebellar disorders
- Undershoots (hypometric)- basal ganglion disorders, parkinsonism, brainstem infarcts, progressive supranuclear palsy
- Glassade- myastenia gravis, cerebellar disorders, internuclear ophthalmoplgid
- Pulsion- infarcts of superior cerebellar artery (contrapulsion), infarcts of posterior cerebellar artery (ipsipulsion)
What is Saccadic Velocity? Norms? Abnormal Results?
- the speed at which the eye moves to obtain a target
- Norm- 330-650 degrees/sec
- Slowing- myastenia gravis, Huntington’s chorea, cerebellar ataxia, CN VI palsy, internuclear ophthalmoplegid, CNS meds, progressive supranuclear palsy affects vertical saccades
- Fast Saccades- opsoclonus, restricted ocular motility
- Asymmetric Velocities- internuclear ophthalmoplegia (normal abduction, slow adduction), CN VI palsy (normal adduction, slowed abduction)
Smooth pursuit (tracking) tests
- Predictable- sinusoidal, ramp
- Non-predictable- rashbass, sum of sines
What is Smooth Pursuit? What does it require?
- eye movements that allow clear vision of objects moving within visual environment
- requires: velocity of eyes match velocity of target, vision must remain clear
What do we look at when analyzing Saccades?
- gaine
- phase
- morphology
- acceleration
- symmetry
What is pursuit?
ability to move eyes to track a target that is moving smoothly
Causes of Symmetric Pursuit Abnormalities
- advanced age
- brainstem disorders
- cerebellar disorders
- cerebral cortical disturbances
- congenital nystagmus
- medications
- inattention
- visual disorders
Causes of Asymmetric Pursuit Abnormalities
- parietal lobe disorders
- frontal lobe disorders
- superimposed nystagmus
What is Optokinetics?
eye movements that allow clear vision of constantly moving objects moving within the visual environment, combines foveal and extrafoveal tracking
What do Optokinetics Require?
- velocity of eyes match velocity of target
- attention to the stimulus
- involves pursuit during initial onset of stimulus and optokintics mechanisms
What do we look at when we analyze Optokinetics?
- symmetric gain
- asymmetric gain
- reversed optokinetics
Causes of Optokinetic abnormalities.
- visual disorders
- pursuit system disorders
- fast-phase disorders
- superimposed nystagmus
- congenital nystagmus
What is Optokinetic Afternystagmus (OKAN)?
nystagmus elicited after sustained (30-60 sec) constant velocity Optokinetics, begins around 1 sec after stimulus ceases
Measurements of Optokinetic Afternystagmus?
- velocity gain
- time constant- amount of time it takes the signal to decrease 63% of the max value
- SCEP- slow cumulative eye position over 45 sec
Positional/positioning tests
- sitting
- dix-hallpike maneuver
- body position
- head position
Types of responses from position tests
- spontaneous- persistent nystagmus unrelated to head and/or body position
- positioning- precipitated by rapid head movements
- positional- result from static position of the head relative to plane of gravity
BPPV
benign paroxysmal positional vertigo
Origins of positioning nystagmus
- cupulolithiasis
- canalithiasis
What is cupulolithiasis?
clot of particulate matter attached to cupula, mass effect
What is canalithiasis?
clot of particulate matter freely mobile in posterior SCC, plunger effect
What direction is geotropic nystagmus?
clockwise
What direction is ageotropic nystagmus?
counterclockwise
What is the Epley Maneuver? Example with a left side problem
- used to roll the oticonia out of the canal and put it back where it should be
- Ex. have patient sit up, turn head to left, lay person down, turn head to right, have person roll onto right side, sit person up
What is Positional Alcohol Nystagmus (PAN) Buoyancy Hypothesis?
differential gravity of ingested compounds have relative diffusion times
Phases of Positional Alcohol Nystagmus
- Phase 1: Absorption by Cupula (PAN 1)
- Phase 2: Resorption by Endolymph (PAN 2)
Central Positional Vertigo
- typically minimal symptoms, with postural imbalance
- onset is typically abrupt, symptoms gradually improve
- freq, assoc. w/mass near 4th ventricle & vestibular nuclei
- differentiate from BPPV by continuous low freq nystagmus- no latency period, and does not fatigue/suppress
Cervical Vertigo
- not simply neck injury associated with rotational vertigo and/or nystagmus -rather, symmptoms generally include: feeling of floating, ataxia of gait, sensation of numbness -various hypotheses
- neuromuscular -neurovascular -mechanical vascular obstruction of vertebral artery interpretation is very difficult
- Classical Assessment:
- from lateral body position, maintain head position while patient turns body to supine. -classic indication: no nystagmus with neck straight and present nystagmus with neck rotation or flexion/extension = positive
CRP
corneo retinal potential- the electrical difference between the voltage at the cornia and the retina
