Download Visual Dysfunction

Dysfunction of the Visual System
SPE 516
What Can Go Wrong
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Acuity
Field
Color Perception
Binocularity
Cortical Visual Impairment
Visual Acuity
What is visual acuity?
How is it measured?
What conditions typically impact visual acuity?
How is acuity measured?
Visual acuity is measured by determined by the smallest object or line which
can be seen clearly and distinguished at a set distance.
Typically expressed as 20/???.
Snellen Acuities.
The higher the bottom number, the worse the vision
Can be converted from 10/200 equivalents
Acuity and Refraction
What is refraction?
The bending of light.
How is it measured?
Diopter = unit of refraction
How Much Refraction
Takes Place in the Normal Eye?
Normal eye generates approximately 60 diopters.
Each of the clear surfaces of the eye contribute at least some refraction.
Amount of Refraction Contributed by Each Structure
Tears
1-2 diopters
Conjunctiva
3-4 diopters
Cornea
Acqueous humor
Lens
Viterous humor
40 diopters
4-5 diopters
5-8 diopters
5 diopters
Conditions Which Cause Refractive Problems
Myopia (nearsightedness)- too much bending power or the eye is too long
Hyperopia (farsightedness)- too little bending power of the eye is too short
Astigmatism (everything’s blurry)
A problem with almost any structure in the eye can limit visual acuity
The Lens
The plasticity of the lens (manipulated by the ciliary muscle and the zonules) allows
for a change in bending power (diopters) by changing its shape –
This allows us to see items closer that are closer than optical infinity.
This ability is lost with lens removal or replacement and later in life due to age Presbyopia
Optical Infinity = 20 feet or 6 meters
Less than Optical Infinity
Other Issues Impacting Acuity
Medication
Fatigue
Illness
Motor Issues
Environment
Light
Color/Contrast
Space
Correction of Refractive Errors
The myopic eye has too much refraction (bending power):
Ex: Normal = 60 and Myopic = 70
The hyperopic eye has too little refraction:
Ex: Normal = 60 and Hyperopic = 50
The presbyopic eye cannot accommodate
Requires addition of near lens or bifocal
Visual Field
170 to 180 degrees
Central and peripheral vision
Central losses are the most serious
Field Losses
Various conditions often result in a certain type of field loss
Most change is the stasis of the eye can impact retina health
Anything that impacts the health of the retina will typically result in field
losses.
Surgical repairs of torn or fallen retinas will result in field losses
Color Perception
Color perception losses vary
Can be total or partial
The condition is typically congenital so the individual may not know what
they are missing.
Typically considered a minor disability – it can have a serious impact on
functioning in school
Color Vision
Common- red-green deficits
Rare-blue yellow deficits
Extremely rare- Achromatopsia- loss of cone receptors resulting in complete
loss of color vision
Most common in boys
Binocularity
image.
Stereopsis – using two eyes to work together to form one three dimensional
Depth Perception – the ability to see the world in three dimensions and to
perceive distance using both binocular and monocular clues.
Balanced Eyes
Esotropia – turning in
Exotropia – turning out
Cortical Visual Impairment
CVI is disturbed or reduced vision due to various brain abnormalities
CVI is a neurological disorder, which results in unique visual responses to people,
educational materials, and to the environment
It is a brain rather than an eye problem
CVI
Individuals with CVI typically have:
Limited functional vision
Fluctuation of visual abilities
Attracted to lights and movement
“look away touch”
Good color vision and preference for certain colors
Difficulty with clutter and complexity of visual targets
Field losses
CVI
The individual may also have ocular impairments
According to APH, CVI is the fastest growing visual impairment diagnosis today
Four major causes -Asphyxia; Brain mal-development; Head injury; Infection