Visual Acuity Flashcards
Minimum detectable resolution
the threshold size of of a spot required to detect its presence against a background
Minimum separable resolution
the least separation between two adjacent points that allows the two to be seen separate.
Recognition test
determine the smallest symbols, letters, or words that can be identified correctly
Visual Acuity
expresses the angular size of the smallest target that can be resolved by the patient
- most useful SINGLE test for determining function of the visual system
- spatial resolving power of the visual system
Snellen Fraction
angular size of optotypes by specifying the test distance and the height of the letters.
VA=(test distance)/(distance at which letters subtend 5 minarc)
Decimal Notation
- reduces snellen fraction to decimalized quantity
- 20/20 is 1.0
- DOES NOT INDICATE TEST DISTANCE
What are the steps to take when a patient cannot see 20/400?
CF,HM, Light Projection, Light Perception
Unaided VA
should be tested before corrected VA
Habitual VA
VA measured when patient is wearing whatever kind of correction they came in with
Corrected VA
VA with best possible refractive correction
Pinhole accuity
Increased depth focus reduces blur due to optical irregularities or refractive error.
- Determines whether reduced VA is due to optical irregularities of uncorrected refractive error.
- can dullen the light so patients with retinal problems can have reduced visual acuity
Distance for near vision accuity
usually 40 cm
Reasons for measuring visual accuity
- refractive and prescribing decisions
- monitor ocular health
- application to vision standards(occupation)
Contrast Sensitivity
-VA is poorer when contrast is lower
Disability Glare
- Light scatters to reduce contrast, which reduces VA
- used to quantify extent to which certain conditions impair vision in this manner
Potential Acuity
can determine visual capability of retina/brain in those with cataracts.
inherited color vision defect
- inherited: inherited genetic defect. congenital and remains constant their whole life(both eyes are the same)
- usually red-green
- cannot be cred, but can be managed
- non pathological
Acquired color vision defect
- caused by accompanying disease/toxicity/trauma.
- may present at any time and may change over time
- usually blue-yellow
- sometimes treatable
- pathological
Types of cones
- blue(s)-shorter wavelength
- green(m)
- Red(l)- longer wavelength
Rods
-do not preferentially detect light of certain wavelengths, but have a specific photopigment that detects light better in dim conditions
Normal Trichromats
have normally functioning cones of all types
Anomalous trichromats
-functional cones of all three types, but one type is sensitive to a different wavelength of light than normal.
Dichromats
have only two types of functional photopigments
Monochromats
have only one type of photopigment
- only rods or a single type of cone function normally
- rod and cone monochromats exist
Protanopia
dichromats that lack L-cone
“red blind”
Protanomalous
anomalous trichromats with functioning L-cone that is abnormal in sensitivity to red
“red weak”
Deuteranopia
Lack of M-cone
“green blind”
Deuteranomalous
functioning M-cone that is abnormal in sensitivity to green
“green weak”
Tritanopia
lacking S-cone
“blue blind”
Tritanomalous
Functioning S-cone that has abnormal senstivity to blue
“blue weak”
X-linked inheritance
L and M cone photopigment genes
Autosomal inheritance
rod and S cone pigment genes
Chromatopsia
abnormal condition in which toxicity causes objects to be seen in a particular color of seem tinged in that color
What are the three things color vision tests specify?
- lighting
- viewing time
- test distance
General guidelines for color testing
- patients SHOULD wear corrective lenses
- each eye is tested separately
- unless just a screening
- Unless occupation requires binocular - testing should be done before administering drugs or using instruments with bright lights
- test should be stored in the dark to avoid fading
- patients should use a brush to prevent oils from getting on the page
Pseudoisochromatic plate test (PIC)
- most common
- quick and easy
- screens for red-green defects
Types of PIC plate tests
- transformation plates-person with color defect reads one figure and those with normal color see another
- vanishing plates: person with color defect cannot read the figure seen by normal vision
- hidden digit plate- person with normal vision fails to read a figure those with a defect can read
- diagnostic plates-figure is readable by those with one type of defect but not by those with another
Arrangement tests
- measures type and severity of defect
- can determine if defect is inherited or acquired
- requires patient to place colors in a sequence based on hue, light, and saturation
- order recorded by number on each sample
- results are plotted and scored
Anomaloscopes
- most accurate
- definitive diagnosis and quantifies condition
- must rotate knobs for each side of a circle until they match in shade and color
- expensive af and not used a lot
occupational vision requirements
driver-very few require color testing. need 20/40
pilot- color defect is okay, but depends on the type and severity BUT YOU CAN
military- US military does not care. some specialties in the military care.
What kind of problems can VA detect?
- refractive error
- optical media
- retina, optic nerve
- interpretive faculty of the brain
- SOME DISORDERS CANNOT BE DETECTED BY VISUAL ACUITY
diffraction pattern
-eye is in ideal focus, point object image is on retina as small circular patch with faint surrounding rings
Optotype
test targets used for VA test
Minimum angle resolution
- expressed in minutes of arc
- indicates the angular size of the critical detail within the just resolvable optotype
- 20/200=10 minarc
Visual efficiency
- used when quantifying vision loss for legal and compensation purposes.
- expanded to include losses of visual field and ocular motility
Measuring specifics
- typically check uncorrected first then corrected
- right eye, left eye, both
- can miss up to half of the letters in the line
-ANYTIME YOU HAVE ABNORMAL VA, YOU HAVE TO ADDRESS IT IN YOU ASSESSMENT AND PLAN
ALTERNATIVE VA TESTS
- Visual evoked potentials- measures electric activity in brain
- preferential looking tests-look towards the most interesting
- optokinetic nystagmus- reaction and movement
- Flash cards/symbols
