Visual Acuity Flashcards
1
Q
What is visual acuity
A
the spatial resolving capacity of the visual system
2
Q
what is clinical visual acuity
A
the measure of the ability of a patient to resolve fine detail
3
Q
What is normal VA limited by
A
the anatomy of the eye
4
Q
what are some factors that limit the eye
A
- density of the photoreceptors in the retina
- diffraction of the eye
- the eye’s optical aberrations
5
Q
why is VA testing special
A
it is the only test done on every patient every time you see him or her
6
Q
what are some important reasons you must take VA
A
- legal
- evaluation of visual function
- detection of visual impariment. low vision
- detection and monitoring of amblyopia
- estimation of refractive error
- detection and diagnosis of disease and necessity of intervention
- monitoring disease
7
Q
what is legal blindness
A
the best corrected visual actuity of 20/200 or less in the better eye, or a visual field of no more than 20 degrees
8
Q
what must you get for a full license in MA in ur VA
A
20/40 in best corrected VA in better seeing eye
9
Q
what number is the snellen VA chart based on
A
1 min or arc
10
Q
what does seeing 1 min arc mean
A
seeing 20/20
11
Q
what is 1 degree equal to in min arc
A
60 min arc
12
Q
what is 1 min arc equal to in sec arc
A
60 sec arc
13
Q
what is 1 degree equal to in sec arc
A
3600 sec arc
14
Q
what is the distance to the fovea to the optic nerve
A
15 degrees
15
Q
what is the visual pathway
A
light, tear film, cornea, anterior chamber, pupil, lens, vitreous, retina, photorecptors
16
Q
where does the eye have the highest resolution ability
A
fovea, center of macula
17
Q
what are the optical limits to normal VA
A
- obtical aberrations
2. diffraction
18
Q
what does a large pupil mean for aberrations and diffraction
A
lower diffraction, more aberrations
-allows more light to retina to reduce diffraction, so resultion limit is aberrations
19
Q
what does a small pupil mean for aberrations and diffraction
A
reduced optical aberrations, resolution is limited
20
Q
what is the optimal pupil size
A
3mm
21
Q
what are the neural limits to VA
A
- photorecptor density and packing
- light/dark adaptations
- other neuronal processes
22
Q
what is predominatnly found in the macular
A
cones
23
Q
what are the only photoreceptors found in the fovea
A
cones
24
Q
where are rods found
A
throughout the peripheral retina
25
where are rods most concentrated at
20 degrees from the fovea
26
where are cones found
macula and fovea (central retina)
27
what part of the eye has the highest density of photorecptors and the highest visual resolution
central retina
28
what is the cones at fovea theoretical limit to resolution
30 sec arc
29
what is the minimum resolvable letter stroke width that corresponds to snellen va of 20/10
0.5 min arc
30
what are the 3 conditions of vision depending on light
1. photopic vision
2. scotopic vision
3. mesopic vision
31
what condition is the bright light conditions with the best VA
photopic
32
waht is photopic vision mediated by
cones
33
what is dim light conditions, night vision.
scotopic
34
what condition has the poorest sensitivity to dim light
photopic
35
what condition has the highest sensitivity for detection of a spot of light
scotopic
36
what is scotopic mediated by
rods
37
what is the sensitivity factor that the visual system alters it to see under scotopic and photopic conditions
a factor of >x10^8
38
how does the eye alter its sensitivity factor to light
1. change pupil size
2. fast regeneration of photopigment
3. other neural processes
39
out of cones and rods which takes longer to adapt to dim light conditions
rods (30-50 min) while cones (5-10 min)
40
at the macula, waht is the ration between cones synapsing to 1 bipolar/ganglion cell
1:1
41
at the retinal periphery, what is the ration of rods to bipolar cells
many rods : 1 bipolar cell
42
what do the photoreceptors synapse with
bipolar and hoizontal cells
43
what do the bipolar cells synapse with
amacrine and ganglion cells
44
where do the ganglion cells exit through
optic nerve
45
where is the blind spot
15 degrees temporal from fovea
46
what are the layers of the eye
retina, choroid, sclera
47
waht is the type of acuity used to resolve 2 objets as separate? ex. grating acuity, landolt c
resolution acuity
48
waht is the acuity used to measure the sensitivity of the eye. what's the smallest object you can see?
minimum detectable acuity
49
what the the minimum detectable misalignment. ex. vernier acuity
hyperacuity
50
what is the smallest obj that you can identiy and name? used in most clinical charts
recognition acuity
51
how is threshold related to sensitivity
threshold=1/sensitivity
52
What is the chart:
- strong serif
- height x5 detail
- based on detail (MAR) of 1 min of arc
- standard dist 20 ft
Snellen (1862)
53
what did sloan propose
m-unit
54
what is this chart:
- bailey lovie layout
- logMAR spacing
- sloan optotypes
- standard sit 4m
current standard
| -ETDRS
55
What are the advantages to hand held charts
- cheap
- readily available
- portable
- free space testing
- more realistic
56
what are the disadvantages to hand held charts
- variations in contrast and illumination
- fade w/ time
- size calibration
- limited optotoes
- need to be used at a std distance
57
what type of chart is an etdrs chart
back illuminated printed chart
58
what are the advantages to a chart projected onto a screen
- can present multiple optotyes
| - lower learning effect
59
which chart do we use in pre clinic
charts projected on a screen
60
how to design a VA chart? (6)
1. type of optotype
2. std test viewing distance?
3. size of optotype
4. progression btwn lines
5. spacing btwn optotypes/lines
f. # of optotypes per line
61
how many cm in an inch
2.54
62
what is 20 ft equal to in in
6.096m
63
when do you stop the pt from reading on a snellen chart
when they get more than half of the ltters on one line wrong
64
what are some disadvantes for snellen
```
not geometrical proportion
glare
-for va worse than 20/80 there are too few optotypes
-projector=multiple optotypes
-different spaces=crowding effect
```
65
what are some advantages for snellen
standarized
based on MAR
projector=multiple optotypes
different spaces=crowding effect
66
what is landolt c
series of rings with a 1 min arc gap
| placed in 1 or 8 positions
67
what is tumbling e
pt says which way e is pointing at
| -use for illiterate pts or children
68
advantages for logma
- geometrica progession based on the log10 of the critical dtail in min of arc
- proportional spacing for all levels of VA
69
advantage for etdrs
-each line differs from the previous line by a size of 0.1 log
70
what is the std distance for etdrs
4m
71
when do you stop the pt from reading for an etdrs chart
when they miss 4/5 letters
72
are etdrs or snellen charts shown to have a better VA score
etdrs
73
what are some common errors for snellen based chart
1. no proper clear instructions
2. not observe pt.
3. not push pt to guess
4. show one line/letter at a time
74
what must you always remember to record for etdrs charts
the sign
75
what is an ico near vision card based on
logMAR
| sloan letters
76
what is the std distance for ico near vision card
40cm or 16 inches
77
what is the standard distance for a runge chart
40 cm or 16 in
78
for runge chart when you record..
make sure to record the line they got ALL 3 LETTERS CORRECT
79
which value do you use to record for runge chart
either snellen equivalent or logmar value
| -be sure to record viewing distance
80
what is the formula for recording amplitude of accommadation
F=100/d (cm)
81
what line do you have your pt look at to measure amp
one line above best near VA
82
what must you round to for amp
nearest 0.50 D
83
what is the formula for minimum amp
15- (age/4)
84
what metrics do you use for amplitude of accomodation
cm
85
at what feet do you do lea cards
10 ft
86
which VA do you check for under 4 years old for lea cards
20/40 or 10/20
87
which VA do you check for over 4 years old
20/36 or 10/16
88
what are we looking for in pediatric screenign
1. amblyopia
2. eye disease
3. any condition that may affect school performance
89
what is crowding
resolution is impaired by the presence of neighboring ojects
90
who are egger's chart good to use for
myopia and astigmatism
91
what kind of lens do you use to correct for myopia
divergent lens
92
what kind of lens do you use to correct for hyperopia
convergent lens
93
why is egger's chart not useful for young hyperopes
bc they have compensation of accomadation. must be 45 years or older to use
94
when must you take pinhole
when VA less than or equal to 20/30
95
how to measure VA in pt with low vision in etdrs and snellen
etdrs @ 4, 2, or 1m
| feinbloom (snellen) @ 10 ft or closer
96
what is the std distance for feinbloom
10 ft
97
when do you stop in feinbloom
when they miss half or more of the letters in a line
98
how to record feinbloom?
distance/letter size
ex. VA cc (Feinbloom) OD 10/180 -1
OS 5/700
99
20/20 is equivalent to how many cm and M
40cm/.4M or 4m/4M
100
what are factors that could affect VA
1. doctor (knowledge, instructions, confidence)
2. test
3. pt (psychological, systemic, ocular)
101
what is differential diagnosis (DDx)
questioning what is affecting visual functioning in order to come up w/ the most likely diagnosis
102
inflammation of cornea
keratitis
103
inflamattion of anterior part of cornea
uvetitis
104
when cornea is not curved, cone like
keraoconus
105
imperfections in media, more prominent w/ a large pupil
aberrations
106
light interferes w/ itself, more prominent w/ a small pupil
diffraction
