Visual Acuity Flashcards
Reasons to measure VA
- establish baseline to follow ocular pathology
- used to predict strength of optical devices to achieve persons goals
To determine eligibility for
- driving
- legal blindness benefits
- school program placement
Quantity vs quality VAs
VAs as traditionally measured only provides a quantity of acuity, under typically optical conditions
Does not give a full picture of a persons visual abiltiy -contrast sensitivity Glare sensitivity VF Motivation
MAR
The smallest angle at which we can see the smallest lines of spaces
For most people, MAR is
1 minute of arc
VA optotypes MAR
5x larger than the AMR, so a 20/20 letter is 5 minarc tall
For any angular visual acuity expression, the MAR is equal in minarc to the inverse of the visual acuity fraction
20/40 MAR= 2 min arc optotype=10 minarc
60min arc =
1 degree
Distance acuity
Flaws with the typical projected snellen chart
- few letters at large optotype sizes
- large gaps between large optotypes
- most people know the largest target is an E
- not much crowding
We need to make every effort to allow the patient to read SOMETHING on the chart
Good distance VA charts
Have optotypes of equal legibility (EDTRS)
Equivalent difficulty on each line (same # of letters with same relative spacing)
0.1 log unit change between lines
ETDRS/Bailey-Lovie/LogMAR chart
In low vision often used on rolling stand to achieve differnt test distances
Commonly used distances are 1, 2, and 4m, but can be testes closer
-but are we really measuring distance VA when we use very close test distances? Yes for THAT patient
M size
Absolute, will stay the same at all distances, just record the distance
Recording VA
Need to record chart used and test distance, even if you convert to a 20 foot equivalent
When used at 2m, the 20 foot conversion is simple, add a “0” to the M value
2m/20M=20/200
The snellen fraction is only valid when the chart is used at 4m (13 feet)
20/200 on a 4m chart is not 20/200 if testes at 2m-it is 20/400, or if at 1m then it is 20/800
Recording on ETDRS
-if every row is 0.1logMAR progression and each row has 5 optotypes, then each optotype can be considered 0.02 logMAR
we can elimate the ambiguity of VA measurements by
- counting total number of optotypes read correctly
- multiply that number by 0.02
- subtract from the logMAR of the starting row
ETDRS clinical tips
Based on referral acuity and/or patient history, decide on a test distance 4m=max VA=2/200 2M=max VA=20/400 1m=max VA=20/800 If 20/800 or worse, use LEA numbers
- If pateitn cannot read at least 7 or 8 optotypes, reduce test distance and start over
- change charts between eyes, especially if your patient memorizes well
- leave room lights up with light cabinet on high
Feinbloom VA
- number based chart
- designed for use at 10 ft
- largest optotype is 10/700 (20/1400), and has only 1 number at the largest size
- useful for nursing home sand other places where a portable chart is needed
LEA numbers chart
-gives results slightly more in agreement with the ETDRS chart than the fleinbloom chart
Projected charts
- using the traditional projected snellen chart is fine, provided the patients VA does not exceed 20/800. After 20/80, there are large gaps in optotype sizes
- the contrast is typically less on a projected chart than a back illuminated chart
- some patients will do significantly better with a back lit or computer based chart, especially with AMD
Computerized charts
- work well because you can change the letters at the larger sizes rather than having only an “E”
- be careful not to only show letters in isolation-as that may artificially inflate acuity
Berkeley rudimentary vision test
- designed so that clinical would no longer have to use finger counting or hand motion
- can quantify up to 20/16000
Finger counting
- should NEVER be used in low vision clinic, or in clinic at all where portable charts are available
- if the patient can see fingers, they can see an optotype
- some estimate finger counting at approximately 20/200-this is NOT a valid measure
- color of hands, color of background, size of hands and length of fingers all affect this measurement
- there is no consistently, so just dont use it
Other acceptable measurements of VA
- hand motion (HM)-if no Berkeley rudimentary vision chart
- light perception with projection (LPP)
- light perception only (LPO)
Fixation
Is the patient viewing centrally or eccentrically
Is it steady or erratic
Head posture
Does the patient turn their head to achieve best vision
What to watch out for when testing VAs in low vision
Watch out for peeking if the patient uses a significant head turn
Recording fixation
If the patient looks away from the letters to see them better, record the direction that the PATIENT IS LOOKING (if they look to the right, record EV at 3:00)
If someone has a scotoma, how will they eccentrically fixate
To the same direction of the scotoma to move it out of the way
Preferential looking tests
Used for kids a lot
Measurement of near VA
Symbols Numbers Letters Words Sentences Paragraphs
Considerations for near testing
- patient should habe appropriate correction, including an add if needed
- maximize likelihood of success-if unsure if patient can read words, start with letters
- start large enough (based on distance VA) that patient has a good likelihood of succeeding
M notation for near vision
- avoids inconsistencies with point and jaeger notation
- allows for variable testing distances
- a 1M letter by definition subtends 5d of arc when located at 1m
Tan 5’=x/1000mm
O.00145=x/1000mm
0.00145 x 1000mm=x
1.45mm=x
Snellen letter acuity equivalent only holds true if
The card is used at the test distance for which it was calibrated
Reduced snellen is an equivalent letter size and must be either tested at
The precise distance the test card was calibrated for, or converted
Absolute measurement of letter size, so we do not need any distance conversions
M notation
the M size of a letter is deterred by
Measuring its height and dividing by 1.45
N system (printers point)
Font size
-helpful in communicating with classroom teachers, parents, TVIs, etc
N=Mx8 (appx)
2M=16 point font
MNRead testing
Measures reading rate (wpm) at differnt print sizes
Critical print size
Minimum print size
Critical Print size on MNRead testing
Last size the patient could read their max reading speed
