Visual Acuity

Question 1

Reasons to measure VA

Answer 1

• 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

Question 2

Quantity vs quality VAs

Answer 2

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

Question 3

MAR

Answer 3

The smallest angle at which we can see the smallest lines of spaces

Question 4

For most people, MAR is

Answer 4

1 minute of arc

Question 5

VA optotypes MAR

Answer 5

5x larger than the AMR, so a 20/20 letter is 5 minarc tall

Question 6

For any angular visual acuity expression, the MAR is equal in minarc to the inverse of the visual acuity fraction

Answer 6

20/40 MAR= 2 min arc optotype=10 minarc

Question 7

60min arc =

Answer 7

1 degree

Question 8

Distance acuity

Answer 8

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

Question 9

Good distance VA charts

Answer 9

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

Question 10

ETDRS/Bailey-Lovie/LogMAR chart

Answer 10

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

Question 11

M size

Answer 11

Absolute, will stay the same at all distances, just record the distance

Question 12

Recording VA

Answer 12

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

Question 13

Recording on ETDRS

Answer 13

-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

Question 14

ETDRS clinical tips

Answer 14

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

Question 15

Feinbloom VA

Answer 15

• 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

Question 16

LEA numbers chart

Answer 16

-gives results slightly more in agreement with the ETDRS chart than the fleinbloom chart

Question 17

Projected charts

Answer 17

• 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

Question 18

Computerized charts

Answer 18

• 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

Question 19

Berkeley rudimentary vision test

Answer 19

• designed so that clinical would no longer have to use finger counting or hand motion
• can quantify up to 20/16000

Question 20

Finger counting

Answer 20

• 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

Question 21

Other acceptable measurements of VA

Answer 21

• hand motion (HM)-if no Berkeley rudimentary vision chart
• light perception with projection (LPP)
• light perception only (LPO)

Question 22

Fixation

Answer 22

Is the patient viewing centrally or eccentrically

Is it steady or erratic

Question 23

Head posture

Answer 23

Does the patient turn their head to achieve best vision

Question 24

What to watch out for when testing VAs in low vision

Answer 24

Watch out for peeking if the patient uses a significant head turn

Question 25

Recording fixation

Answer 25

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)

Question 26

If someone has a scotoma, how will they eccentrically fixate

Answer 26

To the same direction of the scotoma to move it out of the way

Question 27

Preferential looking tests

Answer 27

Used for kids a lot

Question 28

Measurement of near VA

Answer 28

``` Symbols Numbers Letters Words Sentences Paragraphs ```

Question 29

Considerations for near testing

Answer 29

- 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

Question 30

M notation for near vision

Answer 30

- 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

Question 31

Snellen letter acuity equivalent only holds true if

Answer 31

The card is used at the test distance for which it was calibrated

Question 32

Reduced snellen is an equivalent letter size and must be either tested at

Answer 32

The precise distance the test card was calibrated for, or converted

Question 33

Absolute measurement of letter size, so we do not need any distance conversions

Answer 33

M notation

Question 34

the M size of a letter is deterred by

Answer 34

Measuring its height and dividing by 1.45

Question 35

N system (printers point)

Answer 35

Font size -helpful in communicating with classroom teachers, parents, TVIs, etc N=Mx8 (appx) 2M=16 point font

Question 36

MNRead testing

Answer 36

Measures reading rate (wpm) at differnt print sizes Critical print size Minimum print size

Question 37

Critical Print size on MNRead testing

Answer 37

Last size the patient could read their max reading speed