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3F: Low Vision > Visual Acuity > Flashcards

Visual Acuity Flashcards

1
Q

Reasons to measure VA

A
  • 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
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2
Q

Quantity vs quality VAs

A

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
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3
Q

MAR

A

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

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4
Q

For most people, MAR is

A

1 minute of arc

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5
Q

VA optotypes MAR

A

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

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6
Q

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

A

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

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7
Q

60min arc =

A

1 degree

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8
Q

Distance acuity

A

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

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9
Q

Good distance VA charts

A

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

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10
Q

ETDRS/Bailey-Lovie/LogMAR chart

A

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

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11
Q

M size

A

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

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12
Q

Recording VA

A

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

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13
Q

Recording on ETDRS

A

-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
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14
Q

ETDRS clinical tips

A 
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
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15
Q

Feinbloom VA

A
  • 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
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16
Q

LEA numbers chart

A

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

17
Q

Projected charts

A
  • 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
18
Q

Computerized charts

A
  • 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
19
Q

Berkeley rudimentary vision test

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

Finger counting

A
  • 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
21
Q

Other acceptable measurements of VA

A
  • hand motion (HM)-if no Berkeley rudimentary vision chart
  • light perception with projection (LPP)
  • light perception only (LPO)
22
Q

Fixation

A

Is the patient viewing centrally or eccentrically

Is it steady or erratic

23
Q

Head posture

A

Does the patient turn their head to achieve best vision

24
Q

What to watch out for when testing VAs in low vision

A

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

25
Q

Recording fixation

A

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)

26
Q

If someone has a scotoma, how will they eccentrically fixate

A

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

27
Q

Preferential looking tests

A

Used for kids a lot

28
Q

Measurement of near VA

A 
Symbols 
Numbers 
Letters
Words
Sentences 
Paragraphs
29
Q

Considerations for near testing

A
  • 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
30
Q

M notation for near vision

A
  • 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

31
Q

Snellen letter acuity equivalent only holds true if

A

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

32
Q

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

A

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

33
Q

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

A

M notation

34
Q

the M size of a letter is deterred by

A

Measuring its height and dividing by 1.45

35
Q

N system (printers point)

A

Font size
-helpful in communicating with classroom teachers, parents, TVIs, etc

N=Mx8 (appx)
2M=16 point font

36
Q

MNRead testing

A

Measures reading rate (wpm) at differnt print sizes

Critical print size

Minimum print size

37
Q

Critical Print size on MNRead testing

A

Last size the patient could read their max reading speed

3F: Low Vision (16 decks)

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