4 - measuring visual acuity

What are the reasons we test entrance VAs?

1. establish visual acuity prior to exam → legal reasons

2. monitor disease progress/treatment

3. understand level of visual ability/disability

what is Snellen?

test distance/letter (foot) size

What is foot size?

distance at which the target subtends 5' of arc

what notation does logMAR chart use?

M-unit notation

What is 1-M unit?

1 M-unit subtends 5 mins of arc at 1 meter

How do you convert M-unit notation to Snellen?

(test distance (m)/letter size in M-unit notation) x 20 → 20/XX

ex: 0.4 M at 40 cm → 0.4m/0.4M = 1 x 20 → 20/20

How do you get decimal acuity?

divide the snellen fraction

ex: 20/20 = 1

How do you get MAR?

inverse of snellen

ex: 20/20 = 1 min of arc, each sweep of the letter

Each letter is how many logMAR units?

0.02 logMAR units

How should logMAR acuity change if you double the distance?

should be 3 lines worse

ex: if VA is at 20/40 line at 1 m → at 2 m the VA should be (20/50 → 20/63 → 20/80) 20/80 line

How many cycles per degree are in one min of arc?

30 cycles per degree

What is the Snellen equivalent of 1 min of arc (MAR)?

20/20

How do you get from cycles per degree to Snellen?

600/cpd → Snellen denominator

600/Snellen denominator → cpd

example: 20/40 → 600/40 = 15 cpd

What is recognition acuity?

ability to identify a particular symbol

What is resolution acuity?

the ability to resolve a critical element of a stimulus or pattern → aka grating acuity

What is visual ability?

the ability of the eye to see → does not qualify the amount of vision

what is preferential looking?

do they look in the direction of the stimulus

how many correct responses do you need to move on in preferential looking?

only need one correct response to move to more difficult card

what kind of acuity is preferential looking?

resolution acuity

how many correct responses do you need to move on in two-alternative forced choice (2AFC)?

must get 3/4 or 4/4 correct to move on to more difficult level

what is matching?

point to correct optotype using matching card

what kind of acuity is 2AFC, matching and naming?

recognition acuity

What is the order of testing methods for VA (from most to least sophisticated)?

goal is to use the most sophisticated method pt is able to perform!

1. Naming (recognition)

2. Matching (recognition)

3. 2AFC (recognition)

4. Preferential looking (resolution)

What are methods of occlusion for kids?

1. sticky patch

2. occluder glasses

3. hand held occluder

4. patient or parent hand (last resort)

What is resistance to occlusion looking at? How?

patient reacts differently when covering one of their eyes vs the other

signs of interocular acuity difference by looking at difference in behavior (cries or distressed, push away occluder) when covering one eye vs the other

What does resistance to occlusion (RTO) test?

visual ability

How is resistance to occlusion recorded?

No RTO

Equal RTO OD, OS

RTO OD or OS

What does fix and follow test?

visual ability

How is fix and follow performed?

monocular → a large bright (silent) object is moved slowly 1-2 ft from patient

How is fix and follow recorded?

F&F (fixes and follows)

how is central, steady, maintained (CSM) recorded?

-central (C) or eccentric (UC) fixation

-steady (S), or poor (US) fixation

-maintained (M) or unmaintained (UM) → for strab only

What does the 10 BU test measure?

visual ability

How is the 10 BU test performed?

place 10 BU in front of one eye + should see fixation switching between R + L eyes as you move target side to side → eyes moving up + down

at a minimum you should see the eye behind the prism to saccade down to find the target (test both eyes)

how to know reduced acuity when performing 10 BU test?

if eye behind the prism does not move to fixate the target → reduced acuity

What does optokinetic nystagmus test?

resolution acuity → but typically used a visual ability test

When is the OKN response mature?

3-6 months

When is the OKN response present?

at birth

what kind of task is OKN?

motion detection → poor central vision may still respond

how to perform OKN?

spin the drum → a nystagmus response should occur if the child can see the rotation

how to determine visual acuity using OKN?

vary test distance because stripe width of drum fixed

What type of test is teller acuity cards?

preferential looking

what test distances do you test teller acuity?

38 cm, 55 cm, 84 cm

What does teller acuity cards test?

resolution acuity

how to perform teller acuity?

start with easy to see → flip card to confirm

What is the measurement of teller acuity cards?

cpd (cyc/cm)

How many cards does the patient have to get right to move on in teller acuity cards?

1

What is the gold standard VA test for an infant?

teller visual acuity cards

With teller, what is the max difference between eyes in cards you want to see in a patient?

max 1 card difference

What are other preferential looking VA tests similar to teller?

1. Patti Stripes

2. Lea Gratings

3. Richmond face dot paddles

what do you need to make sure to do when using other preferential looking test paddles?

present a grating and blank while remaining expressionless so child looks at paddles instead of face

What do the Cardiff cards test?

resolution acuity

What type of test are the Cardiff cards?

preferential looking

what are Cardiff cards?

pictures instead of gratings → more interesting targets

What is an advantage to Cardiff cards?

vertical eye movement to look → can be easier to determine VA on patients with congenital nystagmus

What are examples of tumbling optotypes?

1. Sjogren hand test

2. tumbling E

3. Landolt C

What do the tumbling optotypes test?

resolution acuity

Why do we avoid using tumbling optotypes on kids?

they dont know laterality (their left vs their right)

What does broken wheel test?

resolution acuity

What type of test is broken wheel?

2AFC

How many do you need to get correct to move on for broken wheel?

≥3/4 correct

Why do we not use the american optical picture chart?

1. not uniform → unequal blur and luminance

2. only measures up to 20/30

What are the gold standard picture charts?

LEA symbols

What does the LEA symbols test?

recognition acuity

what are LEA symbols measured in?

logMAR

What are the types of tests LEA can be used for?

1. naming

2. matching

3. 2AFC

what are Patti pics?

same idea as LEA symbols but with a 5th optotype (adds another symbol, a star)

What does Patti pics test?

recognition acuity

what is Patti pics measured in?

logMAR

What does HOTV test?

recognition acuity

What types of tests are HOTV?

naming or matching → NOT 2AFC

what is HOTV measured in?

logMAR

For amblyopic children under 7, what protocol should we use?

ATS (amblyopia treatment study) HOTV protocol → single, crowded HOTV

for amblyopic children over 7, what protocol should we use?

E-ETDRS protocol → full line

What is similar between the two different protocols for amblyopic VA testing?

both use crowded optotypes and logMAR format

How does the ATS HOTV protocol work?

1. screening test: 1 letter per VA line

2. phase 1: start 2 lines above letter failed in screening → test 3-4 letters per line (one at a time), stop when missed 2 on one line

3. reinforcement: start 3 lines above failed in phase 1 → test 3 lines, one letter each

4. phase 2: start @ last line failed in phase 1, stop after 2 letters missed on line