Visual Acuity Anne Bjerre October 2016 Content Define visual acuity (VA) VA assessment in children VA tests Recording VA threshold Factors influencing VA Visual Acuity (VA) Definition The ability of the individual to discriminate detail Ability to resolve a pattern into spatially separated elements
Denotes the size of detail that can just be resolved by an individual Visual resolution How can a 4 month babys visual acuity be assessed? If you suspect vision might be very poor.. How do parents think the baby sees? Does the baby make eye contact? Does the baby respond to or copy facial expressions?
Does the baby notice if you switch the room lights on/off? Do the babys pupils react to light? Forced choice preferential looking Based on Keeler infants prefer fixating a patterned surface over a blank surface Keeler or Teller acuity
cards 15 cards with black-&white stripes on right or left side 1 blank card 4mm hole in centre Teller Forced choice preferential looking Square-wave gratings of different spatial frequency
Spatial Frequency = c/deg The narrower stripes Higher SF Higher VA threshold Range 0.32-38c/deg 1 spatial cycle = 1 black + 1 white stripe Forced choice preferential looking Method Testing
Design FCPL tested at 57cm Frequency of black stripes given in cycles per centimetre = cycles per centimeter (cpcm) Lea Gratings Paddles Recommended Children < 1 year Children and adults with disabilities Advantage Fast to perform Portable Better fixation in < 1 year old compared to using Cardiff Cards (Mody et al 2012) How can a 1 year toddlers visual acuity(VA) be assessed?
Cardiff Acuity Cards Design Grey cards with familiar pictures Vanishing optotypes Picture beyond acuity threshold = invisible Pictures at top or bottom 3 cards for each acuity level
VA range 6/60-6/6 fixing at 1m 6/120-6/12 fixing at m Cardiff Acuity Cards Method Occlude 1 eye Present cards at eye level Begin with card with widest target (lowest acuity) Observe eye movements Masked
2 out of 3 correct responses How do you assess VA in a 2 old child? Kay Pictures Design Series of pictures Kay Pictures Single pictures of size in flip-over books Based on same principle as Snellen Acuity Test Performed at 6m Kay Picture single logMAR Based on same principle as logMAR Acuity Test Performed at 3m
Kay Picture crowded logMAR 4 pictures of size Linear test Based on same principle as logMAR acuity tests Kay Pictures Method Kay Picture single logMAR Occlude 1 eye Key card 3m Show pictures of size
Choice of 3-4 pictures for each acuity level VA range 1.00 to 0.00 logMAR Kay Picture crowded logMAR 3m 2+ pictures seen show smaller pictures Count number of pictures seen on smallest row of pictures VA range 1.000 to -0.100 logMAR New Kay Picture Test Changed pictures to most easily recognised by 420 children. Comparable and similar repeatability to other visual acuity tests (OConnor et al 2016)
Alternative tests for young children Lea symbols Landolt C E charts Single Sheridan Gardener Lea Symbols Design Based on same principle as logMAR acuity tests Linear test 4 symbols Square, apple
Key card house, circle A. Distance VA test for testing 2-4 year olds B. Distance VA test for testing children 4 years + Lea Symbols Method Test performed at 3m Occlude 1 eye Key card Point to symbols from top of chart Count number of symbols seen on lowest line of symbols
VA range 0.10 to 2.00 logMAR Crowded Single Landolt C/ E charts VA test targets termed optotypes Design Based on grid pattern
5 units high & 5 units wide Method Gap of E & C in 4 ways Up, down, right or left side Indicate direction of E or C Performed at 6m Key card Single Sheridan Gardener Design
Flip-over book with single letters of size 7 letters Matching key card Performed at 6m Patient with poor VA ~ test can be performed at closer distances Linear S.G. chart is also available
O, A, T, X, U, H, V Performed at 6m VA range 6/60 - 6/4 How can you assess a patient with very poor visual acuity? Babies Children/ Adults http://www.ssc.education.ed.ac.uk/resources/pictures/ SSreportimg_27.jpg
Tests rarely used for babies or children Hundreds & Thousands Ffooks symbols Sheridans rolling balls Stycar toys Sjgren hand test Catford Oliver Drum New test for infants Mirror Test Infant
held 20cm from a mirror (on parent lap) Orthoptist decides if infant attends to own reflection Slowly moved further away until fixation lost (i.e. head or eyes turning away) Tested 5 times, low and high value discarded and mean of remaining 3 recorded as result Testing distance use measuring tape (2x actual distance) Better
(62%) attention with wall-mounted (92%) versus hand-held mirror Mirror test normative values Age Teller acuity (FCPL) (c/deg) Snellen equivalent Testing distance x 2 (cm) 95th percentile lower
180.5 Modified from Bowman et al (2010) Any questions? Visual Acuity II Anne Bjerre October 2016 Measurement of visual acuity Visual Acuity (VA)
Ability to resolve a pattern into spatially separated elements Measurement of VA Related to visual angle subtended at the nodal point (N) of the eye Objects subtend different angles depending on distance from eye N Limits of Visual Acuity Optical factors N Diffraction
phenomenon Neural factors Anatomical structure of retinal photoreceptors Foveal region in retina achieve highest visual resolution (VA) due to neighbouring cones are closely together Interactions b/w neurons in retina & visual pathway Factors affecting your VA
Size of pupil Optical aberrations Region of retina stimulated Luminance of the test type Contrast b/w optotypes and background Legibility of letters Familiarity Spacing between letters Factors affecting VA
Best vision at central fovea Retinal sensitivity decreases with distance away from the fovea Patients who cant use their fovea have much poorer VA Factors affecting your VA Hill of Vision Fovea Fixate
centrally Blind spot VA reduces towards periphery Fixate other parts of retina Eccentric fixation Different acuity measures Minimum visible Ability
Minimum detectable (distinguishable) Ability target to perceive the presence of a single target to identify features or internal arrangements in a visible Minimum separable Ability to detect spatial separation between adjacent lines i.e 2 lines are separated by an interval Black-&-white grating tests
Different acuity measures Hyperacuity Ability to perceive the alignment and orientation of linear stimuli Vernier acuity Minimum recognisable Ability to determine the smallest optotypes Symbols, pictures, letters Snellen
and LogMAR charts Snellen Chart Snellen = visual angle subtended at the nodal point of the eye by altering the size of the component parts of a letter Snellen Chart Normal VA ability to discriminate a pattern whose detail subtends a visual angle of 1 minute of arc at the nodal point of the eye
6/6 Numerator (d) = Test distance Denominator (D) = Distance at which each component of a letter subtends 1 minute of arc at the nodal point VA = d/D. Snellen Letter Construction Each component of a letter
subtends 1 minute of arc at the nodal point 1 cone stimulated & 1 cone gap for appreciation Whole letter subtends 5 minute of arc at the nodal point Many can actually discriminate 0.5 min of arc Testing distance 6m
Normal VA level - Snellen 6/6 d/D d = testing distance (6m) D = distance (6m) at which each component of a just resolvable letter is located to subtend 1 minute of arc at the nodal point How large does the letter need to be at 12m to achieve 6/6 and for each component of the letter to subtend 1 minute of arc at the nodal point?
What size is each component of a letter? 6/12 line 6 test distance/ subtend visual angle of 1 min of arc at 12m 6/24 line 6/36 line Snellen Chart Snellen notation 6/60 6/36 6/24 6/18 6/12 6/9 6/6 6/5
Snellen Method Patient seated at 6m Wearing refractive correction Occlude 1 eye Ask patient to read letters Lowest line is recorded Test repeated for other eye Recording
VA cgls R: 6/5 - 2 L: 6/12 Sn sgls R: 6/9 + 1 L: 6/18pt Sn In which situation is visual acuity a) best? b) worst?
Problems with Snellen Some letters easier to read / guess than others Spacing varies - no crowding of top letters Big difference between top row and next row What is logMAR? logMAR =
logarithm of the Minimum Angle of Resolution Minimum Angle of Resolution(MAR) is that subtended by each part of the letter MAR = the angular size of detail within the optotype at threshold LogMAR Tests Introduced by Bailey and Lovie (1976) Aim
Early Treatment Diabetic Retinopathy Study (ETDRS) chart Design a VA chart where the task is the same at each level i.e size is the only variable gold standard test LogMAR charts Five letters per row Between letter & between row spacing proportional to letter
size Constant ratio of size progression (0.1 logMAR steps) Approximately equal legibility of letters at each size level LogMAR Charts Each letter is given a score of 0.02 logMAR One line of letters a score of 0.1 logMAR Scoring: letter-by-letter Bailey-Lovie and ETDRS chart
Gold standard Bailey-Lovie Design Based on British standard letters Testing distance 6m ETDRS Design Sloan letters Testing distance 4m (3m) Convert logMAR to Snellen Snellen LogMAR 6/60 1
6/48 0.9 6/38 0.8 6/30 0.7 6/24 0.6 6/19 0.5
6/15 0.4 6/12 0.3 6/9.5 0.2 6/7.5 0.1 6/6 0.0
6/4.8 -0.1 6/3.8 -0.2 6/3 -0.3 LogMAR Charts Method Termination rule Able to read 3 on 1 line check next line
Recording Patient sees all 5 letters on 0.00 logMAR line viewing with right eye VA sgls R 0.00 logMAR Patient sees 4 of the 5 letters on 0.00 logMAR line viewing with right eye VA sgls R 0.02logMAR Example A patient reads: H, V, Z, D, S, N, C, V, K, D, C, Z, S, H, N, O, N, V, S, R, K, D, N, P, C,
2 letters incorrect on this line. Whole line worth 0.60 But add 0.02 for each letter wrongly named. =0.60 + (0.02 x 2) =0.64 LogMAR Charts Recording Patient wears refractive correction and incorrectly name 3 of the 5 letters on 0.10 logMAR line viewing with left eye 0.10 + (0.02 x 3) = 0.16 VA cgls L 0.16 logMAR Advantages of logMAR Chart distance can be varied Bailey-Lovie chart usually tested at 6 metres
ETDRS logMAR chart usually tested at 4 metres 2 lines difference always means the same Values can be averaged and statistically analysed VA level can be given by the number of letters recognised on a particular line Letter-by-letter score Computerised charts Crowded logMAR acuity test
Glasgow acuity test Design on principle of Bailey Lovie Crowded book Uncrowded book 4 letters inside a crowding bar 2 letters at each acuity level Each letter has a score of
0.025 logMAR One line of letters is a score of 0.100 logMAR. Crowded logMAR acuity test Who is it suitable for? Preschool children 3-5 years old Method
Tested at 3m Practise for near with key card Smallest row of letters/ number of letters seen recorded Sonksen logMAR Test Designed by University College of London Based on principle of ETDRS chart Similar to crowded logMAR test 2 Flip-over booklets 4 letters surrounded by a crowding bar
O, X, H, T, U, V Training booklet Key card Testing distance 3m Near test chart Near VA tests Near VA tested at 33cm or 40cm If distance chart design & luminance levels are comparable to near test design near
provided eye accommodated and refractive correction worn VA score = distance VA score to allow good focus for a retinal image Two different types of near VA tests Near VA Tests Typeset material 1.
Similar to print in newspapers or books arranged in sentences, paragraphs or words Not comparable to distance VA tests Common tests N Test Types (Times Roman Print) Moorfields book Maclure book Based on principle of being equivalent to distance chart Common tests 2.
Reduced Snellen LogMAR chart Near single Kay picture and crowded Kay picture logMAR cards Lea symbols Single S.G. Landolt C & E charts Summary Decide upon suitable test
Which test is suitable for the childs age & level of concentration? Which tests are suitable for patients with very poor vision? Testing Keep the correct testing distance Do not allow peeping
Occluding patch on face not glasses Occlusive sunglasses Test with refractive correction if worn Encourage patient if limited co-operation Record test used, cgls or sgls
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