Visual Acuity Anne Bjerre October 2016
Content Define visual acuity (VA) n VA assessment in children n VA tests n Recording VA threshold n Factors influencing VA n
Visual Acuity (VA) Definition n 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 n Visual resolution
How can a 4 month baby’s visual acuity be assessed?
If you suspect vision might be very poor…. . n n n § § 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 baby’s pupils react to light?
Forced choice preferential looking n Based on Keeler ¨ infants prefer fixating a patterned surface over a blank surface n Keeler or Teller acuity cards ¨ 15 cards with black-&white stripes on right or left side ¨ 1 blank card ¨ 4 mm hole in centre Teller
Forced choice preferential looking n n n Square-wave gratings of different spatial frequency Spatial Frequency = c/deg The narrower stripes ¨ Higher SF ¨ Higher VA threshold n Range 0. 32 -38 c/deg 1 spatial cycle = 1 black + 1 white stripe
Forced choice preferential looking n Method ¨ Testing distance 38 cm ¨ ‘Staircase’ procedure n n n 1 octave: difference 0. 22 & 0. 44 c/deg ½ octave: difference 0. 22 & 0. 33 c/deg Suitability ¨ 8 weeks to 12 months ¨ Patients with mental & physical disabilities
Lea Gratings Paddles n Design ¨ FCPL tested at 57 cm ¨ Frequency of black stripes given in cycles per centimetre = cycles per centimeter (cpcm)
Lea Gratings Paddles Recommended n Children < 1 year n Children and adults with disabilities Advantage n Fast to perform n Portable n Better fixation in < 1 year old compared to using Cardiff Cards (Mody et al 2012)
How can a 1½ year toddler’s visual acuity(VA) be assessed?
Cardiff Acuity Cards n Design ¨ ¨ ¨ n 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 1 m ¨ 6/120 -6/12 fixing at ½m ¨
Cardiff Acuity Cards n 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 2½ old child?
Kay Pictures Design n Series of pictures Kay Pictures Single pictures of ↓size in flip-over books ¨ Based on same principle as Snellen Acuity Test ¨ Performed at 6 m ¨ Kay Picture single log. MAR Based on same principle as log. MAR Acuity Test ¨ Performed at 3 m ¨ Kay Picture crowded log. MAR 4 pictures of ↓size ¨ Linear test ¨ Based on same principle as log. MAR acuity tests ¨
Kay Pictures Method n Kay Picture single log. MAR ¨ ¨ Occlude 1 eye Key card 3 m Show pictures of ↓size n ¨ n Choice of 3 -4 pictures for each acuity level VA range 1. 00 to 0. 00 log. MAR Kay Picture crowded log. MAR 3 m 2+ pictures seen show smaller pictures Count number of pictures seen on smallest row of pictures ¨ VA range 1. 000 to -0. 100 log. MAR ¨ ¨ ¨
New Kay Picture Test Changed pictures to most easily recognised by 420 children. n Comparable and similar repeatability to other visual acuity tests (O’Connor et al 2016) n
Alternative tests for young children Lea symbols n Landolt C n E charts n Single Sheridan Gardener n
Lea Symbols Design n Based on same principle as log. MAR acuity tests n Linear test n 4 symbols ¨ Square, apple n 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 n Method ¨ Test performed at 3 m ¨ 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 log. MAR Crowded Single
Landolt C/ E charts n n VA test targets termed optotypes Design Based on grid pattern ¨ 5 units high & 5 units wide ¨ n Method ¨ Gap of E & C in 4 ways n Up, down, right or left side Indicate direction of E or C ¨ Performed at 6 m ¨ Key card ¨
Single Sheridan Gardener Design n Flip-over book with single letters of ↓size n 7 letters ¨ n n Matching key card Performed at 6 m ¨ n Patient with poor VA ~ test can be performed at closer distances Linear S. G. chart is also available ¨ n O, A, T, X, U, H, V Performed at 6 m VA range 6/60 - 6/4
How can you assess a patient with very poor visual acuity? n Babies n Children/ Adults http: //www. ssc. education. ed. ac. uk/resources/pictures/ SSreportimg_27. jpg
Tests rarely used for babies or children n n n Hundreds & Thousands Ffook’s symbols Sheridan´s rolling balls Stycar toys Sjögren hand test Catford Oliver Drum
New test for infants Mirror Test n Infant held 20 cm from a mirror (on parent lap) ¨ n n n 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 ¨ n Orthoptist decides if infant attends to own reflection use measuring tape (2 x actual distance) Better attention with wall-mounted (92%) versus hand-held mirror (62%)
Mirror test – normative values Age Teller acuity (FCPL) (c/deg) Snellen equivalent Testing distance x 2 (cm) 95 th percentile lower confidence limit (cm) 1. 29 days 0. 33 6/540 15. 5 13 2 weeks 1 6/180 38. 2 24. 5 6 weeks 2 6/90 57. 2 37 11 weeks 3 6/60 72. 5 47 4. 2 months 4 6/45 85. 5 5. 8 months 5 6/36 97. 5 62. 5 17 months 10 6/18 147 93. 5 8 years 30 6/6 279 180. 5 Modified from Bowman et al (2010)
Any questions?
Visual Acuity II Anne Bjerre October 2016
Measurement of visual acuity n Visual Acuity (VA) ¨ n 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 n Optical factors N ¨ Diffraction n phenomenon Neural factors ¨ Anatomical structure of retinal photoreceptors n 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 n n n n 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 n n n Best vision at central fovea Retinal sensitivity decreases with distance away from the fovea Patients who can’t use their fovea have much poorer VA
Factors affecting your VA n n Hill of Vision Fovea ¨ Fixate n n n centrally Blind spot VA reduces towards periphery Fixate other parts of retina ¨ Eccentric fixation
Different acuity measures n Minimum visible ¨ Ability n to perceive the presence of a single target Minimum detectable (‘distinguishable’) ¨ Ability to identify features or internal arrangements in a visible target n 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 n Hyperacuity ¨ Ability to perceive the alignment and orientation of linear stimuli ¨ Vernier acuity n Minimum recognisable ¨ Ability n to determine the smallest optotypes Symbols, pictures, letters ¨ Snellen and Log. MAR 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 n 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 ¨ n Numerator (d) = Test distance n Denominator (D) = Distance at which each component of a letter subtends 1 minute of arc at the nodal point n VA = d/D.
Snellen Letter Construction n Each component of a letter subtends 1 minute of arc at the nodal point n 1 cone stimulated & 1 cone gap for appreciation n Whole letter subtends 5 minute of arc at the nodal point n Many can actually discriminate 0. 5 min of arc Testing distance 6 m
Normal VA level - Snellen n n 6/6 d/D d = testing distance (6 m) D = distance (6 m) at which each component of a just resolvable letter is located to subtend 1 minute of arc at the nodal point
n How large does the letter need to be at 12 m 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 12 m 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 n Method ¨ Patient seated at 6 m ¨ Wearing refractive correction ¨ Occlude 1 eye ¨ Ask patient to read letters ¨ Lowest line is recorded ¨ Test repeated for other eye
Recording n VA cgls R: 6/5 - 2 L: 6/12 Sn sgls n R: 6/9 + 1 L: 6/18 pt Sn In which situation is visual acuity a) best? b) worst?
Problems with Snellen n Some letters easier to read / guess than others n Spacing varies - no crowding of top letters n Big difference between top row and next row
What is log. MAR? n log. MAR = ¨ logarithm n of the 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
Log. MAR Tests n n Introduced by Bailey and Lovie (1976) Aim ¨ n Early Treatment Diabetic Retinopathy Study (ETDRS) chart ¨ n Design a VA chart where the task is the same at each level n i. e size is the only variable “gold standard” test Log. MAR charts Five letters per row ¨ Between letter & between row spacing proportional to letter size ¨ Constant ratio of size progression (0. 1 log. MAR steps) ¨ Approximately equal legibility of letters at each size level ¨
Log. MAR Charts n Each letter is given a score of 0. 02 log. MAR n One line of letters a score of 0. 1 log. MAR n Scoring: letter-by-letter
Bailey-Lovie and ETDRS chart ‘Gold standard’ Bailey-Lovie Design n Based on British standard letters n Testing distance 6 m ETDRS Design n Sloan letters n Testing distance 4 m (3 m)
Convert log. MAR to Snellen Log. MAR 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
Log. MAR Charts Method n Termination rule ¨ Able to read 3 on 1 line – check next line Recording n Patient sees all 5 letters on 0. 00 log. MAR line viewing with right eye n VA sgls R 0. 00 log. MAR n n Patient sees 4 of the 5 letters on 0. 00 log. MAR line viewing with right eye VA sgls R 0. 02 log. MAR
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
Log. MAR Charts Recording n Patient wears refractive correction and incorrectly name 3 of the 5 letters on 0. 10 log. MAR line viewing with left eye n 0. 10 + (0. 02 x 3) = 0. 16 n VA cgls L 0. 16 log. MAR
Advantages of log. MAR Chart distance can be varied ¨ Bailey-Lovie chart usually tested at 6 metres ¨ ETDRS log. MAR chart usually tested at 4 metres n n n 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 log. MAR acuity test n n n Glasgow acuity test Design on principle of Bailey Lovie Crowded book ¨ n Uncrowded book ¨ n n 4 letters inside a crowding bar 2 letters at each acuity level Each letter has a score of 0. 025 log. MAR One line of letters is a score of 0. 100 log. MAR.
Crowded log. MAR acuity test n Who is it suitable for? Preschool children ¨ 3 -5 years old ¨ n Method Tested at 3 m ¨ Practise for near with key card ¨ Smallest row of letters/ number of letters seen recorded ¨
Sonksen log. MAR Test Designed by University College of London n n Based on principle of ETDRS chart Similar to crowded log. MAR test 2 Flip-over booklets 4 letters surrounded by a crowding bar n n n O, X, H, T, U, V Training booklet Key card Testing distance 3 m Near test chart
Near VA tests Near VA tested at 33 cm or 40 cm n If distance chart design & luminance levels are comparable to near test design n ¨ near VA score = distance VA score n provided eye accommodated and refractive correction worn ¨ n 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 Log. MAR chart Near single Kay picture and crowded Kay picture log. MAR cards Lea symbols Single S. G. Landolt C & E charts
Summary n Decide upon suitable test ¨ ¨ n Which test is suitable for the child’s age & level of concentration? Which tests are suitable for patients with very poor vision? Testing ¨ ¨ Keep the correct testing distance Do not allow peeping n n ¨ ¨ ¨ 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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