Visual Acuity Clinical Optometry 1 MOD007288 1 Learning Objectives • Define and understand vision, visual acuity, resolution acuity, minimum angle of resolution. • Understand the principles and use a Snellen Chart. • Understand the principles and use a LogMAR Chart. 2 Learning Objectives • Understand the factors affecting visual acuity measurement. • Be able to record visual acuity appropriately. • Be able to convert various measures of visual acuity. 3 Vision and Visual acuity • “Vision” (V) - Visual standard obtained without a correction. • No spectacles / contact lenses • “Visual Acuity” (VA) - Visual standard obtained with a correction. • With glasses / contact lenses 4 Visual acuity • Visual acuity describes the eye’s ability to discriminate detail in an object Visual acuity(VARes ) 1 • ‘Res’ indicates that the acuity is resolution acuity • ω is angle expressed in arcminutes 5 The Minimum Angle of Resolution • ω > MAR S2 S1 • S1 and S2 are easily resolved • ω < MAR • S1 and S2 are not resolved S1 S2 • S1 and S2 are just resolved as separate S1 S2 • ω = MAR 6 Factors determining resolution acuity • Uncorrected refractive error which causes defocus blur • Image contrast • Diffraction • Aberrations • Imperfections in the ocular media • Imperfections of the eye’s refracting surfaces • Back scatter from the fundus 7 Snellen Acuity 8 The Snellen letter chart • An angular subtense of 1 arc minute forms the basis for the construction of the chart. • This 1 minute of arc angular separation is taken as the “norm” for a measure of visual acuity using letters. 9 The Snellen Letter • The limbs and spaces subtend 1 arc minute (a sixtieth of a degree of angular measurement) at specified distances, and the letter height subtends 5 arc minutes • The original Snellen letter was designed on a 5 x 5 grid and the letters on the chart were serifed. 10 The Snellen letter (5 x 5) 5/ 1/ 11 The Snellen letter chart • The Snellen letter chart consists of a series of rows of letters whose height subtends 5 arc minutes at distances of 6, 9, 12, 18, 24, 36 and 60 metres. • The largest single letter is placed at the top and the number of letters increases as one moves down the chart. 12 Snellen Acuity testing distance in meters VA distance at which the height of letter subtends 5' • This is known as the Snellen fraction or Snellen acuity 13 Snellen Acuity • Usual testing distance is 6m. • If at that distance a patient just resolves a letter that subtends an angle of 5’ at 6m, VA = 6/6 • The limbs of this letter subtend angle of 1’ • The MAR is 1’ 14 6 metre letter 5’ 6m 15 Why a testing distance of 6m? • A testing distance of 6m produces an incident vergence of -1/6 D(-0.17D) at the eye • -0.17D is substantially less than the 0.25D prescribing interval • 6m is therefore effectively infinity 16 Practicalities of 6m testing • 6 metre requires a lot of space. • Most testing rooms are 3 metres and use a mirror to create a virtual image 6 metres away. • Infinity boxes now common too 17 Short room testing 18 Recording the acuity - examples • If the smallest letter a patient reads subtends an angle of 5’ at a distance of 18m (the ‘18m’ letter), VA is recorded as 6/18 • If the patient can just resolve the ’12m’ letter, VA = 6/12 • If the patient can just resolve the ’4m’ letter, VA = 6/4 19 Recording the acuity • The smallest row of letters which the patient can read should always be recorded • If a patient only reads part of a row this must also be recorded as either: • VA of the previous row read correctly plus (+) “the number of letters from the following row read correctly or • VA of the row that is being read minus (-) “the number of letter that were read incorrectly” 20 Recording the acuity • For example: • If a patient reads all letters on the 6/12 line but only 2 on the 6/9 line, this would be recorded as 6/12+2 • If a patient reads 4 letters (out of 6) on the 6/9 line, this would be recorded as 6/9-2 21 Activity • On the following two slides determine what the Snellen acuity is: 22 60 Example Scoring Snellen 36 24 6/? 18 12 9 6 5 4 60 Example Scoring Snellen 36 24 6/? 18 12 9 6 5 4 LogMAR Acuity 25 logMAR chart • Included in computer-based test charts. logMAR charts • Equal number of letters per line (5 letters per line) • Letter spacing on each row is equal to one letter width. The row spacing is equal to the height of the letters on the next row. • Constant change 0.1 log unit between rows of letters logMAR Charts • Each letter in a row is assigned a value of 0.02 log unit • Chart is not truncated at 6/6 • Results usually recorded as logMAR (where 6/6 = logMAR 0) • Chart can be used at different distances (half the testing distance and add +0.30 to the logMAR score) 28 Activity • On the following two slides determine what the logMAR acuity is: 29 Chart conversions 32 Factors affecting Testing Charts 33 Modern test charts • Very few charts use the original Snellen optotypes, but the principal of limb and letter sizes remains the same and the charts are referred to as Snellen • Most modern charts use sans-serif upper case letters constructed on a 5x5 or 5x4 grid A sans-serif letter on a 5 x 4 grid 5/ 1/ The design of modern test types Factors that need to be considered: • letter design and relative legibility • progression of letter sizes • letter and row spacing • contrast between the letters and the background • chart luminance Letter design • British standard BS 4274 -1: 2003 recommends the following 5 x 5 sans-serif letters: CDEFHKNPRUVZ • No letters should be repeated on any one line Progression of letter sizes • Original Snellen chart: • 6, 9, 12, 15, 21, 30 and 60 • Most modern charts: • 5, 6, 9, 12, 18, 24, 36 and 60 • LogMAR charts use a 0.1 log unit interval Letter and row spacing • British Standards (BS 4274 -1: 2003) recommend that: • the gap between the letters should equal the width of the letters • lines of letters should be separated by a gap not less than the letter height in the smaller line or 20 mm, whichever is the smaller Letter contrast • Black letters on a white background is an example of a high contrast target while grey letter on a slightly lighter background describes a low contrast target. DEFNH DEFNH Letter contrast • Contrast of a letter is described by Weber’s definition: • Contrast = Lmax-Lmin/Lmin Lmax Lmin Letter contrast • High contrast letters are easier to read than those of low contrast • The British Standards (BS 4274 -1: 2003) recommendation is a minimum contrast of 0.9 (90%). Chart luminance • British Standards (BS 4274 -1: 2003) recommend that the luminance should be uniform and not less than 120 cd/m2 • The surround luminance level should be as near as possible to the luminance level as the background on the chart. Chart distance • Current British Standards (BSI 4274-1 2003) state that the testing distance should be no less than 4m. Design flaws of the Snellen chart • Some letters are easier to recognise than others. • Only one letter at 6/60 (limited use with low vision px!) • Truncation at 6/6 • Different numbers of letters on each line (more crowding on smaller letters and more difficult to read smaller lines) • No systematic relationship between the spacing between each letter and each row of letters. Other Letter Charts 46 The Landolt ring and Tumbling E • Four alternative forced choice test • Landolt ring (C) -patient is asked to state the position of the gap: up, down, right or left • Tumbling E – patient is asked to the direction the E is pointing: up, down, right or left The Landolt ring and Tumbling E 5' 1' 1' 5' Other charts Lea symbols logMAR chart Elliott, D.B. 2020. Clinical Procedures in Primary Eye Care 5th ed: Elsevier. Recording Visual Acuity 50 Recording Visual Acuity • Snellen (metres) • Snellen (feet) • Decimal • MAR • logMAR 51 Converting Snellen (metres) to Snellen (feet) • 6/6 = 20/20 • Numerator will always be 20 • Multiply denominator by 10/3 • E.g., 6/24 = 20/(24x10/3) = 20/80 52 Converting Snellen (metres) to Decimal • 6/6 = 1.0 • Divide the numerator by the denominator. • E.g., 6/24 = 0.25 53 Converting Snellen (metres) to MAR • 6/6 = 1 MAR • Divide the denominator by the numerator. • 6/24 = 24/6 = 4 54 Converting Snellen (metres) to logMAR • 6/6 = 0.00 • Get the log of the MAR. • E.g., 6/24 • MAR of 6/24 is 4 (previous slide) • Log4 = 0.60 55 Activity • The smallest line of letters that a patient reads at 6m is a row labelled 18 on the chart. • Record the visual acuity, as a Snellen fraction (m and feet) and in decimal notation. What is the MAR? logMAR? Activity Reads a row labelled 18 on the chart • Fraction: VA = 6/? • Decimal: VA = ? • MAR = 1/VA = ? • logMAR = ? • Fraction (feet): VA = 20/? • Fraction: VA = 6/18 • Decimal: VA = 0.33 • MAR = 3’ • logMAR = 0.47 • Fraction (feet): VA = 20/60 Comparison of VA notations MAR (') Decimal acuity Snellen Snellen logMAR acuity at acuity at 6m 20 feet 6/12 1.0 20/200 Comparison of VA notations MAR (') Decimal acuity 0.50 Snellen acuity at 6m 6/12 Snellen logMAR acuity at 20 feet 20/40 0.30 2 1 1.0 6/6 20/20 0.00 10 0.10 6/60 20/200 1.00 Recording the acuity • If a patient’s vision is less that 6/60, this must also be recorded. • Ideally you would have a specialist chart for this – e.g., BRVT • 3m? 1m? CF? HM? LP? • 60 m letter read at 3 m • 60 m letter read at 1 m • Counts fingers at 1m • Hand movements • Light perception • No light perception 3/60 1/60 CF @ 1m HM LP NLP