Visual Acuity PDF

Visual Acuity Gregory M. Fecho, OD Visual Acuity Lecture Overview Background Task-specific classification of VA Optotypes The meaning of 20/20? Other schemes for specifying visual acuity Chart design principles Testing VA Visual acuity calculations VA is an Angular Measurement...

Visual Acuity Gregory M. Fecho, OD Visual Acuity Lecture Overview Background Task-specific classification of VA Optotypes The meaning of 20/20? Other schemes for specifying visual acuity Chart design principles Testing VA Visual acuity calculations VA is an Angular Measurement 𝝷 𝝷 57 Degrees 1 Radian Degrees Radians VA is an Angular Measurement 1 degree 𝝷 60 minutes of arc. Minutes of Arc VA is an Angular Measurement Any letter subtending 5 minutes of arc on the retina is equivalent to 20/20 vision Does not matter the distance the letter is from the surface of the cornea as long as the angle stays 5 minutes of arc. You will have to change the size of the letter to accommodate for keeping the angle the same Visual Acuity What is Visual Acuity? Spatial resolving capacity of the visual system. How “keen” your vision is Angular size of detail resolvable by the patient. 5 minutes of arc = 20/20 vison It is only a small component in the patient’s overall visual function. Visual acuity is not the same thing as vision. Measurement of visual acuity can help us determine if there is a problem with the optical elements of the eye or if there are organic causes to the reduction of acuity. Visual Acuity Limits to the resolving capacity of the Eye: Optical Limits The image of a point source on the retina is NOT a point (!). The point source is imaged as a circular patch containing faint rings around the center image. These rings are a result of diffraction produced by the pupil. This circular patch is referred to as the “Airy disk” Larger Pupil = smaller airy disk Smaller Pupil = larger airy disk Visual Acuity Limits to the resolving capacity of the Eye: Optical Limits Airy Disk 𝛚 is the angular size of the disk in radians 𝛌 is the wavelength of light Angular size of Airy Disk p is the pupil diameter. Smaller pupil size = greater airy disk size The Airy Disk w The smaller the pupil, the larger the disk. Smaller pupil = more diffraction  The Rayleigh Criterion λ βmin = 1.22 With p=4.6mm, p the MAR possible is 0.5 minutes of arc When quality of image only limited by diffraction. 4.6mm Visual Acuity Limits to the resolving capacity of the Eye: Optical Limits Diffractions and Optical aberrations are not the same thing Diffraction Smaller pupil size = less degradation from diffractions (larger airy disk) L Acuity remains relatively constant between pupils sizes of 2.6-6mm. Optical Aberrations: Spherical and Chromatic aberrations (and more) Optical aberrations start to degrade an image beyond 6mm. Visual Acuity Limits to the resolving capacity of the Eye: Optical Limits Refractive Error Light Scattering Light Absorption  forea-center of macula no vascular network in shaped like a parabola because it helps inner layers helps with , focus of light -> nothing there thats not needed with the focus of light Visual Acuity Limits to the resolving capacity of the Eye: Neural Limits Each receptor in the fovea has its own “private line”. Cones in the fovea have a center to center separation of 0.5 minutes of arc. In order to detect two objects as being separated, there needs to be stimulation of two cones separated by one unstimulated cone. need 3 cones to do that & comes get a 1: 1 ratio to ganglion cells  1 arc Visual Acuity # Limits to the resolving capacity of the Eye: Neural Limits Depends on the packing density of the fovea Nyquist Limit: Limit occurs when a sine wave has a spatial period (peak to peak) that is twice the cone distance. l arc theoretical resolving limit of eye at Retina = I : arc Visual Acuity Limits to the resolving capacity of the Eye: Neural Limits For a pupil size of about 2.5 mm, the optical resolution limit is just under 1 minute of arc. The neural resolution is around 0.82 minutes of arc. - 1 minute of arc  20120 becomes an upper limit of normal acuity , more accurate baseline is 2011S Neural limitations have greater effect than Optical limitations 1 arc 2 arc Y j 2.5mm ↓ Visual Acuity Limits to the resolving capacity of the Eye 4-5 million cones 80-110 million rods. Foveal center: rod-free, capillary- free, 0.57mm in diameter = 1° Fovea: 199,000-300,000 mm2 macular degeneration affects fine detail  rods increase in # as you Visual Acuity move away from fored Limits to the resolving capacity of the Eye Temporally or nasally Acuity drops off rapidly as we move further from the fovea. 60% drop off in acuity just 1° from the fovea. Maximum rod acuity about 20/160 - optic nerve Location no rods or cones Physiological blind spot Visual Acuity “Task-Specific” Classification of VA Minimum Distinguishable or Minimum Visible "is the light there or not ? Minimum Separable " "can you see two separate things ? " read ? Minimum Cognizable or Legible " what is the smallest line you can Minimum Discriminable  il How dark is the Visual Acuity object against the Minimum Distinguishable/Visible Acuity background" - Not used clinically Ability of seeing something as being distinguishable from the background Resolution for task depends on if the object is point or line. Point: 10-35” of arc 10-35 seconds of arc Line: 0.44 to 10” of arc 44-10 seconds of arc Measure of the limit to discern small changes in contrast (intensity of target against background), not spatial limit (angular size of target). Visual Acuity Minimum Separable Ability to detect a group of objects as being separate ↑ S Gratings are a good example of this. Resolution limit is 30 cycles per degree so bar width is 1/60th of a degree or 1 minute of arc. > neural limitation of human eye 52 to 64 seconds of arc (around 1 min of arc) No coincidence that this matches nicely with the optical and neural limits of the eye discussed earlier. ↓ ↓ 1/2" ↑ arc arc Visual Acuity Minimum Cognizable or Legible This refers to what we know clinical measurements of VA to be Form Sense: Landolt C or Tumbling E’s True Minimum Legible: Letters and/or numbers. “What is the smallest line of letters you can see on the chart without squinting or straining?” Vernier Acuity · hyper acrity 1 Minimum Discriminable not lined Up Vernier acuity - are the two lines lined up - common hyper acuity task  Keratometry uses lined hyperacuity to measure min and not up max corneal curvature ↓ Visual Acuity VA Tasks Summary Minumum Distinguishable/Visible Acuity Not used clinically. Measure of the limit to discern small changes in contrast, not spatial limit. Minimum Separable Ability to detect a group of objects as being separate Minimum Cognizable or Legible Identification form, letter, number, picture…etc. > clinic use Minimum Discriminable Smallest change in a feature (e.g. a change in size, position or orientation) that one can discriminate. Optotypes Optotypes Landolt C “Landolt Rings” Acuity tests calibrated against charts with these optotypes. External diameter 5 times the stroke width. Inner circle 3 times the stroke width. Opening is 1 stroke width. Patient identifies where the break is (Up, Down, Left, or Right). At or near threshold, the patient does not see the gap, but rather sees a lighter or asymmetrical region of a small blob. This is where the gap is. · detail l ( Opening of "C"(2) Optotypes Tumbling E a E that is presented in different orientations Based on a 5x5 grid · I minute of arc is contained in the detail of the letter & (detail = opening) Optotypes Lea Symbols Useful for children and non-verbal patients Below threshold, all targets look the like small circles. ↳ all look the same Letter Optotypes Most letter optotypes based on a 5x5 unit grid construction The stroke width is 1/5th the total height of the letter. Original Snellen Charts used Serif Fonts. Modern Charts use non-serif letters. Sloan Letters Early British standard optotypes based on 5x4 grid. Later, 5x5 grid letters introduced. Letter Optotypes Sloan Letters Set of 10 letters: C, D, H, K, N, O, R, S, V, Z Based off a 5x5 grid San Serif Selection of letters above are chosen to reduce the variation of legibility so each line can have the same average difficulty. Schemes for specifying visual acuity Snellen Fraction MAR logMAR Decimal M Notation J Notation  What does 20/20 mean? Snellen Fraction - almost always zoft (standard optical infinity) - "20ft sized letter" OR OR ↳ ↳ 20120 (s'arc ex - if apt sees 20180 , & person with 20120 Vision could see that same letter clearly soft away Snellen Fraction 1’ C 1’ 5’ 1’ 1’ 1’ ↑ can distinguish only separate lines with larc VA is an Angular Measurement * MAR: Minimum Angle of Resolution * Reciprocal of Snellen fraction Result gives smallest level of detail patient is able to resolve in minutes of arc ↳ within the letter itself of inverse fraction snellen A > - detail contained within the letter any shellen fraction that equals I What Snellen fraction or acuity would be MAR for 5’ contends to 5'arc (201z0 letter) arc? Answer: 20/100 because 100/20 = 5 logMAR Variance of VA measurements when measured at different retinal eccentricities were virtually constant if measured in log units. clocations) Just noticeable differences are equal in size if the scale is logarithmic. Size progression of some charts have a 0.1 log unit change of MAR so each successive step represents a change in size by a ratio of 1.2589:1 Rounding a bit gives us a MAR size progression if lines by the following: 20120 - 20140 - 1.0, 1.25, 1.60, 2.0, 2.5, 3.2, 4.0, 5.0, 6.3, 8.0, 10, 12.5, 16… Notice that every three lines is a decrease in vision by double. M Notation Distance in meters in which the print subtends 5’ arc on the retina. A 1M print seen at 1 meter subtends 5’ arc on the retina 0.4/0.4M ↓ ↓ test letter size distance both variables in M allows pts to hold near card wherever they want Decimal Acuity This is simply the reduction of the Snellen fraction to a decimal quantity A patient with “normal” 20/20 visual acuity will have a decimal acuity of 1. Decimal acuity can come in handy when comparing one acuity notation to another. For example: A patient with 0.4/0.4M acuity has 20/20 vision and can resolve print that subtends 5’ of arc on the retina. 20140 =. 41. 8 For example: A patient with 1/1M acuity has 20/20 vision and can resolve print that subtends 5’ of arc on the retina. All M and Snellen fractions divide out to 1 so they are all equivalent. 0 4 20 * Anything that divides out to 1 equates to 20/20 vision *. -- T GM X. X = 80 Points not used often Used to specify size of typeset print in the printing industry. 1 point = 1/72 inch. Distance is from the bottom of the descenders (g, p, j, etc) to the top of the ascenders (b, d, j, l, etc). Lowercase are about 1/2 the height. 1.0M size print at near = 1.45mm = 8 points (typical lower case newspaper print.) Jager Notation The letter J followed by a number No standardization of Jager sizes Not used in optometry As much as a twofold difference in letter size for the same Jager size label depending on manufacturer. You may see it in records, don’t use it. Acuity Chart Design Principles Logarithmic Size Progression same thing basically Bailey-Lovie Design Principles Snellen Chart Logarithmic Size Progression Form line to line, there is an increase in size by 0.1 log unit as mentioned before About a 26% increase in size from line to line. Bailey-Lovie Design Principles A logarithmic size progression (constant ratio from one size to the next) The same number of letters at each size level Spacing between letters and between rows that is proportional to letter size depending on letter size Equal (or similar) average legibility for the optotypes at each size level · = roughly similar sloan letters legibility Snellen Chart Original chart had 7 letters 6 on most modern Snellen charts. Single letter at top Differences in letter design and selection (not all sloan) Differences in letter spacing relationships Differences in the number of letters per line Chart used out of tradition 7 Why are there more letters on the bottom vs the top of the chart? Because if there were an equal amount of letters for each line you would need much bigger charts and it is impractical for a lane setup Snellen lovie Near Visual Acuity Demands Real-world examples of Snellen letter sizes: Reduced Snellen Common Usages 20/20 Medication-bottle labels 20/25 Stock market print 20/30 Footnotes 20/40 Telephone directories 20/50 Small column newsprint 20/60 Approximate size of 12 point font 20/80 Children’s books (9-12 years) 20/100 Computer display 20/125 Children’s books (7-8 years) 20/150 Large print books 20/200 Subheadlines 20/400 Newspaper headlines Types of Acuity Charts Snellen Charts ETDRS Chart M&S Chart Preferential Looking PAM Electro Diagnostic Testing Types of Acuity Charts ETDRS Chart (Early Treatment Diabetic Retinopathy Study) · bailey-lovin design Sloan Letters (Letters based on 5x5 Grid) Used as gold standard chart for research: 5 letters per line Specific protocol for scoring acuity: single letter scoring. With single letter scoring, a one line change indicates a meaningful change in visual acuity. 20/20+2 or - 2 etc... With single line scoring, a two line change indicates a meaningful change in visual acuity. 20120 Single letter scoring is more precise, allows you to see visual acuity change more accurately Log scale sizing per line Equal spacing of letters. Same average legibility per line. Types of Charts M&S Chart Types of Charts Preferential Looking If pt can distinguish the grating, their attention will focus to the side of the chart with the grating, if they can’t distinguish the gaze will not focus to one side or the other so you know the patients visual acuity based on the degree of grating presented Used best for non verbal patients Doctor looks through the hole to see where the pt looks, works best & of doctor doesn’t know what side the grating is on Pattern (minimum separable) Types of Charts Potential Acuity Meter (PAM) Two pinpoint coherent lights create interference patterns with various spacing directly on the retina Use for patient with cataracts and patients with amblyopia Creates grating on the retina bypassing everything else to see potential visual acuity Testing Visual Acuity Testing Visual Acuity Distance Acuity Measurement Wash hands and explain purpose of the test: “I am going to be measuring how well you can see at distance and at near” Lighting New Patient Comprehensive Exam Perform Aided and Unaided Distance VA Testing Visual Acuity Distance Acuity Measurement VA sc first (sc means without correction) Use occluder to cover one eye at a time Measure OD first out of convention. Please…..please measure to 20/15. Please. Measuring Visual Acuity What is Normal? Most normally sighted individuals have better than 20/20 vision. 20/20 is the limit on the poorer end of normal vision. logMAR charts used in this study Testing Visual Acuity Distance Acuity Measurement Full chart with 20/50 on top. “Please read the smallest line without squinting or straining” Isolating Lines? Repeat with OS, then OU Perform procedure again aided if patient has spectacles. Perform aided first with cl wearers.  Testing Visual Acuity What if patient cannot see 20/400? Patient Cannot Read 20/400 Letter Do you have Laminated 20/200 > Walking E is preferable to counting fingers Letter? YES NO Because at 10ft the 20/200 letter is actually 20/400 because & Walk the 20/200 Letter up from 10ft. Perform Count Fingers you are 10 ft away from the pt in the exam lane Fail Fail Hand Motion Hand Motion Fail Fail Light Projection, Light Perception Light Projection, Light Perception Testing Visual Acuity Pinhole Acuity If patient is worse than 20/30, perform pinhole acuity. Answers the question: Is the reduction of vision due to refractive blur or pathology? If improvement, refract patient. Pinhole through best habitual correction at DISTANCE ONLY. Testing Visual Acuity ∞ V cc OD: 20/20- OS: 20/60 PH: 20/20 OU: 20/20  Testing Visual Acuity Near Visual Acuity Performed at 40cm with HABITUAL CORRECTION. ↳ whatever pt uses to read Use reduced Snellen card. Lighting Observations Recording: Know how to record in Snellen or M Notation TEST distance (40cm) is very important to measuring accurate visual acuity Testing Visual Acuity Recording for Near. V V 40cm 40cm cc OD: 20/20-2 cc OD: 0.4/0.4M-2 OS: 20/60 OS: 0.4/1.2M Creater 3xorse OU: 20/20 OU: 0.4/0.4M