BEST PPT ON VISUAL ACUITY
- 3. ∗ Visual Acuity is the spatial resolving capacity of the visual system ∗ Expresses the angular size of the detail that can be resolved by the observer Definition
- 4. ∗ Visual function ∗ Refractive status of the eye ∗ Medico legal purposes ∗ Major outcome measure for a treatment ∗ Can be used to explain the patient ∗ Criteria for ∗ person’s fitness to drive ∗ ability to gain entrance into a profession ∗ eligibility to avail Government concessions Uses of VA
- 5. 1. Optical Limitations 2. Neural Limitations Physiological Limitations
- 6. ∗ Focussing Errors ∗ Myopia ∗ Hypermetropia ∗ Astigmatism ∗ Inappropriate correction of refractive error ∗ Chromatic and Monochromatic Aberrations ∗ Pupil size ∗ Small pupil - Diffraction ∗ Large pupil - Aberrations Optical Limitations
- 7. Point Spread Function due to Diffraction AiryDisc
- 8. ∗ Angular size SinΦ= 2.44λ/P ∗ H/F= 2.44 λ/P ∗ H=2.44 λXF/P ∗ H= 1.22 λXF/P ∗ H=1.22 F/P ∗ H= 2.3/P
- 10. ∗ Photoreceptor spacing ∗ Two point images are when the images fall on two cones separated by an unstimulated cone ∗ In the fovea separation between the cones is 2 microns ∗ Hence the spacing between two cones should be 4 microns Neural Limitations
- 11. a) two lines can be resolved b) two lines cannot be resolved and perceived as one thick line
- 12. ∗ Increase in photoreceptor spacing - Poor VA ∗ In the Periphery the cones are less densely packed Neural Limitations
- 13. ∗ Refractive error ∗ Size of the pupil ∗ Illumination ∗ Time of exposure of the target ∗ Area of retina stimulated ∗ State of adaptation of eye ∗ Eye movement ∗ Cognitive status Factors Affecting the VA
- 14. Refractive error ∗ Defocus at the retina ∗ Defocus will blur out ∗ Fine details ∗ Sharp edges ∗ Contrast sensitivity Factors Affecting the VA
- 15. Effect of Blur on Line Spread Function
- 16. Pupil Size ∗ Large pupil-Aberrations (reduces diffraction) ∗ Small pupil-Diffraction (reduces aberrations) ∗ Mid-size pupil-3-5mm optimal ∗ compromise bet. diffraction & aberrations limits Factors Affecting the VA
- 17. Illumination ∗ For recognition tasks VA largely affected by illumination levels ∗ Higher level of VA in photopic conditions - Higher density of cones at fovea Factors Affecting the VA
- 19. Time of exposure ∗ To detect a bright spot, detection is more dependent on brightness of the spot ∗ To detect a line, the acuity is proportional to exposure time ∗ No simple acuity-exposure time relationship to resolve a target Factors Affecting the VA
- 20. Area of retina stimulated ∗ VA greatest at the center of fixation ∗ At 10’ of arc from center of fixation - 25% loss of VA ∗ Negligible loss of VA till 2’of arc eccentricity Factors Affecting the VA
- 21. Change in Visual Acuity with Eccentricity
- 22. State of adaptation of the eye ∗ Higher level of VA achieved when eye is adapted to same level as test luminance (34 cd/m2 to 34,000 cd/m2 ) ∗ Test luminance < 34cd/m2 , adapting to lower luminance achieves better VA Factors Affecting the VA
- 23. Eye Movement ∗ During steady fixation the eye, the eye is in constant motion ∗ Retinal images traverse a distance of 3 min of arc per second Factors Affecting the VA
- 24. ∗ Minimum Distinguishable or Visible Resolution ∗ Minimum Separable Resolution ∗ Minimum Recognizable or Legible Types of Visual Resolution
- 25. ∗ Minimum Distinguishable or Visible Resolution ∗ The minimum detectable resolution is the threshold size of a spot or a line against its background ∗ Contrast discrimination ∗ Visual Fields charting(Eg) Types of Visual Resolution
- 26. ∗ Minimum Separable Resolution ∗ The least separation between two adjacent points or lines that allows them to be seen as separate ∗ e.g. Grating Acuity ∗ Vernier Acuity ∗ Ability to distinguish between two lines placed one of them laterally placed end-to-end with one of them overlapping and displaced, then stereoacuity can be measured Types of Visual Resolution
- 27. ∗ Cycles /degree ∗ 30 cpd ∗ 1 min of arc ∗ Spurious results ∗ Teller acuity cards ∗ Visual evoked potential ∗ Optokinetic Nystagmus Drum
- 30. Vernier Acuity
- 31. ∗ Minimum Recognizable or Legible ∗ Form Sense ∗ Landolt rings where orientation is used to determine the VA ∗ True Minimum Legible ∗ Complex patterns such as letters or numbers used as test stimulus Types of Visual Resolution
- 33. ∗ Optotypes ∗ On grid pattern ∗ Height 5 units ∗ Width 5,4,6 ∗ Serif letters ∗ Sans-serif
- 34. 1. Detection - presence or absence of a test stimulus Task Specific Classification
- 35. 2. Discrimination - separation of points Task Specific Classification
- 36. 3. Recognition - orientation of symbol or ability to pick a particular symbol out of group of symbols Task Specific Classification
- 37. 4. Identification - complex symbols Task Specific Classification
- 38. Designation of Visual Acuity
- 39. ∗ Smallest letter ∗ Longest distance ∗ Angular size – ∗ Several types
- 40. ∗ Snellen Fraction ∗ Decimal Notation ∗ Minimum angle of resolution ∗ Logarithm of the minimum angle of resolution ∗ Visual Acuity Rating ∗ Visual Efficiency
- 41. 1. Snellen Fraction Designation of Visual Acuity VA = Test distance Distance at which letter subtends 5 min of arc
- 42. ∗ Dutch Ophthalmologist - Herman Snellen ∗ Type of minimum legible VA task ∗ Fraction (eg. 6/30 or 20/100) ∗ Numerator - test distance ∗ Denominator - letter size ∗ Letter size - dist. at which height of overall subtends 1 min.of arc Snellen Notation
- 43. ∗ Typical letters on chart - stroke width 1/5th their height ∗ Def. modified - distance at which the limb of the letter subtends 1 min of arc ∗ 20/20, 6/6 = test conducted 20ft/6m and that the limbs of the smallest letters subtend 1 min of arc at 20ft, 6m Snellen Notation
- 45. 5 x 5 grid
- 46. E 5’ 20 ft / 6m h Calculation 1 min of arc
- 47. ∗ Tan 5’ = h/6 ∗ Letter size h = tan 5’ x 6 = 0.000291X5 X 6 = 8.75 mm Width of each stroke = 8.75 / 5 = 1.75 mm (6/6, 20/20 letter) Calculation
- 48. ∗ For 6/60, 20/200 Letter size h = tan 5’ x 60 = 0.0145x 60 = 1.458 x 60 = 87.5mm Calculation
- 49. ∗ Considered to be optical infinity ∗ Definition of refractive error - when accommodation relaxed ∗ Shorter distance - role of accommodation ∗ Longer distance - poor legibility Why 6m or 20ft distance?
- 50. ∗ Eye resolves letter that subtends 5’ of arc at 20ft or 6m ∗ 20/20 can just resolve a 20/30, 6/9 letter from 30ft/9m distance - as it subtends 5’ of arc at 30ft/9m ∗ 20/20 Or 6/6 IS NOT PERFECT VISION??? ∗ BETTER THAN 20/20 OR 6/6 -POSSIBLE ∗ SUPER ACUITY What is 20/20 or 6/6 vision?
- 51. ∗ Does not mean person will not have a functional problem ∗ Contrast sensitivity loss ∗ Color vision loss ∗ Visual Field loss ∗ Does not correlate with REAL WORLD VISION Is 20/20 or 6/6 perfect vision?
- 52. ∗ C and G ∗ F and P ∗ N and H ∗ D and O Commonly confused letters
- 53. ∗ If test chart designed for 6m/20ft is kept at 3m/10ft E.g. 3/60 = 1/20 x 6/6 = 6/120 = 1/20 x 20/20 = 20/400 Conversions
- 54. ∗ A circle with a break in it ∗ Entire diameter of the ring is 5 times the stroke width of the circle so that the diameter is 3 stroke-widths ∗ Break or gap is 1 stroke width wide ∗ Critical detail is 1/5th of the height of the optotype Landolt Rings
- 55. C Landolt C
- 57. ∗ Illiterate chart ∗ 3 limbs of equal length ∗ Presented in various orientation ∗ Patient’s task - identify direction to which the limbs of E point ∗ Four alternative directions - up, down, right and left ∗ School Screening Tumbling E
- 60. Decimal notation ∗ Reduces the Snellen fraction to a decimal quantity 20/20 = 1.0 20/200 = 0.1 ∗ Higher number indicates better VA ∗ Most widely used in European continent ∗ It gives a single number to quantify an angle ∗ It does not indicate the test distance Designation of Visual Acuity
- 61. Minimum Angle of Resolution (MAR) ∗ Expressed in min of arc ∗ Indicates angular size of critical detail with just- resolvable optotype ∗ Dividing letter size by test distance 20/20 ⇒ 1 min of arc 20/200 ⇒ 200/20 = 10 min of arc 20/40 ⇒ 40/20 = 2 min of arc Designation of Visual Acuity
- 62. LogMAR ∗ logarithm of MAR ∗ VA = 20/20, 6/6, the MAR = 1 min of arc Log MAR = log10 (1.0) = 0.0 20/40, 6/6 ⇒ MAR = 2 min of arc, LogMAR = 0.30 20/200,6/6 ⇒ MAR = 10 min of arc, LogMAR = 1.0 Designation of Visual Acuity
- 63. ∗ When VA score >20/20, 6/6 ∗ LogMAR value becomes negative 20/16, 6/4.8 ⇒ MAR = 0.8 min of arc LogMAR = - 0.10 ∗ For charts with size progression ratio of 0.1 log units and 5 letters per row, each letter can be assigned a value of 0.02 on logMAR scale Designation of Visual Acuity
- 64. Visual Acuity Rating (VAR) ∗ VAR = 100 - 50 logMAR ∗ A score of 100 corresponds to 20/20,6/6 ∗ VAR = 50 corresponds to 20/200,6/60 ∗ VAR = 0 corresponds to 20/2000,6/6000 ∗ VAR > 100 when VA > 20/20, 6/6 20/16,6/4.8 ⇒ VAR = 105 Designation of Visual Acuity
- 65. ∗ For charts with size progression ratio of 0.1 log units and 5 letters per row, each letter carries a VAR value of 1 ∗ On the VAR scaling difference of 15 points represents a 2 fold change in MAR ∗ 5-point change - represents a ratio of 5:4 change in MAR Designation of Visual Acuity
- 66. Visual Efficiency ∗ Introduced in 1925 by Snell & Sterling for use in quantifying visual loss for legal and compensation purposes ∗ VE = 1.0 or 100% ⇒ 20/20,6/6 ∗ 20/200,6/60 ⇒ 0.2 or 20% VE = 0.2(MAR-1)/9 Log (VE%) = 2.0777 - 0.777 (MAR) Designation of Visual Acuity
- 67. Gratings and Cycles per Degree (CPD) ∗ Series of light and dark stripes with a sinusoidal luminance profile ∗ Widely used for testing contrast sensitivity ∗ Testing Infants (Teller Acuity) ∗ Size of the grating and thus the VA is of the patient are specified in CPD Designation of Visual Acuity
- 68. ∗ 30cpd would have 60 alternating light and dark stripes per degree of visual angle ∗ The ability to see such a grating ∗ 1 min of arc ∗ 20/20, 6/6 Designation of Visual Acuity
- 69. Snellen Chart ∗ Seven different size levels ∗ One letter at largest size level ∗ The number at each size level increased progressively ∗ 8 optotypes at the smallest size level ∗ Size sequence in ∗ feet 200,100,70,50,40,30 and 20 ∗ metric 60,30,21,15,12,9 and 6 ∗ Modified - 60, 36, 24, 18, 12, 9 and 6 Visual Acuity Chart Design
- 70. Bailey - Lovie Design Principles ∗ Task difficulty same at each size level ∗ Only significant variable - size ∗ Clinical scoring of VA - LogMAR units as well as a method for giving equal additional credit for each additional letter read correctly E.g. ETDRS Visual Acuity Chart Design
- 71. ETDRS chart
- 72. Principles ∗ A logarithmic size progression (constant ratio from one size to the next) ∗ Same number of letters at each size level ∗ Between letter and between row spacing proportional to the letter size ∗ Each (or similar) average legibility for optotypes at each size level Visual Acuity Chart Design
- 73. Log size progression ∗ 0.1 log unit ∗ Each successive step represents a change in size by the ratio 1.2589:1 (approx. 5:4) ∗ For a row of 5 letters each letter can be assigned a value of 0.02 on logMAR scale ∗ A change of 10 increments on this scale ⇒ a change of exactly 10 times ∗ A change of 3 steps ⇒ a change of approx 2 times Design Features of VA Charts
- 74. Number of optotypes at each level ∗ Reliability of VA measure increases with the increased number of letters at near threshold sizes ∗ For 5 letters / rows and 0.1 log unit size progression, SD of letter chart acuity is about 0.028 Design Features of VA Charts
- 75. Spacing between letters and between rows ∗ Spacing between neighboring letters reduce their legibility ∗ VA better with wider spacing ∗ Eye movement control and fixation tremors may contribute to reduction in VA when letters tightly spaced ∗ Influence - more in small print Design Features of VA Charts
- 76. ∗ Printed Panels ∗ Opaque or Plastic ∗ Directly Illuminated ∗ Translucent material ∗ Indirect Illumination ∗ Usual testing distance - 6m or 20ft ∗ Closer test distance ∗ Small examination room ∗ Low vision patient Chart Formats
- 77. ∗ The dist from the pt. to the chart and size of the letter - to determine VA ∗ VA chart designed for 4m ∗ 6/60 (5’ of arc at 4m) ∗ Top row at 4m - 4/40 ⇒ 6/60 ∗ at 1m - 1/60 ⇒ 6/360 ∗ 1/40 ⇒ 6/240 Chart Formats
- 78. ∗ Projector Charts ∗ Angular size of the Optotype ∗ If the projection dist. = observation dist. ∗ Angular size - independent of obs. dist ∗ Proj. dist. ≠ obs. dist. ∗ Letter size proj. should be changed for that dist. Chart Formats
- 79. ∗ Not widely used in clinical practice ∗ For research purposes ∗ Provide means ∗ diff. optotypes ∗ change letter order ∗ change stimulus parameters ∗ contrast ∗ spacing arrangements ∗ presentation time Video Display Charts
- 80. ∗ Moderate photopic luminance ∗ Recommended range ∗ 85 - 300 cd/m2 ∗ Widely used as std - 160 cd/m2 Chart Luminance
- 81. ∗ Alphabets ∗ Numbers ∗ Symbols ∗ Preferential looking Type of Target
- 82. ∗ Std. dist. - 20 ft / 6 m ∗ Mirrors ∗ Low Visual acuity ∗ 3m, 1m, 40cm ∗ Close dist. - add plus lenses (not practiced) ∗ Legibility of the letters not affected ∗ large angular size Test Distance
- 83. ∗ Place occluder in front of one eye ∗ Not hand ∗ Usual practice - right eye first ∗ Begin from larger letter or from the size level little larger than expected ∗ E.g. 20/20 - 20/40 level Procedure
- 84. ∗ Ignore occ. errors if all letters in the next level are read correctly ∗ Encourage to guess - close to threshold level ∗ Testing stopped - missed more than a total of 5 letters ∗ Pin hole acuity ∗ Other eye ∗ B/o acuity Procedure
- 85. ∗ Best Corrected VA ∗ With best ref cx ∗ Provides a benchmark for detecting any future changes of vis. sys Procedure
- 86. ∗ Most widely used ∗ Too coarse to reliably detect small changes in VA ∗ Poor Sensitivity ∗ Partial credit ∗ Adding + or - signs ∗ 20/25+2 (6/7.5+2 ) Row-by-Row Scoring
- 87. ∗ Giving credit for every letter read ∗ More sensitive to changes in VA ∗ Can use + or - signs ∗ Bailey-Lovie Design - the qualifiers (+2,+1,-1,-2) carry same value at all levels ∗ E.g. 20/25+1 , 20/25-1,+2 Letter-by-Letter Scoring
- 88. ∗ If VA in LogMAR ∗ Each letter is assigned a value of 0.02 logMAR ∗ For each additional letter read - 00.02 is deducted from the LogMAR score ∗ If VA recorded in VAR ∗ Each letter is assigned a value of 1 point ∗ Each extra letter adds one extra point to the score Letter-by-Letter Scoring
- 89. LogMAR VAR Correct LogMAR VAR Correct LogMAR VAR 6/15 0.40 80 5 of 5 0.40 80 5 of 5 0.40 80 6/12 0.30 85 5 of 5 0.30 85 5 of 5 0.30 85 6/9.5 0.20 90 5 of 5 0.20 90 5 of 5 0.20 90 6/7.5 0.10 95 1 of 5 0.18 91 5 of 5 0.10 95 6/6 0.0 100 0 0.18 91 4 of 5 0.02 99 6/4.8 -0.10 105 0 0.18 91 3 of 5 -0.04 102 6/9.5+1 6/6 –1,+3 6/6 +2
- 90. ∗ Hearing Impaired ∗ Any Optotypes depending on the pt’s age and ability as any other normal child ∗ Speech Impaired ∗ Ask the pt to write ∗ Matching cards ∗ Sign language with the help of the attendent VA in Other Impairments
- 91. ∗ Hearing Impaired & Speech Impaired ∗ Ask he pt to write ∗ Matching cards ∗ Sign language VA in Other Impairments
- 92. ∗ Intellectually Handicap ∗ If the pt can identify letters or numbers ∗ Symbols ∗ If not possible ∗ Rely on observation skills ∗ Lower the dev level & severe the impairement - greater the dependence on observation VA in Other Impairments
- 93. ∗ Begin examination with non-threatening things (playing with blocks, toys, etc) ∗ For pt. - Game ∗ For Optometrist - Observe pt’s visual behavior ∗ Select procedures appropriate for the dev. age regardless of the Chronological age VA in Other Impairments
- 94. ∗ Forced choice preferantial looking ∗ Teller Acuity ∗ Preferential Looking ∗ OKN ∗ Rewarding VA in Other Impairments
- 95. Reduced Visual Acuity Refractive Errors Non- Refractive Error Media Opacities Retinal Pathology Malingering Amblyopia Optic nerve disorders
- 96. Thank You