VISUAL ACUITY , Basics of vision assessment

Visual acuity (VA) is a measure of the ability of the eye to distinguish shapes and the details of objects at a given distance. It is important to assess VA in a consistent way in order to detect any changes in vision. One eye is tested at a time.

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Indications
To provide a baseline recording of VA
To aid examination and diagnosis of eye disease or refractive error
To assess any changes in vision
To measure the outcomes of cataract or other surgery.
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Equipment
Multi-letter Snellen or E chart
Plain occluder, card or tissue
Pinhole occluder
Torch or flashlight
Patient's documentation.
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Procedure
Ensure good natural light or illumination on the chart. It is important to ensure that the person has the best possible chance of seeing and reading the test chart as treatment decisions are made based on the results of VA testing.
If the test is done outdoors, the chart should be in bright light and the patient in the shade, with enough light to illuminate the patient's face during the test.
Explain the procedure to the patient. Tell patients that it is not a test that they have to pass, but a test to help us know how their eyes are working. Tell them not to guess if they cannot see.
Ensure that any equipment that the patient touches is clean and is cleaned between patients. Infections can be passed between patients if equipment – or the testers' hands – are not clean.
Position the patient, sitting or standing, at a distance of 6 metres from the chart. The patient can hold one end of a cord or rope of 6 metres long to ensure that the distance is maintained
Test the eyes one at a time, at first without any spectacles (if worn).
Note: Some people prefer to always test the right eye first. Others prefer to test the ‘worse’ eye first (ask the patient out of which eye they see best). This ensures that the minimum is read with the ‘worse’ eye, and more will be read with the ‘good’ eye. This means that no letters are remembered, which could make the second visual acuity appear better than it is.

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Visual acuity should be measured from a standard distance, using a standard chart with a white background

Ask the patient to cover one eye with a plain occluder, card or tissue. They should not press on the eye; this is not good for an eye that has undergone surgery. It can also make any subsequent intraocular pressure reading inaccurate and it will distort vision when the occluded eye is tested.
Ask the patient to read from the top of the chart and from left to right. If the patient cannot read the letters due to language difficulties, use an E chart. The patient is asked to point in the direction the ‘legs’ of the E are facing.
Note: there is a one in four chance that the patient can guess the direction; therefore it is recommended that the patient should correctly indicate the orientation of most letters of the same size, e.g. four out of five or five out

1. VISUAL ACUITY Presented by : Dr Mohit Kumar Jha Resident Ophthalmology  

2. BASICS VISION VISUAL ACUITY

3. Visual Acuity can be measured easily it only quantifies high contrast vision loss and therefore doesn't tell you about quality of vision It is the spatial resolving capacity of the visual system which helps the eye to see the fine details

4. TYPES OF VISUAL ACUITY TASKS TARGET DETECTION  Target resolution Target localization Target recognition 

5. TARGET DETECTION TASKS presence or absence of an aspect of the stimuli Test objects and backgrounds are contrasting Landolt C & the illiterate E test task is too locate the gap.

6. TARGET RESOLUTION It is the smallest angular size where the separation between critical elements of a stimulus pattern can be discriminated. Eg. Pair of dots , A grating

7. TARGET LOCALISATION Involves detecting discontinuity in contour , due to the differences in the spatial position of segments of a test object called VERNIER ACUITY

8. TARGET RECOGNITION  Commonly used clinically to measure VA Requires the recognition or naming of a target .                        Eg – Snellen`s letters COGNITION IS INVOLVED

9. CONVERSION OF SNELLEN`S ACUITY TO SPATIAL FREQUENCY Gratings can be used as another method of measuring visual acuity. Visual acuity in Snellen`s notation can be expressed in spatial frequency and vice versa.

10. PHYSIOLOGIC BASIS OF VISION Two points can be recognized as separate when they subtend an angle of one minute of an arc at the nodal point of the eye visual acuity is a measure of the finest detail that can be resolved or recognized by the visual system visual acuity tests in infants check resolution and not recognition therefore amblyopia can go undetected.

11. SNELLEN`S ACUITY CHART Measures distance central acuity designed by Dr Herman snellen in1862 symbols and letters are called optotypes 8 to 9 lines on the chart BLACK capital letters against white background(high contrast) Sans-serif font , equally legible number of letters varies in different lines non uniform degrees in the size of the letters from top to bottom fixed spacing between lines evenly illuminated (34-51 cd/m )

12. SNELLEN`S OPTOTYPES Each letter is designed to fit a 5X5 square the thickness of the dark lines and white spaces between them is equal the height and the width of the letter is five times the thickness of the constituent line at a given distance each letter subtended at an angle of 5 minute at the nodal point and each small square or the thickness of the constituent line subtends at an angle of 1 min at the nodal point

13. METHOD OF TESTING Why is vision recorded at six meter or 20 feet? Because the rays are parallel and no accommodation is exerted and it's simulates Infinity Illumination illuminated chart- dimly lit room non illuminated -chart in the sun, patient in shade one eye at a time don't press, both eyes together repeat with pinhole and spectacle correction

14. How is visual acuity quantified when the patient is unable to see a standard snellen chart ? the best way is to move the patient and chart closer together. note the size of the optotype seen at a particular distance. move 1m closer to chart ( 5/60, 4/60, 1/60 ) hand motions or light perceptions with or without projection. If Topmost Letter is not Visible , move closer 1m at a time ,and if not visible at 1m then we tell patient to count fingers , further hand movement and lastly light perception

15. PERCEPTION OF LIGHT Light source is inferior. Illuminates the superior retina. If patient cannot localize the light inferiorly, PR inaccurate in the superior quadrant .

16. SNELLEN`S FRACTION A visual equity of 6/6- represents the ability of the eye to see one minute of arc VA is equal to distance of the test BY distance at which a person with normal vision can see the optotype   EXAMPLE  6/24  =  6 is the distance of the subject in metres from the vision chart                   24 is the distance in meters at which a person with normal vision sees the                        same autotype                   at least 5 out of 6 letters have to be read to denote is as a snellen`s fraction Reads a line partially record vision of the previous full line add the number of letters read in the subsequent line example a reads full 6/12 line, two letters of 6/9 then visual equity equals to 6/12+2

17. Log MAR ACUITY Logarithm of minimum angle of resolution the logmar scale converts the geometric sequence of a traditional chart to a linear scale logmar charts designed by Bailey and Lovie in 1980 Sans  serified Sloan optotypes equal viewing difficulty each line has five letters and space between the letters is equal to the letter size on that line the distance between letters of a line as well as the space between the lines gradually changes the progression of optotype heights- is geometric, decreasing in 0.1 log unit increments, similarly the space between rows any three line (15 letters )decreases or increases results doubling or half respectively of the visual angle .

18. BAILEY & LOVIE CHART

19. Recording VA in LogMAR Chart Each Letter – 0.02log units Total score per line+ 5x0.02 represents a change of 0.1 log units SCORING METHODS Score of last row where all 5 letters are read Subtract 0.02 log units for every letter read beyond the last row        -   20/30 row = 0.2       - 20/25 row 2x 0.2log/letter = -0.04       Acuity log score = 0.16

20. PIN HOLE TESTING A test performed on a person who has diminished visual equity to distinguish a refractive error from organic diseases the patient looks through it with one eye at a time without wearing corrective lenses light passes only through the center of eyes lens and errors of refraction have no effect while the occluder is used pinhole blocks the peripheral rays only letting those rays which pass through the central portion of the pupil if visual acuity is improved the defect is refractive , if not its organic

21. NEAR VISUAL ACUITY The patient is comfortably seat in a chair and asked to read the near vision chart kept in a distance of 33- 40 cm good illumination drawn over from his or her back preferably over left shoulder  each eye should be tested separately the near vision is recorded as the smallest type that can be read comfortably by the patient as N5, N6, N8, N10, N12, N14, N18, N24, N36 and N48 notation is made as NV= N5 at 30 cm

22. NEAR VISION CHARTS Roman Test Types Snellen`s Near Vision Test Type Jager`s Chart The purpose is to detect people with near vision difficulties e.g  (uncorrected high hyperopia , accommodative dysfunction) In patients over 40 year old the reduce near visual acuity is one of the symptoms of presbyopia.

23. Factors other than diseases that reduce visual activity Uncorrected ametropia Eccentric viewing Decreased contrast Large ( >6mm ) or small ( <2.5mm ) pupil size Young or old age large pupils reduce visual equity due to increased spherical abrasion smaller pupils reduce these optical aberrations but at the expense of increased diffraction optimal 3- 5 millimeter

24. THANK  YOU