Visual acuity

Visaul acuity

1. VISUAL ACUITY DR.ANKITA MAHAPATRA DEPARTMENT OF OPHTHALMOLOGY VSSIMSAR, BURLA

2. VISUAL ACUITY Vision or visual perception : complex integration of VISUAL ACUITY Visual acuity is a measure of the spatial resolution of the eye or, in other words, an estimation of its ability to discriminate between two points.

3. THEORIES OF VISUAL ACUITY 1.RECEPTOR THEORY : If the image fell on two cones separated by an unilluminated cone, then the points of light would be perceived as two distinct sources. In terms of length of the retinal image , two points will be seen clearly when their image size is more than 4.5 μ . ( diameter of individual cone – 1.5 μ ) In terms of Visual angle, two adjacent points seen clearly and discretely when they produce visual angle more than 1 minute. VISUAL ANGLE is the angle subtended at NODAL POINT of eye by an object in visual field.

4. Dimension of the visual angle depends on size of the object and distance from the eye .

5. RAYLEIGH CRITERION Two point sources are resolved from each other when separated by at least the radius of the AIRY DISC .

6. USES : 1. Visual function 2. Refractive status 3. Outcome measure for a treatment 4. medico legal purposes 5. Criteria for : a. Person’s fitness to drive b. Eligibility for entrance into a profession

7. COMPONENTS OF VISUAL ACUITY Ability to detect whether an object is present in an otherwise empty visual field. Clinically measure of Visual acuity. Discrimination of two spatially separated targets. Discrimination of spatial characteristics and identifies the pattern from memory. Ex: identification of faces. Spatial distinction by an observer when threshold lower than ordinary acuity.

8. 1.MINIMUM VISIBLE Minimum visible spatial threshold level depends upon specification of stimulus--- a.size b.shape c.illumination A. black dot against a white background detected if its diameter is of the order of 30 seconds of arc or more. B. Black square against a white background when length of diagonal is 30 seconds. C. Visualization of a thin telegraphic wire against a uniform sky with a thickness of one half second of arc ----- convergence of subthreshold signals from a number of individual retinal elements , yields a suprathreshold level of activity. D. Detection of an illuminated object against a dark background depends on its intensity , not size.

9. 2.RESOLUTION (ORDINARY VISUAL ACUITY ) Discrimination of two spatially separated targets . Minimum separation between two points , which can be discriminated ---- MINIMUM RESOLVABLE Measurement of the threshold of discrimination is an assessment of function of fovea centralis. Normal angular threshold of discrimination for resolution ------- 30-60 seconds of an arc. (minimum angle of resolution).

10. 3.Minimum discriminable (hyperacuity) Ability to determine whether or not two parallel and straight lines are aligned in the frontal plane. ( Vernier acuity ) Threshold value of Vernier acuity : 2-10 seconds of arc .

11. Factors affecting visual acuity

12. Commonly used methods for distant visual acuity, Snellen’s chart Landolt broken ring (or C chart) Tumbling E chart Bailey – Lovie chart / LogMAR chart

13. SNELLEN’S CHART

14. Herman Snellen : A Dutch ophthalmologist who introduced the Snellen chart to study visual acuity in 1862.

15. Principles of Snellen’s acuity : Each letter is designed in a square with sides 5 times the width of letter strokes The breadth of black strokes and white spaces are equal The breadth of line and spaces produce 1’ min of arc at nodal point when viewed from a certain distance Each letter subtends an of 5’ of arc at nodal point when seen at a certain distance VA = DISTANCE OF THE PATIENT FROM THE CHART SMALLEST LINE READ BY THE PATIENT

16. On the 6/6 line each letter is constructed to subtend an angle of 5’ of arc at a distance of 6 meter Other lines are constructed in a similar way, so that letters on the 6/18 line or 6/60 line subtend an angle of 5’ of arc if tested at 18m or 60m from the chart. VA = TESTING DISTANCE DISTANCE AT WHICH LETTER SUBTENDS 5MIN OF ARC

17. LANDOLT’S C- CHART

18. TUMBLING E CHART

19. MINIMUM ANGLE OF RESOLUTION : The denominator in Snellen grading is an indirect measure of the size of the letters read and the angle they subtend. The classic Snellen fraction is the reciprocal of the minimum angle of resolution (MAR)

20. LOG MAR SCALE : A notation of visual acuity that has the same clinically significant difference between each line and allows easy recording of every letter read is the log minimum angle of resolution (MAR) scale. The MAR is arrived at by dividing the denominator by the distance at which the letters were read, i.e. the Snellen fraction is inverted and reduced. A Snellen acuity of 6/12 or 20/40 therefore corresponds to a MAR of 2 minutes of arc. Allows for constant geometric progression over each step. This derivation has been used in the construction of charts such as the Bailey- Lovie chart

21. Minimum angle of resolution Acuity deci m al Acuity Snel l en LogMAR 0.50 2.00 6/3 -0.30 0.67 1.50 6/4 -0.18 0.83 1.20 6/5 -0.08 1.00 1.00 6/6 0.00 1.50 0.67 6/9 0.18 2.00 0.50 6/12 0.30 3.00 0.33 6/18 0.48 4.00 0.25 6/24 0.60 6.00 0.17 6/36 0.78 10.00 0.10 6/60 1.00

22. Procedure for recording visual acuity : Patient seated at 6m distant from the chart .So light rays are parallel and pt exerts minimal accommodation Chart should be properly illuminated The pt is asked to read the chart with each eye separately and VA is recorded Depending upon the smallest line that the pt can read from 6m distance, his/her VA is as 6/6, 6/9, 6/12, 6/18, 6/24, 6/36 and 6/60 If one cannot see the top line from 6m, pt is asked to slowly walk towards the chart till can read the top line Depending upon the distance at which one can read the top line, the vision is recorded as 5/60, 4/60, 3/60, 2/60, 1/60

23. If the patient is unable to read the top line even from 1m, he/she is asked to count fingers (CF) of the examiner His/her VA is recorded as CF-3’, CF-2’, CF-1’ or CF close to face depending upon the distance at which the pt is able to count fingers When the pt fails to count fingers, the examiner should move his/her hand close to pt’s face If pt can appreciate the hand movements, the VA is recorded as HM+ If pt cannot distinguish hand movements, the examiner should test whether the pt can perceive light or not If yes, vision is recorded as PL+ and if not it is recorded as PL- . PL+ pts must be tested with projection of rays to represent quadrants - nasal, superior, temporal and inferior Then VA is recorded as PL+, PR

24. PIN HOLE VISUAL ACUITY If the vision is subnormal, the visual acuity is again determined by asking the patient to read the letters through a pinhole. To determine if a decrease in vision is correctable by lenses

25. BCVA To determine the function of the macula in the best optical conditions, the refraction of the eye must be determined and the visual acuity assessed again in the same way with the correcting glasses in place.

26. Dense cataract LASER INTERFEROMETER forms a diffraction pattern of parallel lines on the retina even through a moderate cataract. The patient is asked to identify the orientation of progressively finer lines, to establish the visual acuity likely to be regained after surgery. POTENTIAL ACUITY METER projects a tiny Snellen chart onto the retina around a lens opacity and the patient is required to read the alphabets

27. VA IN IMPAIRMENTS :

28. YOUNG CHILDREN Maturation of infant visual function has been studied by two techniques, the pattern visual evoked potential and preferential looking behaviour . In children younger than 2 years the VEP test proves more successful .

29. Preferential Looking (Keeler / Teller Cards) Given a choice, an infant prefers to look at patterned rather than unpatterned stimuli. The infant's preference may be quantified by incorporating patterns which vary in stripe width.

30. CARDIFF ACUITY CARDS LINEAR KAY PICTURES

31. NEAR VISUAL ACUITY PROCEDURE : The pt is comfortably seat in a chair and asked to read the near vision chart kept in a distance of 33 – 40 cm Good illumination thrown over from his/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 30cm

32. 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 years old, the reduced near visual acuity is one of the symptoms of presbyopia

33. PHOTOSTRESS TEST The extent of involvement can be assessed clinically by recording vision before and after exposure to a bright light or photostress . The photostress test is performed by covering one eye and asking the patient to read the smallest possible line on the near chart. A bright light is shone into the eye for 15 seconds, following which the patient is asked to read the same line of print and the recovery time noted. The test is repeated with the other eye. In normal people and those with optic nerve disease there is no significant difference in the time taken for the two eyes to recover from the photo stress. In a subject with macular disease the recovery time is prolonged. The test is useful in early macular disease, particularly central serous retinopathy, where there may be minimal deterioration in visual acuity and yet an easily detectable decrease in photoreceptor reserve capacity.

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