Visual acuity and color vision

Aims and Objectives Understand the principles behind vision testing Perform an accurate visual acuity To differentiate between subjects with normal & abnormal color vision

1.Visual Acuity Visual acuity is a measurement of central vision only Assessment of total visual system from cornea to occipital cortex

Why do a visual acuity test? Diagnostic tool Measures progression of disease Evaluates treatment Legal requirement

Vision Testing Tools Vision charts are standardized for size and contrast The Snellen Chart is used for testing distance vision They are designed to be read at 6 metres test types, a series of letters of different sizes constructed so that the top letter is visible to the normal eye by 60 meters& the subsequent lines at 36, 24, 18, 12,9,6&5 meters respectively

E chart For use in illiterate patients Patient matches direction of E with fingers

Distance Correction People who are short-sighted and normally wear glasses or contact lenses should have their visual acuity tested wearing their glasses

Reading glasses (magnifiers) should not be worn during distance testing Note If the subject wears glasses, repeat these observations with & without glasses

Occlusion Each eye needs to be tested separately cover the eye that is not being tested Avoid pressure on the eye

haw to do the test? Using the Snellen Chart Place patient at 6 metres from chart Use adequate illumination Cover left eye Ask patient to read from the top letter Keep going until they cannot read the line clearly and start to make multiple errors.

Visual acuity (V) is recorded according to the ratio: V= d/D V= Visual acuity. d= Distance of the eye from the Snellen’s test. D= Distance at which the eye should be able to read clearly

Normal person should be able to read at least the 7th line (i.e.) V = 6 / 6. If the V is less than 6 /60, walk patient towards the chart about 1 meters. Still can’t read the chart? walk patient towards the chart about anther 1 meters and so on until he can read the top letter. If the top letter is visible at 2 meters; so the V = 2 / 60.

If the Visual acuity of less than 1 / 60 is recorded as: Still can’t read the chart? Ask patient to count how many fingers you are holding up at 1 metre. Keep fingers still. Recorded as Count Fingers If they cannot count fingers see if they can see a moving hand. Recorded as Hand Movements Still no result: can they see a pen torch light : Light perception (LP) Unable to perceive light: No Light Perception (NLP)

Emmetropia Eye considered to be normal “emmetropic” if a parallel light rays from distant object (6 meters or more) in front of the eye, are in sharp focus on the retina when the ciliary muscles is completely relaxed

(Myopia (Near sightedness When the ciliary muscle is completely relaxed, the light rays coming from distant objects are focused in front of the retina due to: a- Usually due to too long eyeball. b- Occasionally result from too much refractive power of the lens system of the eye. Correction BY placing in front the eye a concave lens which diverge the light rays.

2*Hyperopia “far- sightedness” Eye ball that is too short or. Occasionally the lens system too weak when the ciliary muscle is relaxed parallel light rays are not bent sufficiently by the lens system to come to a focus by the time they reach behind the retina. Correction: use convex lenses.

3*Astigmatism the curvature of the cornea is not uniform. light rays are refracted to a different focus, so that part of the retinal image is blurred. Correction: by cylindrical lens

Color Vision

Subjects & methods 1. Subjects. 2.Ishihara Pseudo-Isochromatic palates

White light sensation: due to equal stimulation of all red, green & blue cones. Yet there is no wave length for white light. Yellow color sensation: no wave length for yellow but equal stimulation of red & green cones at the same time. An object appears red when all wave lengths, except red wave length, are absorbed by the object. An object appears white when it reflects all wave lengths. An object appear black when the object absorbs all the wave lengths of light.

color blindness When a single group of color receptive cones is missing from the eye, the person is unable to distinguish some colors from others; the person is especially unable to distinguish red from green and is therefore said to have red-green color blindness. ( 2% red color blindness ( protanope). 6%green color blindness(deuteranope) ( 8% red- green color blindness. blue color blindness very rare, but may have color weakness. The person with loss of Blue cones is called Tritanope

Red-green color blindness It is a genetic disorder that occurs almost exclusively in males. That is, genes in the female X chromosome code for the respective cones Yet color blindness almost never occurs in females because at least one of the two X chromosomes almost always has a normal gene for each type of cone. Because the male has only one X chromosome, a missing gene can lead to color blindness. Because the X chromosome in the male is always inherited from the mother, never from the father

People with defective color vision will read a different number from a normal person on the same plate; in other word color blindness is a defect in the ability to discriminate between light of different wave lengths, it is usually explained by a deficiency of one or more of the cone pigments

procedure 1.The subject is asked to read figures in the available Ishihara Pseudo-Isochromatic palates. 2.In the top chart; the normal person reads (74), while the red green color blind person reads (21). 3.In the bottom chart; the normal person reads (42), while the red blind person reads (2), & the green blind person reads (4).