Visual acuity and color vision

Visual Acuity •Visual acuity is a measurement of central vision. •Assessment of total visual system from cornea to occipital cortex . This test is used for: Diagnosis of the disease. Measures progression of disease. Evaluates treatment.

Snellens` chart

The Snellen Chart is used for testing distance vision, they are designed to be read at 6 metres series of letters of different sizes constructed so that the top letter is visible to the normal eye by 6 meters and the subsequent lines at 36, 24,18, 12,9,6and 5 meters respectively. E chart Patient matches direction of E with fingers(used when the patient can not read or in children)

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

Using the Snellen`s Chart Place patient at 6 metres from the chart Use adequate illumination Each eye needs to be tested separately cover the eye that is not being tested Avoid pressure on the eye Ask patient to read from the top letter Keep going until they cannot read the line clearly and start to make multiple errors

Distance Correction •People who are normally wear glasses or contact lenses should have their visual acuity tested wearing their glasses, i.e. their vision has been corrected •If glasses or contact lenses are prescribed and available, testing without them provides no relevant information

Normal person should be able to read at least the 7th line (i. e 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 another 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 (CF @1m) •If they cannot count fingers see if they can see a moving hand. Recorded as Hand Movements (HM @1m) •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: use concave lens

*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.

*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

Perception of white light About equal stimulation of all the red, green & blue cones, gives one sensation of seeing white. Yet there is no wave length of light corresponding to white instead, white is a combination of all the wave lengths of the spectrum. 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 6%green color blindness 8% red- green color blindness blue color blindness very rare

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

Ishihara Pseudo-Isochromatic palates People with defective color vision will read a different number from a normal person on the same plate

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).