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See Also: Eye examination Visual acuity (VA) is acuteness or clearness of Vision , especially form vision, which is dependent on the sharpness of the retinal focus within the Eye , the sensitivity of the nervous elements, and the interpretative faculty of the brain.Cline D; Hofstetter HW; Griffin JR. ''Dictionary of Visual Science''. 4th ed. Butterworth-Heinemann, Boston 1997. ISBN 0-7506-9895-0 VA is a Quantitative measure of the ability to identify black symbols on a white background at a standardized distance as the size of the symbols is varied. The VA represents the smallest size that can be reliably identified. VA is the most common clinical measurement of visual function. The well-known phrase "20-20 vision" refers to the distance in feet that objects separated by an angle of 1 Arc Minute can be distinguished as separate objects. The metric equivalent is 6-6 vision. HISTORY PHYSIOLOGY OF VISUAL ACUITY To resolve detail, the eye's optical system has to project a focused image on the Fovea , a region inside the Macula having the highest density of Cone Photoreceptor s (the only kind of photoreceptors existing on the fovea), thus having the highest resolution and best color vision. Acuity and color vision, despite being done by the same cells, are different physiologic functions that don't interrelate except by position. Acuity and color vision can be affected independently. The Visual Cortex is the part of the Cerebral Cortex in the posterior (occipital) part of the Brain responsible for processing visual stimuli. The central 10° of field (approximately the extension of the macula) is represented by at least 60% of the visual cortex. Many of these neurons are believed to be involved directly in visual acuity processing. Light travels from the fixation object to the fovea through an imaginary path called the visual axis. The eye's tissues and structures that are in the visual axis (and also the tissues adjacent to it) affect the quality of the image. These structures are: tear film, cornea, anterior chamber, pupil, lens, vitreous, and finally the retina. The posterior part of the retina, called the retinal pigment epithelium (RPE) is responsible for, among many other things, absorbing light that crosses the retina so it cannot bounce to other parts of the retina. The RPE also has a vital function of recycling the chemicals used by the rods and cones in photon detection. If the RPE is damaged and does not clean up this "shed" blindness can result. VISUAL ACUITY EXPRESSION Visual acuity is often measured according to the size of letters viewed on a Snellen Chart or the size of other symbols, such as Landolt C s or Tumbling E. In some countries, acuity is expressed as a Vulgar Fraction , and in some as a Decimal Number . Using the Foot as a unit of measurement, (fractional) visual acuity is expressed relative to 20/20. Otherwise, using the Metre , visual acuity is expressed relative to 6/6. For all intents and purposes, 6/6 vision is equivalent to 20/20. In the decimal system, the acuity is defined as the reciprocal value of the size of the gap (measured in arc minutes) of the smallest Landolt C that can be reliably identified. A value of 1.0 is equal to 20/20. LogMAR is another commonly used scale which is expressed as the logarithm of the minimum angle of resolution. LogMAR scale converts the geometric sequence of a traditional chart to a linear scale. It measures visual acuity loss; positive values indicate vision loss, while negative values denote normal or better visual acuity. This scale is rarely used clinically; it is more frequently used in statistical calculations because it provides a more scientific equivalent for the traditional clinical statement of “lines lost” or “lines gained”, which is valid only when all steps between lines are equal, which is not usually the case. A visual acuity of 20/20 is frequently described as meaning that a person can see detail from 20 feet away the same as a person with normal eyesight would see from 20 feet. If a person has a visual acuity of 20/40, he is said to see detail from 20 feet away the same as a person with normal eyesight would see it from 40 feet away. It is possible to have vision superior to 20/20: the maximum acuity of the human eye without visual aids (such as Binoculars ) is generally thought to be around 20/10 (6/3)however, recent test subjects have exceeded 20/8 vision. Recent developments in optometry have resulted in Corrective Lenses conferring upon the wearer a vision of up to 20/10. Some birds, such as Hawk s, are believed to have an acuity of around 20/2; in this respect, their vision is much better than human eyesight. When visual acuity is below the largest optotype on the chart, either the chart is moved closer to the patient or the patient is moved closer to the chart until the patient can read it. Once the patient is able to read the chart, the letter size and test distance are noted. If the patient is unable to read the chart at any distance, he or she is tested as follows: Many humans have one eye that has superior visual acuity over the other. If a person cannot achieve a visual acuity of 20/200 (6/60) or above in the better eye, even with the best possible glasses, then that person is considered legally Blind in the United States . A person with a visual field narrower than 20 Degree s also meets the definition of legally blind. A person's visual acuity is registered documenting the following: whether the test was for distant or near vision, the eye(s) evaluated and whether Corrective Lenses (i.e. Spectacle s or Contact Lens es) were used:
So, distant visual acuity of 20/60 and 20/25 with pinhole in the right eye will be: DscOD 20/60 PH 20/25 Distant visual acuity of count fingers and 20/50 with pinhole in the left eye will be: DscOS CF PH 20/50 Near visual acuity of 20/25 with pinhole remaining at 20/25 in both eyes with spectacles will be: NccOU 20/25 PH 20/25 "Dynamic visual acuity" defines the ability of the eye to visually discern fine detail in a moving object. MEASUREMENT Visual acuity is typically measured monocularly rather than binocularly with the aid of an optotype chart for distant vision, an optotype chart for near vision, and an to cover the eye not being tested. The examiner may also occlude an eye by sliding a tissue behind the patient's eyeglasses, or instructing the patient to use his or her hand. This latter method is typically avoided in professional settings as it may inadvertently allow the patient to peek through his or her fingers, or press the eye and alter the measurement when that eye is evaluated. # Place the chart at 20 feet (or 6 meters) and illuminate to 480 lux at that distance. # If the patient uses glasses, then the test is performed using them. # Place the occluder in front of the eye that is not being evaluated. The first evaluated eye is the one that is believed to see less or the one the patient says that is seeing less. # Start first with the big optotypes and proceed to the smaller ones. The patient has to identify every one on the line being presented and communicate it to the physician. # If the measurement is reduced (below 20/20) then the test using a pinhole should be done and register the visual acuity using the pinhole. Both measures should be registered, with and without using pinhole. # Change the occluder to the other eye and proceed again from the 4th step. # After both eyes have been evaluated in distant visual acuity, proceed to evaluate near visual acuity placing a modified snellen chart for near vision (such as the Rosembaum chart) at 14 inches (or 35 centimeters). Then repeat the test from the 2nd step. In some cases, binocular visual acuity will be measured, because usually binocular visual acuity is slightly better than monocular visual acuity. MEASUREMENT CONSIDERATIONS Visual acuity measurement involves more than being able to see the optotypes. The patient should be cooperative, understand the optotypes, be able to communicate with the physician, and many more factors. If any of these factors is missing, then the measurement will not represent the patient's real visual acuity. Visual acuity is a subjective test meaning that if the patient is unwilling or unable to cooperate, the test cannot be done. A patient being sleepy, intoxicated, or having any disease that can alter the patient's consciousness or his mental status can make the measured visual acuity worse than it actually is. Illiterate patients who cannot read letters and/or numbers will be registered as having very low visual acuity if this is not known. Some of the patients will not tell the physician that they don't know the optotypes unless asked directly about it. Brain damage can result in a patient not being able to recognize printed letters, or being unable to spell them. A motor inability can make a person respond incorrectly to the optotype shown and negatively affect the visual acuity measurement. Variables such as Pupil size, background adaptation luminance, duration of presentation, type of optotype used, interaction effects from adjacent visual contours (or “crowding") can all affect visual acuity measurement. Visual acuity testing in children See Also: Infant vision The Newborn ’s visual acuity is approximately 20/400 and 20/20 by age 2 years. http://www.uic.edu/com/eye/LearningAboutVision/EyeFacts/BabyEyes.shtml The measurement of visual acuity in infants, pre-verbal children and special populations (for instance, handicapped individuals) is not always possible with a letter chart. For these populations, specialised testing is necessary. As a basic examination step, one must check whether visual stimuli can be fixed, centered and followed. More formal testing using Preferential Looking techniques use ''Teller acuity'' cards (presented by a technician from behind a window in the wall) to check if the child is more visually attentive to a random presentation of vertical or horizontal bars on one side compared with blank page on the other side - the bars become progressively less contrasting, and the endpoint is noted when the child in its adult carer's lap equally prefers the two sides. Other techniques include checking oculomotor responses using a rotating ''' Optokinetic Nystagmus drum''', and electro-physiologic testing using ''' Visual Evoked Potential ''' can be used to moderately estimate visual acuity in doubtful cases and expected severe vision loss cases like Leber's Congenital Amaurosis . "NORMAL" VISION Visual acuity depends upon how accurately light is focused on the retina (mostly the , that is Snellen's chart 20/20 feet, 6/6 meter, 1.00 decimal or 0.0 logMAR. In humans, the maximum acuity of a healthy, Emmetropic eye (and even ametropic eyes with correctors) is approximately 20/16 to 20/12, so it is inaccurate to refer to 20/20 visual acuity as "perfect" vision. 20/20 is the visual acuity needed to discriminate two points separated by 1 arc minute. The significance of the 20/20 standard can best be thought of as the lower limit of normal or as a screening cutoff. When used as a screening test subjects that reach this level need no further investigation, even though the average visual acuity of healthy eyes is 20/16 or 20/12. Some people may suffer from other visual problems, such as Color Blindness , reduced Contrast , or inability to track fast-moving objects and still have normal visual acuity. Thus, normal visual acuity does not mean normal vision. The reason visual acuity is very widely used is that it is a test that corresponds very well with the normal daily activities a person can handle, and evaluate their impairment to do them. 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