Visual Acuity Measurement - 20/20 Vision

Visual Acuity Opt 350  Definition  The power of the eye to see a smallest object or letter clearly and distinguishably at specified distance.  VA is the medical term for sharpness of vision....

Visual Acuity Opt 350  Definition  The power of the eye to see a smallest object or letter clearly and distinguishably at specified distance.  VA is the medical term for sharpness of vision.  It deals with the sharpness or discrimination of central vision, rather than the extent or clarity of peripheral vision. 11/11/2024 2   Visual acuity (VA) is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain.  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.  It is the most common clinical measurement of visual function. In the term "20/20 vision" the numerator refers to the distance in feet from which a person can reliably distinguish a pair of objects.  The denominator is the distance from which an 'average' person would be able to see the letter. (distinguish —the distance at which their separation angle is 1 arc min.) 11/11/2024 3  When to Examine  At the beginning of each examination and for all patients.  When expecting ocular problems or injuries.  Mandatory to qualify for certain occupations with strict vision requirements, such as that of air craft pilot, firefighter… etc 11/11/2024 4  Anatomical Basics  Rods : 120 million/eye, responsible for scotopic vision  Cones: 6 million/ eye, photopic vision and color vision  Visual receptors ( rods and cones) convert the light ( electrical energy) into nerve signals that go to the lateral geniculate body and then into the visual cortex. The visual cortex primarily consists of the primary visual cortex which is responsible for the ability to see and the secondary visual cortex that responsible for recognizing form, color, depth of focus (Stereopsis).  Macula is an area of 5 mm in diameter and lies about 3 mm lateral and 1 mm inferior to the optic disc. The fovea is a depresses area in the center of the macula and measures about 1.5 mm in diameter. 11/11/2024 5   Increasing number of the cones within the macular region will increase the resolution ability, so maximum visual acuity happens within the area of fovea centralis where maximum cones density is recorded. 11/11/2024 6  Physical Basics  Clinically, we measure the VA by having the patient to look at visual targets of various sizes presented on standard visual acuity chart. The targets conform to specific criteria.  The basic target is the basic building block of the typical visual acuity chart. the critical details of the basic target refers to the width of the line used to draw a letter or other visual target.  In general, rather than identify this width in units of linear distance , we use units of visual angle. The critical detail of the basic target is 1 minute of arc ( min arc) or 1 /60. 11/11/2024 7  Hight of a Basic Target The overall height of the basic target is 5 times its critical detail. Therefore, the overall height of the basic target is 5 min arc. In general, for targets on VA charts, the overall height is always 5 times the size of the critical detail. 11/11/2024 8  Visual Angle Definition Visual size Vs Physical size The physical size of an object can be measured in any of a number of units. However, these conventional units are inappropriate if we are concerned with the visual size of an object, since its visual size will vary not only with its physical size but with its distance from the eye. Objects A and B have the same physical size, but because they are at different distances from the eye, they have different visual size and subtend 11/11/2024 different visual angles. 9   We measure the visual size of an objects with units of angular “ subtense”  The angular subtense is simply the angle made by lines connecting the edges of the object to the eye.  Objects with different physical sizes may have the same visual size if they have the same visual angle, see the figure below. b a Object A and B have different physical sizes but the same visual size in that they subtend the same angle 11/11/2024 10  VA Types Distant Pinhole VA Near VA VA 11/11/2024 11  Distant VA Expression  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 visual aids) is generally thought to be around 20/10 (6/3) however, recent test subjects have exceeded 20/8 vision. 11/11/2024 12  The Standard Testing Distance Ideal distance at which to test visual acuity is infinity. However, since it is impractical to build an examining room that long, a compromise was reached and 20 feets or 6 meters, has been defined and widely accepted as optical infinity for purposes of visual acuity measurement. 11/11/2024 13  DVA Chart  In Adults: Snellen’s chart  In children: Teller acuity chart Log MAR chart Allen chart Sheridan Gardiner chart Cardiff’s chart  In Illiterate: Lea chart keeler acuity chart E-chart Picture chart Landolt’s chart 11/11/2024 14  Snellen’s Chart Series of letters of diminishing sizes. Chart is set at 6 meters or 20 feet. Consists of letters or numbers. 11/11/2024 15  Log MAR  LogMAR is another commonly used scale which is expressed as the logarithm of the minimum angle of resolution.  Log MAR 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. 11/11/2024 16 11/11/2024 17  Tumbling E chart 11/11/2024 18  Lea chart 11/11/2024 19  Allen chart 11/11/2024 20  Landolt C chart 11/11/2024 21  Sheridan Gardiner test 11/11/2024 22  Cardiff’s charts 11/11/2024 23  Pelli-Robsin chart 11/11/2024 24  DVA Procedure: 1. Tested with or without glasses. 2. Start with the right eye or the worst eye. 3. Position the ptx at 6 meters or 20 feet from the chart 4. Cover the ptx’s left eye with an occluder or the palm of the hand. 5. Ask the ptx to read the letters from left to right on every other line down the chart until the ptx misses more than half the letters on one of the lines. 6. Repeat the same procedure for the fellow eye. 7. Record the corresponding acuity separately with and without correction. 11/11/2024 25   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: 11/11/2024 26   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.  A person with a visual field narrower than 20 degrees also meets the definition of legally blind. Legally blind patients is NOT the same as blind patient 11/11/2024 27  Near Acuity Test The near acuity test measures the patient’s ability to see clearly at normal reading distance. This test is usually performed if the patient complains of difficulty in reading or doing other close work. 11/11/2024 28  NVA Procedure: 1. Ptx is examined with and without correction. 2. Instruct the ptx to be seated. 3. After the ptx is comfortable seated, printed test card is given to them. 4. Ask the ptx to hold the card at 40 cm. 5. Have the ptx to cover the left eye with an occluder or the palm of the hand. 6. Ask the ptx to read with the right eye the line of the smallest characters legible on the card. 7. Repeat the same procedure with the other eye occluded. 8. Record the near acuity value for each eye separately. 11/11/2024 29  Pinhole Visual Acuity  Small circular disc with small central opening that reduces the peripheral rays and allows the central rays to reach the retina.  To confirm whether below normal visual acuity results from refractive error.  If the pinhole occluder improve a patient’s poor vision , the patient most likely has an uncorrected refractive error.  If the pinhole occluder does not improve poor distance vision, the patient’s problem is probably caused by something other than refractive error, and further tests may wish to performed. 11/11/2024 30  VA Abbreviations  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 or not glasses were used : Distance from the chart D (distant) for the evaluation done at 20 feet (or 6 meters). N (near) for the evaluation done at 15.7 inches (or 40 cm). Eye evaluated OD (Latin oculus dexter) for the right eye. OS (Latin oculus sinister) for the left eye. OU (Latin oculi uterque) for both eyes. Usage of spectacles during the test Cc: (Latin cum correctore) with correctors. sc: (Latin sine correctore) without correctors. Pinhole occluder The abbreviation PH is followed by the visual acuity as measured with a pinhole occluder, which temporarily corrects for refractive errors such as myopia or astigmatism. 11/11/2024 31  Examples:  So, distant visual acuity of 20/60 and 20/25 with pinhole in the right eye will be: D sc OD 20/60 PH 20/25  Distant visual acuity of count fingers and 20/50 with pinhole in the left eye will be: D sc OS CF PH 20/50  Near visual acuity of 20/25 with pinhole remaining at 20/25 in both eyes with spectacles N cc OU 20/25 PH 20/25  "Dynamic visual acuity" defines the ability of the eye to visually discern fine detail in a moving object 11/11/2024 32 11/11/2024 33  Tips  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. We should always know how to deal with them especially children.  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. 11/11/2024 34  Tips  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. 11/11/2024 35  Acuity Tests in Infants and Children  Children presents a special challenge even to experienced examiners. Infants are tested not for visual acuity but for visual function o In children visual acuity is less than the normal adult visual acuity. o At birth VA at average 1.5/60 o 1 month 3/60 o 2 months 6/60 o 3 months 6/36 o 6 months 6/18 o 1 year 6/15 o 2 years 6/12 o 3 years 6/9 o 4 years 6/6  normal adult visual acuity o VA is measured in children using the preferential looking test, pictures test. 11/11/2024 36  Visual Acuity Testing in Children  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 a blank page on the other side — the bars become progressively finer or closer together, and the endpoint is noted when the child in its adult carer's lap equally prefers the two sides. 11/11/2024 37 11/11/2024 38  Normal Vision Defined by Snellen as the ability to recognize an optotype when it subtended 5minutes of arc 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.  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. 11/11/2024 39  Other measurements of VA 1. Vernier acuity: measures the ability to align two line segments. It measures the ability to detect a misalignment or positional offset between visual stimuli, for example between two vertical lines when reading a vernier scale 11/11/2024 40  2. Stereoscopic acuity is the ability to detect tiny differences in depth with the two eyes. For more complex targets, stereo acuity is like normal monocular visual acuity, or around 0.6-1.0 arc minutes, but for much simpler targets, such as vertical rods, may be as low as only 2 arc seconds. Although stereoacuity normally corresponds very well with monocular acuity, it may be very poor or even absent even with normal monocular acuities. Such individuals typically have abnormal visual development when they are very young, such as an eye turn, where both eyes rarely or never point in the same direction and therefore do not function together. 11/11/2024 41 11/11/2024 42  Factors that Affect the Visual Acuity Visual acuity is affected by the following factors: 1. Condition of patient, anger, fear, depression, intelligence, awareness, response 2. Facial expression and symmetry affects muscle of the eye 3. Visual receptor: size and sensitivity. closer to the fovea the density of the cones increase. 4. Optical factors: optical aberration, diffraction, smaller than in the center 5. Illumination: stronger illumination will improve the VA. 6. Pupil size: conversely affect the diffraction. Reduction of the pupil size  increase the concentration of the light in the retina  improve the visual acuity ( pin hole effect). 7. Refractive errors: myopia , hyperopia, astigmatism. 11/11/2024 43  Here are some examples for conditions that affect visual acuity: 1. Optics Refractive problems, myopia, hyperopia, astigmatism Health: irregularity or opacification of the ocular media e.g. cataract 2. Retina: swelling or loss of cells , e.g. macular degeneration 3. Optic nerve or brain loss of function of neurons e.g. stroke tumor miswiring of brain neurons e.g. Amblyopia 4. Beyond the visual system: mental retardation 11/11/2024 44 11/11/2024 45