VISUAL ACUITY

is Visual acuity testing? a very basic and a very important measure of how the visual system functions. is a normal visual acuity reading? What does it tell the doctor? a normal visual acuity reading of 20/20 tells the doctor volumes about how the eye is functioning. In order to see 20/20, the ocular media must be relatively clear, the fovea (central vision) must be healthy, the neural pathway to the brain must be operating well, and the visual cortex of the brain (perception) must functioning. happens if three eye doesn't see 20/20? If the eye does not see 20/20, then it is the doctor's job to figure out why not, and to offer possible remedies. what does visual acuity measure? it only measures the function of the very center of the macula, the fovea centralis. Retinal photo receptors called cones are packed into the fovea, creating the only area of the retina that has sharp vision. The optical system of the eye focus light to a pinpoint in this area. what did Dr. Frans Van De Velde discover with visual acuity testing? using an early scanning laser ophthalmosope, was able to project letters directly onto the retina with a microperimeter. Studying normal eyes, he found that visual acuity drops off rapidly as an acuity stimulus is moved away from the fovea. The macular area immediately surrounding the fovea has an acuity of 20/70. Moving outward from the macula, visual acuity drops to 20/400. The retina outside of the fovea is populated with what? rod photoreceptors and a few cone receptors The rod cells are good at detecting what? motion and low levels of light what are the cone cells associated with? color vision and sharp visual acuity The visual function of the outer retina is tested with what? what does it do? a visual field analyzer, which tests how sensitive the retina is to different levels of light intensity. The results of visual acuity tests are usually noted with what? a large "V", "VA", or "Va". The result for the right eye is identified with "OD", which is an abbreviation for the Latin term "oculus dexter" (right eye). The result for the left eye is identified with "OS" (oculus sinister). Distance visual acuity can be tested with what? "CC" (with correction), or "SC" (without correction). If the correction is a contact lens, what is used as the identifier? "CL" is often used as the identifier. "With correction" usually means what? with glasses "Without correction" usually means what? no optical aide (no glasses), including no contact lenses Near vision may be identified by what? a large "V" with the word "near", or perhaps with a large "N" Near vision can be noted how? in Snellen acuity (e.g. 20/20), or the Jaeger notation may be used (e.g. J1). Your doctor will have his/her own way of notation that may or may not match what is presented here. Here is a common example: V CC OD 20/20 OS 20/30 N SC OD J1 OS J2 The most common visual acuity test uses what? how is it used? the familiar Snellen acuity chart. The chart is designed to be viewed at a distance of 20 feet. Optically, light rays from an object 20 feet away are considered to be parallel. Parallel light does not require accommodation for the emmetropic eye to focus. The chart can be projected onto a screen. If the room is not 20 feet long, then a mirror system can be used to reflect the image a total of 20 feet. The Snellen notation, e.g. 20/20, should not be thought of as what? what does it represent? a fraction. The first number means that the tested eye has to be at a distance of 20 feet in order to see this size letter. The second number means that a "normal" eye can see this size letter at the indicated distance. For example, a visual acuity of 20/80 means that the eye tested has to be 20 feet away to see a 20/80 sized letter. A normal eye can see a 20/80 size letter at a distance of 80 feet. One goal of a complete eye examination is to determine what? the best corrected visual acuity (BCVA) Normal visual acuity is generally accepted as 20/20, although many people can see how well? 20/15, and a few can see 20/10. If the eye cannot see 20/20 with a glasses or contact lens correction, then a refraction is required to determine what? the BCVA The term "BCVA" is usually not found in what? where is it found? an examination record, but it is often used in medical literature If the eye sees less than 20/20 with correction, then the doctor will look for the BCVA in what? the refraction results. protocol for Distance acuity with correction (CC): If the patient needs glasses or contact lenses for the best distance vision, then the doctor will always want this measurement. The exception is visual acuity testing after cataract or refractive surgery, as these procedures will likely change the refractive status of the eye. protocol for Distance acuity without correction (SC): If the patient has no glasses or contact lenses, then this will obviously be the default test. If the patient needs glasses for distance, then the doctor may want this measurement if the patient is being seen for the first time, but the doctor may not require this measurement for subsequent visits. This measurement may be needed for a driver's license vision test. It is a good idea to perform this measurement if the patient says he/she sees better in the distance without his/her glasses. protocol for Distance acuity with contact lenses (CL): This measurement will be needed if the exam includes a contact lens evaluation. If a contact lens evaluation is not part of the examination, then the doctor may view this measurement as optional if the patient has both glasses and contact lenses protocol for Near vision with correction: If the exam is a complete eye examination, then this measurement is important. Your doctor may not require this measurement for follow-up and post-op exams protocol for Near vision without correction: If the patient needs glasses for reading, then your doctor likely will not need this measurement. The exception is the patient who says he can read better without his reading correction. When testing Snellen visual acuity, the opposite eye is covered, usually with an occluder that is held by the patient. It is conventional to start the examination with the right eye, although there are reasons to sometimes start with the left eye. If the right eye has particularly poor vision, you may want to start with the left eye so the patient can become oriented to the chart. The examiner should watch the patient at all times to make sure the opposite eye remains occluded. Most patients don't try to cheat. They usually don't realize that the other eye is peaking. If glasses are worn, watch to make sure that pressure applied to the glasses with the occluder does not misalign the glasses. If the acuity chart is an actual chart that hangs on a wall, how does the lighting need to be? then the room light will need to be on for testing. Your procedure in this regard will be up to your doctor. It is best to be consistent. If the acuity chart is projected, how does the lightning need to be? then the room light may be off, on, or dimmed somewhere in between. Your procedure in this regard will be up to your doctor. It is best to be consistent. Some eyes will see better with the light how? what is an example? dimmed. A common example is an eye with a significant cataract. does the level of light within the room affect contrast? The level of light within the room will affect contrast, which does have an effect on visual acuity does the light level affect pupil size? The light level will also affect pupil size, which can also have an effect on visual acuity. The visual acuity projector will have controls that allow you to do what? isolate a line of letters or even a single letter at a time how should letters be presented in VA testing and why? Letters should not be isolated for acuity testing, they should be presented as a whole chart or as a single line. This is particularly important when testing children. An eye with amblyopia will have better vision when tested with single letters. With a line of letters, the amblyopic eye experiences the so called "crowding phenomenon", which decreases visual acuity. If you have an idea of what the end result of VA will be, what can you do? you can skip lines on the chart and start testing near your acuity estimate. For example, when checking a young person with no visual complaints, you can probably start with the 20/25 or 20/30 line. What should you have the patient do, even if they protest that the letters are blurry. Encourage the patient to guess Have the patient continue reading lines and letters until when? when more than half of the letters are named incorrectly What is the final visual acuity? the smallest line of letters for which more than half of the letter are named correctly what is it good practice to note in VA testing? It is good practice to note how many letters are read correctly. For example: 20/30 +2 means that all of letter on the 20/30 line were read correctly and 2 letters on the 20/25 line were read correctly 20/60 -1 means that all the letter on the 20/60 line were read correctly, except for one. Near vision is checked with what? a near vision card. Pictured is a Rosenbaum pocket vision screener, which is a commonly used near card. How should near vision be conducted with the near vision card? Normally the card is held by the patient at a distance of 14 to16 inches. The patient is normally checked with whatever he usually uses for reading (bifocals, readers, or no correction). A reading light should be used, angled in a manner that will not cause glare on the card. The opposite eye is occluded, either by the patient or by the examiner. With the Rosenbaum card, have the patient read the numbers on the right side of the card. Just as with the Snellen chart, the idea is to read the smallest line possible. how should near vision card reading be recorded? The result can be recorded as "distance equivalent" (20/25, etc.), or by Jaeger notation. Jaeger 1 is equivalent to 20/25, Jaeger 5 is equivalent to 20/60, and so forth. Other less common notations are "14/14", which is similar to 20/20, only in inches, and "N5" which a point size notation. What is the "E game", or the "tumbling E" vision test? designed for those who don't know the English alphabet. There are also charts with all numbers. This test is especially useful for children who do not yet know the alphabet. The patient indicates which way the legs of the "E" point. Children can be asked to hold up three fingers and to point with the fingers which way the "E" points. what is the "broken ring" chart? It is similar to the E game. The patient indicates where the break in the ring is (up, down, left, right). How are preschool aged children VA tested? This is a pre-school vision test. There are also "Allen cards" that have one picture on each card, to make it easier for the child to be attentive to one picture. The child says the name of the picture. This test is for the child who cannot understand how to play the E game. Allen picture cards are held close to the child until he/she becomes familiar with them. Then they are held farther and farther away until the images cannot be consistently recognized. The size of the pictures is equivalent to 20/30 vision (at a distance of 20 feet). Therefore, if the child can recognize the pictures at a maximum distance of 10 feet, then the vision is recorded as 10/30. This can be converted to 20/20 equivalent be multiplying by 2, to get 20/60. Be sure to note that Allen cards were used. Research has shown that the best visual acuity of a baby in the first month of life is what? about 20/120 at four months of age what is the VA? about 20/60 at eight months of age what is the VA? it has developed to about 20/30 How do you generally assess VA in infants? A gross estimation of visual performance can be made by observing the fixation and following behavior of the infant eyes. An eye with normal visual development should be able to fixate on an object, maintain fixation on the object, and follow the object as it moves in space. Binocular testing is done while looking for signs of strabismus, and the fellow eye is occluded for monocular testing. A note is made in the record regarding the ability to "fix and follow". is a more sophisticated infant visual acuity examination possible? if so with what and how is it done? Teller acuity cardshave alternating black and white bars that attract the attention of the infant, causing the seeing eye to initiate and maintain fixation. The bars vary from widely spaced (gross acuity) to more narrowly spaced (better acuity). The background is gray. If the eye cannot differentiate the bars, then the pattern fades into the background and fixation is not stimulated. The pattern is on only one end of the card, so that the pattern can be presented from either side by rotating the card. Each card is presented, starting with the larger pattern, and the infant eyes are observed for fixation. The card with the smallest pattern that holds fixation is a measure of the visual acuity. This type of testing is usually only done for research as testing can take from 10 minutes to more than a half hour. What is the optokinetic drum? a gross test of visual function that can be used with infants. The drum has alternating black and white stripes. The drum is slowly rotated in front of the infant. The eyes of an infant with a functioning visual system will follow the stripes with a jerky nystagmus pattern. how do you assess vision using Count fingers (CF)? Hold up fingers, starting at 4 feet away, and move your hand closer until the number of fingers being held up can be identified. For example, if the eye can see to count fingers at 2 feet, then the visual acuity is recorded as "CF @ 2 ft.". how do you assess vision using Hand motion (HM)? If the eye cannot count fingers, then wave your hand in front of the eye. If the eye can see hand motion, then the visual acuity is recorded as "HM". how do you assess vision with Light perception (LP)? If the eye cannot see hand motion, then use a penlight, or muscle light and shine it into the pupil. If the light can be seen, then determine if the eye can tell what direction the light is coming from (left, right, up, down). If the eye can tell what direction the light is coming from, then record visual acuity as "LP with projection". If the eye can see light, but cannot tell which direction it is coming from, then record visual acuity as "LP without projection". how do you assess vision with No light perception (NLP)? Light perception is a very important measurement. If the eye cannot see light, then there is very little, or no hope for improvement. You can be sure of no light perception if you use a very bright light, such as the light from an indirect ophthalmoscope. If no light can be see, then record visual acuity as "NLP". Many retina specialists, who deal with low vision patients often, prefer to obtain the best Snellen visual acuity in low vision situations by doing what? bringing the Snellen chart closer than 20 feet. What kind of chart will you need for testing the VA of the low vision patient? If you have a chart hanging on the wall, you can simply take the chart off the wall and hold it closer to the patient, or ask the patient to move closer to the chart. If you have a projected Snellen chart, then you will need a handheld chart. You can buy a cardboard Snellen chart, or you can buy a Fonda Low Vision Chart, which is very similar. Once you have a chart, you can then run some copies on your copier, and then you will have charts for each exam room. A standard Snellen chart is about 11 inches wide and 22 inches long, so you will need to copy the chart in 2 sections since your paper is 8.5 x 11 inches. You will not need to copy the smallest letters, because if the patient can see those then he can see the big E on the wall chart. What is the procedure for testing the VA of the low vision patient? Procedure: The room light will need to be as bright as possible. Start by holding the chart 5 feet away. The fellow eye is occluded. Just as with a regular Snellen acuity test, have the patient read the smallest line possible. If no letters an be seen at 5 feet, then hold the chart at 2 feet. If nothing can be seen at 2 feet, then hold the chart at 1 foot. There is nothing magic about the distance. Meters can be used also, for example, a one meter distance and then a half meter distance. Once you have the distance and the smallest line read, you will then need to calculate/record the visual acuity. There are two options, depending upon the preference of your doctor. What is the first option? The first option is to simply record the VA in regular notation. For example, the patient can read the 20/80 line at a distance of 5 feet away. The visual acuity can be recorded as 5/80. The problem with this method is that there is no standard distance. The patient may see 5/80 on one visit and 3/60 on the next visit. It is not immediately obvious which is the better visual acuity. Once you have the distance and the smallest line read, you will then need to calculate/record the visual acuity. There are two options, depending upon the preference of your doctor. What is the second option? The other option is to convert to 20/xx equivalent. This is accomplished by dividing the test distance into 20, and they multiplying the result by the line read. For example, the smallest line our patient can read is the 20/80 line at a distance of 5 feet away. As discussed, the regular notation would be 5/80. Divide 5 into 20 to get a result of 4. Now multiply 80 by 4 to get 320. The 20/xx equivalent of 5/80 is 20/320. To avoid having to do calculations all the time, decide on 2 standard distances to use. A good choice is 5 feet and 2 feet. You can then record the standard results on your charts, to be read and recorded after testing. Here are the calculations below. For example, next to the 20/80 line on your chart you would mark, in small figures, 20/320 and 20/800. If the 20/80 line is the smallest line read, you would record 20/320 if you are 5 feet away, and 20/800 if you are 2 feet away. how do countries that use the metric system notate their VA testing? use metric notation for visual acuity. The standard Snellen distance for the metric system is 6 meters. Therefore, "normal" visual acuity is 6/6. A visual acuity of 20/40 is equivalent to 6/12 in meters, because the distance is doubled. A visual acuity of 20/30 is equivalent to 6/9, because the distance is half again as much, and so forth. What is pinhole VA testing? a method of estimating corrected visual acuity. For example, if a patient has lost her glasses and you want to estimate the results you should get with a refraction, the pinhole will give you this estimate. What does pinhole VA testing tell you? Good pinhole acuity tells you that blurry vision is due to a refractive problem. Poor pinhole acuity does not necessarily tell you that blurry vision is not due to a refractive problem. Poor pinhole acuity may be due to the patient not executing the measurement properly. How does pinhole VA testing work? The pinhole works by eliminating all rays of light going to the macula except for the central rays, which are not refracted by the optics of the eye. The concept is illustrated below. The image on the left is a schematic of a nearsighted eye. Most of the light rays going into the eye focus in front of the retina and then diverge to a blur circle on the retina (blue lines). However, the central rays (red line) shoot straight through and focus on the fovea. The image on the right illustrates the pinhole occluder blocking all but the central rays of light. Thus, visual acuity is improved. how is pinhole VA notated? Pinhole acuity is recorded in a manner similar to below: V CC OD 20/200 PH= 20/30 can you pinhole with correction? The pinhole occluder can be useful for testing acuity with or without glasses correction, or with contact lenses. what is pinhole acuity only used to test? usually only used to test distance vision, although it is possible to test near vision. A eye may have more than one vision affecting condition, such as a cataract and macular degeneration. If the eye were to have cataract surgery, how much vision could be gained? Is it even worth having cataract surgery at this time? what was designed to answer these questions? These are questions that the Potential Acuity Meter (PAM) was designed to answer. how does the potential acuity meter work? The PAM attaches to the slit lamp microscope. It uses a very small beam of light to project a visual acuity chart onto the retina. The idea is that the examiner can move the beam around in the dilated pupil and find an area through which the beam can pass through the cataract with minimal interference. The patient is simply asked to read the letters on the chart image. If the smaller letters can be read, it is a good indication that vision would improve significantly with cataract surgery. how does the PAM fit onto the Haag Streit slit lamp? with a post that fits into the central shaft. The cover plate has to be removed first. how do you work the potential acuity meter? The PAM is turned on by a switch on the control panel. The PAM test is best performed in the dark, so there is a "face illum." dial that provides some light to guide the placement of the beam, which is very narrow. The focus of the projected chart can be tweaked by using the diopter knob. The operator cannot see the chart, so the patient must give you feedback about the focus.