Reading Between the Lines (Visual Acuities) Learning Objectives 8/25/2010

Transcription

1 Reading Between the Lines (Visual Acuities) Debbie L. Hettler, OD MPH FAAO Clinical Director, Associated Health Education Office of Academic Affiliations (14) VA Central Office 810 Vermont Ave NW Washington, DC Phone 202/ Fax 202/ Learning Objectives Attendees will understand the concept of performing visual acuities. Attendees will have beginning competency at performing basic acuity measurements of an easy patient. Attendees will be able understand the concept of acuities in special populations. 2 Here is the test to find whether your mission on earth is finished: If you're alive, it isn't. Richard Bach, writer (b. 1936) 3 1

2 Vitals for the Eye Baseline Vitals for the Eye Legally necessary first test Similar to BP, Pulse, HR for Primary Care 4 Visual Acuities OD-Right OS-Left With and Without correction Distance Near Pinhole Low Vision 5 DEFINITION Acuities measures the ability of each eye to see a defined target at a specific distance. Acuity is expressed in terms of the Snellen Fraction. The greater the distance from an object to the eye, the smaller the visual angle subtended by that object at object at that eye. The denominator is the letter or number size (ie the distance at which the smallest letter read subtends angle of 5 seconds. Visual acuity tests measure the patient's ability to see fine visual detail. The Snellen chart consists of optotypes, specially formed letters of the alphabet arranged in rows of decreasing letter size. The sizes of the letters are standardized so that the letters in each row should be clearly legible at a designated distance to a person with normal vision. 6 2

3 Abbreviations The abbreviation cc signifies with correction (eye glasses or contact lenses) sc signifies without correction. Oculus Dexter is the latin for right eye. Oculus Sinister is the latin for left eye. Oculi Uterque or uniter is latin for both eyes. 7 Why is 20 Feet used for Distance? The distance of 20 feet is used because it approximates optical infinity. Light rays coming from this distance and beyond are considered to be parallel, so that the emmetropic eye need not accommodate to focus them on the retina. If a shorter distance were used, a myopic eye might test at normal acuity. Sometimes acuity is expressed in metric terms 20 feet equals about 6 meters so an acuity of 20/40 would be expressed as 6/12. An acuity is not a fraction but merely a comparison of the distance from which a person with normal visual acuity can read the same characters. 8 Examples The line 20/20 defines the line read by average normal people at 20 feet from the chart. 6/6 is the metric equivalent. The line read by average normal people at 400 feet is 20/400. The numerator (or top number) is the testing distance. The larger the denominator, the poorer the vision in the measured eye. 9 3

4 10 Light Intensity-Contrast Visual resolving power increases with light intensity (within limits. The level of Test screen illumination should be between 20 and 80 footcandles. Higher contrast is needed with advancing age or lower levels of luminance. 11 Snellen Equivalents 20/?? For legal purposes acuities should be converted to Snellen equivalents. Record the actual measurement and the equivalent acuity in parentheses.i.e. e 10/60 (20/120). 12 4

5 Low Vision Acuities Counting fingers (CF) Hand Movement (HM) Light perception (LP) No light perception (NLP) 13 PROCEDURE Acuity is taken with glasses. Acuity is taken without glasses Acuity is taken at distance. Acuty is measured at near. Acuity is measured one eye at a time Acuity is measured with both eyes open. Acuity is measured monocularly and binocularly. 14 The patient faces the chart at a distance of 20 feet, covers the left eye, and the right eye open to read the chart. The same procedure is repeated, with the right eye occluded. The patient reads successively smaller lines. 15 5

6 Patients are placed at a specified distance, generally 20 feet, from the charts and are asked to read aloud the line of smallest letters they can discern. Encourage guessing. 16 The right eye should be tested first because less confusion results in recording information about the two eyes if a sequence is followed habitually. 17 Make sure the eye being occluded is completely covered and that the occluder is not touching the eye. 18 6

7 Observe the patient during the test to be sure the patient is not peeking around the occluder. 19 Ask the patient to read lines until the patient misses more than half the letters on one of the lines. 20 Are we still here.? 21 7

8 Tumbling E chart ask the patient to indicate the symbols visible on the smallest line by stating the direction or pointing the fingers in the direction of the three spokes of the E point-left, right, up or down. Ask the patient to begin reading the line with the smallest letters legible. Ask the patient to read the letters from left to right on every other line down the chart until the patient misses more than half the letters on one of the lines. 22 Flash cards and symbol books 23 LOW VISION ACUITY Alternative methads of measuring acuities include, Number of fingers being held up in front of him, 5' Light projection-where the penlight is located and when it is on Light Perception i.e. the light is on or off 24 8

9 Feinbloom Low Vision Chart The tester starts with the biggest number at the furthest distance in feet that a patient can see. By turning the pages until the patient can no longer distinquish the numbers. Visual acuity can be defined. This test is generally done at distances of 10 feet or less, and maybe recorded at 10/700 ie 700 at 10' or 5/250 at 5'. A patient who can see your fingers can respond to a chart acuity at the same distance. Tell the patient to to move the eye around" to see the figures on the chart better. Typically testing is performed at 10 feet, unless the acuities are below 20/400 then closer distances of 5 or 2 to 2 1/2 feet will provide better results. 25 Low Vision Acuity Flip Chart 26 Procedure With the Distance Acuity ETDRS Chart The rear illuminated Ferris-Bailey ETDRS visual acuity charts are designed to be used at 4, 2, or 1 M. They are available in three randomly arranged letter charts, a number, HOTV, and Landolt C configurations. Visual acuity can be noted in the metric, Snellen, or LogMar systems with this chart. Visual acuity is taken without correction at 4 M in the right eye with the left eye occluded and repeated for the left eye with the right eye occluded. Binocular visual acuity is also taken. The chart should be moved in to 2 M or even I M when the top line cannot be seen. Correction is to be used in cases with significant refractive error such as high myopia or aphakia. The examiner also should instruct the patient in viewing eccentrically during the taking of visual acuity. This procedure introduces the patient to the techniques of eccentric viewing while also demonstrating that the vision may improve if the fixation changes from a central to eccentric area. Peeking, unintentional though it may be, frequently results in misleading visual acuity measurements. The examiner should therefore closely watch the patient's eyes even when the eye is covered with a paddle occluder or clip-on occluder (e.g., Halberg, Bernell). Recording of visual acuity is noted as test distance over letter size in meters. Visual acuity, for example, is noted as 4/20-2, 2/32-1, or 1/40. To find the 20-foot Snellen equivalent, multiply the numrator and denominator by 5,10,or 20 for the 4-,2-, and 1-meter test distances. The Designs for Vision number chart is still of value in estimating visual acuity less than 1/40 (20/800). When acuity falls below that which can be recorded on standard eye charts, the examiner should test for light projection, light perception, or no light perception. The upper range for successful visual rehabilitation with low vision optical devices is 1/40. Cogenital or determined low vision patients, however, often succeed with low vision aids when the visual acuity falls below this range. 27 9

10 NEAR ACUITY Near examinations, care must be taken to provide comfortable reading light on the test material, usually in a ratio about three fold greater than the room light. Near acuity notations should be based on the Snellen principle of a 1 minute arc of minimal resolution 28 Near point Card 29 PINHOLE A pinhole is a small device that allows the physician to see if refractive error is he cause of the patient's decreased vision. The patient is asked to look through this device and ascertain if his or her visual acuity improves. If so, the patient needs a change in his or her eyeglass prescription. If no improvement can be obtained the patient may have another problem. For pinhole acuities, the patient can look through any of the pinholes on a multihole paddle

11 True or False? If a patient has to guess then the acuity is not valid. It is OK if a patient squints to read the chart. A patient with bad vision is cheating if he uses his side vision. 31 Thanks for my time in Missouri! 32 11