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Module 28 |
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Module 28: |
Binocular Vision |
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Section 2: |
Stereo Tests and the Four Diopter Base-Out Prism Test |
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Stereo Tests Stereo tests are used to measure the highest level of binocular vision, stereopsis. These tests are used most often in the vision assessment of children. If it can be demonstrated early in the evaluation process that a child has good stereopsis, then it can be concluded that the child fuses, has good binocular vision, and has straight eyes (at least part of the time). These conclusions can be made because stereopsis is not possible without them. If a child has poor stereopsis or no stereopsis, it is important to find out why. A lack of stereopsis may be due to a condition (such as ambylopia) that is treatable if the child is still in the developmental stage of visual function (generally under age 7). Commonly used stereo tests have rows of animals and graded circles for testing the level of stereopsis. They usually include a large test object such as a fly or a butterfly.
Young children may only respond to the large object and/or the rows of animals. Older children can skip the animals and go directly to the circles, but many like the entertainment value of the fly. Your goal is to determine the level of stereopsis. The large fly or butterfly is the easiest to stereo. The "A" row animals are the easier ones to stereo; the "C’ row the hardest. Of the circles, group 1 is the easiest to stereo; group 9 hardest. Stereopsis is measured in minutes, or seconds, of arc. This has to do with the angle of separation created by the distance between the eyes and the distance from the eyes to the subject. The smaller the angle is, the more difficult it is to perceive stereopsis, and the better the patient’s ability is. That is, 80 seconds is better than 140 seconds. Results should be recorded in seconds of arc if possible. Alternatively, if one type of test is used exclusively, you can record the results as the number correct, e.g. 6 of 9 circles. Most of these tests are standardized for 16 inches, normal reading distance. The test card is held perpendicular to the visual axis and over the shoulder lighting is preferable. These tests use polarized glasses. If the patient wears glasses, have her place the polarized glasses over her correction. If the patient has bifocals, make sure the patient is looking through the bifocal. Do not rush the patient, the stereo effect is not always readily apparent. Younger children can be asked to "touch (or pick) the wings of the fly". Older children are simply asked to identify the animal or circle that stands out from the rest. Some of the more common tests are listed below with the catalog blurb from stereooptical .com.
Stereo Fly Test: Recognized as the standard for Stereo Depth
Perception testing. This test evaluates both gross and fine depth
perception utilizing the famous fly image. Stereopsis from 3500 to 40
sec of ARC is measured.
Stereo Randot Test: Tests the ability to identify six geometric forms
from random dot backgrounds. Figures cannot be identified without
glasses to discourage guessing. 500 to 20 sec of ARC
Stereo Random Dot E: The only, easy-to-administer preferential
looking stereopsis test at 480 and 120 Seconds of ARC. Test designed
for 6 months to 5 years of age or any nonverbal patient.
Asked to identify the card with the raised E.
I know some of you have lost that little card that tells what the seconds of arc are for each test, so I have listed them for one of the most common tests, the stereo fly test.
Stereo butterfly or fly top of upper wings: 2000 seconds bottom of lower wings: 1150 seconds tip of abdomen: 700 seconds Animals A row: 400 seconds B row: 200 seconds C row: 100 seconds
Circles 1. 800 seconds 2. 400 seconds 3. 200 seconds 4. 140 seconds 5. 100 seconds 6. 80 seconds 7. 60 seconds 8. 50 seconds 9. 40 seconds
The Four Diopter Base-Out Prism Test This test is useful for detecting fusion or suppression in what appear to be "straight" eyes. It is an objective test that can be used on a cooperative young child who may not understand the stereo acuity test. It is also useful for the patient you may suspect is "faking" a negative stereo test. The test is based on the observation that a prism put in front of either eye of a fusing pair will cause an eye movement. The prism causes the image to move away from the fovea. The eye must move to regain foveal fixation. As the name implies, a four diopter base-out prism is placed over the right eye first, as the patient fixes on a distant target. A normal response is a small convergence movement by each eye. If the prism is placed over a suppressing eye, that eye will not move. A fusing eye will move toward the nose. |
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