Binocular vision is a remarkable faculty that provides precise depth perception, singularity of vision and compensation for what would otherwise be blind spots in the field of vision. Children with binocular vision disorders may not even be aware of their vision problems. Fortunately there are simple tests than can be performed in the doctor's office or on location at school or at home to determine whether both eyes are aligned normally and are synchronised by tandem movement. One of the most common of these tests is the Stereo Fly Test.
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The Stereo Fly Test is the most common of the Stereo Tests. These types of tests measure stereopsis, which is the highest level of binocular vision. It is important to catch binocular vision problems early in a child's visual development because many of the causes of poor stereopsis, such as amblyopia, can be treated if recognised early enough.
The Stereo Fly Test usually comprises rows of animals which younger children can readily identify. The animals are divided into three rows, designated A, B and C, with the A row being the easiest for children to differentiate. The test will also include graded circles for testing older children. These circles are divided into nine groups with group 1 being the easiest for older children to distinguish. The test also commonly features a large fly or a butterfly as a test object.
The test administrator will hold the card 16 inches away from the test subject. This is considered the average distance for reading. Lighting will be provided over the test subject's shoulder. The test administrator will hold the card perpendicular to the test subject's visual axis.
The Stereo Fly Test for binocular vision typically requires the test subject to wear special polarised glasses and look at the images printed on the test card. The patient is asked to identify certain objects on the card while the card is positioned at different angles. Young children who may not be able to communicate verbally may be asked to point to specific objects, such as a butterfly's wings. Older children will be asked to verbally identify which object stands out.
The Stereo Fly Test can help to evaluate gross stereopsis and fine depth perception. Grading is measured in seconds of arc, which is a unit of angular measurement (one minute of arc is equal to 1/60 degree of a circle). By discovering at which second of arc the test subject is able to distinguish stereopsis, the vision-care specialist can determine whether the test subject's binocular vision is normal or, if it is not, what level of correction is needed.
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