Your eyes need to work together as a team in order to see clearly. Part of any comprehensive eye examination will include a test to determine how well your eyes are functioning together. The simplest and most common way to check eye alignment and how your eyes work as a pair is with a cover test.
A cover test is an eye exam that determines if a patient has any ocular deviation. Essentially, the test is measuring eye alignment and trying to figure out whether one eye is working harder than the other is. During a cover test, one eye is covered while the other eye focuses on an object within the room. The test is repeated for both eyes at distances close-up and far away.
Cover tests allow eye doctors to observe how much each eye has to move after being uncovered in order to focus on an object. If eye alignment is outside of the normal limits, treatment may be necessary.
Cover tests can be used to detect eye turn, or strabismus; a condition where the eyes are not properly aligned. Eventually, strabismus can lead to amblyopia (lazy eye), as well as poor depth perception and problems with binocular vision.
Cover tests are part of all comprehensive eye examinations given to young children because they are at a higher risk of developing problems with eye alignment. Amblyopia is currently the leading cause of vision loss in children.
There are two types of cover tests performed during eye examinations: the unilateral cover test and the alternating cover test.
During the unilateral cover test, you are asked to focus on a distant object while the doctor covers each of your eyes in turn. If either of the uncovered eyes has to move to focus on the object, this may be evidence of strabismus.
The second part of the exam is the alternating cover test. You are asked to focus on an object while the eye cover is switched from one eye to the other. If the doctor detects eye movement after the eye cover is removed, this is an indication of phoria. A significant amount of phoria can lead to eyestrain and/or double vision.
If you are experiencing vision problems, be sure to schedule a comprehensive exam with your eye doctor.