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Eye and Vision Care

Cover Test



I`m 19 years old and I had a strabismus surgery at the age of 7. According to my doctors the surgery was pretty successful and the strabismus was gone. But it caused amblyopia to the affected eye which could not be entirely reversed. I feel that I see mainly with my other eye. Every year I have my routine eye examination because I also have myopia and I wear glasses. During this examination my doctor performs the cover-uncover test during which I notice that the amblyopic eye tends to move a bit in order to catch the item that seems to "move" when the other eye is covered. My doctor insists that strabismus is gone for good. Could the positive cover test be attributed to amblyopia?


Amblyopia can be caused by two conditions:

1) An eye turn (strabismus). If someone has an eye that turns in towards the nose constantly (esoptropia) or out towards the temple (exotropia) that patient will rely on the non-turned eye to do the fixating. Many times this occurs early in life, and the brain is very good at suppressing (does not pay attention) the image from the turned eye. This prevents double vision. Unfortunately since the brain is not interpreting that image, the visual cortex of the brain never fully develops, which may cause amblyopia. If that person did not patch the good eye, allowing for the turned eye to be used, amblyopia will occur.

2) The other condition occurs when the prescription is very different in one eye as compared to the other eye (anisometropic amblyopia). In this condition the patient relies on the eye that has the more in-focus image. This type of patient typically uses both eyes at the same time, but since the image in one eye is very out of focus - again the visual cortex of the brain does not develop properly. This condition can be fixed with the appropriate prescription and many times with patching.

-Important to note: For either condition listed above, if the patching does not occur before age 8, it may be unlikely if the amblyopia can be treated to 20/20 vision. It is also important to note that the eye is fine, but it is actually the brain that did not develop due to a lack of an in-focus image.

Surgery for strabismus: This is a procedure that is commonly performed to help align turned eyes. If you had a long standing eye turn, your binocular vision may not be fully functional, and most likely never will be. (Ask your doctor whether you have global or random dot stereopsis.) Typically, however, this is not a problem.

The way you are describing the cover / uncover test, I think you have a small mis-alignment of the eyes (successful surgery, however, is within 5-6 degrees of "zero" misalignment). This movement you experience could be the result of a small amount of residual strabismus. Another possibility is a phoria - which is a misalignment of the eyes, only when one eye is occluded, meaning that if both eyes are open, they both fixate the same target. When one eye is covered, the covered eye will deviate towards the nose or out towards the temple. This condition can also cause the image to move.  (Ask your doctor whether your eye alignment issue is a phoria or a tropia.)

Hopefully this answers your question.

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Response by:

Aaron  Zimmerman, OD, MS Aaron Zimmerman, OD, MS
Clinical Associate Professor of Optometry
College of Optometry
The Ohio State University