The cover-uncover test describes that when the cover is removed from in front of the eye, movement of the eye may occur.
Each eye is covered and then uncovered in turn, and the examiner should be able to detect the deviated eye by observing refixation immediately in the just uncovered eye.
This movement indicates either a tropia or a phoria. Therefore, the cover test must done first to see if a tropia is present; if there is no tropia present, then movement of the eye when the cover is removed indicates a phoria. With a phoria, the eyes are straight, but due to interruption of binocular vision, a deviation of the eye may occur. A phoria is a latent strabismus or misalignment that is only present when binocular fusion is suspended or interrupted.
Subtle phorias may be detected by the alternate cover test.
Although the duration of the cover varies (from 2 to 10 seconds), it is likely that a duration of 5 seconds is adequate1. (Use of a translucent occluder, which is opaque to the patient but transparent to the examiner, allows movements of the eye under cover to be observed.)
The test is done in the same way as the cover test, except that attention is now directed to the eye that has been occluded by the cover, just as the cover is pulled away.
For example:
- Both eyes open, and fixating on the distant target. The eyes will look straight.
- Now cover the Left eye ..while Right eye fixates
- After about 5-10 seconds....uncover the Left eye...
- Observe the Left eye on removing the cover: if it moves to take up fixation as you uncover it, BUT the Right eye DOES NOT move..and both eyes are now straight, that is a phoria.
(vv)CoverTestEsoExo.mp4(tt)
(vv)AcuteESO.mp4(tt)
From: Trobe, J https://collections.lib.utah.edu/ark:/87278/s6wf01vp