The prerequisite for all cover tests is the presence of foveal fixation, which requires binocular fusion. In a tropia, fusion has been broken down due to muscle weakness or restriction, and this results in a manifest strabismus or misalignment of the eyes that is always present.
A phoria may be detected by looking at the occluded eye during the cover-uncover test, or when doing the alternate cover test when synchronization is broken between the two eyes.
Typically, the standard approach to detecting an abnormality of ocular motility would be:
- Do the cover test to detect any tropia that may be present. Typically the cover test and cover-uncover test are performed together.
- Do the alternate cover test to quantify any phoria or tropia that may be present.
Cover test: this is a test for tropia.
In the cover test, a tropia is observed in the uncovered eye, since it is the uncovered eye which potentially may move when the other eye is covered. A patient with a tropia starts out with a deviated eye and ends up (after the test) with either the same or the opposite eye deviated (if the opposite eye is the deviated one, the condition is termed alternating heterotropia).
In the cover test, the examiner pays attention to the uncovered eye.
(When the examiner shifts focus to the covered eye and watches what happens as the cover is removed from in front of that eye, that is the cover-uncover test. In that test if there is movement as the cover is removed, and no tropia was demonstrated by the cover test, that movement is a phoria.)
1. First, the patient is asked to fixate on a distant object, about 5–6 m away.
2. Then, the examiner covers one eye for one to two seconds, and looks for correction movements of the uncovered eye. (often called a corrective "flick" movement).
If there is no shift in fixation, this is because:
-The patient has no misalignment, however, this needs to be confirmed by seeing the same thing when the opposite eye is covered, OR
-The uncovered eye is the fixating or preferred eye in the setting of a tropia.
If the eye moves in order to take up fixation, the examiner must work out the nature of the tropia from the direction of the corrective movement:
If the uncovered eye moves:
(a) from the inside outward, an esotropia is present: the eye was too far in.
(b) from the outside inward, an exotropia is present: the eye was too far out.
(c) from above downward, a hypertropia is present" the eye was too high.
(d) from below upward, a hypotropia is present: the eye was too low.
3. The other eye is then examined, the examiner making sure to wait a few seconds before doing this, in order not to suspend fusion, since that would allow for a phoria to manifest.
For example (see Figure 1):
- Cover, say, the Left eye...
- Did the Right eye have to move from the inside to fix on the target ?
- YES....Then this is a right esotropia.
- Then uncover the Left eye, and continue to observe the Right eye.
- The Right eye will deviate again, as the Left eye now takes up fixation.
(vv)Naming Conventions.mp4(tt)
From: Root T. Ophthalmology Lecture - Tropias & Phorias (part 1/2) Retrieved from: https://www.youtube.com/watch?v=dRYBOBSyzAU
(vv)Cover Test.mkv(tt)
(vv)Smaller Tropia Cover Uncover.mp4(tt)
(This guide uses the Strabismus Simulator)
(vv)All3.mp4(tt)
From: Koklanis C. Cover Test. From: https://www.youtube.com/watch?v=UZts69B02sA