The Alternate Cover Test
Phoria vs Tropia

• The Alternate Cover Test can be used to quickly tell whether a patient is ortho or if the patient has a deviation. It is particularly useful in uncovering phorias because the technique does not allow the patient to establish binocular fixation. It is not always easy for the beginner to differentiate between a phoria and a tropia with the alternate cover test. The cover test can be used for confirmation.

• In this test, you watch the eye being uncovered to look for movement.

What is the difference between the Cover-Uncover Test and the Alternate Cover Test?

• The Cover-Uncover test allows the patient to establish binocular fixation if possible. Of course, if there is binocular fixation, then there is no tropia. The cover test is used to detect binocular fixation and rule out a tropia.

• The uncover part of the test is used to disrupt fusion (binocular fixation) and hopefully reveal a phoria. The problem is that once fusion is established a covered eye will not always immediately drift off to its resting position when fusion is disrupted. Sometimes you have to leave the cover in place for an extended period of time until the eye "relaxes" into the phoric position.

• This problem is solved by using the Alternate Cover (cross-cover) Test. Instead of allowing the eyes to be simultaneously uncovered for a period of time as with the cover-uncover test, in the alternate cover test the cover is moved from one eye to the other, preventing fusion. As a result, the eyes move to their "resting" positions relative to one another and the appearance of movement is accentuated during the test.

1. Either eye can be covered with an occluder  to start the test. Pause for a while with the cover in place over the eye to allow time for the eye to "drift".

2. Quickly move the cover to the other eye. Pause again to allow the uncovered eye time to pick up fixation
3. Quickly move the cover back to the other eye. Pause again to allow the uncovered eye time to pick up fixation.
4. Repeat the procedure in a swinging fashion,  observing the eye that is being uncovered.
5. Any movement of  the eyes to pick up fixation indicates that the patient has a muscle imbalance.  As discussed in the previous sections,  the direction of movement indicates the type of deviation (eso, exo, or hyper).

Final step:  After you have been able to observe any eye movement present,  the final step is to remove the cover, allowing binocular fixation to re-establish,  if possible.  This step helps you to tell the difference between a phoria and a tropia.  The following discussion only applies if you have observed eye movement during the alternate cover test.  If both eyes remained fixed on the target during the alternate cover test,  then the patient was orthophoric (does not demonstrate a deviation of eye alignment)

If the eye being uncovered moves to take up fixation and the fellow eye remains fixing, then you have been observing a phoria. If the eye being uncovered does not move to take up fixation (it remains in the deviated position), or if the fellow eye moves to a deviated position, then you have been observing a tropia. The topia is further identified by which eye it is that is deviating (right, left, or alternating). Confirmation can be made by performing the cover test.