The Cover Test
Phoria vs Tropia
The cover test is a simple procedure, using only the occluder, that is used to detect the presence of a a fixed eye muscle imbalance known as a tropia.
While the fellow eye fixes on a visual target, the tropic eye deviates either toward the nose (esotropia), temporally (exotropia), superiorly (hypertropia), inferiorly (hypotropia), or a combination of a horizontal and a vertical deviation.
An alternating tropia occurs when a patient always has a deviating eye, but can and does fixate with either eye. For example, the patient may be fixing with the right eye and the left eye may be deviating outward (XT). A moment later the patient may switch fixation to the left eye and the right eye will move to an outward deviation.
It is possible that an obviously tropic eye will not move to take up fixation. This typically occurs when the vision is poor in the deviating eye. The eye cannot see the fixation target, so it doesn't pick up fixation. In these cases the tropia is usually so obvious that a cover test is not needed.
When a patient is using binocular vision, fusion
When performing the cover test, our job as technicians is to answer the question: Is there a tropia present? Yes or no. If the answer is yes, we should identify (name) the tropia. The Uncover Test and the Alternate Cover Test will be used to detect a phoria.
Have the patient view a distant target (the test is also performed at near).
While you observe the right eye, use the occluder to cover the left eye. One of two things will happen:
If the right eye does not move and remains fixed on the visual target, then the right eye is not tropic.
If the right eye is observed to move in order to take up fixation, then the eye is tropic and the tropia is identified according to the direction the eye moved from, and which eye it is that moved. A shift is evidence that the uncovered eye was not regarding the target with its fovea while both eyes were viewing
Remove the occluder from the left eye and allow the patient's gaze to stabilize on the target.
Now cover the right eye with the occluder while you observe what happens to the left eye. As discussed above, the left eye will either remain straight, or it will move from a deviated position to take up fixation.
In practice the cover and uncover tests are performed together. ie. cover left eye, watch right eye for tropia. Uncover left eye, watch left eye for phoria. Rpt in other eye.