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In general, healthy eyes are aligned and are able to work together to give you a clear and single vision. When the brain has trouble combining the two views, this can result in double vision or other visual disturbances.
Abnormalities of eye function that affect human performance are commonly associated with conditions such as traumatic brain injury, stroke, neurodegeneration, and demyelination.
On this episode of CITV, we discuss the single cover test, the cover/uncover test, and the alternate cover test and how to interpret your findings.
– On today’s episode of CITV, we’re gonna be discussing the cover-uncover test, as well as the alternate cover test. Stay tuned.
– In general, healthy eyes are aligned are able to work together to give you clear and single vision. Since our eyes are situated on either sides of our head, the image each eye sees is slightly different. But the brain combines these two images into one clear image. This is known as binocular vision. Now there are sometimes, many cases actually, where the eyes are not aligned with each other and the two eyes may see slightly different images. So when the brain has trouble combining the two views, this could result in double vision or other types of visual disturbances. Abnormalities of eye function that affect human performance are commonly associated with conditions such traumatic brain injury, stroke, neurodegeneration, and demyelination. Balance, stance, gait, coordination and, as well as neuromuscular integrity, are affected by, dependent on eye movements, so it is important that our treatment plans incorporate the phenomenology of eye movements such that each person will benefit from a unique and specifically designed therapeutic program. To create these unique programs, we must know what we’re seeing during examination and how to interpret those results, which is why having a firm grasp on examination procedures such as the cover-uncover test is important. Before we go into the testing, let’s talk about the difference between the tropia and the phoria. A tropia is when the eye is always deviated. This can be revealed either from cardinal directions of gaze if it is a large angle tropia or with the unilateral cover test. A phoria, on the other hand, is only sometimes deviated and can be seen when we break fusion or synchronization between the two eyes or when the patient is fatigued. We can see this by looking at the occluded eye during the cover-uncover test or when doing the alternate cover test and we break synchronization between the two eyes.
– So the three tests we’re going to go over today are the single cover test, the cover-uncover and the alternate cover test. Let’s go over the techniques for each examination procedure. First we’re going to do the single cover test. The single cover test is used to determine if there is a heterotropia, or a tropia, which is a manifest strabismus or misalignment of the eyes that is always present. We observe this by looking at the uncovered eye. If a tropia is present, the uncovered eye will have to move to correct itself and pick up fixation when the other eye is covered. As one eye is covered with the occluder, the examiner will watch carefully for any movement of the uncovered eye. Such movement will indicate a presence of a heterotropia. The first eye is covered for approximately one to two seconds. As this eye is covered, the uncovered eye is observed for any shifts and fixations.
– It’s gonna look like this. We’re gonna cover her left eye and I’m gonna observe her uncovered eye. So as I go in for two seconds. Again, we’re observing the uncovered eye. I’m gonna remove the occluder and again, looking for any changes in only the uncovered eye. Let it resynchronize. One to two seconds to break synchronization. Again, uncover for one to two seconds. If there is no shift in fixation it could mean two things. One, the patient is orthotropic and has no misalignment, but this must be confirmed by seeing the same thing when the opposite eye is covered. Or two, the uncovered eye is the preferred, or fixating eye, in the setting of a heterotropia. Next, the opposite eye is covered in a similar way for approximately one to two seconds making sure to wait a few seconds after covering the prior eye as not to suspend fusion and allow for a phoria to manifest. Any shifts of the unoccluded eye is then noted. Again, this test is done on both sides, both eyes.
– So there are a few possibilities that you can see with the single cover test. So if you cover up one eye and you see shift inward, that’s gonna indicate an exotropia. Now if the uncovered eye shifts outwards, or laterally, that’s gonna indicate an esotropia because it started inwards and when you covered it, it fixated by coming out. If the uncovered eye shifts downwards, it’s gonna indicate a hypertropia. If the uncovered eye shifts upwards, it indicates a hypotropia. Again, because the eye is starting down in a hypotropia position and when I cover it and force it to have fixation, it moves up to look at the target. Now let’s look at the cover-uncover test. Remember that the cover-uncover test is the most important test for detecting the presence of a strabismus and for differentiating a heterophoria from a heterotropia. The cover-uncover test is performed the same way as the single cover test, but attention is now turned to the eye that has been occluded as the occluder is pulled away. What we’re gonna do is simply use the occluder to cover one eye, observe the covered and uncovered eye, and then test the other side. If we observe movements of the uncovered eye, these movements are considered a heterotropia. If we observe movements of the covered eye, these movements are considered a heterophoria. We already observed the uncovered eye during our single cover test, so let’s now observe the movements of the covered eye. With a movement of the non-covered eye assumed to be absent, movement of the covered eye in one direction just after the cover is applied and then movement in opposite direction as the cover’s removed indicates that a heterophoria becomes manifest only when binocularity is interrupted. Think of it as when you put the occluder over the eye, you are giving the eye an opportunity to go into its relaxing position, so it deviates into its resting spot, but then when we uncover it and ask for it to kick back on binocular vision, it will move right back to the correct position. So in summary, if a patient has a heterophoria, the eye will be straight both before and after the cover-uncover test and the deviation appears due to the interruption of binocular vision. In a patient with a heterotropia, however, we’ll start out with a deviated eye and end up with either the same eye or opposite eye deviated after the test. Now if it’s the opposite eye deviated, we’re gonna call that an alternating heterotropia. Some patients can have straight eyes and start out with a heterophoria before testing, but with prolonged interruption of binocular vision during the testing, a manifest heterotropia can occur.
– So let’s see what this looks like in clinical practice. For this particular test, I’m gonna use a frosted occluder, a little different than the previous occluder. The reason we’re using a frosted one, is this allows me to see under the covered or occluded eye while still preventing the patient from foveating on a target. So we’re gonna see what their eye does when it loses a target that they’re looking off into the distance. So she’ll be looking straight ahead. I’m gonna cover her left eye and in this scenario, remember, we’re really focusing a little bit more on the covered eye. I’m seeking for any movements there. I’m gonna uncover, again looking at the covered and uncovered eye. I’m gonna switch over. Prevent it from foveating. Looking to see if there’s any movement of the covered eye. Gonna remove and I’m looking at, again, the covered and uncovered eye. Any movements of either eye is noted for this particular test.
– Lastly, let’s look at the alternate cover test. The alternate cover test is used to bring out the full deviation by bringing out any phorias present as well as any tropias observed on a single cover testing by breaking binocular vision. The test involves holding the occluder over one eye for several seconds to suspend binocular fusion then shifting the occluder to the other eye and rapidly alternating between eyes with the occluder, not allowing the patient to become binocular with their vision. The eye under the occluder is observed as the occluder is removed and is placed on the other eye in order to determine the direction of deviation.
– So let’s see what this looks like in clinical practice. Now remember, the key is that you’re gonna observe the eye that was just covered in your uncovering and you’re gonna quickly go back and forth between both eye. So it’d look like this. I’m gonna cover her left eye for a couple seconds. I’m gonna quickly move to her right eye, but I’m observing her left eye as I switch. Couple seconds there and then I switch, always observing the eye that I just uncovered for any movements.
– Now there are a few possibilities that you can see with this. Now if you see that there is an outward, or a lateral, refixation of the eye as you remove the occluder, that’s gonna be known as an esodeviation. If there is an inward refixation, that would be considered an exodeviation. Now, if there is a vertical refixation movement, it’s either gonna represent a hyper or a hypodeviation. Remember you can have both vertical and horizontal deviations present. Let’s look at some examples that we would see while doing the single cover test, the cover-uncover test, and the alternating cover test. The first example that we’re showing you is what we would see in a normal patient. We would cover one eye and then the patient would have no movements of their eyes or refixations. Again, this would be a normal response where the patient’s fixating on a target ahead. We cover their eye, and we see no movement of their eyes. The second example would be an example of what we would see on a single cover test with a bright exotropia. What’s gonna happen here is we’re gonna cover the left eye and we’re gonna see the right eye refixate or come medially to center, to pick up the vision. This would be an example of a right exotropia. Now, this would be an esotropia. Here the eye is coming from the center and it’s moving laterally as we cover one eye. We cover the eye and the eye moves laterally to pick up fixation. This would be a hypertropia, so the eye is starting upwards and then it’s moving downwards as we cover the eye. Then a hypotropia would be the last one. Okay, so now let’s look what happens during the cover-uncover test when we’re examining the covered eye, rather than examining the uncovered eye as we did in the single cover test. In this example, we’re gonna place our occluder over the eye and we’re gonna see that the eye moves medially when the cover is placed over the eye. This is an example of an esophoria. When we break binocular vision, the eye moves to its resting position, which is inwards. Remember, this a phoria and not a tropia. When you remove the cover, they’ll go back to getting fixation and will come back to the center. This is another example. We’re gonna cover the eye and we’re gonna see now that the eye deviates outwards when vision is broken. With this, we have an exophoria and then when we remove the occluder, it’s gonna go back to the center. Now let’s look at the alternate cover testing. So we’re just gonna do some examples to see what happens as we break binocular vision as we quickly go back and forth between the eyes. So we start with covering the left eye and then we switch over to the right and we see that the eye deviates laterally. This would be an example of an esodeviation of the left eye. Here’s another example. We have the left eye covered. We come over and we see the eye move medially. This is an example of an exodeviation. Another. This would be a hyperdeviation. Then one more, a hypodeviation. We hope you enjoyed this episode of CITV.
– If you have any particular question for us or any examination procedure that you’d like more information on, feel free to email us at email@example.com and we’ll try to tackle it for a future episode of CITV. Thank you and catch you next time.