| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (CL) Class of 2022, Baylor College of Medicine, Houston, Texas |
| Transcript |
Today we're gonna be talking about the cover test, both the alternate cover test and the regular cover test. And as you know, what we're trying to differentiate is: are the two eyes aligned on the target? And so the fovea in the back of the eye is seeing the target, and if both eyes are on the target, when we do a cover test, if I cover one eye this fellow eye, in this case the left eye, won't move. And if I cover the left eye and the right eye is on the target, the left eye won't move. And what that tells us is when we do a cover test on either eye that we are straight, and our word for straight is ortho. So orthodontics - straight teeth; orthopedics - straight bones. But this is orthotropic. So if we are orthotropic, we are straight. And tropism means "move towards." So if the eye is turned in, we call that esotropia; if it's turned out we call it exotropia; and if it's turned up we call it hypertropia. And so when we do the cover test on an eye that's esotropic, so in this example esotropia, if I cover the fixing eye the non-fixing eye, the esotropia eye, has to turn out to take up fixation. So when we see esotropia, we do; a cover test, we can tell that the eye was turned in because it has to turn out. And likewise if the eye is exotropic and we cover the fixating eye, the eye that is uncovered now has to move in to take up fixation, and that means in exotropia that the eye is moving in, and likewise with; hypertropia the eye has to move down. So we can tell if someone has a tropia which is a manifest deviation (manifest means it's visible all the time). Obviously it assumes that the eye can move and that both eyes can see. So if an eye is blind it won't move to pick up fixation because it can't see. However, a manifest deviation is detected on the cover test, and that's what we were using to detect a tropia, whether it's eso-, exo-, or hypertropia. The alternate cover test breaks the fusion. So when we do alternate cover tests, underneath the cover one of the eyes, when it loses fixation, might drift. And so as opposed to the cover test, which is used to measure tropia, the alternate cover test breaks the fusion and can measure the entire deviation including phorias. So a phoria is not a manifest deviation. A manifest deviation would be called a tropia. A phoria is a tendency for the eye to drift, but what's keeping the eyes straight is fusion. When you disrupt that fusion with the alternate cover test, you can bring out a phoria. And if someone has a phoria that we brought out and they can't fuse the image, they might intermittently become tropia. So as opposed to esotropia, exotropia, and hypertropia, esophoria we just write as "E," exophoria just write as "X," and hyperphoria we just write as "H." And if they become intermittently tropic with the alternate cover test and now they have double vision and they're unable to fuse, and that we call an intermittent esotropia. And we signify that by putting these parentheses around the "T." And it might be intermittent exotropia or an intermittent hypertropia. So in summary, both the cover test and the alternate cover test are taking advantage of the fact that both of our foveas want to be maintained on the target. If we have a cover test on both eyes that shows no shift, then we have no manifest deviation, no tropia. If the cover test shows no shift we still have to do an alternate cover test to determine whether there is a; deviation that is being held in check by fusion. We break the fusion with the alternate cover test, and if they become tropic and diplopic during that portion of the test then they are intermittently tropic. If they have no deviation and it immediately snaps back into primary, that's a phoria. So patients with phoria don't have double; patients with manifest tropias have diplopia; and patients with intermittent tropias have intermittent diplopia. And you need to be able to tell the difference between a tropia, a phoria, and an intermittent tropia ; using the cover test and the alternate cover test. |
