Cover Tests
Foveal fixation in each eye, patient attention and cooperation, and the ability to make eye movements are all necessary for cover testing. If the patient is unable to maintain constant fixation on an accommodative target, cover tests should not be used. There are 3 main types of cover tests: cover-uncover, alternate cover, and simultaneous prism and cover. All can be performed at distance or near fixation.
The monocular cover-uncover test is the most important test for detecting manifest strabismus and for distinguishing a heterophoria from a heterotropia (Fig 7-1; Video 7-1). As the target is viewed, one eye is covered and the opposite eye observed for any movement, which would indicate a heterotropia. The occluder is then removed. If there is no movement of the noncovered eye when the occluder is introduced, movement of the covered eye in one direction with application of the occluder and then in the opposite direction (a fusional movement) with removal of the occluder would indicate a heterophoria. If the patient has a heterophoria, the eyes will be straight before and after the cover-uncover test; the deviation appears during the test because of interruption of binocular vision. A patient with a heterotropia, however, starts with a deviated eye and, after testing, ends with the same eye or—in the case of an alternating heterotropia—the opposite eye deviated. In patients with intermittent heterotropia, the eyes may be straight before testing but become dissociated after occlusion.
VIDEO 7-1 The cover-uncover test.
Animation developed by Steven M. Archer, MD, and Kristina Tarczy-Hornoch, MD, DPhil.
Access all Section 6 videos at www.aao.org/bcscvideo_section06.
The alternate cover test (Fig 7-2A; Video 7-2) detects both latent (heterophoria) and manifest (heterotropia) deviations. As the patient views the target, the examiner moves the occluder from one eye to the other, observing the direction of movement of each eye when it is uncovered. Because this test disrupts binocular fusion, dissociating the eyes, it does not distinguish between latent and manifest components. Testing should be performed at both distance and near fixation.
VIDEO 7-2 The alternate cover test.
Animation developed by Steven M. Archer, MD, and Kristina Tarczy-Hornoch, MD, DPhil.
In the prism alternate cover test, prisms of varying amount are held over one eye or both eyes during alternate cover testing; the amount of prism that neutralizes the deviation, such that eye movement is no longer seen as the occluder is moved from one eye to the other, represents the magnitude of the deviation (Fig 7-2B; Video 7-3). It may be necessary to use both horizontal and vertical prisms. This test measures the total deviation (heterotropia plus heterophoria).
VIDEO 7-3 The prism alternate cover test.
Animation developed by Steven M. Archer, MD, and Kristina Tarczy-Hornoch,
MD, DPhil.
Two horizontal or 2 vertical prisms should not be stacked; such stacking can induce significant measurement errors. Deviations larger than the largest-available single prism are best measured by placing 1 prism in front of each eye, although this is not perfectly additive either. A horizontal prism and a vertical prism may be stacked over the same eye, however. Plastic prisms should always be held with the back surface (closest to the patient) in the patient’s frontal plane. If the head is tilted, the prisms must be tilted accordingly. With incomitant (paretic or restrictive) strabismus, the primary and secondary deviations are measured by holding the prism over the paretic or restricted eye and the sound eye, respectively.
The simultaneous prism and cover test (Video 7-4) measures the manifest deviation during binocular viewing (only the heterotropia). The test is performed by placing a prism in front of the deviating eye and covering the fixating eye at the same time. The test is repeated using increasing prism powers until the deviated eye no longer shifts. This test has special application in monofixation syndrome. Under binocular conditions, patients with this syndrome often use peripheral fusion to exert some control over their deviation. The heterotropia alone is smaller than the total deviation (heterotropia plus heterophoria) measured by the prism alternate cover test. The simultaneous prism and cover test provides the best indication of the size of the deviation under real-life conditions.
VIDEO 7-4 The simultaneous prism and cover test.
Animation developed by Steven M. Archer, MD, and Kristina Tarczy-Hornoch, MD, DPhil.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.