Eye movement capability, image formation and perception, foveal fixation in each eye, attention, and cooperation are all necessities for cover testing. If a patient is unable to maintain constant fixation on an accommodative target, the results of cover testing may not be valid, and this battery of tests therefore should not be used.
There are 3 types of cover tests: the cover-uncover test, the alternate cover test, and the simultaneous prism and cover test. All can be performed with fixation at distance or near.
The monocular cover-uncover test is the most important test for detecting the presence of manifest strabismus and for differentiating a heterophoria from a heterotropia (Fig 6-1). As 1 eye is covered, the examiner watches carefully for any movement in the opposite, noncovered eye; such movement indicates the presence of a heterotropia. With movement of the noncovered eye assumed to be absent, movement of the covered eye in one direction just after the cover is applied and a movement in the opposite direction (a fusional movement) as the cover is removed indicate a heterophoria that becomes manifest only when binocularity is interrupted. If the patient has a heterophoria, the eyes will be straight before and after the cover-uncover test; the deviation that appears during the test is a result of interruption of binocular vision. A patient with a heterotropia, however, starts out with a deviated eye and ends up (after the test) with either the same or the opposite eye deviated (if the opposite eye is the deviated one, the condition is termed alternating heterotropia). Some patients may have straight eyes and start out with a heterophoria prior to the cover-uncover test; however, after prolonged testing—and therefore prolonged interruption of binocular vision—dissociation into a manifest heterotropia can occur.
The alternate cover test (prism and cover test) measures the total deviation, regardless of whether it is latent or manifest (Fig 6-2). This test does not specify how much of each type of deviation is present (ie, it does not separate the heterophoria from the heterotropia). This test should be done at both distance and near fixation. Once dissociation is achieved, the amount of deviation is measured using prisms to eliminate the eye movement as the cover is alternately switched from eye to eye. It may be necessary to use both horizontally and vertically placed prisms. The amount of prism power required is the measure of deviation. Two horizontal or 2 vertical prisms should not be superimposed on each other because this can induce significant measurement errors. Their values cannot be directly added. A more accurate method for measuring large deviations is to place prisms in front of each eye, although it should be noted that these are not perfectly additive either. However, it is acceptable to superimpose a horizontal prism on a vertical prism before the same eye if necessary.
Whereas the alternate cover test measures the total deviation (heterophoria and heterotropia), the simultaneous prism and cover test is helpful in determining the actual heterotropia when both eyes are uncovered (heterotropia alone). The test is performed by covering the fixating eye at the same time the prism is placed in front of the deviating eye. The test is repeated using increasing prism powers until the deviated eye no longer shifts. This test has special application in monofixation syndrome, which may include a small-angle heterotropia. Patients with this condition may reduce the amount of deviation measured in the alternate cover test by exerting at least partial control over a coexisting heterophoria through peripheral fusion when both eyes are open. In this instance, the simultaneous prism and cover test measures the amount of heterotropia in a deviation that has a superimposed heterophoria. This test may be useful in assessing the deviation under real-life conditions with both eyes viewing.
, GuytonDL. Ophthalmic prisms. Measurement errors and how to minimize them.1983;90(3):204–210.