The primary purpose is to detect a tropia (a deviation when both eyes are open) and/or a phoria (a latent deviation that manifests when binocular fusion is disrupted). Ideally performed with best correction, as the patient must have adequate vision to fixate on a target.
Full range of ocular motility, vision adequate to see the target of fixation, foveal fixation in each eye, and patient cooperation. This test should be performed before the alternate cover test (see below).
Ask the patient to fixate on a nonaccommodative target at a distance (e.g., a letter on the vision chart).
Cover one of the patient’s eyes while observing the uncovered eye. A refixation movement of the uncovered eye indicates the presence of a manifest deviation (tropia). Repeat the procedure, covering the opposite eye. A shift in fixation may not occur if the uncovered eye is the preferred or fixating eye. Prisms may be used to quantify the observed deviation.
If there is no movement of either eye, the eyes are aligned with both eyes open (no tropia).
Ask the patient to fixate on an accommodative target nearby. Both eyes are tested at a near distance in the manner described previously.
In the absence of a tropia, the alternate cover test may be used to reveal any latent deviation that occurs with the interruption or suspension of binocular fusion (phoria). When it has been determined by the cover/uncover test that a tropia exists, the alternate cover test may be used to dissociate the two eyes and further quantify the total deviation (manifest tropia and latent phoria combined). The alternate cover test does not distinguish manifest from latent deviations.
Same as for the cover/uncover test above.
Ask the patient to fixate on a nonaccommodative target at a distance. To make certain that he or she is fixing on the target, ask that the letters be read or the picture described.
Repeatedly cover one eye and then quickly move the cover to the other eye. The eye being uncovered may be noted to swing into a position to refixate on the target, indicating the presence of a deviation. Then repeat the test at a near distance.
ALTERNATE COVER TEST WITH PRISM
Measures the size of the total deviation, regardless of whether a phoria or tropia is present.
To measure a deviation, prisms are placed in front of one eye with the prism base placed in the direction of the eye’s refixation movement. While continuing to alternately cover, as described above, increase the prism strength until eye movement ceases. The strength of the weakest prism that eliminates eye movement during alternate cover is the amount of deviation.
Measurements may be done for any direction of gaze by turning the patient’s head away from the target while asking him or her to maintain fixation on it (e.g., the right gaze is measured by turning the patient’s head toward his or her left shoulder and asking the patient to look at the target).
In general, measurements are taken in the straight-ahead position (both at a distance and nearby), in right gaze, left gaze, downgaze (the head is tilted up while the patient focuses on the target), upgaze (the head is tilted down while the patient focuses on the target), and with the patient’s head tilted toward either shoulder. Measurements are often taken both with and without glasses in the straight-ahead position.