unilateral cover test

The unilateral cover test, usually performed as the cover-uncover test, is an objective test that detects a heterotropia and separates it from a heterophoria. The examiner watches the eyes while the patient holds fixation on a target and each eye is covered and uncovered in turn.^1,2

In the cover phase, the examiner covers one eye and watches the other eye. If the uncovered eye moves to take up fixation, a heterotropia is present; adduction of that eye indicates an exotropia, abduction an esotropia, and downward movement a hypertropia.³ If neither eye moves when the fellow eye is covered, the patient has a phoria.²

In the uncover phase, the examiner removes the cover after 2 to 3 seconds and watches the eye that was covered. A refixation movement as the cover comes off shows a heterophoria, with adduction indicating exophoria and abduction esophoria.³ In a patient with a heterotropia, the uncover phase also shows whether the tropia is unilateral or alternating.² The cover-uncover test comes before the alternate cover test, because occlusion can turn an intermittent tropia into a constant one.² The alternate cover test then dissociates the eyes and reveals the total deviation.³