Cover and Uncover Tests
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The cover and uncover tests help to further classify concomitant squints. These tests are not for patients with incomitant squints. Comitant squints that are present at all times of the day are called tropias (manifest squints), while those are only present when the binocular fusion is broken are called phorias (latent squints).
Technique
An object to focus on is held in front of the patient who is instructed to focus on it.
Cover test
One eye is completely occluded for several seconds and the uncovered eye is observed for movement as it focuses on the object. This eye is then covered and the other eye is observed for movement. Movement of the eye outwards confirms that there is an esotropia (the eye was turned inwards initially) and vice versa for exotropia. If a tropia is identified then the uncover test is not done.
Uncover test
One eye is completely occluded for several seconds and then uncovered. Observe for movement upon uncovering the eye. Movement of the eye outwards on uncovering confirms an esophoria (the eye was turned inwards when binocular fusion was broken), and vice versa for exophoria.
Clinical importance
Tropias are clinically important conditions as their presence indicate that the eyes are constantly misaligned. As they usually occur during early childhood, the brain may compensate for confusion by suppressing the vision of the misaligned eye. This causes poor development of vision in the affected eye and is termed amblyopia. Phorias are not clinically important because misalignment is only present occasionally. The eye tends to drift inwards or outwards when binocular fusion is weak. This is common during major illness or when tired at the end of the day. With age, binocular fusion weakens and patients may have increasing episodes of phoria.
Common mistakes in examinations
Performing the cover and uncover test at the same time. It is best to perform the cover test for each eye first and then perform the uncover test for each eye afterwards to avoid confusion.
