Cover test
A cover test is an objective determination of the presence and amount of ocular deviation. It is typically performed by orthoptists, ophthalmologists and optometrists during eye examination
Eye examination
An eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes....


The two primary types of cover tests are the alternating cover test and the unilateral cover test (cover-uncover test).

The test involves having the child (typically) focusing on a near object. A cover is placed over an eye for a short moment then removed while observing both eyes for movement. The "lazy eye" will wander inwards or outwards, as it begins to favour its perceptive visual preference. The process is repeated on both eyes and then with the child focusing on a distant object.

The cover test is used to determine both the type of ocular deviation and measure the amount of deviation. The two primary types of ocular deviations are the tropia, also known as Strabismus
Strabismus is a condition in which the eyes are not properly aligned with each other. It typically involves a lack of coordination between the extraocular muscles, which prevents bringing the gaze of each eye to the same point in space and preventing proper binocular vision, which may adversely...

, and the phoria. A tropia is a constant misalignment of the visual axes of the two eyes, i.e. they can't point the same direction. A phoria is a latent deviation that only appears when fixation is broken and the two eyes are no longer looking at the same object.

The unilateral cover test is performed by having the patient focus on an object then covering the fixating eye and observing the movement of the other eye. If the eye was esotropic, covering the fixating eye will cause an outwards deviation and vice versa. The alternating cover test is used to detect heterophoria, where direct cover test will be normal for both eyes.
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