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 examinations.

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, 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 used to determine whether the deviation is a phoria or tropia, and the alternating cover test then used to measure the amount of deviation, usually with the aid of loose prisms.

[edit] See also

• Hirschberg test
• Strabismus

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