Cover test

A cover test or cover-uncover 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 test involves having the patient (typically a child) 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 misaligned eye will deviate inwards or outwards. 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 and the phoria. A tropia is a misalignment of the two eyes when a patient is looking with both eyes uncovered. A phoria (or latent deviation) only appears when binocular viewing 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 exotropic, covering the fixating eye will cause an inwards movement; and esotropic if covering the fixating eye will cause an outwards movement. The alternating cover test, or cross cover test is used to detect total deviation (tropia + phoria).

The cover test is regarded as an essential examination in investigating strabismus. It is simple to undertake, does not require great amounts of skill by the examiner and is objective in nature. To ensure that the test is executed accurately and that the maximal amount of information is obtained, it is paramount that the appropriate fixation targets are used for near (approximately 33 cm), distance (3m) and far distance (>3m) and also that a cover consisting of a black ‘paddle’ is used. A translucent occluder paddle may also be used. This enables the examiner to see the eye’s position behind the occluder.

Many characteristics of a strabismus can be gained from performing the cover test.

In a Manifest Strabismus it can tell us the following:

- The type of deviation: whether it be eso, exo, hyper, hypo or cyclo tropia.
- The size of the deviation: slight, small, moderate or large
-Speed to take up fixation: if the eye takes up fixation fast it means there is good vision in that eye
-Accommodation on the deviation
- Nystagmus
-Dissociated vertical deviation (DVD)
- Incomitance – deviation angle varies in each position of gaze

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Wikipedia