How to carry out a cover test.
A cover test is a test performed on the eyes to check for a deviation. It is mostly carried out by Orthoptists which you will find, working in the ophthalmology department in the hospital. A cover test is a basic test which can give you a lot of information about your patient’s eyes; from the type of deviation to an indication of their vision in each eye.
A cover uncover test is performed to check for a manifest deviation. The practitioner asks the patient to look at the target (near or distance) and uses their occluder to cover one eye. While covering that eye they must keep an eye on the uncovered eye to see if there is any movement. This movement would be the eye taking up fixation to look at the target. The practitioner then uncovers the eye and sees if the uncovered eye moved back to its original position. This is then repeated on the same eye. This time looking at the eye under the cover while removing the occluder. This informs the practitioner of a latent deviation. This is then all repeated this for the second eye.
Alternate cover test
An alternate cover test is to give an indication of the true size of the deviation, may it be manifest or latent. This is done with the patient looking at the preferred target and the practitioner using their occluder once again to cover one eye. This time instead of uncovering the eye the occluder is switcher to the second eye. This is done quickly so the eyes are not given a chance to fixate in the time it takes to uncover the one eye and cover the second. You then swing the occluder back to the originally covered eye. This is done a few times while the practitioner looks at the eye under the cover each time to check for a deviation. The movement of the occluder is almost pendula.
Step 1. Perform cover test to a non- accommodative target.
A pen torch is a common non-accommodative target used by many orthoptists. Ask your patient to look at the pen torch while you use your occluder to carry out a cover/uncover to check for a manifest deviation. You then want to carry out an alternative cover test to either check the true size of the manifest deviation, if found or to check for a latent deviation. The results are noted.
Step 2. Perform an accommodative near cover test.
The budgie stick is used as a common accommodative target for a near cover test. An appropriate target is picked for the patient e.g. The letter chart on the flip side of the budgie stick is used for older children and adults, the budgie or any other small picture that is interesting is used for a young child and a small toy may be used for a baby.
With the appropriate target in one hand at a good distance from the patient (normally 33cm), the practitioner uses the occluder to perform a cover and uncover to check for a manifest deviation when accommodating at near. An alternate cover test is then carried out. The results are noted.
Step 3. Perform an accommodative distance cover test.
The test chart is a very common distance accommodative target used by many. Again, an appropriate target must be selected for the patient. The test chart is good for anyone why knows their letters such as an adult or older child. Larger pictures or toys in the distance can be used for younger children.
With the patient looking into the distance (6m) at an appropriately picked target the cover test is done. A cover uncover is carried out followed by an alternate cover test. The results found are recorded.
When recording the results: if there is a manifest deviation, it should be stated which eye this was seen in, the size and type of deviation should be recorded and the recovery speed of the eye that is deviating.
Exotropia – when doing a cover and uncover the eye that is uncovered will move towards the nose to fixate on the target. When the occluder is removed from the neighbouring eye the uncovered eye will move back out to its original position.
Exophoria – when doing a cover and uncover the eye under the cover will move in towards the nose. When the occluder is removed the eye that was under the cover will be seen moving back out to its original position.
Esotropia – when doing a cover and uncover the eye that is uncovered will move away from the nose to fixate on the target. When the occluder is removed from the neighbouring eye the uncovered eye will move back in to its original position.
Esophoria – when doing a cover and uncover the eye under the cover will move out away from the nose. When the occluder is removed the eye that was under the cover will be seen moving back in to its original position.
Both exo and eso deviations can be measured using a prism bar and an alternate cover test. As the alternate cover test dissociates the eyes it helps to bring out the true size of the deviation allowing a more accurate measurement to be done.