Cover testing is the gold standard objective method for determining the presence, type, and amount of ocular misalignment (strabismus). It is typically performed by the ophthalmologist, orthoptist, or ophthalmic technician. The comprehensive cover test assessment is multifaceted. Cover testing can be done to measure a deviation at near, which requires an accommodative target held at a distance of 33cm for fixation. It can also be done to measure a deviation with distance fixation.
Single cover test (cover-uncover test)
The single cover test is generally performed first. This is done by using an opaque or translucent (fogged) occluder to occlude one eye. The occluder is simply held in front of the eye for a few seconda and then removed. If a tropia is present in the non-occluded eye, the examiner will observe the contralateral eye move to pick up fixation when the fixing eye is occluded. If not, the eye will remain stationary. The presence of ANY movement on a single cover test indicates a tropia. In the case of a phoria, or latent deviation, the examiner observes the eye underneath the occluder. In the case of a phoria, the uncovered eye does not move however the eye that is under the cover will deviate when occluded and return to a straight position when the occluder is removed. The cover-uncover test can differentiate whether the misalignment is a phoria or a tropia.
Alternate cover test
The alternate cover test is performed after the single cover test. For this test, the examiner occludes one eye and then the other, switching the occluder back and forth to occlude the eyes without allowing the patient to fuse in between occlusion. The alternate cover test is the most dissociative cover test and measures a total deviation, including the tropic plus the phoric/latent component. When performing the alternate cover test it is important to hold the occlude over each eye for at least a few seconds, in order to allow the non-occluded eye enough time to pick up fixation. In general, the faster the eyes are able to recover when the occluder is switched, the better the control of the deviation.
Alternate prism cover test
The alternate prism cover test is similar to the alternate cover test, with the addition of a prism held over one eye to quantify the misalignment. Prisms should be held in the “frontal plane” so that the rear face of the prism is parallel to the plane of the patient’s face. The examiner then performs the alternate cover test while holding prism over the deviated eye until the misalignment is neutralized. The misalignment is quantified with the size of the prism (measured in Prism Diopters) which is required to neutralize the deviation.