How to Perform a Basic Cover Test in Ocular Misalignment or Strabismus

Christopher Kirkpatrick, MD and Tony Klauer, orthoptics trainee

April 24, 2015

If video fails to load, use this link.

Video Transcript

The following is a brief instructional video tutorial on the appropriate method to perform a cover test in the evaluation of a patient for ocular misalignment or strabismus.  The patient should be seated and asked to fixate at distance on an accommodative target. The patient should be wearing their best correction for their refractive error. If the patient is adopting an alternate head position such as a chin up, chin down, face turn or head tilt, they should be placed into a forced primary or straight head position.

The single cover test is a test is used to determine if there is a heterotropia or tropia, which is a manifest strabismus or misalignment that is always present. The first eye is covered for approximately 1-2 seconds. As this eye is covered, the uncovered eye is observed for any shift in fixation. The occluder is then removed and any refixation movements are then noted under binocular conditions. If there is no shift in fixation it can mean two things:

1. The patient is orthotropic or has no misalignment, but this needs to be confirmed by seeing the same thing when the opposite eye is covered; or
2. The uncovered eye is the fixating or preferred eye in the setting of a heterotropia.

Next, the opposite eye is covered in a similar fashion for approximately 1-2 seconds making sure to wait a few seconds from the prior eye as not to suspend fusion and allow for a phoria to manifest. Any shift in fixation of the unoccluded eye is then noted. If the unoccluded eye shifts in, or medially, in a temporal to nasal fashion when the opposite eye is occluded – this means that there is an exotropia, such as in this case. If the unoccluded eye shifts out, or laterally, in a nasal to temporal fashion when the opposite eye is occluded – this means that there is an esotropia. If the unoccluded eye shifts down when the opposite eye is occluded – this means that there is a hypertropia. If the unoccluded eye shifts up when the opposite eye is occluded – this means that there is a hypotropia.

If a tropia is present – the angle of deviation can be determined by performing a simultaneous prism cover test. This test is performed by placing a prism of varying strength prism that is oriented in the appropriate direction for the deviation over the non-fixating eye while simultaneously placing an occluder over the fixating or preferred eye until there is no shift in fixation or the deviation is neutralized. This test is typically only used to measure small angle tropias with the idea of not dissociating the patient and bringing out an underlying phoria. It is important that when performing a single cover test that occlusion of each eye is brief as not to suspend binocular fusion and dissociate the patient as this may make the deviation appear larger by superimposing a phoria on top.

The next test is a cover uncover test. This is a that test is performed to determine if there is a heterophoria or phoria, which is a latent strabismus or misalignment that is only present when binocular fusion is suspended or interrupted. This test is performed in the same manner as the single cover test except that attention is turned to the eye that has been occluded as the occluder is pulled away.  If the uncovered eye does not show a fixation shift as the occluder is placed, but as the occluder is pulled away, the covered eye shows a refixation movement once binocular conditions are restored – this represents a phoria.

The next test is the alternate cover test. This test will allow the full deviation to be measured as it will bring out any phoria that is present in addition to the tropia determined on single cover testing by suspending binocular fusion. This test is performed after single cover testing as it is the most dissociative of cover tests. This test involves covering one eye and holding the occluder for several seconds to suspend fusion, then shifting the occluder to the other eye and rapidly alternating back and forth without allowing the patient to become binocular and being careful to always keep one eye occluded. The eye under the occluder is observed as the occluder is removed and placed over the fellow eye in order to determine the direction of deviation. If there is an outward, or lateral, refixation in the nasal to temporal direction – this represents an eso deviation. If there is an inward, or medial, refixation in a temporal to nasal fashion – this represents an exo deviation. If there is a vertical refixation movement – this represents either a hyper or hypo deviation. There can be both horizontal and vertical deviations present at the same time.

The full angle of deviation can be determined by performing an alternate prism cover test.  This test is performed by placing prism of varying strength that is oriented in the appropriate direction for the deviation over the non-fixating eye while performing the alternate cover test until there is no longer a shift in fixation in either eye. This is when the deviation has been neutralized. For an eso deviation, a base out prism should be used. For an exo deviation, a base in prism should be used. For a hyper deviation, a base down prism should be used. For a hypo deviation, a base up prism should be used. Horizontal prism and vertical prisms can be stacked over the same eye, but two prisms that are oriented in the same direction cannot be used over the same eye. This test can be performed in all 9 cardinal gaze positions and head tilts if the clinical situation dictates. This is done by adjusting the patient's head position while asking them to maintain fixation on the distance accommodative target. This same test can also be performed at near, by having the patient fixate on an accomodative near target. Again, making sure they are in their best correction for near.

The Parks-Bielschowski 3-step test is a helpful test in determining the etiology of a vertical deviation with or without any horizontal deviation. This test is performed in a similar fashion as the alternate prism cover test. The deviation is first measured in primary gaze. The deviation is then measured in both lateral gazes by turning the patient's head the left while maintaining fixation on the same target to simulate right gaze and similarly with the head turned to the right to simulate left gaze. The deviation is then measured with the face pointed straight ahead, but the head tilted to both the right and left.

  Share this page: