Everything You Need To Know About Cover Testing

7:00 AM

To fully investigate visual problems, we need to establish an idea of how the eyes behave both independently and together.  Checking visual acuity is not enough; a person may have clear vision but still suffer from double vision or ocular strain because the eyes aren’t working efficiently together.  Today we break down one of  Dr. S’s most essential tests in her normal battery of VT work-up items: the cover test.  “You can get so much information when doing a cover test, especially if you take the time to examine the effect of fatigue.”

Cover Testing

What it Tells You:  Ocular alignment and how much stress the visual system can take before binocularity (the ability to use both eyes together) breaks down.  Another way to think of this is how fragile your patient’s binocularity is, or the likelihood that the patient experiences diplopia or fatigue with fine detail tasks in distance and/or at near.

Strabismus is the scientific word for an ocular misalignment where the two eyes do not naturally
focus on the same central point.  The eyes will point in different directions at the same time for any givenlength of time – be it constantly or only under certain activities. Other layman terms often associated are“squint” or “cross-eyed”

  • The term tropia is used to define the direction of the strabismus (or misalignment).  For example, an exotropia means that the misalignment occurs due to one eye deviating outwards.  An esotropia is a deviation inwards.  Vertical misalignment is also possible: hypertropia means deviated upward and hypotropia means deviated downward.


  • A phoria is a misalignment that occurs only after the eye has been fatigued; it is a tendency for the eyes to deviate in any direction – vertically or horizontally - but the visual system is able to compensate well enough for the deviation that it does not manifest as a tropia.  A phoria will often present with visual, mental, or physical fatigue.  In medical literature you may see the term “latent deviation,” meaning that the misalignment is not always present.  This person does not have strabismus (the eyes are correctly aligned), but due to ocular stress and fatigue the eyes may become temporarily misaligned. Phorias can be labelled as exophoria (outward deviation) or esophoria (inward deviation).
Cover Testing is used to uncover the presence of tropias and phorias.

Equipment You Need:

  • Use a fine detail target and have the patient wear their appropriate glasses for the distance or near target to achieve best vision
    • at distance you can isolate a 20/30 size letter or picture on the chart (or larger if their acuity is reduced)
    • at near you can use:
      • a small letter on a card or target
      • a small or fine detail point on a picture ---“Look at the elephant’s trunk”
      • or the patient’s own reflection in a Wolff wand – great for younger patients!
  • an Occluder or some way to isolate the two eyes (be that a dark piece of paper or even your hand that can be moved in front of each eye if you are in need of a quick assessment)
How to Perform: Cover Testing is typically done at both near (around 40 cm) and far (with a target set for 20 feet) to assess ocular alignment.  Testing at near is very important – the patient may achieve ocular alignment when they are looking at a distance, but when they move to doing near work their eyes may have much more difficulty.  It is quite common to see larger misalignment or deviations with near testing than you would find with distance testing, especially if there is a concurrent accommodation (or focusing) issue.  In addition, if a patient presents with a distance complaint, careful near testing may reveal why the eyes are experiencing distance blur or diplopia (more to come on this in future posts!).

In cover testing you are looking for ocular movement to determine the presence and direction of tropia or phoria.  If the eye moves inward after being uncovered (or moves outward when covered, asviewed with a translucent occluder or by peeking behind the occluder), then you have an exo deviation.  If the eye moves outward after being uncovered, then you have an eso deviation.


1. Unilateral Testing:

  • Occlude one eye, and observe the uncovered eye for movement
    • If the uncovered eye moves toward the patient’s nose (inward) when covering the fellow eye, an exotropia is present. 
    • If the uncovered eye moves away from the patient’s nose (outward) when covering the fellow eye, an esotropia is present.
  • Quantifying the angle using prism is necessary, as the angle may change with fatigue.
  • Qualifying the tropia as constant or intermittent, or monocular or alternating is also important
  • Ask yourself: with an alternating tropia, which eye does the patient tend to fixate with?
  • Repeat on the left eye.
**If you note movement on Unilateral Testing, you have found a tropia

2. Alternate Testing:

  • Cover the right eye for 3 seconds, and then switch to cover the left eye.  
  • Repeat, waiting 3 seconds in between movements to allow time for the eyes to re-find your test target.
  • Immediately observe the uncovered eye for movement.
    • if the eye moves “with” your occluder you have an exo deviation.
    • if the eye moves “against” your occluder movement in eso deviation
**If you note movement on Alternate Testing, you have found a phoria

3. Repeat Unilateral Testing

  • If you truly have a phoria, you will see misalignment on the alternating cover test, but the patient will have normal alignment when you repeat the unilateral testing.  If instead you now see misalignment on the unilateral testing, this person actually has a tropia that maynow be easier to detect since the eyes are more fatigued
Check out this great video featuring abnormal cover test results and perfect technique!

Tips from Dr. S:

  • When using prism to quantify angles of deviation, careful attention is needed to observe smaller angles.  Sometimes the angle of deviation is so small that you cannot observe it, but the patient will report a “jump” when switching the paddle from one eye to another, or when uncovering the eyes completely. The direction in which their target is jumping will let you know how the eye is deviating.  For instance, if during an alternating cover test that appears ortho, but the patient reports the target “jumps” to the left when you go from covering the right eye to covering the left eye, an exophoria is present due to the report of crossed diplopia.
  • If you don't see any movement on unilateral testing, but vision is reduced in one eye, you can perform a quick Bruckner/Hirschberg test with your direct scope and immediately see the amblyopia or strabismus.  Sometimes after strabismus surgery, the deviation is SO SMALL, but the strabismus still remains so their vision will be less than 20/20. 

Take Home:

The key hallmark of cover testing is revealing and differentiating strabismus (full time deviations) from phorias (latent deviations).  Alternate cover testing cannot differentiate phoria from tropia, so you have to perform unilateral testing to determine that difference.  Unilateral testing can also
tell you if you have an alternating tropia, or a single eye misalignment.  Patients with constant alternating tropias may have more equal acuitieswhen comparing the right and left eyes, but they still suffer from loss of depth perception because they can’t have binocularity.  In the case of a (constant) one-eye deviation, that eye is at risk for developing amblyopia, which includes reduced vision, as well as reduced accommodation and ocular motor deficiencies..  Vision can’t develop clearly in a constantly strabismic eye because the image is being constantly suppressed and ignored by the brain. With constant alternating deviations, each eye is getting visual use during the day at some point, so visual information is still being processed by both eyes.  That is why it may be less likely to have strabismic amblyopia in a patient with constant alternating strabismus than in a unilateral trope where the deviated eye is never being used.

Determining the type of ocular misalignment helps your doctor develop the best plan for achieving better visual performance.  A patient with a large unilateral strabismus may prefer to pursue strabismus surgery to help gain alignment, along with strabismus vision therapy to gain visual function and binocularity after surgery.  Conversely, a patient with only a phoria or an intermittent tropia is much less likely to be a candidate for surgery because their eyes aren’t always misaligned and the patient may be able to learn to expand their range of fusion without surgery in most cases. There is even a risk in surgeries in patients with intermittent tropias that what binocularity does exist could be eliminated after surgery due to the common subtle vertical deviations and nystagmus than can occur after a strabismus surgery.  The “VT sandwich” is a great approach: your vision therapy optometrist and your strabismus surgeon will work together as a team to achieve best visual outcomes by employing vision therapy before surgery to establish awareness, limit suppression, and increase visual acuity, and if possible to pinpoint a centration point for the eyes to focus in space together. Then surgery will be performed with the aim of moving the deviant eye to that centration point to achieve the best possibility for ocular alignment.  After surgery vision therapy can help the patient fine tune their ocular function and work towards achieving the chief goal: (functional) binocularity.

Phorias are very common in my average patient base – if you sometimes see double or feel your eyes fighting to stay focused together after long hours reading or on the computer, you probably have a phoria.  Patients with phorias can learn how to better control their ocular muscles to keep their eyes aligned, even with fatigue.  And that is just one example of what vision therapy can help a person achieve!

Dr. S is a residency trained optometrist and COVD fellow specializing in Vision Therapy

You Might Also Like


  1. My daughter has cross eye and she will be 3years old by December.
    What can be done to correct it at this early stage.

    1. Good news! You are catching your daughter's eye turn at an early age. Many times patients with an eye turn may have a high uncorrected prescription, so full time glasses wear may be a necessary treatment. In some cases glasses correction and patching for amblyopia correction aren't enough, and surgery is required to realign the eye's muscular alignment. Your eye doctor will be better able to find the proper treatment for your child's individual case.

  2. Hello, why did you say "**If you note movement on Alternate Testing, you have found a phoria" but then you say "Alternate cover testing cannot differentiate phoria from tropia, so you have to perform unilateral testing to determine that difference?" Why is the unilateral cover test the only test that can differentiate between a tropia and a phoria? Thanks

    1. Hi! Thanks for your question! If you just do alternate cover testing alone, both a tropia and a phoria will move, so this test alone can't distinguish the problem. Unilateral cover test will only move on a tropia. Patients with a phoria have fusion, so they will look normal on unilateral testing, but their system is inefficient so it shows as movement on alternating cover testing. So in the case of a phoria, you would see no movement on unilateral testing, but then you would see movement on the alternating. If you see movement on both unilateral and alternating, you have a tropia. Hope that helps!

  3. Excellent and helpful post.I am so glad to left comment on this. This has been a so interesting ..I appreciate your effort..