The cover test: cover–uncover, alternate cover and prism cover test
Quick answer
The cover test detects and measures strabismus. In the cover–uncover test you cover one eye and watch the other: if the uncovered eye moves to take up fixation, there is a manifest deviation (a tropia). If no tropia is seen but the covered eye moves back into line when the cover is removed, there is a latent deviation (a phoria). The alternate cover test breaks fusion to reveal the total deviation, and adding prisms until the movement stops (the prism cover test) measures it in prism dioptres.
Open the simulatorBefore you start
- Check that the patient can see the target with each eye on its own.
- Use an accommodative target, such as small letters or a detailed picture — not a light — first at near (33–40 cm) and then at distance (6 m).
- Test with the patient's glasses on if they wear them, with the head straight.
Cover–uncover test: is there a tropia or a phoria?
- Cover the right eye and watch the left eye. If it moves to take up fixation, the left eye has a tropia.
- Remove the cover, let the patient fixate with both eyes, then cover the left eye and watch the right eye.
- Read the direction of the refixation movement: moving outward means the eye was turned in (esotropia); inward means exotropia; downward means hypertropia; upward means hypotropia.
- If neither eye moves when the other is covered, there is no tropia. Now watch the eye behind the cover as you remove it: a recovery movement back to fixation means a phoria (outward recovery for esophoria, inward for exophoria).
Alternate cover test: the total deviation
Move the occluder directly from one eye to the other several times, holding it for a couple of seconds on each eye so that the eyes never fuse in between. Watch each eye as the cover leaves it. Any movement shows the total deviation — tropia plus phoria. The alternate cover test on its own cannot tell a tropia from a phoria; that is the job of the cover–uncover test.
Prism cover test: measuring the angle
- Hold a prism in front of one eye while you perform the alternate cover test.
- Point the prism apex toward the direction of the deviation: base-out for eso deviations, base-in for exo deviations, and base-down in front of a hypertropic eye.
- Increase the prism power until the refixation movement disappears. That prism is the deviation in prism dioptres (Δ).
- The simultaneous prism cover test — placing the prism over the deviating eye at the same moment the fixing eye is covered — measures the tropia alone, without the phoria.
- Record near and distance measurements, with and without glasses.
Recording the result
| Notation | Meaning |
|---|---|
| ET / XT | Constant esotropia / exotropia |
| E(T) / X(T) | Intermittent esotropia / exotropia |
| E / X | Esophoria / exophoria |
| RHT / LHT | Right / left hypertropia |
| ET′, XT′, E′, X′ | The prime mark means the measurement was taken at near |
Common mistakes
- Using a penlight as the fixation target: it does not control accommodation, so eso deviations can be missed or underestimated.
- Covering too briefly: a phoria needs a few seconds under the cover to show.
- Watching the covered eye during the cover step instead of the fellow eye.
- Testing only at near or only at distance: many deviations differ between the two.
Practice it on the virtual patient
- Open esotropia and cover the fixing eye: watch the deviated eye move outward to take up fixation.
- Compare with esophoria: no movement on cover, but a recovery movement when you uncover.
- Try exotropia, intermittent exotropia and hypertropia to see each direction of movement.
- Open the cover test panel, add prisms during the alternate cover test and read the deviation in prism dioptres.
Frequently asked questions
What is the difference between a tropia and a phoria?
A tropia is a manifest misalignment that is present while both eyes are open. A phoria is a latent tendency to misalign that fusion keeps under control; it shows only when fusion is broken, for example under a cover.
Which eye do you watch during the cover test?
In the cover step you watch the uncovered, fellow eye. In the uncover step you watch the eye that has just been uncovered. In the alternate cover test you watch each eye as the cover leaves it.
What does it mean if the eye moves outward on the cover test?
If the uncovered eye moves outward to take up fixation, it was turned inward, so the patient has an esotropia. An inward refixation movement means an exotropia.
What is a prism dioptre?
One prism dioptre (1Δ) deviates a ray of light by 1 cm at a distance of 1 m. For small angles, about 1.75 prism dioptres equal 1 degree.
Related guides
- Hirschberg and Krimsky tests: measuring strabismus with the corneal light reflex Read the corneal light reflex, convert millimeters to prism dioptres and neutralize with prisms.
- Ocular motility and the H-test: ductions, versions and the nine positions of gaze Trace the H, know which muscle each gaze position tests and interpret limited movements.
- Third, fourth and sixth cranial nerve palsies and the three-step test Recognize each ocular motor palsy, run the three-step test and spot the red flags.