This patient has a left exotropia. In the cover-uncover test you always watch the eye that is not covered. Covering the right eye forced the left eye to take over the target. The left eye had to swing inward to get there. An eye only swings in if it was already resting outward, away from the nose. Resting outward is what exo means, so the left eye is the turned eye.
Now pick the ending. The left eye was turned while both eyes were open, before any occluder was used. A deviation you can see with both eyes open is a tropia, a manifest deviation. A phoria is a latent deviation. It stays hidden because the brain fuses, or merges, the two eye images into one. A phoria only appears once fusion is broken, which is the job of the alternate cover test. The textbook says to run the cover-uncover test first, because the alternate cover test cannot tell a tropia from a phoria. The prefixes are fixed: eso is in, exo is out, hyper is up, and hypo is down. Cover testing is also preferred over the corneal light reflex tests, because a light reflex can be thrown off by an odd-shaped pupil or by unusual retinal anatomy. Repeat each cover a few times for accuracy, and test at near as well as distance.
Take one sentence into the lane: the uncovered eye moves the opposite way from where it was resting. Moves in means it was out. Moves out means it was in. Moves down means it was sitting up. Running and naming the cover-uncover test is a core part of the COA ocular motility content area.
Incorrect Answers
- A. A right exotropia would make the right eye swing in when the left eye was covered. The right eye never moved.
- B. A right esotropia would make the right eye swing out when the left was covered. Again, the right eye stayed still.
- D. A left esotropia rests inward, so it must move outward to reach the letter. This eye moved inward instead.
Testing Pearls
- Cover one eye and watch the other: movement of the uncovered eye means a tropia.
- The uncovered eye moves opposite its rest position: in means exo, out means eso.
- Tropia is manifest and always present; phoria is latent and hidden by fusion.
- Do the cover-uncover test first; the alternate cover test cannot separate tropia from phoria.
- The prism and alternate cover test measures the deviation in prism diopters.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 9, Ocular Motility.
