Cover test is done to determine…
- presence of deviation (phoria or tropia)
- laterality/which eye (unilateral or alternating)
- frequency of the deviation (constant or intermittent)
- direction of the phoria or tropia
- magnitude (size) of the deviation
orthophoria
the perfect alignemtn in the absence of a stimulus for fixation
heterophoria/phoria
the tendency for the eyes to deviate when fusion is blocked
-a small phoria is common in most people; overcome by fusion
phoria with “posture” slightly inward
esophoria
phoria with “posture” slightly outward
exophoria
decompensating phoria
when motor function is no longer enough to overcome a phoria. discomfort or diplopia
heterotropia/tropia
- manifest deviation
- visual axes of both eyes do not intersect at fixation
- leads to diplopia or suppression of the deviated eye
reasons for tropia
- aniosmetropia
- abnormality of the extraocular muscles or strabismus
- eye disease
anisometropia
difference in refractive errors between the two eyes
cover-uncover test (unilateral cover test)
- presence of deviation (phoria or tropia)
- laterality/which eye (unliateral or alternating)
- frequency of the deviation (constant or intermittent)
- this test done FIRST
Which test should be done first?
cover-uncover test
-want to make sure you don’t break fusion
alternating cover test
determines:
- direction of phoria or tropia
- magnitude (aize) of the deviation
At what distance is the cover test done?
at distance and near
“Cover” in the cover-uncover
- show if deviation is phoria or tropia
- evaluates the visual axis of each eye when both eyes are viewing a target
- look at eye that is NOT covered
phoria
the visual axis of both eyes are aligned on the target with both eyes open
tropia
the visual axis of both eyes is aligned on the target while the other is misaligned with both eyes open. Now it has to move to pick up fixation
“uncover” in cover test
- shows if a tropia is alternating or unilateral (laterality)
- shoes if a tropia is constant or intermittent (frequency)
unilateral tropia
R or L tropia is when one eye usually fixates when both eyes are open
-the tropic eye only fixates when the preferred fixating eye is occluding
alternating tropia
pt able to keep either eye on a target with both eyes open
equipment for cover test
- VA chart
- near point
- occluder/paddle
- prisms
- habitual correction
- well lit room
how do you explain purpose of cover test to patients?
“I want to make sure your eyes are aligned properly
How to do cover test
- sit or stand in a way that you can see the eys but does not block pts fixation. Hold occluder and prisms
- tell pt to look at target and keep it clear
- determine if pt is able to fixate with the right eye when the left eye is covered, and then evaluate if the left eye is able to fixate when the right eye is covered
- observe any movement as soon as the left eye is covered
- allow 2-3s for eyes to ressume relationship with each other
- repeat a few times
- repaet with other eye
How to do cover test at distance
-at distance, isolate letter that is 1 or 2 lines larger than the best corrected VA in the poorer seeing eye
How to do a cover test at near
- hold target at 40cm
- leter 1-2 lines larger than BCVA of poor eye
- target of comparable size. pt can hold target
