Cover Test Flashcards
Cover test is done to determine:
Presence of a 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
what is orthophoria?
The perfect alignment in the absence of a stimulus for fixation
Eyes are aligned.
What is heterophoria (or just phoria)?
The tendency for the eyes to deviate away when fusion is blocked.
Small phoria is common in most people, overcome by fusion.
“Posture” slightly inward
Esophoria
Posture slightly outward
Exophoria
What is a decompensating phoria?
When motor fusion is no longer enough the overcome a phoria.
Could result in discomfort or diplopia
Heterotropia/tropia is….
a manifest deviation.
Visual axes of both eyes do not intersect at fixation.
Leads to diplopia or suppression of the deviated eye
Tropia could be due to:
Anisimotropia (difference in refractive errors between the two eyes)
Abnormality of the extraocular muscles or strabismus
Eye disease
Cover-Uncover Test (Unilateral Cover Test)
Helps differentiate:
- Presence of a deviation (phoria or tropia)
- laterality/which eye (unilateral or alternating)
- frequency of the deviation (constant or intermittent)
This is done first.
Alternating Cover Test
Used to determine:
- direction of phoria or tropia
- magnitude (size) of deviation
Is a cover test done at distance or near?
BOTH
The “Cover” in Cover-Uncover
Used to show if the deviation is a phoria or a tropia.
Evaluates visual axis of each eye with both eyes are viewing a target.
While using a Cover, it will be a Phoria if…
The visual axis of both eyes are aligned on the target with both eyes open
While using a Cover, it will be a Tropia if…
The visual axis of one eye is aligned on the target while the other is misaligned with both eyes open
The “Uncover” in Cover-Uncover
shows is a tropia is alternating or unilateral tropias
*focus on the eye that is not occluded to determine if it picks up fixation
Unilateral tropia
When one eye usually fixates when both eyes are open
Alternating tropia
When the pt is able to keep either eye on a target with both eyes open
Equipment for cover test
Visual acuity chart for fixation at distance
Near point target for fixation at near
Occluder/paddle
Horizontal and vertical prism bars or loose prisms
Habitual correction (CT will be done w/ and w/out HC in some cases)
Well lit room
How to explain the purpose of a cover test to a patient:
“I want to make sure your eyes are aligned properly”
Distance Cover Test
Isolate a letter that is one or 2 lines larger than the best corrected VA in the poorer seeing eye
Could be between 20/25 and 20/40
Near Cover Test
Hold an accommodative target at 40 cm
It could be a letter that is one or two lines larger than the BCVA of the poorer seeing eye
Could be a target of comparable detail/size (about a 20/30 size)
Patient may hold target.
When the occluder is removed, how long should you wait for allow both eyes to resume their normal relationship?
About 2-3 seconds
If there were no movements seen when either eye was covered, the patient has a ______
Phoria
Alternating Cover Test
Determines the direction and the size of the deviation
Measuring the deviation
Put prism in front of one eye and watch the other
Continue to move the occluder back and forth while increasing the prism until you no longer see movement
The deviation is neutralized at the amount of prism where both eyes appear aligned and there is no more movement on alternate CT
Prisms to neutralize:
- Exo
- Eso
- Hypo
- Hyper
- Ortho
- BI
- BO
- BU
- BD
- Verify using BI and then BO
Recording a Cover Test
Indicate if it was done with prescription (cc) or without correction (sc)
Record distance (D) and near (N) findings separately
Record the amount of prism that was required to neutralize the deviation
Record the direction of deviation
- E for eso
- X for exo
- RH for right hyper, LH for left hyper
- Ortho for no deviation
Record if it’s phoria/tropia
- R/L/Alt
Record an intermittent tropia with a parentheses (T)
Record the amount of prism measured in Prism Diopter
Comitancy
Deviation size remains same (or within 5pd) in all positions of gaze
Implying no muscles are underacting or overacting
Noncomitancy
Deviation size changes in different positions of gaze
Due to an overaction or underaction of one of multiple muscles
how to correctly determine comitancy
CT is done in all positions of gaze
Hirschberg test
Rough objective estimate and measurement of a deviation/strabismus.
Useful in young and uncooperative patients
Penlight is held straight in front of patients face, pt is instructed to fixate on the penlight with both eyes open
In the absence of strabismus, the corneal reflex should be centered in both eyes. In the presence of strabismus, the reflex will not be in the center of the deviated eye.
For 1 mm of estimated deviation, it is approximately a deviation of 22 prism diopters
Krimsky Test
This is the combination of the Hirschberg test with prisms to determine the angle of deviation.
Prisms are placed in front of the fixating eye until the corneal reflexes are symmetrical.
Base Out prism is used for esotropia Base In prism is used for exotropia
In the Prism Reflection Test, the prism is held in front of the deviated eye until the corneal reflex is symmetrical.
