Binocular Vision Lecture 4

Binocular Vision Lecture 4

• Objective measurement of eye deviations is the focus
• Students will be able to perform and interpret cover tests
• Including the Brückner test, Hirschberg method, and Krimsky test

Introduction

• Heterophoria: Dissociated (passive) eye position differs from associated (active) position
• Associated position: When both eyes are fixated on a target
• Dissociated position: When one eye is covered
• Sensory fusion is prevented, and fusional vergence doesn't occur when one eye is covered
• Covering an eye allows assessment of heterophoria as the eye moves from associated to dissociated position

The Cover Test - When used

• Used in every sight test
• Objective method to assess heterophoria and heterotropia
• Differentiates between phoria and tropia

Cover-uncover and Alternating Cover

• Two parts to the cover test
• Part 1 (cover-uncover): Identifies tropias
• Part 2 (alternating cover): Identifies phorias

Set up – Distance Cover Test

• Room lights on for proper visibility
• Explain the test's purpose; eye muscle function
• Test should be performed with and without spectacles
• Determine fixation target (e.g., letter on line above worst eye's vision, or spotlight if < 6/18 vision)
• Patient should fixate throughout the test
• Patient's gaze should be in their primary position

Conducting the Procedure - Cover/Uncover Test

• Determining right eye tropia: Cover left eye; observe response of right eye; hold cover 3 seconds. If right eye doesn't move = no tropia; if moves = tropia.
• Determining left eye tropia: Cover right eye; observe response of left eye; hold cover 3 seconds. If left eye doesn't move = no tropia; if moves = tropia.

Investigation of a Heterotropia

• One eye fixates; other deviates
• Large deviations are visible without a cover test

Classifying Your Findings

• One eye fixates; other eye deviates
• Large deviations are visible even without a cover test (examples of left exotropia shown)

What to Expect When No Tropia Is Present

• When cover test moves over left eye, right eye does not move
• When cover test moves over right eye, left eye does not move

Right Exotropia

• Right eye abducts when left eye fixates
• Right eye moves inwards when cover test moves over left eye
• Right eye doesn't move when cover test moves over right eye

Left Esotropia

• Left eye adducts when right eye fixates
• Left eye abducts to take up fixation when the cover test covers the right eye

Left Hypertropia

• Left eye is superior to the right eye when fixated
• Left eye moves down to take up fixation when the cover moves over the right eye

Left Hypotropia

• Left eye is inferior to the right eye when fixated
• Left eye moves up to take up fixation when the cover moves over the right eye

Alternating Tropia

• Either eye can fixate; generally one will be preferred
• When cover is over the left eye, the right eye moves to take up fixation
• Right eye maintains position until the right eye is covered

Investigation of a Heterophoria

• Active position of the eye doesn't match the passive position

If no tropia, then determine if a phoria is present.

• Phoria is when eye's active position differs from its passive position
• Uncovered eye is in its active position
• Covered eye moves to a passive position (this is phoria)

Orthophoria - What Happens Under the Cover

• Passive and active eye positions match
• The eye under the cover will not move

Exophoria - What Happens Under the Cover

• Eyes are more divergent in passive position
• Covered eye moves outward

Esophoria - What Happens Under the Cover

• Eyes are more convergent in passive position
• Covered eye moves inward

R/L Hyperphoria - What Happens Under the Cover

• Eyes are vertically misaligned in passive position
• Right eye moves up (or down) if its covered
• Left eye moves down (or up) if its covered

L/R Hyperphoria - What Happens Under the Cover

• Eyes are vertically misaligned in passive position.
• Right eye moves down if its covered
• Left eye moves up if its covered

So How Do We Investigate a Phoria?

• Occluders are opaque; frosted occluders help with this
• Methods include looking behind the occluder during the cover/uncover test; asking patient about target movement; performing an alternating cover test (best method)

Alternating Cover Test

• Same set up as the cover/uncover test
• Patient determines presence of phoria by watching eye movements (3 second intervals) with swapping of eyes

Orthophoria - What is Seen on Alternating Cover Test

• Eye under the cover does not move

Exophoria - What is Seen on Alternating Cover Test

• Covered eye moves to its divergent passive position
• Eye returns into position once cover is removed

Esophoria - What is Seen on Alternating Cover Test

• Covered eye moves to its convergent passive position
• Eye returns into position once cover is removed

R/L Hyperphoria – What is Seen on Alternating Cover Test

• Right eye moves down (or up), left eye moves up or down to retake fixation

L/R Hyperphoria – What is Seen on Alternating Cover Test

• Right eye moves up (or down), left eye moves up or down to retake fixation

Distance Cover Test

• Conducted without and with corrective lenses
• To examine binocular vision in absence of corrective lenses/specs
• Patient may be wearing prism without knowing it
• Determining effect of refraction on binocular vision system

Set up – Near Cover Test

• Similar set up to the distance cover test
• Patient looks at a near target
• Fixation target can be chosen; letter on line above worst visual acuity using a "budgie stick"; spotlight

Near Cover Test

• Conducted without and with corrective lenses
• To examine binocular vision in absence of corrective lenses/specs
• Patient may be wearing prism without knowing it
• Determining effect of refraction on binocular vision system

Things to Watch Out For - Herrings Law

• Herrings law: When an impulse to move one eye is sent, the other eye reacts in the same direction.
• Large phorias may make the eye appear to have a tropia.
• This information will help determine if there was a true tropia or a false one.

Things to Watch Out For - Pseudostrabismus

• Pseudostrabismus is often seen in children.
• The epicanthus (skin fold over the inner canthus) can make it look like strabismus; no strabismus is present.

Other Objective Tests

• Brückner test: Compares the red reflexes in both eyes
• Hirschberg test: Measures corneal reflexes to ascertain directionality of eye deviation(1mm = 20 degree)
• Krimsky test: Modified Hirschberg; adds prisms to measure deviation accurately.

Recording a Phoria

• Direction (eso, exo, R/L, L/R)
• Magnitude (Prism dioptres)
• Fixation distance (distance, near)
• Compensation (good, moderate, poor)

Recording a Tropia (the minimum)

• Which eye (and laterality)
• Direction (eso/exo/hyper/hypo)
• Magnitude (prism dioptres)
• Fixation distance (near/far)

Determining the Size of the Deviation

• Experienced practitioners estimate deviation size.
• Practices to estimate include using a ruler for patient and friend, asking the patient to describe the target to see any deviations; use of prism bars

Cover Test 'Phi' Movements

• Adds subjectivity to cover test.
• Targets move in opposite direction to determine phoria
• Can help to identify small phorias

Conclusion

• Important to understand and execute these tests correctly, and understand how to spot false positives when performing assessments