Fixation Disparity

Question 1

What is fixation disparity?

Answer 1

Where 2 visual axes fail to intersect the object of regard under associated conditions in the presence of a fusional vergence response

All vergence components are present

Images do not fall on corresponding point but are imaged within Panum’s areas.

Question 2

Describe Eso and Exo fixation disparity?

Answer 2

Eso- over converged

Exo- underconverged

Image may still be single if point is within PFA

Question 3

How are fixation disparities usually measured in general?

Answer 3

Majority of visual field is binocular, two small marker are viewed monocularly

Polarized glasses allow one to be viewed by right eye and the other to be viewed by left.

Question 4

Describe the mallet unit.

Answer 4

Red lines are monocular, everything else is binocular and good stimulus to fusion
No FD if lines are on top of each other
Uncrossed- ESO FD
Crossed- EXO FD
If neither bar lines up with the middle X both eyes have FD

Question 5

What is only one red line is seen during mallett unit?

Answer 5

One eye is being suppressed

Question 6

How do we measure the deviation with mallett unit? What is this value called?

Answer 6

Introduce prism until the images are lined up
Place prism in front of affected eye
The amount of prism that eliminates FD is associated phoria

Question 7

Describe the vectograph slide.

Answer 7

The polarized test works similarly to mallett unit

One vertical and one horizontal seen by OD, other set seen by OS

Question 8

How can we measure fixation disparity?

Answer 8

FD is the angle between where the eye is pointing and where it should be pointing (angle alpha)

Can be measured using separation of vertical bars and viewing distance

Question 9

What is the speedy disparometer?

Answer 9

Allows for measurement of FD in minutes of arc

Letter present around to provide good accommodative stimulus

knob on the back allows the bars to be separated until patient reports they are aligned, FD can be read off back of scale

Question 10

What is a Saladin card?

Answer 10

Allows for measurement of FD

Contains polarized targets
See what circle the patient sees that the line are lined up then read results off the back

Question 11

Describe the Wesson card.

Answer 11

Postcard sized card for FD measurement
Ask which colored lines the black arrow is lining up with (colored lines seen by one eye, black arrow seen by the other)

Performed at 40cm or 25cm

Question 12

Describe the Hoya “Eye Genius”.

Answer 12

Measures FD then converts this into associated phoria
Patient presses buttons until lines line up

Question 13

Can multiple of these FD tests be performed to provide similar results? What does this tell us about the reliability of these tests?

Answer 13

No
95% limit of agreement were different from each other

Instruments should not be used interchangeably

Clinical tests may not apply to real life conditions

Question 14

Why do we test FD?

Answer 14

To assess vergence response when all components of vergence are present (tonic, proximal, accommodative, disparity)

Close to how the patient functions in the real world with all aspects present

Question 15

What are the limitations of tests for heterophoria, vergence ranges, NPC, vergence facility, AC/A that FD covers?

Answer 15

Heterophoria: dissociated, so no disparity vergence

Vergence ranges: measures change in vergence while accommodative stimulus is constant

NPC: may not reflect vergence response for sub-maximal targets.

Vergence facility: accommodative stimulus is constant

AC/A: only tests a single vergence component (AC)

Question 16

True/False: there are no other clinical tests that assess the complete vergence response at naturalistic stimulus levels other then FD.

Answer 16

True

Question 17

What do patient with associated phoria report?

Answer 17

Decreased binocular cortical responses
Symptoms attributable to decompensated heterophoria
Reduced binocular VA
Reduced reading speed
Elevated contrast sensitivity thresholds

Question 18

What does the fixation disparity curve determine?

Answer 18

Assess the vergence response over a range of vergence demands
Determine the type and magnitude of correction (prism, lens, or VT) for an oculomotor anomaly
Determine whether a heterophoria is fully compensated

Question 19

How are FD curves plotted?

Answer 19

By determining the magnitude of FD following the introduction of various BO and BI prisms to stimulate convergence and divergence (most patient get more eso with BI and more exo with BO)
X- prism power
Y- FD (minutes of arc)

Question 20

Describe the values of the Y-intercept and X-intercept on a typical FD curve.

Answer 20

Y-intercept: FD with no prism in place

X-intercept: prism power required to reduce FD to zero (associated phoria)

Question 21

What do the values for Slope and Center of Symmetry (CS) represent on a typical FD curve?

Answer 21

Slope: minutes of arc/prism diopter, usually measured using points at 3 BI, 0, and 3 BO

CS: flattest central portion of the curve (doesn’t coincide with Y-intercept)

Question 22

When are symptoms associated with the y-intercept according to Sheedy and Saladin?

Answer 22

If FD greater than or equal to 6 min of arc EXO
Or greater than or equal to 4 min of arc ESO

Question 23

Describe a Type 1 curve.

Answer 23

Shows increasing eso FD with BI (divergence) and increasing exo FD with BO (convergence)

Found in about 60% of population

Asymptomatic

Question 24

Why do patients have exo FD with BO?

Answer 24

Introducing BO prism associated stimulates convergence. Most patients will not converge the full amount of the prism, but will converge just enough to place image in PFA leaving small vergence error.

(Think of exo as underconverged rather then diverged)

Question 25

Why are patients eso with BI prism?

Answer 25

BI prism under associated stimulates divergence. Most patients will not diverge the full amount, will diverge just enough to pace image in PFA, leaving a small vergence error (eso FD)

Question 26

Describe a type 2 curve. How common is it? What do the results indicate?

Answer 26

Increasing eso FD with BI prism (divergence) (difficulty diverging, esophoria)
Minimal change in FD with BO prism (patient is good at converging)

25% of population

Question 27

Describe a type 3 curve. What do the results indicate? How common is it?

Answer 27

Increasing exo FD with BO prism (convergence) (difficulty converging, exophoria)
Minimal change in FD with BI prism (good at diverging)

10% of population

Question 28

Describe a type 4 curve. Is there any treatment? How common is it?

Answer 28

Shows minimal change in FD for both BO and BI prisms

No consensus on management

Rare 5% of patients

Question 29

How can we use FD curve to prescribe prism?

Answer 29

Prescribe the associated phoria (x-intercept)
Or
Prescribe prism that corresponds to the center of the flat region of the curve (flat is good)

Question 30

What are 3 treatments for FD?

Answer 30

Prisms shift FD curve to the right or left (BI shifts to the right)
Plus and minus lenses shift the FD curve down or up (plus add shift downward, reduce eso fixation disparity (reduce AS))
Vision training flattens the cure (makes vergence more accurate)

Question 31

Should we perform fixation disparity, dissociated phoria or both?

Answer 31

Both, if you want to know if the dissociated phoria is fully compensated (how much disparity vergence the patient is exerting)

Question 32

At distance is there a a correlation between FD and dissociated phoria?

Answer 32

There is no correlation between FD and dissociated phoria b/c there is limited spread of dissociated phorias at distance (most patients are close to orthophoric)

Question 33

When should you test FD?

Answer 33

If the patient has symptoms that may be due to an abnormal vergence response

When patient’s visual task place high demand on vergence system (magnitude or duration)
Occupational or avocational visual demands

When measuring the impact of VT