Fixation Disparity

What is fixation disparity?

2 visual axes fail to intersect the object under associated conditions

During fixtion disparity testing what is being tested?

all vergence components (tonic, proximal, accommodtive, and disparity)

Fixation disparity exist when what happens?

images of a binocularly fixated object do not fall on corresponding retinal points but are imaged within Panum's area

-target is still imaged as single

What is Panum's area in the fovea?

6-10 minutes of arc

What type of deviations tend to be present when looking at fixation disparity?

very small deviations (less than 6-10 minutes of arc)

-almost like a small tropia that is not visible with cover test

Is the target seen as single or double in fixation disparity?

single

What type of fixation disparity exists when the eyes are over converged?

Eso fixation disparity

What type of disparity exists when the eyes are under converged?

Exo fixationdisparity

What is done to test fixation disparity?

-majority of the visual field is viewed binocularly to provide strom fusional stimulus

-two small areas are veimed monocularly and patient is asked to judge the alignment of these polarized markers

What is mallett unit?

test for fixation disparity

-two red strips are polarized one seen by the left eye and one seen by the right

-remainder of chart (consits of letters) is viewed binocularly providing a good stimulus to fusion

What does patient have to wear inorder for most fixation disparity testing?

polarized filters

What is the result if the 2 red lines are exactly one beneath the other in malet unit?

no fixation disparity is present

Which line do we assume is seen by the right and left eye for horizontal fixational disparity in Mallett unit?

-the top bar is seen by the right eye

-the bottom bar is seen by the left eye

Which line do we assume is seen by the right and left eye for vertical fixation disparity in mallet unit?

-left bar is seen by OD

-right eye is seen by OS

For Mallet unit if the OD image is to the right of the OS image what type of fixation disparity is there?

eso

For Mallet unit if the OD image is seen to to the left of the OS image what type of fixation disparity is there?

Exo

Does the fixation disparity have to be just in one eye or can it be in both? How can you tell?

it can be in one or in both

-you can tell by asking the patient if each line is seen directly in the center above/below the X or if they are off to the side

-only one is off to side that means deviation is only in one eye

If the left bar in seen higher in Mallett unit what is the fixation disparity?

L hyper fixation disparity

If the left bar (seen OD) is seen higher and the right bar (seen OS) is seen lower what is the fixation disparity?

L hyper fixation disparity that is present in both eyes

Can you detect incyclo and excyclo deviations with Mallett unit?

yes if patient sees one or both bars tilted at all

What is the patient only sees one of the two bars in Mallett unit?

this means that the patient is suppressing an eye

How can you measure fixation disparity withMallett unit?

add prism infront of the deviated eye

What is assocciated phoria?

the amount of prism that eliminated the fixation disparity

Which eye should the prism be placed infront of for fixation disparity?

if only one eye has the FD then just infront of that one eye

What is vetrograph slide?

another way to measure fixation disparity very similar to mallett

-uses a small central target with four lines around the outside

-top and line on left are seen by the right eye

-polarizers are used

What is angle alpha?

the angle between the where the eye is pointing and where it should be pointing

-another way to measure fixation disparity

How is angle alpha determined?

you can more the bars of Mallett until they are aligned

What is sheedy disparometer?

device that is hung on the near rod to measure fixation disparity in minutes of arc

-letters are presented around the chart to act as good fusional stimulus

-knobs on the back of the chart are used to adjust the alignment of the bars until the patient reports they are aligned

What is Saladin card?

card is put in place of the near card and can be used to test...

-fusional disparity

-dynamic ret

-maddox rod

-logMAR VA

-vergence ranges

What is Wesson card?

postcard sized witha central colored square used to measure fixation disparity

-the black arrow is seen by one eye and the colored eyes are seen by the other eye

-depending on which line the arrow lines up with will determine the amount of deviation

What is Hoya EyeGenius?

measures fixation disparity and coverts this into associated phoria based on the registration analysis

-not found in the US

Can you compare the data from two different fixational disparity tests?

No they are not comparable and the instruments can not be used interchangably

Why do we test fixation disparity?

assess the vergence response when all 4 components are active

-closer to real world conditions

Don't other tests measure the same information as fixation disparity?

Yes but only in isolation, fixation disparity testing is the only test that can look at all 4 vergence responses at the same time

Who should you test fixation disparity on?

patient who report...

-decreased binocular cortical responses

-symptoms attributed to decompensated heterophoria

-reduced binocular VA

-reduced reading spead

-elevated contrast sensitivity

Is there a relationship between symptoms and associated phoria?

more sever symptoms are correlated to a higher degree of horizontal associated phoria

What is a fixation disparity curve?

used to assess the vergence response over a range of vergence demands

Why do we obtain fixation disparity curve?

-to determine the type and magnitude of correction for oculomotor anomaly

-to determine whether the heterophoria is fully compensated for

How is fixation disparity curve obtained?

by placing different amount of BI and BO prism infront of patient to stimulate different vergence demands

Is base in convergence or divergence? What about base out?

divergence

-base out convergence

What does the y intercept of fixation disparity curve tell you?

magnitude of fixation disparity with no prism in place

What does the x intercept of fixation disparity curve tell you?

prism power requird to reduce fixation disparity to zero

-measures the associated phoria

What does the slope of fixation disparity curve tell you?

change in fixational disparity in minutes of arc per prism diopter

Where is the slope of fixation disparity curve measured?

typically between 3 prism diopters base in, zero, and 3 prism diopters base out

What is the center of symmetry for fixation disparity curve?

the flattest central portion of the curve

Does the center of symmetry always align with the y intercept?

NO

For a typical fixation disparity curve, Sheedy and Saladin determined that deviations above what would lead to symptoms?

-more that 6 minutes of arc exo

-more than 4 minutes of arc eso

What is the shape of a type 1 curve?

-increasing eso FD with increasing base in prism

-increasing exo FD with increasing base out prism

What percent of the population has type 1 FD curve?

about 60%

Do patients with a type 1 FD curve usually have symptoms?

NO

Why is type 1 FD curve most common?

most people will do just enough to see the target as single and clear

-this means BO prism will makes them underconverged and BI prism will make them over converged

What is the shapes of type 2 FD curve?

-increasing eso FD with increasing base in prism

-Minimal change in FD with increasing BO prism

What percent of the population has type 2 FD curve?

25% of population

What is indicated by a type 2 curve?

patient is good at converging accurately with minimal error

Is a flat line good for FD curves?

yes

What type of deviation do most patient with type 2 curve have?

esophoria due to difficulty diverging with BI prism

What is the shape of type 3 FD curve?

-increasing exo with BO prism

-minimal change in FD with BI prism

What percent of the population has a type 3 curve?

about 10%

What is indicated by a type 3 FD curve?

the patient is good at diverging

What type of deviation do patientw with a type 3 curve usually have?

exophoria due to difficulty converging with the BO prism

What is the shape of a type 4 FD curve?

minimal change in deviation with BI and BO prism

What percent of population has a type 4 FD curve?

about 5%

Do patients with type 4 curve have symptoms?

yes a lot of them do

-the reason for this is unknown and there is little consensus on how to treat them

How do you determine how much prism do you prescribe for fixation disparity?

determine the FD curve then...

1) prescribe the associated prism

2) prescribe the prism that corresponds to the center of the flat region of the curve

What do prisms do to the FD curve?

shift the curve to the left or right

What do lenses do to the FD curve?

shift the curve up or down

What do plus lenses do to the FD curve?

shift the curve down

What do minus lenses do to the FD curve?

shift the curve up

What can we do if we want to flatten the patients FD curve?

vision therapy training

How do you decide which treatment is best for a patient with symptoms associated with fixation disparity?

the shape of their FD curve

What treatment tends to be prescribe for type 1 curves? (can’t find in slide)

plus and minus lenses or VT

What treatment tends to be presribed for type 2 curves?

Base in prism

What treatment tends to be presribed for type 3 curves?

Base out prism

What does adding base in prism do to the FD curve?

shifts to the right

What does adding base out prism do to the FD curve?

shifts to the left

What should we really measure fixation disparity or phoria?

ideally both so you can determine if the dissociated prism is compensated for

How can you determine how much disparity vergence a patient is exerting?

subtract fixational disparity from phoria measurment

What will happen if a patient is compensation 100% for their phoria with disparity vergence?

fixation disparity will be ortho

If a person has an ESO phoria at near what type of fixation disparity are they most likely to have?

eso

If a person has an EXO phoria at near what type of fixation disparity are they most likely to have?

exo

Can you determine what type of fixation dispatiry a patient will have based on their distance phoria?

NO

When should you test fixation disparity?

-patient has symptoms that may be due to abnormal vergence response

-patients visual tasks place high demands on vergence

-monitoring the impact of vision therapy

What is better at predicting the presence of symptoms at near: associated phoria or dissociated phoria?

associated phoria (for both eso and exo patients)

Waht is the only clinical test that will measure associated vergence response?

fixation disparity

What can help you determine what type of treatment for vergence issues?

fixation disparity