Fixation Disparity Flashcards
What is fixation disparity?
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.
Describe Eso and Exo fixation disparity?
Eso- over converged
Exo- underconverged
Image may still be single if point is within PFA
How are fixation disparities usually measured in general?
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.
Describe the mallet unit.
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
What is only one red line is seen during mallett unit?
One eye is being suppressed
How do we measure the deviation with mallett unit? What is this value called?
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
Describe the vectograph slide.
The polarized test works similarly to mallett unit
One vertical and one horizontal seen by OD, other set seen by OS
How can we measure fixation disparity?
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
What is the speedy disparometer?
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
What is a Saladin card?
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
Describe the Wesson card.
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
Describe the Hoya “Eye Genius”.
Measures FD then converts this into associated phoria
Patient presses buttons until lines line up
Can multiple of these FD tests be performed to provide similar results? What does this tell us about the reliability of these tests?
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
Why do we test FD?
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
What are the limitations of tests for heterophoria, vergence ranges, NPC, vergence facility, AC/A that FD covers?
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)
True/False: there are no other clinical tests that assess the complete vergence response at naturalistic stimulus levels other then FD.
True
What do patient with associated phoria report?
Decreased binocular cortical responses
Symptoms attributable to decompensated heterophoria
Reduced binocular VA
Reduced reading speed
Elevated contrast sensitivity thresholds
What does the fixation disparity curve determine?
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
How are FD curves plotted?
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)
Describe the values of the Y-intercept and X-intercept on a typical FD curve.
Y-intercept: FD with no prism in place
X-intercept: prism power required to reduce FD to zero (associated phoria)
What do the values for Slope and Center of Symmetry (CS) represent on a typical FD curve?
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)
When are symptoms associated with the y-intercept according to Sheedy and Saladin?
If FD greater than or equal to 6 min of arc EXO
Or greater than or equal to 4 min of arc ESO
Describe a Type 1 curve.
Shows increasing eso FD with BI (divergence) and increasing exo FD with BO (convergence)
Found in about 60% of population
Asymptomatic
Why do patients have exo FD with BO?
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)
Why are patients eso with BI prism?
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)
Describe a type 2 curve. How common is it? What do the results indicate?
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
Describe a type 3 curve. What do the results indicate? How common is it?
Increasing exo FD with BO prism (convergence) (difficulty converging, exophoria)
Minimal change in FD with BI prism (good at diverging)
10% of population
Describe a type 4 curve. Is there any treatment? How common is it?
Shows minimal change in FD for both BO and BI prisms
No consensus on management
Rare 5% of patients
How can we use FD curve to prescribe prism?
Prescribe the associated phoria (x-intercept)
Or
Prescribe prism that corresponds to the center of the flat region of the curve (flat is good)
What are 3 treatments for FD?
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)
Should we perform fixation disparity, dissociated phoria or both?
Both, if you want to know if the dissociated phoria is fully compensated (how much disparity vergence the patient is exerting)
At distance is there a a correlation between FD and dissociated phoria?
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)
When should you test FD?
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
