Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5...
Transcript of Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5...
![Page 1: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/1.jpg)
Outline:
Fixation Disparity I Definitions & Purpose
Retinal slip
Micro Strabismus
Closed-loop error
Prevents decay of vergence
II. Sign Convention
III. Measurement with Nonius lines
III. Factors influencing magnitude of FD
Phoria
Prism
Lenses and Accommodative vergence
IV. Provable test- Forced duction FD curve
Prims induced
Associated phoria
slope
relation to prism adaptation
Lens induced
V. V. Relation to AC/A ratio
VI. VI. Vertical Fixation Disparity
![Page 2: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/2.jpg)
Four Maddox Components of Convergence
Tonic
Proximal
Fusional
Accommodative
![Page 3: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/3.jpg)
Fusional vergence component is a buffer that takes up
the slack left by the other four components.
The “slack” or residual error of the sum of tonic,
proximal and accommodative vergence is measured as
the phoria.
Stress is assumed to be related to the amount of fusional
vergence effort under binocular viewing conditions.
Prism adaptation reduces the demand on fusional
vergence by increasing tonic vergence under binocular
viewing conditions.
![Page 4: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/4.jpg)
Shortfall of the Maddox classification in the classical
case analysis:
The Maddox classification assumes that the phoria,
measured under monocular conditions, equals the
residual error under binocular viewing conditions.
Maddox does not consider prism adaptation, that
reduces the demand on fusional vergence under
binocular viewing conditions.
![Page 5: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/5.jpg)
Instead of assuming the dissociated phoria equals the
demand on fusional vergence under binocular viewing
conditions, why not measure the stress on fusional
vergence directly under binocular viewing conditions?
How can we assess the demand under binocular
viewing conditions?
Fixation disparity is a small error under binocular
viewing conditions that is proportional to the demands
on fusional vergence.
![Page 6: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/6.jpg)
Fixation Disparity is a steady-state error- Room
temperature thermostat analogy. Under binocular viewing
conditions, it keeps the eyes from drifting back to their
position of rest.
Fixation disparity increases when we fuse with the phasic
component of disparity vergence. It is analogous to
trying to heat a room with open windows.
Fixation disparity decreases when we fuse with adaptable
tonic vergence. This is analogous to heating a room with
sealed windows.
![Page 7: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/7.jpg)
Fixation Disparity - Definition
• Fixation disparity is the difference between the
convergence angle under binocular viewing and the angle
subtended by the target at the centers of rotation
Fixation Disparity
Fixation Plane
Fixation Plane
Convergence
Point
ESO Fixation Disparity EXO Fixation Disparity
![Page 8: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/8.jpg)
![Page 9: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/9.jpg)
Fixation disparity: A small error in convergence or
eye alignment in the presence of feedback from
disparity.
Phoria A vergence error without fusion (i.e. when one
eye is occluded- i.e. open-loop or without feedback
from disparity). AKA dissociated phoria
Synonyms-
Retinal slip
Micro- strabismus
![Page 10: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/10.jpg)
Sign Convention:
Eso (+) excessive convergence error
Exo (-) excessive divergence error
Units of measurement:
Minutes of arc.
Measurement tools using Nonius lines:Nonius lines are
a measure of Dichoptic Vernier Acuity
Mallett Unit- Measures the amount of prism needed to
reduce fixation disparity to zero (associated phoria)
Disparometer- Measures the fixation disparity directly.
Demonstrations:
![Page 11: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/11.jpg)
Nonius lines are adjusted to lie on each fovea and their offset
indicates the small vergence error, i.e. Fixation Disparity
![Page 12: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/12.jpg)
Factors that influence the magnitude of FD.
1) Phoria Eso FD increases with Eso phoria
Exo FD doesn’t increase with Exo phoria
1) Prisms (Base-out ∆ reduces Eso and increases Exo)
(Base-in ∆ reduces Exo and increases Eso)
2) Lenses (Plus lenses reduce Eso and increases Exo)
(Minus lenses reduce Exo and increase Eso)
Forced Duction (prism induced) Fixation Disparity Curve:
A provocative tests of prism adaptation.
Measure changes in fixation disparity with added prisms.
.
![Page 13: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/13.jpg)
Phoria Status and Fixation Disparity
Three things are of interest to us…
1. The Y-intercept (Habitual FD)
2. The X-intercept (Associated Phoria)
3. The slope (∆∆∆∆y / ∆∆∆∆x)
![Page 14: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/14.jpg)
Associated phoria- the amount of prism that reduces
fixation disparity to zero- good index of how
much prism to prescribe.
Dissociated phoria- the amount of prism that
reduces the phoria to zero under open-loop
conditions. Standard clinical phoria measure.
![Page 15: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/15.jpg)
Four categories of forced duction Fixation Disparity Curves
Most common
Adapts well to
Base In prism
Adapts well to
base-out prism
Adapts
equally to
Base in and
out prism
Rare anomalous
response found in
strabismus
20%
75%
5%
![Page 16: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/16.jpg)
Lens induced FD Prism induced FD
Plot lenses against prisms that produce the same fixation disparity
Estimate the AC/A
under binocular
conditions.
![Page 17: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/17.jpg)
Vertical Fixation Disparity induced with vertical prism
![Page 18: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/18.jpg)
Could there be Cyclo vergence fixation disparity?
Yes, but its not measured clinically.
![Page 19: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/19.jpg)
![Page 20: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/20.jpg)
![Page 21: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/21.jpg)
![Page 22: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/22.jpg)
Measuring Fixation Disparity
• Measured using Nonius lines
• View of right and left eyes’ separated using red-green goggles or polaroid goggles
• Patient makes qualitative judgment about whether the upper line is to the right or to the
left of the lower line
• The perceived location of the upper line is opposite to the deviation of the eye
Nonius
lines
![Page 23: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/23.jpg)
![Page 24: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/24.jpg)
Measuring Fixation Disparity
Mallett Unit
Demonstrate fixation
disparometer
Demonstrate fixation
disparity software
![Page 25: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/25.jpg)
Prism induced fixation disparity curve
![Page 26: Outline: Fixation Disparity I Definitions & Purpose ...schorlab.berkeley.edu/passpro/Lecture 5 slides.pdf1) Phoria Eso FD increases with Eso phoria Exo FD doesn’t increase with Exo](https://reader034.fdocuments.us/reader034/viewer/2022052012/602951468bdbca70c66e46bc/html5/thumbnails/26.jpg)