VS 117- Co-ops
3/12/98
V. Ko

Ch 21 -- Fixation disparity

Fixation disparity--
During attempted binocular fixation, the object of regard is not imaged on the 2 foveas. Fixation Disparity is a small error or lag of vergence response

Sometimes the vergence leads and sometimes it lags just like the accommodative response can lead or lag. The direction of FD (eso or exo) depends on the direction of the phoria, which pushes fixation around.

Fixation disparity sign convention:
eso (+): eyes don't diverge enough; residual convergence
exo (-): eyes don't converge enough; residual divergence

Measurement of FD:
Nonius lines-- two vertical lines are seen dichoptically; one line is seen by the left eye and the other line is seen by the right eye; you adjust
the horizontal position of the 2 lines until they are perceptually aligned (one over the other) but they are not necessarily physically aligned. The physical offset of the lines equals the vergence error of the two lines of sight. This is used to measure fixationdisparity.

Synonyms for fixation disparity (FD):
1. retinal slip-- during fusion your eyes don‚t quite align the target on correstponding points(the two foveas; the binocular image is slipping away from
corresponding points; the slippage is within Panum's Fusional Area. FD is
much less than Panum‚s Fusional Area.
2. microstrabismus-- very small eye turn; you can‚t see it with direct observation; The magnitude of FD varies from 1 min to 20 min of arc or about 1/3 of a degree. When measured with the cover tests give
How do we distinguish between fixation disparity and a true strabismus. In the case of true strabismus, the eyes deviate the same amount whether tested monocularly or binocularly. In contrast, FD is a small deviation when measured binocularly but when one eye is covered it deviates a lot to get to the phoria. The magnitude of the phoria is much greater than FD

Two types of vergence have different affects on fixation disparity:
1. fusional vergence-- fast; aligns the eyes quickly; not much durability
2. tonic vergence-- adapts; slowly shifts phoria to new convergence demand and is very durable

FD is related to the effort of fast fusional vergence. It provides us with a clinical way to assess the stress on fast fusional vergence that has not been relieved by tonic vergence. If a patient has lots of tonic vergence, the eyes will hold alignment without much effort and there's not much FD. If a patient has little tonic adaptable vergence then fast fusional vergence must sustain eye alignment and there will be retinal slippage or FD. Slippage is related to the stress placed on fast fusional
vergence.
Physical Analogy: We have a room that loses heat through an open door. The room temperature will drop until it drops below the setting of the thermostat. The furnace produce heat proportional to the drop in room temperature below the thermostat setting. Heat is produced by the furnace until the rate of production equals the amount of heat loss through the door (equilibrium). Then there is a steady state or equilibrium where the furnace is on by a constant amount and the room temperature remains stable. This is a feedback system that produces an error between the room temperature and the setting on the thermostat. This error will activate the heater just like FD will activate fusional convergence. FD is a misalignment of the 2 retinal images that stimulate an increase in fusional vergence innervation. This innervation matches the rate at which fusional vergence normally relaxes when one eye is occluded. Relaxation of fusional vergence--cover one eye, eye drifts back to phoric posture. Without fixation disparity, fusional vergence is unstable--we need FD to hold eyes in fixed position close to perfect binocular alignment
Big FD--system is stressed out
To correct, we need to reduce the phoria (error) or source of stress.
Clinically, we monitor how stressed the fusional vergence system is.

FD changes with heterophoria. Plot of fixation disparity as a function of exo and exo phoria.
· 1:1 line--1 min of arc of eso FD=1 prism diopter of esophoria. Esophores
have little or no tonic divergence to overcome esophoria. Fusional
vergence helps diverge the eyes.
There is a constant FD independent of the magnitude of exo phoria--Exophores adapt away their phoria under binocular viewing conditons. They have lots of tonic convergence to overcome exophoria. No stress is placed on the fusional vergence system.
Clinically, we use fixation disparity as a provacative test. We change the phoria with prisms and see how FD varies as we change the phoria increases. In the lab, we will see how FD changes with change in phoria produced by prisms and lenses. A base out (BO) prism stimulates convergence and produces exophoria. It
also cures esophoria by making the patient less eso and more exo. A BO
prism causes the image to move in toward your eyes. The eyes try to turn
in to see the target, the eyes don't turn in enough, so the eyes are divergent relative to the convergence demand and the error or phoria is in the exo or divergent direction. For example, a person wearing a 20 BO prism converges only 19 prism diopters. This person will have a slight divergence error (slight exofixation disparity).

FD = binocular vergence response - binocular vergence stimulus
FD is like a phoria (one eye occluded) but under binocular viewing conditions.

In the forced duction FD curve (Ch. 21, pg. 3 in the reader), the y-intercept is the habitual FD, what a person normally has without any prism. The x-intercept is the associated phoria, the amount of prism required to make FD = 0. (Optometrists often give patients this prism to relieve symptoms.) A flat slope indicates that the patient is helped by tonic vergence adaptation and can adapt to keep phoria constant. A steep slope indicates that the patient is not helped by adaptable tonic convergence and relies on fusional vergence. The patient has difficulty fusing and may have diplopia and red eyes. Dissociated phoria is the phoria. You occlude one eye and dissociate binocular vision. Dissociated phoria is greater than associated phoria (2 eyes open, feedback present for disparity) in all people.

Eating up the prism (a clinical description of prism adaptation)--refers to the tonic system adapting away the prism, so you get no change in fixation disparity as you add prism in front of a patient's eye; this usually takes 2-3 minutes

There are four characteristic shapes of forced duction FD curves (Ch. 21, pg 5)· Type I: symmetrical slopes for responses to BI and BO prism; characteristic of 20% of the normal population · Type II: steep slope on BI side and shallow slope on BO side; tonic prism adaptation response to exophoria produced by BO, but little tonic adaptation to esophoria produced by BI; most common FD curve; characteristic of 70% of the normal population · Type III: mirror image of Type II; steep slope on BO side and shallow slope on BI side; these people can adapt to BI since they diverge easily, but they lack divergent adaptation and show lots of exophoria while wearing BO; characteristic of 10% of normal population · Type IV: anomalous, unstable binocular vision problem; corresponding points of the two retinas shift in time; marginal strabismus; very rare; less than 1% of normal population

In lab, we will also add plus and minus lenses to stimulate accommodative
convergence and divergence.
(-) lens: stimulates accommodative convergence; esophoria
(+) lens: relaxes accommodative convergence, so the eyes diverge; exophoria

We can plot FD responses to lenses and to prisms and use these graphs to derive a plot of prism diopters vs. diopters. The slope of this plot gives us AC/A. (Ch. 21, pg 6) Because the same slope is derived from fixation disparity and changes in the phoria stimulated by monocular accommodation, this indicates that accommodative vergence works under binocular viewing conditions.

Foveal fusional stimuli produce smaller FD (similar to normal FD under normal conditions) than large peripheral stimuli which exaggerate FD.

Vertical vergence is also associated with vertical FD. Vertical FD curves have a smaller range and are linear. Most of vertical vergence is produced by tonic vergence adaptation. Examples of some plots are on pg 7 of Ch 21.

Clinical tests to measure FD:
1. Disparometer--the most precise way to measure FD; uses peripheral fusion stimulus, polaroid glasses; nonius lines are perceptually aligned. We used this in lab.
2. Mallett unit --uses foveal fixation stimulus, red-green glasses; nonius lines are physically aligned and patient reports where the lines are perceptually. You prescribe enough prism or lenses to cause the nonius lines to come into alignment (associated phoria).

Review for midterm:
Simple optometer
produces changes in image size when focus is changed
the plus optometer lens is placed in the spectacle plane which is 1.5 cm in front of eye
You should use the Gaussian equation (1/f = 1/i + 1/o) to calculate image distance (or use V = P + U from optics) for the simple optometer
Badal optometer
procudes constant image size when focus is varied
the plus optometer lens is placed so that its posterior focal pt is coincident with the anterior focal plane of the eye (approximately the spectacle plane)
use CF = P - P2 * d ; (d = distance of object to lens in meters, CF is in
diopters)
take 1/CF to get image distance in meters (or find image distance from lens using same eqation as for simple optometer, but remember to add the distance between the lens and the spectacle plane,
that is, add the focal length of the lens)
also remember CF = AR + RE + L