Assigned Readings: Griffin; pp 126-132
Recommended Reading:
Schor and Ciuffreda; Vergence Eye Movements: Ch. 14 pp. 465-485; Ch. 15 pp. 521-539
I. Introduction: Fixation disparity has gone by several terms since it's introduction nearly a century ago; with retinal slip being the most common term used in the "early days" of studying this binocular phenomenon . Fixation disparity, however, is the most appropriate and descriptive term used. Linkage of the words "slip" or "mistake" do not coincide with recent experimental evidence regarding this topic. No matter what it is called, fixation disparity measurement is a valuable clinical tool with a high diagnostic value (see the handout by Sheedy for specific evidence). To understand the consequences of a particular fixation disparity result, it is critical to analyze (as we did for vergence testing) the demands on our binocular system created by each step of these procedures.
A. Definition of terms
1. Definition of fixation disparity: small (< 30 minutes of arc) discrepancies between the vergence demand of the target and vergence response of the oculomotor system.
a. it must not exceed Panum's fusional area.
b. fixation disparity can be placed on a continuum, with "zero" fixation disparity at one end of the line to strabismus at the other.
c. this is also called the Y-intercept
2. Definition of associated phoria: the amount of prism required to reduce a fixation disparity to zero.
a. this is also called the X-intercept
b. the AO vectographic slide has a test to measure the associated phoria at distance
c. this value is most often used when quantifying the prismatic correction to be incorporated.
3. Definition of fast vergence movements: it is the first responder to the presence of vergence disparity. It makes rapid corrections for disparity errors created when a particular target is initially viewed. It also can't stand the heat of the binocular kitchen, because it's response decays in short order (10-15 sec)
a. in fact, it has been stated by several investigators that the stress and eventual overload of the fast vergence mechanism results in changes of fixation disparity
b. relief of the stress on this fast vergence response is necessary
i. the response decays in short order, resulting in increased fixation disparity, if not dealt with
ii. the fast vergence mechanism must be "spared" for use in subsequent vergence demand changes
4. Definition of slow vergence movements: this system responds to the effort of the fast vergence movement mechanism. It is not nearly as "leaky" and serves as "911" for the vergence control system.
a. it is believed to be responsible for what is called prism adaptation.
b. it is believed to be responsible for the slope of the fixation disparity function (discussed later)
B. Functional interpretations of fixation disparity:
1. Slop in vergence posture: a rather discredited theory which states that fixation disparity is the result of random error of the vergence system. If this were the case:
a. both eso and exo fixation disparities should be found in 50% of the population
b. the fixation disparity would change randomly on a particular individual
2. Response to physiological stress: Here, it is proposed that fixation disparity results from binocular stress that the vergence system cannot overcome. The very presence of fixation disparity, then, was considered to be indicative of a binocular vision system under stress. Although several clinical situations fit this model (see below), it is far too rigid to predict many aspects of fixation disparity.
a. fixation disparity usually exceeds normal levels when a binocular vision dysfunction is present
b. fixation disparity changes in response to prismatic demand
c. fixation disparity will change depending on the size of the stimulus
d. fixation disparity will change with movement of the stimulus inwar
3. A purposeful "wake up call" to the vergence system: this theory, which states that fixation disparity can play an active role in "nudging" the vergence system, to provide a constant driving force to maintain a desired vergence posture. This theory has recent experimental evidence in it's support. Many studies indicate that the fusional vergence system is leaky, that the response will decay over a short period of time. This model proposes that a fixation disparity is an intentional adjustment that constantly drives the vergence system. Using this model, it is possible for a patient to respond in two very different ways in order to set a fixation disparity purposefully to maintain a desired vergence position
-let us assume the pt. is 15Æ exophoric @ near.
a. the pt. responds to the near target (in this case a disparometer target) with fusional convergence. This, according to a proposed mechanism, leads to:
increased convergent accommodation (via CA/C) * reduction of the accommodative response (to maintain clarity of the target) *relaxation of accommodative convergence (via the AC/A ratio) * increased exophoria ** increased EXO fixation disparity
- it may be beneficial to set this exo fixation disparity to drive (nudge) the positive fusional vergence system into a desired position (ie overcome the phoria)
- the exo fixation disparity creates a posture close to the limits of Panum's fusional area; this drives fusional convergence
-the exo fixation disparity serves as a constant reminder to the fusional vergence system, which, as reported earlier is a "leaky" system
-let us assume the pt. is 15Æ exophoric @ near.
b. the pt. responds to the vergence demand (disparometer target) and ultimately reaches an eso fixation disparity. There are few attempts to develop a successful mechanism behind this particular response. The benefits for maintaining this eso posture may be:
- the eso fixation disparity creates a posture that allows a more central position within Panum's fusional area
- the eso fixation disparity attempts to compensate for a convergence system that the pt. "no longer trus
II. The Effects of Vergence Demand and Prism Adaptation On Fixation Disparity
A. Under most circumstances, it is expected that vergence demand has an effect on fixation disparity measurements.
1. During a NPC measurement, it has been shown that a "normal" patient will have increased exo fixation disparity, as the maximum convergence response is made.
2. It should not be too surprising, then, that fixation disparity would also change as vergence demand is applied. The direction and magnitude of this change, however, CANNOT be accurately predicted by Von Graeffe phoria results.
a. at least 25% of the patient population will have an associated phoria (the amount of prismatic demand necessary to reduce a fixation disparity to zero) and Von Graeffe phoria in opposite directions
b. These results (Sheedy and Saladin) indicate that fixation disparities must be conducted as an independent test of vergence function
3. Measurement of the change in disparity as prismatic demand (BO and BI) can be plotted. This plot has significant diagnostic value and is called a (forced duction fixation disparity curve). It is a plot of the disparity measurements through a slew of prismatic demand ranging from 9 BI to 9 BO (usually in three prism diopter steps).
a. points along this curve include:
i. the fixation disparity
ii. the associated phoria
iii. the limits of fusional ranges (BO and BI)
- testing should stop in a particular direction once diplopia is reported
- testing should stop in a particular direction once suppression is reported
b. the general shape and position of the curve is also important
i. Ogle defined four distinct types of curves.
ii. more recent investigations indicate an association between curve type and asthenopic complaints
iii. the slope of the curve in its central region has been postulated to indicate the amount of prism adaptation present (see below)
B. Slow fusional vergence responses are believed to be responsible for prism adaptation. For the most part, prism adaptation is a component of normal, complaint free binocular vision. It also may be a component of a patient who has successfully completed vision therapy or a patient who has well entrenched adaptations to a binocular vision dysfunction.
1. It can be defined as the amount of vergence response there is to prism application.
2. The vergence response may persist from a few seconds (ie 40 sec) to a few hours after removal of the prism, depending on the amount of time the prism was worn.
3. It has been postulated that patients who tend to prism adapt have fairly flat slopes to the central portion of their forced duction fixation disparity curves (ie. as low amounts of BI or BO prism is applied, their disparity measurement remains relatively constant).
4. In general, prism adaptation is seen as a positive sign. In fact it has been proposed as an important outcome of successful vision therapy.
a. these pts. are usually free from symptoms of asthenopia.
b. these pts., however, would not respond favorably to the application of prism to neutralize a phoria or fixation disparity
5. In general, a lack of prism adaptation indicates a binocular system under uncompensated stress.
a. these pts. are more likely to have symptoms of asthenopia.
b. these pts. would respond favorably to the application of prism to neutralize a phoria or fixation disparity
6. Perhaps prism adaptation can best be described in an example:
a. assume a pt. has 6Æ exophoria at near.
b. a 6Æ BI prism is applied and the pt is allowed to wear the prism under binocular viewing conditions for a few minutes
c. repeated phoria measurements (through the prism) would remain constant (6Æ EXO) at near
d. immediately on removal of the prism, the patient would be 12 XP’. This adaptation response would decay over the next few minutes.
7. To complete this clinical picture, one may draw a continuum (yes ...another one) from complete prism adaptation at one end to NO prism adaptation at the other.
8. Example of lack of prism adaptation:
a. assume a pt. has 6Æ exophoria at near.
b. a 6Æ BI prism is applied and the pt is allowed to wear the prism under binocular viewing conditions for a few minutes
c. a repeated phoria measurement (through the prism) would change to orthophoria at near
d. immediately on removal of the prism, the patient returns to 6 XP’
III. THE PROCEDURE:
The Disparometer is the most commonly used clinical instrument to measure fixation disparity. It consists of two 1.5 degree circular targets containing two oppositely polarized nonius lines. The circular border provides a fusion lock. The upper circle measures vertical fixation disparity. The right line is seen by the right eye. The lower circle measures horizontal fixation disparity. The top line is seen by the right eye. The straight lines separating the nonius lines prevents vergence eye movements to fuse the nonius lines. The letter charts on the face of the Disparometer provide accommodative controllers.
- The Phoropter
A. Place the Disparometer on the nearpoint rod at 40 cm and aim the overhead light at the fiber optic elements.
B. Place the patient behind the phoropter with habitual Rx or subjective Rx and the Risley prisms before each eye set at 0.
C. Set the Disparometer to 0 with the knob on the back.
D. Show the patient what "perfect alignment" looks like.
E. Place polarizing filters in the phoropter O.U.
- Vertical Fixation Disparity
A. Have the patient read the small letters on the sides of the top (horizontal nonius lines) section of the disparometer.
1. Since it is assumed that accommodation remains constant for this procedure, it is best to have the patient frequently check the letters to maintain a constant accommodative response during testing
B. Have the patient attend the upper circle.
C. Ask if both the horizontal nonius lines can be seen simultaneously.
D. If there is no suppression, ask if the lines appear horizontally straight across (as they appeared without the polarizing filters).
E. If they are not aligned, and the right line is lower than the left line, it indicates a right hyper fixation disparity.
1. You then must turn the knob on the back of the disparometer counterclokwise (to the R Hyper scale).
2 When you reach a point where the patient sees the nonius lines alligned, it is the vertical fixation disparity. RECORD THIS VALUE.
F. If they are not aligned, and the right line is higher than the left line, it indicates a left hyper fixation disparity.
1. You then must turn the knob on the back of the disparometer clockwise (to the L Hyper scale).
2 When you reach a point where the patient sees the nonius lines alligned, it is the vertical fixation disparity. RECORD THIS VALUE.
- Vertical Associated Phoria
A. Have the patient read the small letters on the sides of the top (horizontal nonius lines) section of the disparometer.
1. Since it is assumed that accommodation remains constant for this procedure, it is best to have the patient frequently check the letters to maintain a constant accommodative response during testing
B. Have the patient attend the upper circle.
C. Ask if both the horizontal nonius lines can be seen simultaneously.
D. If there is no suppression, ask if the lines appear horizontally straight across (as they appeared without the polarizing filters).
E. If they are not aligned, place increasing vertical prism in the neutralizing direction before one of the eyes until the nonius lines are aligned.
1. If the right line is lower relative to the left line, this is right hyper fixation disparity, so add base-down OD.
2. If the right line is higher relative to the right line, this is left hyper fixation disparity, so add base-down OS or base-up OD.
F. Record this as the vertical associated phoria.
- Horizontal Associated Phoria
A. Have the patient read the small letters on the sides of the bottom (horizontal nonius lines) section of the disparometer.
1. Since it is assumed that accommodation remains constant for this procedure, it is best to have the patient frequently check the letters to maintain a constant accommodative response during testing
B. Have the patient attend the lower circle.
C. Ask if both the vertical nonius lines can be seen simultaneously.
D. If there is no suppression, ask if the lines appear vertically alligned (as they appeared without the polarizing filters).
E. If they are not aligned, place increasing vertical prism in the neutralizing direction before one of the eyes until the nonius lines are aligned.
1. If the upper line is displaced to the right of the lower line, this is an ESO fixation disparity, so add base-out O.U.
2. If the upper line is displaced to the left of the lower line, this is an EXO fixation disparity, so add base-in O.U.
F. Record the value of prism required to allign the nonius lines as the horizontal associated phoria.
- Horizontal Forced Vergence Fixation Disparity Curve
A. Place the patient behind the phoropter with habitual Rx or subjective Rx and the Risley prisms before each eye set at 0.
B. Set the Disparometer to 0 with the knob on the back.
C. Have the patient read the small letters on the sides of the bottom (horizontal nonius lines) section of the disparometer.
1. Since it is assumed that accommodation remains constant for this procedure, it is best to have the patient frequently check the letters to maintain a constant accommodative response during testing
D. Ask the patient the relative location of the upper line.
1. If it appears to the right, this is eso fixation disparity. Turn the knob in the eso direction until the patient reports alignment of the nonius lines.
2. If it appears to the left, this is exo fixation disparity. Turn the knob in the exo direction until the patient reports alignment of the nonius lines.
3. Record this fixation disparity along the y-axis of the graph.
4. Return the Disparometer to the 0 position.
E. Introduce vergence demand by placing 3 pd base-in prism split between the eyes.
1. Measure the fixation disparity induced.
2. REMOVE RISLEY PRISM.
3. Record fixation disparity on the graph.
4. Return the Disparometer to the 0 position.
E. Repeat with 3 base-out prism split between the eyes.
F. Continue alternating between base-in and base-out in 3 prism diopter steps up to 9 pd.
G. Diplopia or suppression marks the end of the curve.
H. Note instability and movement of the nonius lines.
I. Determine a 3-point central slope of the forced vergence fixation disparity curve (between 3 base-in and 3 base-out).
IV. Important Parameters to Be Aware Of During Fixation Disparity Testing:
A. Take all measurements rapidly. Have the pt. respond as quickly as possible.
B. Make sure the pt. maintains clarity of the accommodative checks (on the disparometer)
1. It is assumed that the net accommodative response remains constant throughout a particular test procedure.
2. Failure to monitor accommodative response can taint data.
C. Remove the Risley prism between measures and return the disparometer to zero.
1. This prevents a prism adaptation after effect as testing proceeds from one prism amount to another.
2. It may be ideal to wait for the fixation disparity to return to the original amount before continuing testing (this is rarely, if ever done, however).
1) A patient has a 10 exophoria at near. From this value, it is assumed that this patient will have an exo fixation disparity and an associated phoria of 7.5 pd BI. Is there anything wrong with assumption?
2) Another patient with a 10 exophoria at near enters your office. This time you perform a full fixation disparity analysis. The results are provided below.
0 pd: 5 MAR Exo 6pd BI: 0 MAR Exo
3 pd BI: 3 MAR Exo 6pd BO: 9 MAR Exo
3pd BO: 7 MAR Exo 9pd BI: 2 MAR Eso
9pd BO: 14 MAR Exo
A. What is the patient's fixation disparity?
B. What is the patient's associated phoria?
C. What is the slope of the fixation disparity function?
D. What could be said about this patient's prism adaptation?
3. Why is it important to remove the Risley prisms between each measurement in a fixation disparity analysis?