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Chapter 21
FIXATION DISPARITY (ACCURACY OF VERGENCE)

Key words: disparity vergence, prism, lens, eso, exo

Outline
XII. Near Response: Pupil constriction, Accommodation and Vergence. 

Part IV: Accuracy of Vergence (Fixation Disparity)



Introduction
As discussed in Ch. 19, the four Maddox components of vergence are tonic vergence, disparity vergence, accommodative vergence, and proximal vergence.  The main strength of disparity vergence is its speed or rapid control.  Its main disadvantage is its lack of durability.   Disparity vergence is described as a leaky integrator.  It aligns the eyes but it doesn't hold their position well.  It requries a constant error or stimulus disparity (fixation disparity) to continue stimulating vergence to overcome the phoria.  Fixation disparity keeps generating more vergence innervation to compensate for the decay of fusional vergence innervation.

If we mainly relied upon fixation disparity to overcome the phoria we would have considerable eye strain.  However eye strain is relieved with adaptable tonic vergence.  The stress associated with disparity vergence is manifest as a small residual error of convergence known as fixation disparity.

 

 

Fig 21.1
Nonius misalignment and fixation disparity

Fixation disparity (FD) is the difference between the convergence angle under binocular viewing and the angle subtended by the target at the centers of rotation.  It is a failure of the two visual axes to simultaneously intersect the object of regard during attempted binocular fixation.  It is also referred to as a micro strabismus because the eyes are not binocularly aligned during attempted fusion.  Retinal slip is another synonym.  The main difference between a strabismus and FD is that the patient is not fusing with a strabismus but are with FD.  Fixation disparity is the residual error of a partially corrected phoria.  Usually a large phoria underlies a small fixation disparity.  FD is like the tip of the iceberg above the surface of the water and the phoria is the whole iceberg.

Measuring Fixation Disparity
Sign convention: FD can be horizontal or vertical and probably torsional but torsion is never measured clinically.  Horizontal FD are eso or exo corresponding to + and - respectively.  FD is measured directly with nonius lines that are physically aligned and the subject notes their perceived misalignment or indirectly with nonius lines that can be adjusted until they appear vertically aligned.  Tests using the direct measures include the Mallett unit and Wesson Card.  Both of these present fixed vertically aligned nonius lines that appear offset to the patient if there is a small FD.  Usually the patient makes qualitative judgments about whether the upper line seen by the right eye is to the right or left of the lower line.  The perceived location of the upper line is opposite to the deviation of the right eye.  If it is seen to the right by the right eye, then the right eye is pointing leftward or eso.  The converse is true for a leftward perception.  Many clinicians then place prisms before the patients eyes to reduced the perceived offset to zero.  The amount of prism that nulls the fixation disparity is called the associated phoria because it is a measure of the underlying stress or phoria present during binocular viewing that causes the fixation disparity to appear.  The associated phoria is distinguished from the more conventional dissociated phoria which is measured with one eye occluded.

Indirect measures are made with devices that allow the clinician to readjust the alignment of nonius lines until they appear aligned to the patient.  One such device is the Disparometer.  It contains many possible offsets of the nonius lines that can be selected by rotating a wheel behind the instrument.   The patient views the polarized nonius lines that are centered in a field of binocular print.  The wheel is  rotated until the lines appear vertically aligned.  This is a very sensitive binocular vernier task.  It is important to ask the patient to keep the letters clear at all times to prevent accommodative fluctuations from causing fluctuations of accommodative vergence.
 
 
Factors Influencing the Magnitude of Fixation Disparity

Phoria
Fixation disparity varies with the magnitude of the phoria.  This has been demonstrated with measures of FD with the Disparometer in many patients with different phorias.  FD can be plotted on the vertical axis and phoria on the horizontal.  When this is done it can be seen there is a linear increase of eso fixation disparity with esophoria which equals one arc min FD for every prism diopter of esophoria.  However on the right there is a shallow plateau of eso FD with increasing exophoria.  Most patients with exophoria have small amounts of FD but those with esophoria have large amounts of FD.  This is because it is more difficult to diverge the eyes and overcome an eso than it is to converge the eyes and overcome an exo.  Clinically you observe this with the forced duction or vergence test.  The base-out convergence range is at least twice as big as the divergence base-in range at far.  (They are about the same at near.)

 

Fig 21.2
Fixation Disparity curve.

 


Prism (and the Forced duction FD test)
It is possible to measure the relation between phoria and FD in a single patient by placing prisms before the eyes, causing the phoria to vary optically.  For example, you can make someone more eso by stimulating divergence of the eyes with base-in prism.  (Students are often confused by this because they remember that eso-phoric patients are corrected with base out, not base in.  But we are not correcting esophoria, we are increasing it with base-in prism.)  This prism stimulates divergence of the eyes.  If the eyes don't diverge enough there is a residual convergence error that is manifest as eso FD.  Using similar logic, it can be shown that base-out prism produces exophoria and exo FD.

The fixation disparity can be plotted as a function of added base-in prism on the left and base-out prism on the right.  As expected, base-in induces eso FD and base-out induces exo FD.  This produces a sigmoidal function of FD plotted as a function of prism value.  There are three key points on the FD curve.  (See Fig 21.2 above)  These are the Y intercept that represents the habitual FD when no prism is worn, the X intercept which is the associated phoria or the amount of prism that reduces FD to zero and the slope of the function between the X and Y intercept which represents how adaptable the vergence system is to prism.  It shows how well the tonic system is able to relieve the effort produced by the phasic disparity vergence system that is stimulated by prism.  The relationship between the amount of FD and the balanced activity of tonic and phasic vergence is based on the differences in ability of phasic and tonic vergence to sustain responses.  Phasic is fast but it is not sustained.  Tonic is slow but it is easily sustained.  When phasic vergence is active, the vergence response tends to slip or go into error which stimulates further effort to maintain the response.  When the tonic response is present, vergence doesn't decay or slip and no extra effort is needed to maintain the response.  When the tonic adaptation is incomplete, some fusional vergence is necessary, and fixation disparity acts as a stimulus to evoke the necessary fusional vergence needed to align the eyes precisely with the target of regard.  FD is a reflection of the amount of fusional vergence that is active in the total vergence response.
 
The amount of eso and exo FD induced by base-in and base-out prism varies between subjects and the response patterns fall into four general categories.  (See Fig 21.3 below)

 

 

Fig 21.3
Four general categories (types) of FD response patterns.

These categories describe the degree of phasic and tonic activity of the vergence system for a given individual. The type I FD curve has symmetrical slopes for responses to base-in and base-out prism.  In contrast, type II curves have steep slopes on the base-in side and shallow slopes on the base-out side.  This indicates that there is a tonic prism adaptation response to the exophoria produced by base-out prism but that there is little tonic adaptation to the esophroia produced by the base-in prism and the phasic disparity vergence system is left to maintain eye alignment as best it can.  It uses FD as an additional stimulus to help maintain its response.  Type III curve is the mirror image of type II.  Here the slope is steep on the base-out exo side and shallow on the base-in eso side.  Interestingly these patients adapt readily to base-in prism, they diverge easily but they have trouble adapting to base-out prism and show lots of exophoria while wearing base-out. The type IV curve is anomalous.  It has a very limited range due to the failure of vergence to respond to moderate amounts of prism.  When large prism is placed before one of these patients, they become strabismic.

In summary, we can think of the forced duction fixation disparity test as a provocative test similar to tests of interocular pressure after drinking water or tests of blood pressure and pulse rate after exercise.  We want to know how the oculomotor system handles the stress of fusional disparity stimuli.  If it copes well by adapting tonic vergence, there is little residual phoria, but if it is unable to fuse without extreme effort there is residual fixation disparity.  We can estimate the underlying stress by measuring the associated phoria.  This is a measure of the amount of fusional vergence yet to be adapted.  The slope of the function between the x and y intercepts indicates how readily the vergence adapts to varying amounts of prism.  If it cannot change its adapted state quickly, the slope will change steeply from one prism value to the next.  However if the response is rapid and complete, the slope will be very flat.  A flat slope is a sign of a highly adaptable system. A steep one is a sign of someone who needs to learn to adapt using orthoptics eye exercises.

Lenses - Accommodative Vergence
Horizontal phoria can also be changed with binocular accommodation because of the associated stimulation of accommodative convergence.  Minus lenses stimulate additional convergence, which produces an esophoria and associated eso fixation disparity.  Similarly, plus lenses relax accommodation and produce an exophoria which is associated with exo-fixation disparity.  If the AC/A ratio is known, it is possible to predict the equivalent lens and prism that will produce the same change in fixation disparity and the change in phoria that produces it.  You can also predict the amount of lens that will reduce fixation disparity to zero once you know the associated phoria and the AC/A ratio.

 

Fig 21.4
Fixation Disparity responses to Lenses (B) and Prism (A), and Deviation of Associated AC/A ratio (C).


Vertical Fixation Disparity
Vertical vergence is also associated with vertical fixation disparity.  It differs from the horizontal FD curves in that vertical FD curves are straight linear functions.  This indicates that most of vertical vergence is accomplished with vergence adaptation and that there is very little or limited fast phasic vertical vergence.  Clinically this is seen by the very small amplitude of vergence ranges measured with vertical prism duction test.  The average amplitude is only 1-2 prism diopters whereas horizontal vergence amplitudes are easily 10 prism diopters.  Larger ranges of vertical prism vergence are possible if more time is given to fuse and adapt the vertical phoria.

 

 

Fig 21.5
Vertical fixation disparity curves.


Review Questions:

  1. List two synonyms for fixation disparity.
  2. Define the associated phoria.
  3. What distinguishes types I, II and III forced duction fixation disparity curves?

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