During patient examinations, fusional vergence measurements are often performed at distance and near, using loose or rotary prisms in free space or through a phoropter, or both. Because fixation disparity measures assess ocular alignment through an extended range of prism powers, rotary prism measurement of vergence ranges through the phoropter can frequently be eliminated from the test sequence when forced vergence fixation disparity curves are measured. Loose prism measurements, which are presented in discrete steps, provide a useful indication of fusional ranges as well as recovery ability.

Increasing amounts of prism can be used to change fusional vergence demand while fixation disparity is monitored. Graphic representation of the results portrays what has come to be called the forced vergence fixation disparity curve. As vergence demand is altered, the amount of fixation disparity may change. Usually, an increase in base-out prism increases exo-fixation disparity and an increase in base-in prism increases eso-fixation disparity.¹⁰ Curves with steep slopes, large associated phorias, and substantial fixation disparities are found more frequently when testing symptomatic patients.¹¹ Asymptomatic patients usually have type I curves. Other curve types are frequently associated with patients who have large dissociated heterophorias (type II eso, type III exo) or unstable binocularity (type IV).^6,12 Figure 15.1 illustrates distance and near-horizontal forced vergence fixation disparity curve types found in the examination of clinic populations.¹³

Most people adapt more to one type of lateral prism (base-in or base-out) than the other. These variations in adaptation determine the shape of the curve.¹⁴ Type I curves result from approximately equal adaptation to base-in and base-out prism stress. However, properly differentiating type I from type II curves often requires
that the base-out end of the range be carried to the full extent of the disparity (fusional) vergence range,¹⁵ so that fusional (disparity) convergence and fixation disparity relationships are fully evaluated. Type II curves originate from more adaptation to base-out prism and less adaptation through base-in prism. Persons who adapt more completely to base-in than base-out prism have more fixation disparity with base-out prism, and have a type III curve.

Center of symmetry (CS) refers to the area of the forced vergence fixation disparity curve where vergence adaptation occurs most readily to changes in disparity (fusional) vergence⁶ (Fig. 15.2).

The associated phoria is defined as the amount of prism required to reduce fixation disparity to zero.⁶ The associated phoria (x-intercept, Fig. 15.2) is the point usually measured clinically. Figure 15.2 shows the location of the associated phoria plotted on a forced vergence fixation disparity graph. The magnitude of the associated phoria is generally significantly less than the dissociated phoria for exophoric patients and is often greater for esophoric patients.⁶ In addition, there are occasional patients who have paradoxical responses where, for example, an exophoric patient has an eso-associated phoria (see Chapter 16—Fig. 16.9).⁶ Associated phoria magnitude is influenced by proximal vergence and vergence adaptation.^16,17 In addition to vergence adaptation, factors such as suppression and reduced peripheral fusion¹⁷ may influence the associated phoria magnitude by limiting responses to vergence stimuli in the peripheral retina.^6,18

The associated phoria is determined at distance using a Mallett unit, American Optical (AO) vectographic chart, or Bernell lantern and neutralizing any reported misalignment of the lines with appropriate prism. The same concept applies to near testing, where the associated phoria can be determined using the AO near vectographic card, Borish card, or near the Mallett unit (Fig. 15.3).

The associated phoria is only one aspect of the fixation disparity curve. Although measurement of the associated vertical phoria is the method of choice for determination of vertical prism corrections (Chapter 14), the lateral associated phoria is not as clinically useful. To utilize fixation disparity assessment for prescription of horizontal prism, it is necessary to consider the lateral associated phoria along with other forced vergence
fixation disparity curve parameters (slope, curve type, and fixation disparity).³ Use of only the lateral associated phoria is not adequate, because there is then a tendency to overestimate needed prism corrections for patients with esodeviations.

Indeed, for esophoric patients, it is often useful to start prism analysis through prism base-out equal to one-third of the distance esophoria. Thus, when there is distance esophoria of 6 Δ, a prism of 2 Δ base-out can be used as a starting prism, and the vergence ranges, distance associated phoria/fixation disparity, and stereopsis can be assessed to determine whether the prism is the appropriate amount. More normal vergence ranges, a zero-associated phoria, or improved stereopsis through the prism indicate that the chosen prism power is appropriate; often the patient will also report that distance visual acuity is more clear. Starting prism determination with one-third of the distance esophoria allows a more rapid clinical determination of the prism needed. The distance phoria is used because the near phoria is more influenced by the accommodative convergence to accommodation (AC/A) ratio, and added lenses are used to alter this relation, whereas the distance phoria can only be treated by prism or vision therapy.

Fixation disparities are small misalignments of the visual axis (usually less than 6 seconds of arc) under binocular conditions.¹³ Fixation disparity exists when images of a bifixated object are not on exactly corresponding points, but remain within the Panum area. The retinal images are seen singly, even though the foveas may have a misalignment of up to 30 seconds.¹⁰ The amount of misalignment (fixation disparity) is dependent on the size of the Panum area. When lenses or prism or both are added and the eyes maintain binocular fixation, there is frequently increasing misalignment of the visual axis (still maintained within the Panum area).⁶

The forced vergence fixation disparity curve is graphically represented by plotting the change in fixation disparity with change in lens or prism.⁶ The actual fixation disparity measurement, which is not generally measured clinically except by instruments designed for the purpose (such as the Disparometer, Woolf card, or Wesson card), is the point where the curve crosses the y-axis (Fig. 15.2). Typically the fixation disparity is in the same direction as the heterophoria¹; however, there are patients (who are typically exophoric) who have paradoxical patterns where there is an eso-fixation disparity when fusing but an exo-dissociated phoria measured by cover test or Maddox rod (see Chapter 16, Fig. 16.8).

The slope, which can easily be determined using a forced vergence fixation disparity graph, can be estimated by calculating the change in fixation disparity between prism demands of 3 Δ base-out and 3 Δ base-in. When a patient has a flat fixation disparity curve, prism prescriptions based on shifting the CS (midpoint of the flattest portion) of the curve (Fig. 15.2) toward the y-axis are generally more successful in reducing symptoms, enhancing binocularity, and yielding lower prism prescriptions than those based on either dissociated or associated phoria measurements. These prescriptions optimize responses of vergence adaptation to convergence and divergence responses.¹⁴

Patients with steep curves can often be treated using vision therapy. When the curves do not flatten with vision therapy, the patient may be viewed as being resistant to the development of vergence adaptation.¹⁴ For these patients, prism can be prescribed, based on the associated phoria (prism to reduce fixation disparity to
zero). Measures of the CS of forced vergence fixation disparity curves, the fixation disparity, and the associated phoria allow analysis of tonic disturbances of vergence adaptation.