Interactions Between Accommodation and Vergence
Binocular vision dysfunctions occur because of excessive tonic vergence; abnormal interactions of vergence, proximal vergence, and accommodation1; and/or deficient vergence (prism) adaptation.2 Analysis of tonic vergence disorders3 and accommodation/vergence interactions2 suggests that classic analysis techniques are often not sufficient. More than one analysis system has traditionally been used to determine whether existing binocular deficiencies are related to existing symptoms.4 Because of the deficiencies in current analysis techniques, we introduced integrative analysis in Chapter 2 of this book.
Although many areas remain to be fully explored, in this chapter we discuss binocular vision from a theoretical and practical clinical viewpoint to introduce the concepts behind integrative analysis. Effects of the magnitude of proximal vergence and the influence of the depth of focus of the eye, lag of accommodation, and tonic vergence are considered and related to a dual interactive model of accommodation and vergence. We discuss many tests of binocular function that are currently performed clinically and relate them to the model. In addition, we suggest new tests that may be used in the future to provide diagnostic information (e.g., measurement of the convergence accommodation to convergence [CA/C] ratio and/or proximal vergence).
Analysis Approaches
TRADITIONAL ANALYSIS TECHNIQUES
Three techniques have been used for the analysis of examination data to determine whether vergence or accommodative deficiencies exist in sufficient magnitude to result in symptoms. The graphical method of analysis5 emphasizes the role of the vergence system in the etiology of symptoms. The analytical method emphasizes the role of accommodation6 and the normative method is not selective.7
Graphical Analysis
Graphical analysis is based on the hierarchy of vergence mechanisms originally described by Maddox. Graphical analysis is designed to predict how tonic, accommodative, and fusional or disparity vergence8 will result in the final eye position. Proximal vergence is generally not represented graphically.9 Conventional graphical analysis is based on the following:
Distance dissociated heterophoria
Accommodative convergence to accommodation (AC/A) ratio
Positive relative convergence
Negative relative convergence
Amplitude of accommodation
Using these parameters, the zone of clear single binocular vision is graphed10 to visually represent the ranges of stimulus values through which the accommodative or vergence system or both can maintain binocular vision (Fig. 16.1).
The relationship of the heterophoria and AC/A ratio to relative vergence measurements is used to determine visual efficiency and, subsequently, to plan therapeutic intervention. In classic methods, the vertical lines of the zone are the reciprocal of the stimulus AC/A and represent the vergence limits of clear single binocular vision.10 Proponents of graphical analysis imply that deficits or excesses of tonic and accommodative vergence are compensated by disparity (fusional) vergence, and that excessive demands on disparity (fusional) vergence cause asthenopic symptoms.11 Diagnostic criteria, such as those of Sheard12 and Percival,13 have been adopted to determine lens or prism corrections that reduce the disparity (fusional)
vergence demand in amounts needed to minimize symptoms. Unfortunately, using graphical analysis, it is sometimes difficult to identify the underlying problem when a purely accommodative dysfunction exists.
vergence demand in amounts needed to minimize symptoms. Unfortunately, using graphical analysis, it is sometimes difficult to identify the underlying problem when a purely accommodative dysfunction exists.
 ▪ Figure 16.1 By using the parameters of distance dissociated heterophoria, accommodative convergence to accommodation, positive relative convergence, negative relative convergence, and amplitude of accommodation, the zone of clear single binocular vision is graphed to visually represent the ranges of stimulus values through which the accommodative or vergence system or both can maintain binocular vision. |
Analytical Analysis
The physiologic basis of analytical analysis is that faulty accommodation forces the visual system to compensate, resulting in the development of a vergence dysfunction.14 The aspect of accommodation emphasized in this analysis is posture (lag). Aspects such as facility, sustaining ability, velocity, and amplitude are not typically considered. Further, nearly all anomalies (up to 95%) are given an accommodative basis, even when other causes often seem equally or, in some cases, even more likely.
Normative Analysis
Normative analysis involves determination of how individual test results (phorias, vergence and accommodative amplitudes, accommodation/vergence interactions) deviate from clinical norms.15 Normative analysis, which is most accurate when diagnosing a single problem, is not as accurate for diagnosis when there is excessive tonic vergence combined with abnormal vergence/accommodation interactions. Multiple interactions can be analyzed using a mechanistic approach16 with partial success.
Key Concepts
The flaw with current systems that are used to analyze the results of binocular visual function tests is that many important accommodation and vergence interactions can only be adequately evaluated under binocular conditions. For example, demands on disparity (fusional) vergence during binocular viewing may be dramatically different from predictions of stress on sensory and motor fusion based on measures of the dissociated heterophoria.17 Variations between results determined using monocular and binocular testing may be caused by two components neglected in the Maddox hierarchy—vergence (prism) adaptation2 and CA/C ratio—and an additional one not usually considered in any analysis, proximal vergence.
DISSOCIATED VERSUS ASSOCIATED TESTING
Binocular testing provides a more complete picture of the interactions between binocular components than traditional systems that compare various monocular (dissociated) measures. The fixation disparity curve, measured at distance as well as near fixation distances, provides a binocular test that allows the clinician to determine a treatment that results in optimal binocularity. With the fixation disparity testing described in Chapter 15, residual misalignment can be directly determined; it is unnecessary to assume that a measured latent neuromuscular bias (heterophoria) also exists and causes symptoms noted during binocular viewing. These tests are useful clinically to determine prism prescriptions for patients who have esophorias or hyperphorias and as a means to monitor vision therapy programs.
THEORETICAL INTERACTIONS
A previously published representation18 of the interactions of accommodation and vergence is shown in Figure 16.2. The lower block diagram represents the components that describe vergence responses, and the upper section represents the accommodative system. The convergence and accommodative systems interact through separate crosslinks. Because of these crosslinks, innervation to convergence drives accommodation through the CA/C, just as innervation to accommodation drives convergence through the AC/A. Proximal effects are input into each section of the system before the crosslinks.
The model in Figure 16.2 has significant implications for normal binocular vision, and we have considered these in developing the integrative analysis system (Chapter 2). In designing integrative analysis, we have incorporated implications about interactions between accommodation and convergence described by the model in Figure 16.2, tonic vergence, the depth of focus and lag of accommodation, and research implications concerning the influences of proximal vergence.19 Certain assumptions must be made about the model so that predictions about the accommodative and vergence systems can be made. For example, at this time, considerations are limited to static situations in which constant stimuli are held at a fixed position. This makes the potentially complex interactions somewhat less complicated. However, even with these limitations, the results apply to a large number of clinical and real-life situations.
Application of the model to measurement of heterophorias, fixation disparity, associated phoria, and the zone of clear single binocular vision helps explain many binocular vision responses seen during clinical patient care, including why presbyopic patients are generally asymptomatic despite loss of accommodative vergence. The following sections describe how the model applies to many of the examination and diagnostic techniques we suggest in Chapter 1. Additionally, we discuss areas where further clinical research needs to be performed and how the model might direct such research.
Tonic Vergence
Tonic vergence represents the eye position that results in the absence of disparity, blur, and proximal stimuli. Tonic vergence can be measured directly by incorporating pinhole apertures during distance von Graefe phoria testing. The disparity and accommodative systems are open loop,* and proximal input is absent because fixation is at distance. Repeatable measures can be made because of the stable nature of tonic vergence.
Testing tonic vergence is typically not done in a clinical setting, and, for most patients, there is probably little need to add this testing. In normal adults, tonic vergence is only approximately 2 Δ more convergent than the distant phoria, and the distribution of tonic vergence is similar to the leptokertotic distribution of the distance phoria, where the peak is 1 Δ exo ± 2. Thus, for normal adults, tonic vergence outside the range of 2 Δ exophoric or 1 Δ esophoric is abnormal. As will be seen in subsequent discussions, the model in Figure 16.2 suggests that significant tonic vergence has a major effect on interactions between accommodation and vergence.20 For patients who have large esophorias at distance, the assessment of tonic vergence may yield some useful diagnostic information. For example, a substantial difference between the distance phoria and the tonic measure would suggest a need to increase the amount of base-out prism incorporated in the prescription.
DEPTH OF FOCUS AND LAG OF ACCOMMODATION
Determination of the role played by accommodation requires knowledge of the amount of accommodation (or accommodative effort) used.21 To maximally relax accommodation at distance, refraction is generally done to achieve best visual acuity with the maximum plus (or least minus) lenses possible. The maximum plus refraction places distant objects at the farthest focus point from the retina. Because of the refractive technique and the depth of focus of the eye, small accommodative stimuli do not affect accommodative activity22 with distance fixation. As an object is moved closer, the blur circle moves through the limits of the depth of focus, and no change in accommodation occurs because no appreciable blur results until the object focus goes beyond the depth of focus of the eye.
 ▪ Figure 16.2 The lower block diagram represents the components that describe vergence responses, and the upper section represents the accommodative system. The convergence and accommodative systems interact through separate crosslinks of convergence accommodation and accommodative vergence. Proximal effects are input into each section of the system before the crosslinks. Innervation to convergence drives accommodation through CA/C, just as innervation to accommodation drives convergence through AC/A. AC/A, accommodative convergence to accommodation; CA/C, convergence accommodation to convergence. (Reprinted with permission from Wick B, Currie D. Dynamic demonstration of proximal vergence and proximal accommodation. Optom Vis Sci. 1991;68(3):163-167. Copyright © 1991 American Academy of Optometry.) |
 ▪ Figure 16.3 The distance depth of focus combined with the normal lag of accommodation of 0.50 D or more when accommodating on near objects results in only approximately 1.50 D of accommodative change when fixation is changed from distance to 40 cm. (Reprinted with permission from Wick B. Clinical factors in proximal vergence. Am J Optom Physiol Opt. 1985;62(1):119. Copyright © 1985 American Academy of Optometry.) |
Clinicians typically think in terms of the stimulus rather than the response to accommodation. However, the preceding discussion suggests that the accommodative response is generally significantly smaller than the stimulus. For approximately the first 0.75 D of accommodative stimulus, there is no accommodative change because of the refraction and the depth of focus of the eye.22 The distance depth of focus, combined with the normal lag of accommodation of 0.50 D or more when accommodating on near objects,23 results in only approximately 1.50 D of accommodative change when fixation is changed from distance to 40 cm24 (Fig. 16.3). This is significantly less than the 2.5 D accommodative stimulus.
The average lag of accommodation is between 0.25 and 0.50 D for children and young adults. Determination of the accommodative lag is done routinely in clinics using monocular estimation method (MEM) retinoscopy. MEM retinoscopy is very useful for evaluating the accommodative response of patients who complain of near blur or other symptoms of accommodative dysfunction. A finding of a high lag of accommodation during MEM retinoscopy suggests a tentative power for a near addition or a need to prescribe accommodative therapy, or both. An excess of accommodation on MEM retinoscopy (lead of accommodation) directs treatment to techniques that maximally relax accommodation, such as the prescription of near plus additions, which reduce accommodative demand to zero, or vision training emphasizing plus acceptance, or both. Integrative analysis routinely includes MEM retinoscopy and thus incorporates many of the effects of accommodative lag and the depth of focus of the eye.
Proximal Vergence
Proximal vergence contributes up to 70% of the vergence demand for near tasks25 and is greater when measured under binocular conditions.24,26 Measurements of proximal vergence velocity using infrared limbal sensing have found that the mean peak velocities for proximal convergence and divergence (66 and 39 degrees per second, respectively)27 are substantially faster than disparity vergence velocities of 14 and 10 degrees per second.28 The magnitude and velocity of proximal vergence responses suggest that this component is a major contributor to the total vergence response when looking from distance to near. Proximal effects are included as the initial entry in the vergence/accommodation interactive model shown in Figure 16.2.
Disparity vergence has been shown to have a shorter latency and more rapid course than accommodative vergence.29 As a result, most clinicians feel that disparity vergence initiates the near vergence response. However, there are a number of situations in which disparity cannot serve as the initiator of near vergence. For example, if a near object located such that it is seen by only one eye is to be fixated, an eye movement and a head movement will be required for bifoveal fixation. In this situation, knowledge of the object location and “awareness of nearness” (proximal vergence) probably provide the initial vergence and accommodative component. Another
frequent situation involves copying from a blackboard in school. The images of the object are frequently located so far in the peripheral retina that disparity cues are not applicable30 and, again, proximal vergence is more likely to be the initial vergence component. Because binocular proximal effects make up a large portion of the near demand,24 they can move the system within the ranges of foveal vision, where disparity vergence and blur-driven accommodation can fine-tune the response.
frequent situation involves copying from a blackboard in school. The images of the object are frequently located so far in the peripheral retina that disparity cues are not applicable30 and, again, proximal vergence is more likely to be the initial vergence component. Because binocular proximal effects make up a large portion of the near demand,24 they can move the system within the ranges of foveal vision, where disparity vergence and blur-driven accommodation can fine-tune the response.
Proximal vergence can be measured clinically by incorporating pinholes (to eliminate accommodative vergence) and measuring the heterophoria at different distances using the von Graefe technique (to eliminate disparity vergence). Changes in the proximal stimulus are introduced using targets located at 2.0, 1.0, 0.5, and 0.25 m. The change in angle that is measured provides an estimate of proximal vergence. For an optimal stimulus to proximal vergence, the patient must be fully aware of the target location. This is accomplished by having the patient view the targets under natural viewing conditions before recording the measurements and by having him or her hold the target during the nearer measurements. Although these proximal measures are not currently used in routine clinical testing, they may provide useful information for clinical management. For example, in preliminary studies, deficient proximal vergence has been linked to the prolonged blur that some patients report after reading.31
Binocular Vergence Interactions
The relationships considered in the preceding section become somewhat more complicated when considering interactions between vergence and accommodation. For example, consider the situation in which no output is needed from accommodation (such as when a pinhole is placed in front of the eyes to greatly increase the depth of field). This eliminates the effects of accommodative lag and depth of focus and reduces accommodation caused by blur to zero. Thoughtful clinicians will recognize that accommodation caused by blur is also zero in a common physical condition—absolute presbyopia when there is physiologically no accommodation. Analysis of the situation in which there is no blur-driven accommodation using the model described in Figure 16.2 helps explain why presbyopic patients are routinely asymptomatic when classic analysis systems frequently predict binocular distress.32 The effects of interactions between accommodation and vergence are often deleterious; presbyopia removes this interactive problem, and patients are commonly asymptomatic.
CONVERGENCE ACCOMMODATION
Measurement of the CA/C ratio provides information concerning the strength of the crosslink from the vergence system to accommodation. The model of Figure 16.2 shows why interactions between vergence and accommodation, via the CA/C ratio, complicate findings of classic graphical analysis. For example, suppose that vergence measures were made in a patient with a zero CA/C ratio (so that changes in vergence did not affect accommodation), while proximal vergence was held constant. Clinically, this measurement is called relative vergence or vergence free of accommodation. Under these conditions, graphical analysis techniques suggest that disparity vergence equals the dissociated phoria. However, except in presbyopia, the CA/C ratio is seldom zero, and changes in vergence are accompanied by changes in accommodation, forcing reflex accommodation to change to compensate for vergence accommodation. As a result, the relationship between the dissociated phoria and disparity vergence is not adequately predicted by classic methods of analysis.
Clinical research on the CA/C ratio indicates a linear relation, although as extremes of the vergence stimulus are reached, the range becomes nonlinear—possibly due, in part, to the decrease in pupil size and increased depth of focus that accompanies increased vergence.33 Because there is generally very little difference between the vergence stimulus and the vergence response, there is very little difference between the stimulus and the response CA/C ratios. For young adults, the CA/C ratio is about 0.5 D per meter angle (a meter angle [MA] is determined by dividing the interpupillary diameter, in millimeters, by 10 and expressing the value in prism diopters [see Chapter 1]); for clinical purposes, the average value of an MA is about 6 Δ. The CA/C ratio is inversely related to age (Fig. 16.4).
The CA/C ratio can be measured clinically using pinholes before each eye or using a “blur-free” grating target (DOG, or difference of Gaussian) (Fig. 16.5). These techniques open the accommodative system loop so that stimulation of accommodation by vergence is completely effective. The clinician who wishes to assess the CA/C ratio can use a Wesson DOG card34 and perform MEM retinoscopy, with bifixation on the central bright target region, while the patient fuses disparity stimulation of 12 Δ base-in, 0 Δ, and 12 Δ base-out. The MEM findings can be determined at each vergence level, the change averaged (assuming linearity), and the CA/C ratio computed.
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