Normal AC/A Conditions: Fusional Vergence Dysfunction, Basic Esophoria, and Basic Exophoria

Introduction

This chapter discusses the characteristics, diagnosis, and management of nonstrabismic binocular disorders associated with a normal accommodative convergence to accommodation (AC/A) ratio. Although there are some significant differences among these conditions, they are grouped together based on the classification system described in Chapter 2. The unifying characteristic of the conditions described in this chapter is the normal AC/A ratio. This similarity is an important one because it is the basis for the development of the management plan. Specifically, the AC/A ratio is the major factor that determines the sequence of management decisions in patients with heterophoria (Chapter 3). Consequently, certain general treatment strategies are shared by all binocular conditions associated with a normal AC/A ratio. However, there are also important differences among these conditions. After a review of general principles that apply to all normal AC/A disorders, each condition is described separately to highlight the differences in characteristics, diagnosis, and management.

The specific conditions that are discussed in this chapter are fusional vergence dysfunction, basic exophoria, and basic esophoria.

Overview of General Management Principles for Heterophoria Associated with Normal AC/A Ratio

Table 11.1 lists the various sequential management issues presented in Chapter 3. The specific sequence in which these factors should be considered is primarily dependent on the AC/A ratio. For binocular vision disorders associated with a normal AC/A ratio, the specific management sequences are listed in Tables 11.2, 11.3, 11.4.

Table 11.1 SEQUENTIAL CONSIDERATIONS IN THE MANAGEMENT OF NONSTRABISMIC BINOCULAR ANOMALIE

Optical correction of ametropia	Vision therapy for amblyopia
Added lens power	Vision therapy for suppression
Horizontal prism	Vision therapy for sensory motor function
Vertical prism	Surgery
Occlusion for amblyopia

Table 11.2 SEQUENTIAL CONSIDERATIONS IN THE MANAGEMENT OF FUSIONAL VERGENCE DYSFUNCTION

Optical correction of ametropia	Vision therapy for sensory motor function
Vertical prism	Added lens power
Occlusion for amblyopia	Horizontal prism
Vision therapy for amblyopia	Surgery
Vision therapy for suppression

Table 11.3 SEQUENTIAL CONSIDERATIONS IN THE MANAGEMENT OF BASIC ESODEVIATIONS

Optical correction of ametropia	Vision therapy for amblyopia
Vertical prism	Vision therapy for suppression
Added lens power	Vision therapy for sensory motor function
Horizontal prism	Surgery
Occlusion for amblyopia

Table 11.4 SEQUENTIAL CONSIDERATIONS IN THE MANAGEMENT OF BASIC EXODEVIATIONS

Optical correction of ametropia	Vision therapy for suppression
Vertical prism	Vision therapy for sensory motor function
Added lens power	Horizontal prism
Occlusion for amblyopia	Surgery
Vision therapy for amblyopia

In contrast to conditions with low AC/A ratios, in which the use of added lenses is not very helpful, in conditions with a normal AC/A ratio, added lenses are sometimes useful. As a result, the consideration of the use of added lenses is moved closer to the top of the list in Tables 11.3 and 11.4.

Optical correction of ametropia still remains the first issue that a clinician should consider. As stated in Chapter 3, the first consideration for all patients with accommodative and nonstrabismic binocular anomalies is optical correction of ametropia. In normal AC/A cases, lenses may have a moderate effect on the size of the deviation. Prescription for hyperopia in basic esophoria or for myopia in basic exophoria will generally be helpful.

When considering the final prescription, it is important to first determine whether a vertical deviation is present. We suggest prescribing for vertical deviations as small as 0.5 Δ and basing the prescription on fixation disparity assessment (Chapter 15).

As discussed in earlier chapters, a key difference between the sequential management of esophoria and exophoria is the differential effectiveness of both horizontal prism and vision therapy for these conditions. Horizontal prism tends to be more effective for esophoria than for exophoria. Therefore, the use of prism is an earlier consideration for basic esophoria than for basic exophoria. Fusional vergence dysfunction is, by definition, associated with very small magnitude horizontal phorias. As a result, the use of horizontal prism is never indicated for this condition. Vision therapy tends to be more effective in exophoria than esophoria. Vision therapy, without the assistance of horizontal prism, will often be sufficient to treat basic exophoria. These differences are reflected in the sequential management considerations in Tables 11.2, 11.3, 11.4.

Although amblyopia is uncommon in nonstrabismic binocular vision anomalies, it will occur if the phoria is associated with a significant degree of anisometropia. In such cases, one of the early considerations should be treatment of amblyopia using occlusion and vision therapy. The use of occlusion and specific vision therapy procedures for the treatment of the amblyopia and any associated suppression always needs to be considered immediately after prescribing for the anisometropia and considering prism to compensate for a vertical phoria. In cases of basic esophoria, basic exophoria, and fusional vergence dysfunction associated with anisometropia, we recommend part-time occlusion. Several (2 to 3) hours of occlusion, using an opaque patch along with active amblyopia therapy, are usually sufficient to resolve the amblyopia. Complete details regarding the evaluation and management of anisometropic amblyopia are provided in Chapter 17.

In most cases, however, amblyopia will not be present in nonstrabismic normal AC/A binocular disorders. Thus, after consideration of ametropia and prism, vision therapy for suppression and sensory and motor improvement is the next treatment issue.

The final sequential management consideration listed in Tables 11.2, 11.3, 11.4 is surgery. Surgery is never necessary for fusional vergence dysfunction because it is always associated with a very small phoria. For basic esophoria and exophoria, surgery is also unlikely to be necessary. Generally, a combination of nonsurgical approaches will be effective. However, when the magnitude of the deviation is large and all nonsurgical approaches have been unsuccessful in relieving the patient’s symptoms, surgery may occasionally be helpful.

PROGNOSIS FOR THE TREATMENT OF BINOCULAR VISION CONDITIONS ASSOCIATED WITH NORMAL AC/A RATIOS

All of the conditions described in this chapter can be very effectively treated using the management sequence we have suggested. Although there has been little research investigating the treatment of basic esophoria, there is no reason to believe that the combined use of lenses, prism, and vision therapy for basic esophoria should be any less effective than for other esodeviations, such as convergence excess and divergence insufficiency. Daum¹ studied the efficacy of vision training for improving the fusional vergence ranges of 34 asymptomatic adult subjects. He demonstrated significant increases in both positive fusional vergence (PFV) and negative fusional vergence (NFV), using tonic- and phasic-type activities. Thus, there is support for the use of vision therapy to improve NFV in basic esophoria.

Vision therapy has also been shown to be highly successful for basic exodeviations. Daum² found that 96% of his patients with basic exodeviations achieved either a total or partial cure with vision therapy. In addition, recent randomized clinical trials have demonstrated that vision therapy is highly successful at improving PFV.³^,⁴^,⁵^,⁶ Several authors have also investigated the effectiveness of vision therapy for fusional vergence dysfunction and general skills cases. Hoffman, Cohen, and Feuer⁷ reported a 94% success rate for normalizing binocular findings and eliminating symptoms in such cases. Grisham⁸^,⁹ studied subjects with fusional vergence dysfunction and was able to demonstrate that vision therapy can be used to normalize vergence facility.

SUMMARY OF KEY POINTS IN TREATING PHORIA PATIENTS ASSOCIATED WITH NORMAL AC/A

Because of the normal AC/A ratio, added lenses have a moderate effect on the angle of deviation. For basic esophoria, added lenses may be quite helpful, although horizontal prism and vision therapy are often necessary as well. For basic exophoria and fusional vergence dysfunction, vision therapy tends to be the principal treatment modality.

Fusional Vergence Dysfunction

BACKGROUND INFORMATION

Unlike most of the other nonstrabismic binocular vision conditions described in this text, fusional vergence dysfunction is not part of Duane classification. Duane classification and description of binocular vision disorders centers around the AC/A ratio and the magnitude of the phoria at distance or near. It is primarily a classification of disorders in which the deviation is larger or smaller than the average 4 to 6 Δ of exophoria at near and orthophoria at distance. A patient who is symptomatic and has a phoria that falls within the normal range at both distance and near does not fit into any category in Duane classification.

Does such a condition exist? Are there patients who are symptomatic (after correction of refractive error) despite having normal phorias at distance and near and normal accommodative function? Many authors have described such a problem, although they have not consistently used the term fusional vergence dysfunction.⁷^,⁸^,⁹^,¹⁰^,¹¹^,¹² Schapero¹⁰ described the characteristics of the 10 basic visual training problems encountered in optometric practice. His ninth category is a condition in which tonic vergence is normal (normal phoria at distance), the AC/A ratio is normal, and both the PFV and NFV are low. He suggested that the etiology in such cases may be a sensory fusion problem, such as uncorrected refractive error or aniseikonia, a small vertical deviation, or some underlying systemic disease process. His recommendation is to rule out systemic disease and eliminate any obstacle to sensory fusion using lenses and prism. He stated that, in most cases, elimination of the sensory disturbance will lead to improved motor fusion without vision therapy.

Hoffman, Cohen, and Feuer⁷ described a condition called a general skills case. According to the authors, this is a condition in which there is a small esophoria or exophoria with restricted fusional vergence ranges, inadequate saccades and pursuits, suppression, and normal accommodation. Other authors have essentially described the same set of characteristics and have used terms such as inefficient binocular vision¹³ or sensory fusion deficiency.¹²

Grisham⁸^,⁹ used the term fusional vergence dysfunction to describe a binocular vision disorder in which there are deficiencies in fusional vergence dynamics. In a laboratory setting, he used a haploscope to evaluate vergence latency, velocity, and tracking rate for both PFV and NFV. He was able to differentiate a group of patients with no significant phoria at either distance or near who exhibited deficits in response latency and velocity. He also demonstrated that one can identify such patients clinically by performing vergence facility testing. This test is described in Chapter 1.

We agree that clinically there is a condition in which the AC/A is normal, the phorias are within expected values for both distance and near, and the fusional vergence findings are restricted in both positive and negative directions. Sometimes, as Grisham⁹ described, the smooth fusional vergence ranges are normal and only vergence facility testing is a problem. These patients also tend to have central suppression and complain of symptoms characteristic of other binocular vision disorders. We feel that the term used by Grisham,⁹ fusional vergence dysfunction, best describes this condition, and it is the term that we use in this text.

Fusional vergence dysfunction is an important diagnostic category, and it is a condition that can easily be missed or dismissed by clinicians not aware of this disorder. For example, a patient presents with asthenopic symptoms; no refractive error; normal eye health, accommodative function, and AC/A ratio; and distance and near phorias that are both within the normal range. In this situation, many clinicians may not even perform vergence testing because the phoria is normal at all distances. Given these findings, a clinician might tell this patient that there is no vision problem present to explain the symptoms. This presentation, however, is characteristic of fusional vergence dysfunction, and the diagnosis requires administration of both smooth vergence and vergence facility testing.

Scheiman et al¹⁴ studied 1,650 children (ages 6 to 18 years) and found a prevalence of only 0.6%. Porcar and Martinez-Palomera¹⁵ studied a university population and found a prevalence of 1.5%. The more recent population-based Binocular Vision Anomalies and Normative Data (BAND) study¹⁶ reported the prevalence of nonstrabismic anomalies of binocular vision among schoolchildren in rural and urban Tamil Nadu. In four schools, two each in rural and urban arms, 920 children in the age range of 7 to 17 years were included in the study. Comprehensive binocular vision assessment was done for all children including evaluation of vergence and accommodative systems. The prevalence of fusional vergence dysfunction was 0.8%.

In another cross-sectional study¹⁷ of 1,201 high school students aged 13 to 19 years, the prevalence fusional vergence dysfunction was 3.3%. Garcia-Munoz et al¹⁸ only found 0.6% of the study population (university students ages 18 to 35 years old) with fusional vergence dysfunction.

Based on these studies, it appears that this condition is considerably less common than convergence insufficiency or convergence excess.

CHARACTERISTICS

Symptoms

Most symptoms (Table 11.5) are associated with reading or other close work. Common complaints include eyestrain and headaches after short periods of reading, blurred vision, sleepiness, difficulty concentrating, and loss
of comprehension over time. Some patients with fusional vergence dysfunction are asymptomatic. As discussed in Chapters 9 and 10, this absence of symptoms may be due to suppression, avoidance of near visual tasks, a high pain threshold, or occlusion of one eye when reading. Clinicians should always inquire about avoidance of reading or other near tasks if a patient with fusional vergence dysfunction reports an absence of symptoms. Avoidance is as important a reason for recommending therapy as any of the other symptoms associated with fusional vergence dysfunction.

Table 11.5 SYMPTOMS AND SIGNS OF FUSIONAL VERGENCE DYSFUNCTION

Symptoms
All of these symptoms are associated with reading or other sustained near work:
Eyestrain	Problems with reading comprehension
Headaches	Excessive tearing
Inability to attend and concentrate	Blurred vision
Signs
Normal AC/A ratio
Normal phoria at distance and at near
Reduced smooth positive and negative fusional vergence at distance and/or near
Reduced positive and negative vergence facility
Low positive relative accommodation
Low negative relative accommodation
Fails ±2.00 on binocular accommodative facility testing
Passes monocular accommodative facility
Normal amplitude of accommodation
Intermittent central suppression
AC/A, accommodative convergence to accommodation.

Signs

See Table 11.5 for the signs of fusional vergence dysfunction.

Refractive Error

Although a clear trend has not been identified in the literature in regard to refractive error, in our experience most people with fusional vergence dysfunction do not have a significant degree of refractive error.

Characteristics of the Deviation

Patients with fusional vergence dysfunction have phorias that fall within the normal range, based on Morgan’s table of expected findings. They are generally ortho at distance and have a small degree of exophoria or esophoria at near.

AC/A Ratio

A normal AC/A ratio is present in fusional vergence dysfunction.

ANALYSIS OF BINOCULAR AND ACCOMMODATIVE DATA

Because fusional vergence dysfunction is a disorder in which both PFV and NFV are reduced, it is important to perform all tests that either directly or indirectly assess vergence ranges and facility. Thus, direct tests of vergence amplitudes, such as smooth positive and negative vergence testing at distance and near, will tend to be lower than expected. If these findings are normal, it is important to evaluate vergence facility. It is possible to have normal smooth ranges and abnormal vergence facility.

Indirect tests of vergence, such as the negative relative accommodation (NRA), positive relative accommodation (PRA), and binocular accommodative facility (BAF), may also be reduced. In contrast to the other binocular conditions described in previous chapters, both the NRA and PRA will be low, and the patient will have difficulty with both plus and minus on BAF testing. Again, this result suggests problems with both PFV and NFV. Of course, low findings with the NRA, PRA, and BAF could also suggest an accommodative facility disorder. It is important, therefore, to perform monocular accommodative facility (MAF) testing. In fusional vergence dysfunction, the results on this test will be normal, confirming that the problem is in the area of vergence, not accommodation.

It is also common to find central intermittent suppression during binocular vision testing. This type of suppression can be detected during phoria, vergence, and BAF testing.

DIFFERENTIAL DIAGNOSIS

See Table 11.6 for the differential diagnosis of fusional vergence dysfunction.

Figure 2.2 illustrates the clinical reasoning process we suggest relative to the diagnosis of fusional vergence dysfunction. Because these patients tend to be symptomatic and the phoria is normal at all distances, the most likely hypothesis is an accommodative disorder. If accommodative testing is normal, the fusional vergence dysfunction becomes a likely hypothesis. We also suggest ruling out fixation disparity, latent hyperopia, cyclophoria, a small vertical deviation, and aniseikonia. Once these etiologic factors have been eliminated, fusional vergence dysfunction becomes the most likely diagnosis.

Table 11.6 DIFFERENTIAL DIAGNOSIS OF FUSIONAL VERGENCE DYSFUNCTION

Functional Disorders to Rule Out	Serious Underlying Disease to Rule Out
Accommodative infacility	Systemic disease causing debilitation
Latent hyperopia	Medication
Vertical or cyclodeviation
Fixation disparity
Aniseikonia

One condition that can be confused with fusional vergence dysfunction is accommodative infacility. As discussed in Chapter 12, accommodative infacility is a condition that has many of the same symptoms and signs as fusional vergence dysfunction. The key differential sign is that in fusional vergence dysfunction, all monocular accommodative testing is normal.

TREATMENT

We recommend the management sequence listed in Table 11.2.

Lenses

The symptoms commonly found in fusional vergence dysfunction may also be present in patients with uncorrected refractive error or latent hyperopia. It is therefore important to eliminate latent hyperopia as a cause, by performing a cycloplegic refraction, and to correct any significant refractive error.

Prism

If a vertical deviation is present, we recommend that vertical prism be prescribed before vision therapy begins. The most effective method for determining the amount of vertical prism is the associated phoria, which can be measured with any fixation disparity device (Chapter 15).

The use of horizontal relieving base-in prism is not necessary in fusional vergence dysfunction because the phoria is normal at both distance and near.

Vision Therapy

A vision therapy program for fusional vergence dysfunction generally requires between 12 and 24 in-office visits, if vision therapy is office based. The total number of therapy sessions depends on the age of the patient and his or her motivation and compliance. Motivated adults can sometimes successfully complete vision therapy for fusional vergence dysfunction in 10 to 12 visits.

Specific Vision Therapy Program

All of the vision therapy techniques recommended next are described in detail in Chapters 6, 7, 8.

Phase 1

This first phase of therapy is designed to accomplish the objectives listed in Table 11.7 under Phase 1. As discussed in earlier chapters, it is important to develop a working relationship with the patient during the first few sessions and make the patient aware of the various feedback mechanisms that will be used throughout therapy. The basic approach used for fusional vergence dysfunction is to work toward establishing both normal PFV and NFV ranges and facility. Because it is generally easier to work with PFV, we suggest starting the therapy using convergence techniques.

The first goal of the therapy, therefore, is to teach the concept and feeling of converging. The patient should be able to voluntarily converge and diverge to any distance from 5 cm (2 in.) to 6 m (20 ft). Once the patient can voluntarily initiate a controlled convergence movement, the other goals of the vision therapy program become much easier to accomplish. Three commonly used procedures to accomplish this first objective are the Brock string, bug on string, and the red/green Barrel card.

Table 11.7 OBJECTIVES OF VISION THERAPY FOR FUSIONAL VERGENCE DYSFUNCTION

Phase 1

Develop a working relationship with the patient
Develop an awareness of the various feedback mechanisms that will be used throughout therapy
Develop voluntary convergence and divergence
Normalize positive and negative fusional vergence amplitudes (smooth or tonic vergence demand)
Normalize accommodative amplitude and ability to stimulate and relax accommodation

Phase 2

Normalize positive fusional vergence facility (jump or phasic vergence demand)
Normalize negative fusional vergence facility (jump or phasic vergence demand)

Phase 3

Develop ability to change from a convergence to a divergence demand
Integrate vergence procedures with changes in accommodative demand
Integrate vergence procedures with versions and saccades

Simultaneously try to normalize PFV amplitudes. The initial goal is to reestablish a normal vergence range for smooth- or tonic-type vergence demand. A smooth vergence demand is easier for the patient to accomplish in the early part of a vision therapy program (Chapter 5). Equipment that can be used to accomplish this objective includes the variable tranaglyphs, variable vectograms, and the Variable Prismatic Stereoscope from Bernell. These three devices can be used to create a smooth, gradual increase in convergence demand. As the patient begins to make progress with PFV, begin working with NFV as well, using the same techniques.

By definition, patients with a diagnosis of fusional vergence dysfunction do not have accommodative problems. Nevertheless, accommodative training techniques are often useful in the initial phase of therapy because they can aid in the process of establishing the feeling of looking close and far and of converging and diverging. Certainly, if the patient has an accommodative disorder in addition to the fusional vergence dysfunction, accommodative techniques are important. Accommodative techniques are described in Chapter 7. Lens sorting, loose lens rock, and Hart chart procedures are commonly used in this first phase of therapy.

Endpoint. Phase 1 of therapy ends when the patient can:

Demonstrate voluntary convergence
Fuse to about 30 base-out and 15 base-in using a tranaglyph or other comparable technique
Complete 12 cpm of accommodative facility with +2.00/−2.00 lenses using a 20/30 target.

A sample vision therapy program for Phase 1 is summarized in Table 11.8. This program includes several techniques that can be used by the patient at home to supplement the in-office therapy.

Table 11.8 SAMPLE VISION THERAPY PROGRAM FOR FUSIONAL VERGENCE DYSFUNCTION

Phase 1

Sessions 1 and 2

In-office

Discuss nature of vision problem, goals of vision therapy, various feedback cues, importance of practice
Brock string
Lens sorting
Loose lens rock (begin with plus if accommodative excess, with minus if accommodative insufficiency)
Tranaglyphs or vectograms: convergence
Begin with a peripheral target such as tranaglyph 515 or the Quoit vectogram
Computer Orthoptics Random Dot program: convergence home therapy

Home therapy

HTS Vergence and Accommodation programs
Brock string

Sessions 3 and 4

In-office

Bug on string
Loose lens rock
Tranaglyphs or vectograms: convergence and divergence
Use targets with more central demand (Clown, Bunny tranaglyphs; Clown, Topper vectograms)
Computer Orthoptics Random Dot program: convergence and divergence

Home therapy

HTS Vergence and Accommodation programs
Loose lens rock

Sessions 5 through 8

In-office

Barrel card
Voluntary convergence
Loose lens rock
Tranaglyphs or vectograms: convergence and divergence
Use even more detailed targets such as tranaglyph Sports Slide and Faces targets and the Spirangle vectogram
Computer Orthoptics Random Dot program: convergence and divergence

Home therapy

HTS Vergence and Accommodation programs

Phase 2

Sessions 9 and 10

In-office

Tranaglyphs or vectograms with modifications to create jump vergence demand: convergence
Nonvariable tranaglyphs: convergence
Binocular accommodative therapy techniques: use any of the binocular techniques listed earlier with ± lenses

Home therapy

HTS Vergence and Accommodation programs

Sessions 11 and 12

In-office

Tranaglyphs or vectograms with modifications to create jump vergence demand: convergence and divergence
Aperture Rule: convergence and divergence
Binocular accommodative therapy techniques: use any of the binocular techniques listed earlier with ± lenses

Home therapy

HTS Vergence and Accommodation programs

Sessions 13 through 16

In-office

Aperture Rule: convergence and divergence
Eccentric Circles or Free Space Fusion cards: convergence and divergence
Computer Orthoptics Random Dot Vergence program: both divergence and convergence
Binocular accommodative therapy techniques: use any of the binocular techniques listed earlier with ± lenses

Home therapy

HTS Vergence and Accommodation programs
Eccentric Circles or Free Space Fusion cards

Phase 3

Sessions 17 through 20

In-office

Tranaglyphs or vectograms with Polaroid or red/green flippers
Eccentric Circles or Free Space Fusion cards
Computer Orthoptics Random Dot Vergence program: step-jump vergence

Home therapy

HTS Vergence and Accommodation programs
Eccentric Circles or Free Space Fusion cards: convergence

Sessions 21 and 22

In-office

Tranaglyphs or vectograms with Polaroid or red/green flippers
Eccentric Circles or Free Space Fusion cards
Lifesaver cards
Computer Orthoptics Random Dot Vergence program: jump-jump vergence

Home therapy

HTS Vergence and Accommodation programs
Eccentric Circles or Free Space Fusion cards: divergence

Sessions 23 and 24

In-office

Tranaglyphs or vectograms with Polaroid or red/green flippers
Eccentric Circles or Free Space Fusion cards with rotation and versions
Lifesaver cards with rotation and version
Computer Orthoptics Vergence program with rotation

Home therapy

HTS Vergence and Accommodation programs
Eccentric Circles or Free Space Fusion cards: divergence/convergence

HTS, home therapy system.

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