High Accommodative Convergence to Accommodation Conditions: Convergence Excess and Divergence Excess
This chapter discusses the characteristics, diagnosis, and management of nonstrabismic binocular disorders associated with a high accommodative convergence to accommodation (AC/A) ratio. As described previously, 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 high AC/A ratio. However, there are also important differences among patients with high AC/A ratios who have convergence excess and those who have divergence excess. After a review of the general principles that apply to all high 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 the various forms of convergence excess and divergence excess.
Overview of General Management Principles for Heterophoria Associated with High AC/A Ratio
For binocular vision disorders associated with a high AC/A ratio, the specific management sequences are listed in Tables 10.1 and 10.2. The major difference that distinguishes high AC/A problems from conditions associated with low and normal AC/A ratios is the relative effectiveness of added lenses in effecting a change in the size of the heterophoria. An example of this is the patient with convergence excess described in Case 10.1.
Case 10.1
A 10-year-old boy presented with complaints of eyestrain, blurred vision, and inability to concentrate when reading after 10 minutes. These problems had been bothering him since the beginning of the school year. The refraction was +1.00 D OD and OS, the distance phoria was 4 esophoria, and the near phoria was 20 esophoria (interpupillary diameter [IPD] = 58 mm). The calculated AC/A ratio in this case is 8:1. Near point testing through the subjective revealed the following:
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Because the AC/A ratio is high, it is important to prescribe for ametropia in this case. Prescribing +1.00 will reduce the near phoria to about 2 esophoria. Analysis of the near point data indicates that all of the direct and indirect measures of negative fusional vergence (NFV) are low. The use of added plus lenses in this case is indicated and will eliminate the remaining esophoria, balance the NRA/PRA relationship, and normalize the MEM and base-in findings. The near point analysis in this case suggests that a +0.75 to a +1.00 add is appropriate.
Table 10.1 SEQUENTIAL CONSIDERATIONS IN THE MANAGEMENT OF CONVERGENCE EXCESS | ||||||||||
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Table 10.2 SEQUENTIAL CONSIDERATIONS IN THE MANAGEMENT OF DIVERGENCE EXCESS | ||||||||||
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Another example of the effectiveness of added lenses in high AC/A cases is illustrated in Case 10.2.
Case 10.2
A 4-year-old girl was brought for an examination because her parents had noticed that her right eye was drifting out toward the end of the day and when she was tired. They had noticed it for the last year and felt that the proportion of time that the eye turned was increasing. The refraction was +0.25 OD and OS. The cover test at distance was an intermittent 25 Δ, right exotropia (deviates 10% of the time). At near, the cover test was 5 exophoria. The calculated AC/A ratio is 13:1.
The ametropia is insignificant in this case, but the use of added lenses can be helpful in the treatment of this patient. Because of the high AC/A ratio, glasses can be prescribed to help control the deviation. The use of −1.50 OU would significantly reduce the angle at distance. Because it could also lead to a high degree of esophoria at near, investigation of the use of a bifocal is also appropriate. In this case, the use of −1.50 with a +1.00 add OU is a valuable optical management technique because of the high AC/A ratio. If this child were older, vision therapy would probably be adequate to treat the divergence excess. In preschool children who may be difficult to treat using vision therapy, added lenses are an important temporary treatment option.
Optical correction of ametropia still remains the first issue that should be considered. However, as evidenced by Case 10.2, the consideration of added lenses is close to the top of the list in Table 10.1. In high AC/A cases, correction of the refractive error may be helpful for two reasons. The first is that the presence of an uncorrected refractive error may create an imbalance between the two eyes, leading to sensory fusion disturbances, or may create decreased fusional ability because of blurred retinal images. The second reason is that, because of the high AC/A, correction of the ametropia may have a beneficial effect on the magnitude of the deviation.
When considering the final prescription for these patients, it is important to first determine whether a vertical deviation is present. As discussed in previous chapters, correction of even small amounts of vertical deviations can have a positive effect on the horizontal deviation. We suggest prescribing for vertical deviations as small as 0.5 Δ (Chapter 14).
A key difference between the sequential management of convergence excess and divergence excess is that esophores are more likely to benefit from horizontal prism than exophores. The use of base-out prism is an early management consideration in convergence excess, but the use of base-in is near the bottom of the list for divergence excess (Tables 10.1 and 10.2). Because of the effectiveness of added lenses, prism is rarely necessary for convergence excess associated with normal tonic vergence (orthophoria at distance). An example of the value of base-out prism in the treatment of convergence excess, however, is illustrated in Case 10.3.
Case 10.3
A 21-year-old woman presented with a chief complaint of intermittent diplopia when driving and when reading. The diplopia was worse when reading, and she also complained of eyestrain when reading for more than 15 minutes. Although she had always had these problems, they seemed worse since she started a new job in which she worked at a desk 8 hours a day doing paperwork. The cycloplegic refraction revealed plano OD and OS. The cover test was 8 esophoria at distance and 16 esophoria at near. The calculated AC/A ratio is 9.4:1 (IPD = 62 mm).
This is a case of convergence excess with high tonic vergence or a moderate degree of esophoria at distance. Because there is no refractive error, the use of lenses in a distance prescription is not an option. The patient does have a high AC/A ratio, and the near deviation can be decreased easily using added lenses. However, a prescription such as plano with a +1.50 add would only eliminate the patient’s near complaints. She would still experience diplopia when driving. The use of base-out prism is an important option in this type of case. The magnitude of the prism should be determined based on fixation disparity assessment (Chapter 15). A final prescription in this case might be as follows:
OD: plano, 2 base-out
OS: plano, 2 base-out
+1.00 add
Thus, in cases of convergence excess associated with high tonic vergence, horizontal prism is an important consideration. When convergence excess is associated with a normal or low tonic vergence, prism is generally not necessary.
Although amblyopia is uncommon in nonstrabismic binocular vision anomalies, it can occur if the phoria is associated with a significant degree of anisometropia. Although anisometropic amblyopia is typically shallow (about 20/60 to 20/80), 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 convergence excess or divergence excess associated with anisometropia, we recommend part-time occlusion. Several (2 to 4) hours of occlusion using an opaque patch along with active amblyopia therapy is usually sufficient to resolve the amblyopia. Details regarding the evaluation and management of anisometropic amblyopia are provided in Chapter 17. In almost all cases, however, amblyopia will not be present in either convergence or divergence excess. Thus, after consideration of ametropia, added lenses, and prism, vision therapy is the next treatment issue.
In many cases of convergence excess, the use of added lenses and prism will be sufficient to successfully treat the patient. If NFV is severely reduced, or the magnitude of the esophoria is very large, or if the patient remains uncomfortable even after wearing the glasses, vision therapy should be recommended. In contrast, vision therapy is the primary treatment option for divergence excess. In general, vision therapy is more effective for divergence excess, whereas base-out prism and added plus lenses tend to be more effective for convergence excess.
The final sequential management consideration listed in Tables 10.1 and 10.2 is surgery. Convergence excess can almost always be successfully managed with a combination of nonsurgical methods. Divergence excess may at times present with a very large-magnitude exotropia at distance. When the size of the deviation is greater than 30 to 35 Δ, surgery is sometimes necessary to supplement other nonsurgical approaches.
PROGNOSIS FOR BINOCULAR VISION DISORDERS ASSOCIATED WITH HIGH AC/A RATIOS
The prognosis for successful treatment of convergence excess is excellent. In many cases the use of lenses, added lenses, and prism will be sufficient. If the patient is still symptomatic after these other interventions, vision therapy can be used and will generally lead to success. It would be rare, therefore, to be unable to successfully treat a patient with convergence excess. Failures with these patients are almost always associated with refusal to wear glasses or poor compliance with vision therapy.
The treatment of convergence excess with vision therapy has received some attention in the literature. In a record review of 12 patients with convergence excess who underwent vision therapy, Shorter and Hatch1 found that
8 (66%) of 12 patients reported improved symptoms, and 5 (62.5%) of the 8 patients with complete data showed increased NFV. The changes were not statistically significant, however. Grisham et al2 and Wick3 each reported a case of convergence excess that showed increased NFV and reduced symptoms after vision therapy. Ficcara et al4 performed a retrospective review of 31 patients (mean age 15.9 years) with convergence excess. The mean number of vision therapy visits was 19.4. There was a significant reduction in symptoms and significant improvement in NFV and PRA. The authors found that the most important factor in determining success was the magnitude of the near phoria prior to vision therapy. Gallaway and Scheiman5 also performed a retrospective analysis of 83 consecutive patients treated with vision therapy for convergence excess. In contrast to the study by Ficcara et al,4 which took place at an optometry school clinic, this study consisted of private practice patients. Thus, the testing was standardized within a practice, and two clinicians performed all the measurements. Statistically significant changes were found in direct and indirect measures of NFV, and 84% of patients reported total elimination of symptoms. Vision therapy does appear to be a viable alternative for patients with convergence excess.
8 (66%) of 12 patients reported improved symptoms, and 5 (62.5%) of the 8 patients with complete data showed increased NFV. The changes were not statistically significant, however. Grisham et al2 and Wick3 each reported a case of convergence excess that showed increased NFV and reduced symptoms after vision therapy. Ficcara et al4 performed a retrospective review of 31 patients (mean age 15.9 years) with convergence excess. The mean number of vision therapy visits was 19.4. There was a significant reduction in symptoms and significant improvement in NFV and PRA. The authors found that the most important factor in determining success was the magnitude of the near phoria prior to vision therapy. Gallaway and Scheiman5 also performed a retrospective analysis of 83 consecutive patients treated with vision therapy for convergence excess. In contrast to the study by Ficcara et al,4 which took place at an optometry school clinic, this study consisted of private practice patients. Thus, the testing was standardized within a practice, and two clinicians performed all the measurements. Statistically significant changes were found in direct and indirect measures of NFV, and 84% of patients reported total elimination of symptoms. Vision therapy does appear to be a viable alternative for patients with convergence excess.
In contrast to the success of added lenses with convergence excess, divergence excess responds best to vision therapy. Many studies have evaluated the efficacy of vision therapy for divergence excess. Goldrich6 reported on the success of vision therapy in a sample of 28 divergence excess patients. He developed criteria for excellent, good, fair, and poor outcomes. To be placed in the excellent category, a patient had to be free from asthenopia, have a phoria at all times, and have normal binocular findings. Placement in the good category meant that the patient was also free from asthenopia and had a phoria at all times, but could have deficiencies on some binocular test findings. A fair result meant that an intermittent strabismus was occasionally observed on cover testing, and a poor result suggested that little improvement had occurred. Twenty patients (71.4%) achieved an excellent rating, and three patients (10.7%) had a good rating. Thus, in 82.1% of the patients, vision therapy was successful in eliminating the intermittent strabismus and asthenopia. For the subjects in the excellent category, the mean number of therapy visits was 20.2 sessions, and for the good category, 28.3 sessions. Only 1 patient was rated poor after treatment.
Pickwell7 reported on the results of vision therapy on 14 divergence excess patients; 10 patients achieved a satisfactory level, 2 patients showed measurable improvement, and 2 others discontinued therapy before completion. Daum8 did a retrospective study of 18 divergence excess patients. The duration of treatment was unusually short, only 5.2 weeks, which raises questions about the meaning of his treatment results. However, he did suggest several interesting points relative to prognosis. He found that success was significantly better in subjects who had lower angles of deviation and no vertical deviation.
Other authors who have studied the effectiveness of vision therapy for intermittent exotropia did not differentiate divergence excess from other types of intermittent exotropia. Although this makes the results more difficult to analyze, the results still have relevance for understanding the effectiveness of vision therapy for divergence excess. Divergence excess is the most common type of intermittent exotropia for which surgery is likely to be recommended. As a result, it is reasonable to suggest that many of the patients reported in the following studies had divergence excess strabismus.
In a study of 37 exotropes, Sanfilippo and Clahane9 found an excellent result in 64.5%, a good result in 9.7%, and a fair result in 22.6%. Only 1 patient (or 3.2%) had a poor result. The authors considered 64.5% to be “cured,” and 32.3% to have immediate improvement in status. They also provided useful data about various factors that influenced the effectiveness of treatment. Amblyopia, a constant deviation, noncomitancy, and a vertical component were negative factors.
Cooper and Leyman10 reported on a retrospective study of 182 intermittent exotropes treated with orthoptics alone. They found a good result in 58.7% and a fair result in 38.4%. Only 5.6% of their sample failed to make significant progress with orthoptics. Coffey et al11 reviewed 59 studies of intermittent exotropia treatment and compiled pooled success rates. They calculated the following pooled success rates: 28% for over minus therapy, 28% for prism therapy, 37% for occlusion, 46% for surgery, and 59% for vision therapy. Cooper and Medow12 also reviewed the literature and concluded that divergence excess in patients younger than 6 years should be treated cautiously so as to reduce or eliminate the possibility of developing amblyopia or permanent loss of stereopsis. They suggested various nonsurgical intervention approaches such as patching, minus lens therapy, and home-based antisuppression treatment initially. Only if the deviation persists or increases should surgical intervention be considered. They suggested that in children older than 6 years, vision therapy is the treatment of choice unless the deviation is large (>35 Δ).
In a recent prospective study by Ma et al,13 the authors evaluated changes in the Office Control Score after office-based vergence/accommodative therapy for intermittent exotropia in children 12 to 17 years old. They enrolled 14 Chinese participants, ages 6 to 17 years, with intermittent exotropia (excluding the convergence insufficiency type). All participants were treated with 12 weeks of office-based vergence/accommodative therapy. Therapy included vergence, accommodative, antisuppression, and monocular fixation in binocular field techniques. The primary outcome measure was the change in the Office Control Score from the baseline visit to the
13-week outcome visit. In this selected group of children with intermittent exotropia, 12 weeks of office-based vergence/accommodative therapy with home reinforcement resulted in statistically and clinically meaningful improvement in distance control of the exodeviation.
13-week outcome visit. In this selected group of children with intermittent exotropia, 12 weeks of office-based vergence/accommodative therapy with home reinforcement resulted in statistically and clinically meaningful improvement in distance control of the exodeviation.
Thus, although the current literature lacks a high-quality randomized clinical trial, it does support the effectiveness of vision therapy in the treatment of divergence excess and, when compared to the cure rates for surgery described later in this chapter, suggests that vision therapy should be the first treatment option. It is important to keep in mind some of the negative prognostic factors suggested by the studies described earlier. Negative factors include a large angle of deviation (>35 Δ), a large vertical component, and a noncomitancy.
SUMMARY OF KEY POINTS IN TREATING PHORIA PATIENTS ASSOCIATED WITH HIGH AC/A
The primary determinant of the management sequence of high AC/A binocular vision problems is the effectiveness of added lenses. Because of the high AC/A ratio, added lenses have a significant effect on the angle of deviation and are, therefore, an important early treatment consideration. When esophoria is present at distance and correction of hyperopia is not sufficient to decrease the phoria to a manageable level, base-out prism is sometimes useful. At times, the use of lenses, added lenses, and prism will not be enough to restore comfort, and vision therapy is necessary.
Convergence Excess
Convergence excess is a condition in which there is an esophoria at near, orthophoria or low to moderate esophoria at distance, reduced NFV, and a high AC/A ratio. Of the various nonstrabismic binocular vision problems seen in clinical practice, convergence excess is one of the most common. Hokoda14 found a prevalence rate of 5.9% in a population of symptomatic individuals seeking vision care. In contrast, 4.2% were found to have convergence insufficiency. Scheiman et al15 also found a higher prevalence of convergence excess than convergence insufficiency. They performed a prospective study on 1,650 children between the ages of 6 and 18 years and found a prevalence of 8.2%. In a university population, Porcar and Martinez-Palomera16 found a 1.5% prevalence of convergence excess.
CHARACTERISTICS
Symptoms
Most symptoms are associated with reading or other close work. Common complaints include eyestrain and headaches after short periods of reading, blurred vision, diplopia, sleepiness, difficulty concentrating, and loss of comprehension over time (Table 10.3). Some patients with convergence excess are asymptomatic. This 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 convergence excess reports an absence of other symptoms. Avoidance is often as important a reason for recommending therapy as any of the other symptoms associated with convergence excess.
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 convergence excess reports an absence of other symptoms. Avoidance is often as important a reason for recommending therapy as any of the other symptoms associated with convergence excess.
Table 10.3 SIGNS AND SYMPTOMS OF CONVERGENCE EXCESS | ||||||||||||||
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Signs
Signs of convergence excess are listed in Table 10.3.
Refractive Error
Convergence excess may be associated with hyperopia. This is a desirable characteristic. Because of the high AC/A ratio, correction of the hyperopia will lead to a decrease in the magnitude of the esophoria at near and at distance. As we stated earlier, one of the primary reasons for a lack of success in the treatment of convergence excess is the patient’s refusal to wear glasses. Although this is very rare, there is another treatment alternative that can be considered in such cases.
When all efforts to have the patient wear eyeglasses or contact lenses fail, pharmacologic treatment is a last resort that can be attempted. Because of the side effects and complications associated with these drugs, this approach should only be used when the patient is either very symptomatic or the deviation is intermittent and the proportion of time the eye deviates is significant and is increasing. Pharmacologic treatment involves the use of echothiophate iodide (Phospholine Iodide) drops or diisopropyl fluorophosphate ointment (DFP). Both are anticholinesterase agents that cause miosis and ciliary spasm. This reduces or eliminates the need for accommodative effort and thereby leads to less accommodative convergence and reduced esophoria.
Echothiophate iodide solution comes in concentrations of 0.03%, 0.06%, 0.125%, and 0.25%. We recommend using 0.03% echothiophate iodide solution once a day (at night) for 1 week. The use of Tylenol for the first week helps reduce the headaches associated with the ciliary spasm, which occur initially. After the first week, increase the concentration to 0.06%, and reevaluate the patient’s status in 2 weeks. Side effects and complications associated with the use of echothiophate iodide include headaches, reversible iris cysts, cataracts, and a greater risk of retinal detachment. Concurrent use of 2.5% phenylephrine has been shown to minimize the formation of iris cysts.
DEP ointment is also an anticholinesterase that can be used to treat convergence excess. A 0.25-in. strip of 0.025% ointment is applied every night. Tylenol should be used the first week or two to reduce the headaches associated with ciliary spasm. Side effects are similar to those described for echothiophate iodide.
When either drug is used, monthly reevaluations should be scheduled to monitor the patient for any side effects or complications and to assess the effect of the treatment. If symptoms have decreased or the proportion of time the deviation occurs is significantly reduced, the treatment can be continued with monthly reevaluation.
Characteristics of the Deviation
Patients with convergence excess generally have greater esophoria at near, a high AC/A ratio, and decreased NFV.
Some authors have suggested that a 10 Δ difference from one distance to another is a useful guideline. Rather than depend on this guideline, we find it more useful to think about the difference one would expect based on the presence of a high AC/A ratio. Because an AC/A ratio of greater than 7:1 is considered high, as little as a 3 Δ difference between distance and near would be sufficient to fit the diagnosis of convergence excess.
Clinicians should use their judgment and generally rely on the other characteristics, in addition to the magnitude of the angle at distance and at near, to reach a diagnosis. For instance, the near deviation may be an intermittent or constant strabismus versus a phoria at distance. This finding would also suggest a diagnosis of convergence excess. Thus, a comparison of the proportion of time the deviation is present, as well as the magnitude at distance and at near, is an important part of the diagnostic process.
AC/A Ratio
A high AC/A ratio (=7:1) is always present in convergence excess. This is well accepted, based on the calculated AC/A, and is an important factor when treatment is considered.
ANALYSIS OF BINOCULAR AND ACCOMMODATIVE DATA
All direct tests of NFV tend to be low in convergence excess (Table 10.3). This includes step, smooth, and jump vergences. In addition, all tests that indirectly assess NFV (Table 10.3) will be low as well. Tests performed binocularly with minus lenses evaluate the patient’s ability to stimulate accommodation and control binocular alignment using NFV. Two examples are PRA and BAF testing with minus lenses. A characteristic finding in convergence excess is a report of diplopia, rather than blur, as the endpoint on PRA and BAF testing. In fact,
it is important to specifically ask about diplopia when performing these tests on a patient suspected of having convergence excess.
it is important to specifically ask about diplopia when performing these tests on a patient suspected of having convergence excess.
A low finding on either PRA or BAF testing may be due to an inability to stimulate accommodation or to reduce NFV. The differential diagnosis is based on assessment of accommodation under monocular conditions. An easy and helpful technique is to simply cover one eye after the patient reports blur on the PRA test. If the blur continues, the problem is usually accommodative (accommodative insufficiency or ill-sustained accommodation). If the patient’s vision clears, the problem is associated with binocular vision (NFV). Normal monocular accommodative ability on other tests suggests reduced NFV.
Another important indirect test of NFV is MEM retinoscopy. It is not unusual to find an abnormal result on this test in convergence excess. An MEM finding of greater plus than expected suggests that the patient is using as little accommodation as possible to decrease the use of accommodative convergence. This reduces the amount of esophoria and the demand on NFV.
In some instances of convergence excess, a low-to-moderate degree of esophoria is present at distance as well. This is due to a moderate to high degree of tonic vergence. In such cases, in addition to the low NFV at near, the distance findings will be low as well.
DIFFERENTIAL DIAGNOSIS
It is important to rule out serious underlying etiologies in all cases of convergence excess. Differential diagnosis (Table 10.4) depends very much on the nature of the patient’s symptoms. Typically, convergence excess presents with long-standing chronic complaints. The health history is negative, and the patient is not taking any medication known to affect accommodation. Convergence excess associated with serious underlying disease has an acute onset, and medical problems or neurologic symptoms are usually present. The primary functional disorders that must be differentiated from true convergence excess are basic esophoria, divergence insufficiency, and esophoria at near secondary to accommodative anomalies.
Convergence excess is considered to be a benign condition, with no serious consequences other than the visual symptoms listed in Table 10.3. It is relatively easy to differentiate from other binocular vision disorders associated with esophoria, such as basic esophoria (equal deviation at distance and at near) and divergence insufficiency (greater esophoria at distance). Convergence excess must also be differentiated from esophoria at near secondary to an accommodative anomaly, requiring a careful analysis of all accommodative and binocular vision data. Cases 2.1 to 2.4 in Chapter 2 are examples of the analytic process the clinician must follow.
Convergence excess or esophoria at near can also be associated with more serious underlying conditions. A condition called spasm of accommodation or convergence can occur, and one resulting clinical finding may be esophoria at near. Accommodative spasm can be functional, but it may also be caused by more serious underlying disease. Some of the more common causes include local inflammation and central nervous system lesions. Ocular inflammation such as scleritis, iritis, and uveitis can cause uniocular accommodative spasm and esophoria. This suggests that slit lamp evaluation is an important test in the differential diagnosis of convergence excess.
Central nervous system disorders such as sympathetic paralysis and syphilis may also lead to accommodative spasm and esophoria. In addition, a variety of drugs may produce bilateral accommodative spasm and esophoria. Some of the more common drugs that can produce these effects include eserine, pilocarpine, excessive doses of vitamin B1, and sulfonamides.
Table 10.4 DIFFERENTIAL DIAGNOSIS OF CONVERGENCE EXCESS | |||||
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When managing a case of convergence excess that is thought to have a functional basis, if symptoms and findings do not improve as expected, it is wise to reconsider the etiology of the condition.
TREATMENT
We recommend following the management sequence listed in Table 10.1.
Lenses
In all cases of binocular and accommodative dysfunction, the first management consideration is correction of any significant refractive error. With convergence excess, it is important to prescribe maximum plus if a significant degree of hyperopia is present (+0.50 or greater). When dealing with convergence excess associated with high tonic vergence, a cycloplegic examination should be performed before determining the prescription.
Added Lenses
Because of the high AC/A ratio, the use of added plus lenses at near is highly effective in cases of convergence excess. Chapter 3 discussed the important clinical data that are used to determine whether additional plus should be prescribed. Although the AC/A ratio is the key finding, it is important to consider all of the data listed in Table 10.5.
How Much Additional Plus Should Be Prescribed?
When prescribing added plus lenses, the objective is to determine the lowest amount of plus that will eliminate the patient’s symptoms and normalize optometric data. A variety of methods have been suggested for calculating the amount of additional plus to prescribe for patients with convergence excess. Some of the more popular methods are analysis of the NRA/PRA relationship, MEM retinoscopy or other near point retinoscopy, use of the AC/A ratio, and fixation disparity analysis. We advocate the use of a group of findings rather than relying on any one test. As discussed in Chapter 2, reliance on any one test may be misleading at times. The optometric data listed in Table 10.5 can be used to determine the amount of plus to prescribe.
An example of this is Case 10.1, described earlier in this chapter. After prescribing +1.00 to correct the hyperopia, this patient was still 3 esophoric at near, with a low PRA, reduced base-in at near, an MEM finding of +1.25, and diplopia with minus lenses during BAF testing. Both the NRA/PRA (NRA, +2.50; PRA, −1.00) relationship and MEM retinoscopy suggest a prescription of an additional +0.75 to +1.00 for near. In addition, the AC/A ratio suggests that the near phoria should be about 3 exophoria with this prescription.
Prism
If a vertical deviation is present, we recommend that vertical prism be prescribed. 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).
Because of the high AC/A ratio, the use of lenses is so effective that horizontal prism is rarely necessary except for convergence excess associated with high tonic vergence (moderate to high esophoria at distance). When a moderate to high degree of esophoria is present at distance, base-out prism should be considered. The decision to prescribe base-out prism should be based on the presence or absence of distance-related symptoms. If a prism prescription is being considered, fixation disparity testing is the most effective method for determining the amount of horizontal prism (Chapter 15).
Table 10.5 CONSIDERATIONS FOR PRESCRIBING ADDED PLUS LENSES | ||||||||||||||||||||||||||||||
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Vision Therapy
If NFV is severely reduced, if the magnitude of the esophoria is very large, or if the patient remains uncomfortable even after wearing glasses, vision therapy should be recommended. A vision therapy program for convergence excess generally requires 12 to 24 office visits. If refractive correction and added lenses are used, the number of sessions may be less. The total number of therapy sessions also depends on the age of the patient and his or her motivation and compliance.
Specific Vision Therapy Program
Phase 1
This first phase of therapy is designed to accomplish the objectives listed in Table 10.6 under Phase 1. As discussed in Chapter 9, vision therapy requires communication and cooperation between the therapist and patient, and it is important to develop a working relationship with the patient during the first few sessions.
The first goal of therapy is to teach the concept and feeling of diverging and the ability to accurately diverge. The patient should be able to voluntarily converge and diverge to any distance from 5 cm (2 in.) to 6 m (20 ft). Commonly used procedures to accomplish this first objective are the Brock string and bug on string.
Convergence excess patients generally have very limited base-in blur, break, and recovery findings. Therefore, another objective of the first phase of vision therapy is to normalize NFV 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. Such a demand allows the patient to begin the procedure with accommodation and convergence at the same plane. A divergence demand can then be slowly introduced, which requires the patient to hold accommodation at 40 cm and move the convergence plane further away.
Another advantage of beginning with smooth vergence procedures is that, in some cases, the introduction of any divergence is enough to cause suppression or diplopia. Smooth vergence techniques provide a starting point for therapy with such patients. If the patient is unable to fuse any divergence demand, the procedure can begin with a convergence demand. For example, a variable tranaglyph can be set at 10 base-out and then gradually reduced to zero. This approach at least allows the patient to get started and experience some success. The change from 10 base-out to zero can be viewed as divergence therapy relative to the starting point. Speed is of little importance initially. Rather, we just want the patient to be able to maintain fusion as the divergence demand is slowly increased. Equipment that can be used to create a smooth, gradual increase in divergence demand includes the variable tranaglyphs, variable vectograms, and the Variable Prismatic Stereoscope.
In some cases of convergence excess in pre-presbyopes, an accommodative problem may also be present. If so, the final objective of the first phase of therapy is to normalize accommodative amplitude and the ability to stimulate and relax accommodation. If, however, accommodative function is normal, there is generally no need to spend much time working with the accommodative system. Accommodative techniques can be found 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:
Accurately diverge using the Brock string to 3 m (10 ft).
Fuse to about 15 base-in at 40 cm using a tranaglyph or other divergence technique.
Complete 12 cpm of MAF with +2.00/−2.00 lenses using a 20/30 target.
Table 10.6 OBJECTIVES OF VISION THERAPY FOR CONVERGENCE EXCESS | ||||
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A sample vision therapy program for Phase 1 is summarized in Table 10.7.
Phase 2
This second phase of therapy is designed to accomplish the objectives listed in Table 10.6 under Phase 2. Once smooth NFV is normalized, phasic or jump vergence demand should be emphasized. Variable tranaglyphs and vectograms can still be used. However, the specific modifications to create a step vergence demand (described in Chapter 6) must be implemented. These include the following:
Changing fixation from the target to another point in space
Covering and uncovering one eye
Loose prism or flip prism
Flip lenses to create a step change in accommodative demand, requiring a compensatory vergence change to maintain fusion
Two different tranaglyphs set up in a dual Polachrome illuminated trainer
Polaroid or red/green flippers
Other valuable techniques at this stage are the nonvariable tranaglyphs, the Aperture Rule, Eccentric Circles, Free Space Fusion cards, Lifesaver cards, and the Computer Orthoptics Jump Vergence program.
In contrast to Phase 1, in which speed was not a factor, during this second phase of therapy the emphasis should be on the qualitative aspects of fusion rather than magnitude. It is important to increase the speed of the fusional vergence response and the quality of the recovery of fusion.
A second objective of this phase of therapy is to begin working with positive fusional vergence (PFV) amplitudes. The same techniques used in Phase 1 to work with NFV are repeated for PFV. Finally, during the end of this phase of therapy, begin to incorporate PFV facility-type techniques, using the same procedures listed for jump vergence demand for NFV.
Table 10.7 SAMPLE VISION THERAPY PROGRAM FOR CONVERGENCE EXCESS | ||||||||||||||||||||||
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