General Treatment Modalities, Guidelines, and Prognosis
This chapter provides an overview of general management strategies for accommodative, ocular motor, and nonstrabismic binocular anomalies. Chapters 9, 10, 11, 12, 13 provide specific details on each type of accommodative, ocular motor, and binocular vision disorder.
The treatment model we present is based on the information in Chapters 1 and 2 regarding diagnosis, analysis, and classification. To use the information in this chapter as a general guide in treating a specific binocular vision case, it is first necessary to determine, from the clinical evaluation, whether there is an exophoria, orthophoria, or esophoria at distance (low, normal, or high distance tonic vergence) and whether it is associated with a low, normal, or high accommodative convergence to accommodation (AC/A) ratio. In addition, it is important to determine whether the grouped data suggest a problem in a particular area. In regard to accommodative anomalies, it is important to ascertain whether the difficulty is in stimulation of accommodation, relaxation of accommodation, or both. Once this information is known, you will be able to apply the general treatment model presented in this chapter.
A primary objective of the model we present is to emphasize the significance of considering all treatment options for every accommodative, ocular motor, and binocular vision anomaly encountered. There are a limited number of management options for any patient with these disorders. When managing such patients, it is best to acquire the habit of always considering each option and then either using or rejecting that management for a particular patient. This approach will ensure that no management option has been ignored and should lead to more frequent and rapid success. For instance, several of the treatment options discussed later (i.e., occlusion, vision therapy for amblyopia and anomalous correspondence, and surgery) are rarely necessary when managing accommodative and nonstrabismic binocular vision problems. In heterophoria cases associated with anisometropia, however, occlusion and amblyopia treatment will often be necessary. In very rare instances, when dealing with exceptionally large-magnitude heterophorias, surgery may be necessary. Thus, it is prudent to always consider all treatment options for all cases.
We also stress the order in which different treatment procedures should be considered for efficient and successful management of any specific type of accommodative, ocular motor, or binocular anomaly case. Sequencing decisions are based on the characteristics of the condition being considered. Specific sequencing for each classification described in Chapter 2 is covered in detail in Chapters 9, 10, 11, 12, 13.
Determination of “Successful Treatment”
For accommodative and nonstrabismic disorders, the chief concern of the patient is overcoming symptoms or some visual performance deficiency, such as limited ability to maintain clear vision while reading. The goal of the practitioner is to improve certain visual functions in some measurable way. The criteria that practitioners use to define functional correction of these cases therefore involve some combination of decrease in symptoms and improvement in measured visual functions. After treatment, the patient should feel that his or her original symptoms have been eliminated or significantly improved. In addition, accommodative and binocular test findings should fall within the parameters consistent with the expected findings discussed in Chapter 1. Investigators have developed symptom and quality of life questionnaires that have been used for research purposes and can now be used in optometric practice, allowing clinicians to reliably measure symptoms before and after treatment.
SYMPTOM AND QUALITY OF LIFE QUESTIONNAIRES
Garcia-Munoz et al1 conducted a literature review of articles published between 1988 and 2012 that analyzed any aspect of the symptomatology associated with accommodative and nonstrabismic binocular dysfunctions.
They found a wide disparity of symptoms related to accommodative and binocular dysfunctions in the scientific literature, most of which are associated with near vision. Only two of the questionnaires were validated (Convergence Insufficiency [CI] Symptom Survey [CISS] and Conlon). They were unable to identify specific questionnaires for other anomalies. The questionnaires most commonly used in studies they retrieved were the CISS, followed by the 19-item College of Optometrists in Vision Development Quality of Life (COVD-QOL) questionnaire and the Conlon survey. The authors, however, failed to retrieve studies that used a symptom scale known as the Vision Quality Scale (VQS).2 This symptom scale has been validated.
They found a wide disparity of symptoms related to accommodative and binocular dysfunctions in the scientific literature, most of which are associated with near vision. Only two of the questionnaires were validated (Convergence Insufficiency [CI] Symptom Survey [CISS] and Conlon). They were unable to identify specific questionnaires for other anomalies. The questionnaires most commonly used in studies they retrieved were the CISS, followed by the 19-item College of Optometrists in Vision Development Quality of Life (COVD-QOL) questionnaire and the Conlon survey. The authors, however, failed to retrieve studies that used a symptom scale known as the Vision Quality Scale (VQS).2 This symptom scale has been validated.
Convergence Insufficiency Symptom Survey
The Convergence Insufficiency Treatment Trial (CITT) investigator group conducted a series of studies to develop a symptom questionnaire for use as the primary outcome measure in a series of CITT studies. This questionnaire, the CISS, is the first standardized instrument that has been shown to be valid and reliable for measuring the type and frequency of symptoms before and after treatment for patients with CI.3,4,5,6,7 It can be used in clinical practice to compare symptoms before and after optometric intervention for patients with CI and other binocular vision and accommodative disorders.
The CISS allows a two-factor analysis of symptoms: first, whether the symptom is present, and second, how frequently the symptom occurs. The questionnaire consists of 15 items (Fig. 3.1). The patient chooses one of five possible answers (never, infrequently, sometimes, fairly often, always). Each answer is scored from 0 to 4, with 4 representing the highest frequency of symptom occurrence (i.e., always). The 15 items are summed to obtain the CISS score, with the lowest possible score being 0 (totally asymptomatic) and the highest possible score being 60 (most symptomatic). For children aged 9 to 17 years, a symptom score of 16 or higher on the CISS has been found to differentiate children with symptomatic CI from those with normal binocular vision.
For adults (18 and older), a symptom score of 21 or higher has been found to be significant. Research using this survey has demonstrated that a score below 16 for children and below 21 for adults or a change of 10 or more points is clinically significant.
For adults (18 and older), a symptom score of 21 or higher has been found to be significant. Research using this survey has demonstrated that a score below 16 for children and below 21 for adults or a change of 10 or more points is clinically significant.
 ▪ Figure 3.1 Convergence insufficiency symptom survey (CISS). |
The CISS has been used in multiple randomized clinical trials to study the effectiveness of vergence/accommodative therapy (VAT) for improving symptoms in children and young adults with symptomatic CI.8,9,10,11,12 In some cases, significant differences in the change in the CISS score have been found between the active treatment and placebo groups demonstrating that vision therapy effectively improves symptoms.8,12 In other studies, a significant difference in the change in the CISS score was not found, although clinical measures were significantly different between the active treatment and placebo groups.10,13 For example, in the Convergence Insufficiency Treatment Trial—Attention and Reading Trial (CITT-ART) improvements in both the near point of convergence and positive fusional vergence (PFV) were found. Although the mean CISS reduction of 11.2 points in the VAT group was statistically significant and clinically meaningful, it was considerably less than the 22.6 and 14.8 point improvement found in the previous two CITT studies. Compared with the large-scale CITT, the mean improvement in the VAT group was 3.7 points less (14.8 vs. 11.1, respectively) and in the placebo therapy group was 2.5 points greater (7.8 vs. 10.3, respectively), resulting in a minimal and nonsignificant treatment group difference (1.5 points).
Whether this finding is simply due to chance or whether there is another explanation is not certain. Using the CISS, a subjective questionnaire, as a visually related outcome measure for children has raised concerns from some authors. It has been pointed out that children’s responses on the CISS in regard to the occurrence of symptoms “when reading or doing close work” may depend on the type of near activity that the child is envisioning when queried14 (e.g., doing homework, playing videogames, or interacting with smart devices) or whether the reading is for pleasure or required for school.14 It is possible that the increasing use of electronic devices may have impacted the validity of the CISS since its development. Another potential issue is that comorbid ocular (e.g., dry eye, allergies) or even nonocular conditions may lead to positive responses on the CISS,15,16 which could account for the lack of a relationship between the severity of clinical signs and level of symptoms as reported by our group17 and others.18,19,20 It may be necessary to reconsider the use of the CISS in its current form as an outcome measure for studies of CI, and we suggest caution when using the CISS as a primary outcome measure for vision therapy effectiveness studies.
College of Optometrists in Vision Development Quality of Life Outcomes Assessment
The COVD-QOL Outcomes Assessment is a clinical survey instrument that can also be used to evaluate changes in symptoms before and after treatment (Fig. 3.2). It was originally developed with 30 items21,22 but a 19-item version has been developed to make the survey more efficient to use.23,24 Both the original and short forms of the COVD-QOL have been shown to be reasonably reliable assessment tools for children and adults.21,23,24 The short form of the assessment consists of 19 items. For each item, the patient selects from the following five possible response options: never, once in a long while, sometimes, a lot, or always. The options are scored from 0 for never to 4 for always. Thus, the highest score (most symptomatic) is 76, and the lowest score is 0. Previous research indicates that a score of 20 or greater suggests that the patient has significant symptoms. Several authors have reported on the use of the COVD-QOL short form to evaluate symptoms before and after vision therapy25,26,27 and found the assessment to be a valuable clinical tool.
Conlon Survey17
The Conlon survey was developed to analyze the symptoms associated with any type of visual anomaly, including accommodative and binocular dysfunctions, and has shown to be a reliable and valid measure of visual discomfort for adults. The survey uses a Likert scale with four response choices. Each item is scored between 0 and 3. Total score ranges from 0 to 69 points, with three groups defined—Low discomfort: score 0 to 24; Moderate discomfort: 25 to 48; High visual discomfort: 49 to 69.
Vision Quality Scale
The questionnaire of McKeon et al, known as the VQS, determines the presence of symptoms by a 9-item quality of vision questionnaire. This questionnaire was developed as part of the National Eye Institute grant to analyze symptoms of patients with intermittent exotropia and validated by comparison to an established quality of life questionnaire. It has been used in multiple studies.28,29,30,31,32 The vision symptoms addressed include headaches, eye strain, dry eyes, blurred vision, difficulty reading, and asthenopia. These symptoms are clinically associated with many accommodative and vergence dysfunctions such as CI or excess, divergence insufficiency or excess, and accommodative insufficiency, excess, or infacility. It is a criterion referenced test that has been
used on several thousand subjects in research studies. Based on validation study results, patients scoring more than 84 are considered asymptomatic and those scoring less than 71 (1 standard deviation below the mean) are considered symptomatic. This survey is particularly useful for intermittent exotropia.
used on several thousand subjects in research studies. Based on validation study results, patients scoring more than 84 are considered asymptomatic and those scoring less than 71 (1 standard deviation below the mean) are considered symptomatic. This survey is particularly useful for intermittent exotropia.
 ▪ Figure 3.2 College of Optometrists in Vision Development Quality of Life (COVD-QOL). |
We recommend that clinicians use one of these four assessment tools to evaluate patient symptoms before and after treatment for accommodation, binocular vision, and eye movement problems.
General Treatment Sequences for Accommodative and Nonstrabismic Binocular Vision Anomalies
In this system for treating phorias, the size of the AC/A ratio (low, normal, or high) determines the specific treatment sequence. The direction of the distance phoria and the analysis of the grouped data determine certain particulars of treatment, such as whether base-out or base-in prism should be prescribed and the nature of the vision therapy recommended.
SEQUENTIAL MANAGEMENT APPROACH
1. Optical correction of ametropia
2. Added lens power
3. Prism
4. Occlusion
5. Vision therapy
(a) Amblyopia
(b) Suppression
(c) Anomalous correspondence
(d) Sensory motor function
6. Surgery.
Optical Correction of Ametropia
The first consideration for all patients with accommodative, ocular motor, and nonstrabismic binocular anomalies is optical correction of ametropia. Prescription of lenses to correct the refractive error is generally not thought of as management for accommodative and binocular problems because we so routinely prescribe lens corrections. However, such prescriptions are often so essential in the management of these conditions that it is wise to routinely think of correction of ametropia as the first consideration.
SIGNIFICANT DEGREES OF REFRACTIVE ERROR
As a general rule, it is advisable to first prescribe for any significant refractive error. Table 3.1, based on the Orinda study,33 lists criteria for significant refractive error. It is important to view these criteria only as guidelines. With any given patient, a variety of factors must be considered. Several underlying assumptions form the basis for the approach of first considering management of refractive error.
The presence of an uncorrected refractive error may:
Result in either underaccommodation or overaccommodation, leading to disorders of accommodative function
Result in a high phoria and an unusual demand of either negative fusional vergence (NFV) or PFV
Create an imbalance between the two eyes, leading to sensory fusion disturbances
Create decreased fusional ability as a result of blurred retinal images.
The strategy of first prescribing for significant refractive error is, therefore, based on the assumption that there may be a cause-and-effect relationship between refractive error and accommodative and binocular vision anomalies. Dwyer and Wick34 reported improvement in binocular function that occurred one or more months after prescription of an initial spectacle correction for 143 nonstrabismic patients who had a refractive error and either a vergence anomaly, an accommodative anomaly, or both. Most corrections were low to moderate in power, essentially following the guidelines in Table 3.1. Recovery of normal vergence and accommodative function varied according to refractive error type, direction of astigmatic axes, age, and vergence anomaly. These results support the strategy of first prescribing for significant refractive error.
There are some differences in refractive errors between patients with esodeviations and exodeviations. Esodeviations tend to be associated with greater amounts of hyperopia, whereas exodeviations tend to be associated with myopia. By prescribing for the refractive condition, we are attempting to minimize a possible underlying etiologic factor.
Table 3.1 GUIDELINES FOR SIGNIFICANT REFRACTIVE ERROR | ||||||||||
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When deciding on a prescription for ametropia, it is also important to consider and understand that the correction influences the vergence posture of the eyes through the AC/A ratio. As a result, we generally recommend prescribing maximum plus for esodeviations and minimum plus for exodeviations. Optical correction may also have a negative effect on binocular vision. An example is an exophoric patient with uncorrected hyperopia of 2.00 diopters (D). With correction of the ametropia, he or she may experience asthenopia and diplopia. Another common example is that of an esophoric patient with 4.00 D of uncorrected hyperopia. With correction, this patient may become exotropic. Management of these patients is more complex and requires the use of additional steps described next.
If a significant refractive error is present, the patient is generally asked to wear the prescription for 4 to 6 weeks, at which point another evaluation is performed to reassess the status of accommodative and binocular function. In some instances, the previously detected disorders will have resolved and no additional treatment will be necessary. If accommodative, ocular motor, or binocular disorders still persist after wearing the prescription, additional treatment alternatives must be considered.
A typical example would be a patient who presents with a history of eyestrain associated with visually demanding tasks. The evaluation reveals +2.00 D of uncorrected hyperopia in both eyes, along with an accommodative insufficiency. Appropriate management would be a prescription for the hyperopia and a reevaluation in 4 to 6 weeks to reassess the status of accommodation. If the patient continues to feel uncomfortable and the accommodative problem is still present, additional treatment would be prescribed. Occasionally, however, management of a significant refractive error will lead to resolution of the secondary accommodative or binocular anomaly.
CYCLOPLEGIC REFRACTION
Static retinoscopy and a dry subjective refraction are sufficient to determine the refractive error in most cases. When esophoria is present or latent hyperopia is suspected, a cycloplegic refraction may be helpful. For children younger than 3 years, 1 gtt of 0.5% cyclopentolate hydrochloride repeated in 5 minutes is the recommended concentration and dosage. For children aged 3 years and older, the dosage is the same as for younger children, whereas the recommended concentration is 1.0%. After waiting approximately 40 minutes, retinoscopy is performed. To determine the final refractive correction, the following issues must be considered:
Tonus of the ciliary muscle: If full cycloplegia is achieved, then the normal tonus of the ciliary muscle will also be relaxed. Thus, more plus will be found than can be prescribed. If we know that complete cycloplegia has been achieved, about 0.75 D should be subtracted from the net finding.
Type of refractive error: In myopes, it is usually not necessary to subtract the 0.75 D of plus, whereas in hyperopia it is necessary.
Binocular status: If esophoria or intermittent esotropia is present, maximum plus should be considered.
SMALL DEGREES OF REFRACTIVE ERROR
Far less agreement exists about the management of small amounts of refractive error. This would be defined as a refractive error less than the values listed in Table 3.1. An example would be a patient presenting with a history of eyestrain associated with reading, with the refraction as follows:
OD: + 0.25 – 0.05 × 90
OS: + 0.25 – 0.05 × 90
The question the clinician must answer is whether such a refractive error could be the cause of the patient’s discomfort. This decision should be based on additional testing and analysis of accommodative and binocular data.
Two scenarios generally occur. First, the patient may also present with significant accommodative and binocular problems. Assume this patient also had a near point of convergence of 6/12 in., orthophoria at distance, and 12 exophoria at near, and that the PFV group of data strongly suggested decreased PFV. In the presence of additional data, the low refractive error becomes significant only if the clear retinal images achieved through refractive correction will improve fusion and assist in management.
Another possible situation would be a patient with the low refractive error listed earlier and all accommodative and binocular testing within the expected values. In this case, the clinician may be left with no other possible visual basis for the patient’s discomfort and must make a decision about prescribing for the low refractive error. It is wise, in such a situation, to ask additional questions about the nature of the symptoms to clarify whether there truly appears to be a relationship between the use of the eyes and the discomfort. If based on
this additional questioning there seems to be a relationship, prescription for the low refractive error may sometimes be helpful—especially if small astigmatism corrections against the rule or oblique axis are present. In our experience, however, there is often an accommodative, ocular motor, or binocular vision disorder present in addition to the low refractive error. It is very unusual to find a low refractive error in isolation that accounts for the significant symptoms.
this additional questioning there seems to be a relationship, prescription for the low refractive error may sometimes be helpful—especially if small astigmatism corrections against the rule or oblique axis are present. In our experience, however, there is often an accommodative, ocular motor, or binocular vision disorder present in addition to the low refractive error. It is very unusual to find a low refractive error in isolation that accounts for the significant symptoms.
Other authors have addressed the issue of prescribing for low refractive errors.35,36,37,38 Blume38 reported that symptoms induced by low refractive error include slightly blurred vision, headaches, and ocular discomfort associated with activities such as reading and other near work. Case reports35,36,37,38 have demonstrated the positive effect of prescribing for low refractive errors. A critical analysis of these case reports, however, indicates that an assessment of accommodation and binocular vision either was not performed or was not reported in the majority of cases. This lack of data about accommodative and binocular function makes it difficult to interpret these reports.
MANAGING ANISOMETROPIA AND ANISEIKONIA IN NONSTRABISMIC BINOCULAR VISION DISORDERS
Anisometropia is defined as a condition in which the refractive status of one eye differs from that of the other. A difference of 1 D or more in the sphere or cylinder is considered clinically significant. The critical underlying concept that should be considered when deciding about correction of anisometropia is that clear retinal images facilitate fusion. The general rule, therefore, is to fully correct the anisometropia. If the patient is amblyopic, the underlying cause of the amblyopia is the uncorrected refractive error. Unless the refractive error is corrected, there is little reason to expect maximal improvement in acuity, accommodative response, and binocular skills. A possible exception is a patient who might become more symptomatic if corrected. An example is an elderly patient who requires a large increase in cylindrical correction in one eye. For such patients, consider reducing the prescription. In all other cases, fully correct the anisometropia.
There are two additional concerns that one must take into consideration when prescribing for anisometropia. The first issue is the possibility of inducing aniseikonia. Aniseikonia is defined as a condition in which the ocular images are unequal in size or shape or both. The different image sizes induced by the prescription can cause symptoms and affect sensory fusion. This topic is covered in depth in Chapter 19. Although aniseikonia may occur occasionally in clinical practice, it affects only a small percentage of patients with anisometropia. The decision that must be made is whether to prescribe spectacle lenses or contact lenses. Knapp law provides guidelines and suggests prescribing eyeglasses for aniseikonia secondary to axial length differences and contact lenses for anisometropia caused by refractive differences. As most anisometropia is due to axial length differences, eyeglasses would be the method of choice according to Knapp law.
Clinically, however, we find that this is not the case. The reason for this is the second problem associated with the correction of anisometropia. An anisometropic prescription will always cause prismatic differences for the patient as he or she looks from one position of gaze to another. The greater the degree of anisometropia, the larger the prismatic differences. This creates a motor fusion problem, placing a demand on horizontal and vertical fusional vergence. Whereas aniseikonia only occurs in a small percentage of patients, this motor problem affects all anisometropes after correction with eyeglasses. As a result, contact lenses should be considered the treatment method of choice in cases of anisometropia.
Added Lens Power (Minus and Plus)
The other primary use of lenses in the treatment of accommodative and binocular disorders is to alter the demand on either the accommodative or binocular system. The important clinical data used to determine whether such an approach will be effective are listed in Tables 3.2 and 3.3. The idea that optometric case analysis should be based on groups of data (Chapter 2) applies to decision making about the possible effectiveness of additional lenses as well. Table 3.2 lists the eight findings that should be considered when trying to determine whether added plus lenses should be prescribed, and Table 3.3 lists the findings that should be considered when trying to determine whether added minus lenses should be prescribed.
The primary test finding that helps determine the effectiveness of added lenses is the magnitude of the AC/A ratio. If the AC/A ratio is greater than expected, the use of added lenses will generally be an effective approach. A high AC/A ratio suggests that a very large change in binocular alignment can be achieved with a small addition of lenses. A low AC/A ratio indicates that the use of lenses will have little desirable effect. When the AC/A ratio is in the normal range of 3:1 to 7:1, the other data in Tables 3.2 and 3.3 must be taken into consideration before
determining the potential value of prescribing added lenses. It is important to understand the effect that plus or minus lenses will have on all examination findings. Tables 3.4 and 3.5 provide examples of these effects. If one keeps in mind the effect that a prescription of additional plus or minus will have on all of the different diagnostic tests, it becomes easier to make decisions about appropriate treatment for any particular patient.
determining the potential value of prescribing added lenses. It is important to understand the effect that plus or minus lenses will have on all examination findings. Tables 3.4 and 3.5 provide examples of these effects. If one keeps in mind the effect that a prescription of additional plus or minus will have on all of the different diagnostic tests, it becomes easier to make decisions about appropriate treatment for any particular patient.
Table 3.2 CONSIDERATIONS FOR PRESCRIBING ADDED PLUS LENSES | ||||||||||||||||||||||||||||||
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Table 3.3 CONSIDERATIONS FOR PRESCRIBING ADDED MINUS LENSES | ||||||||||||||||||||||||||||||
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The most common example of the effectiveness of the use of lenses in the absence of refractive error is convergence excess. In such a case, the patient will generally have no significant phoria at distance and a moderate to high degree of esophoria at near. The findings in the NFV group will be low, suggesting decreased NFV, and the AC/A ratio is typically high. These findings suggest that a significant change could be achieved in the amount of esophoria at near simply by prescribing plus lenses for near. If the patient has
12 Δ of esophoria at near, for example, with base-in at near of 4/6/2 and an AC/A ratio of 10:1, an add of +1.00 would be expected to have a considerable beneficial effect. In this case, the add would result in a near point phoria of about 2 esophoria, and the base-in range measured through this add would be expected to increase as well. If, however, the clinical data are somewhat different and the patient has moderate esophoria at near with a low AC/A ratio, then the use of added lenses may not be sufficient to lead to a resolution of the patient’s complaints.
12 Δ of esophoria at near, for example, with base-in at near of 4/6/2 and an AC/A ratio of 10:1, an add of +1.00 would be expected to have a considerable beneficial effect. In this case, the add would result in a near point phoria of about 2 esophoria, and the base-in range measured through this add would be expected to increase as well. If, however, the clinical data are somewhat different and the patient has moderate esophoria at near with a low AC/A ratio, then the use of added lenses may not be sufficient to lead to a resolution of the patient’s complaints.
Table 3.4 EXAMPLE OF THE EFFECT OF PLUS LENSES ON TEST RESULTS | ||||||||||||||||||||||
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Table 3.5 EXAMPLE OF THE EFFECT OF MINUS LENSES ON TEST RESULTS | ||||||||||||||||||||||
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The classic example of the ineffectiveness of the use of lenses in the absence of refractive error is CI. In such a case, the distance phoria is insignificant, whereas a moderate to large exophoria may be present at near along with a low AC/A ratio, a receded near point of convergence, and low PFV group data. The use of lenses—in this case, to achieve a desirable change in the near phoria—would not be expected to be helpful. For instance, one might consider the use of additional minus at near. If the patient has 12 exophoria at near with an AC/A ratio of 2:1 and base-out at near of 2/4/-2, the use of -1.00 or even -2.00 at near would have little effect on the exophoria or base-out relationship. Thus, because of the low AC/A, the use of lenses in this situation would not be an effective strategy.
The use of added plus or minus lenses is particularly helpful for the conditions listed in Table 3.6. Prescription guidelines for prescribing added plus lenses are based on the information in Table 3.2. This table lists all of the findings from the optometric evaluation that contribute to the final decision about prescribing added plus. The concept, stressed in Chapter 2, that groups of data should be analyzed rather than a single isolated finding applies to the issue of added plus lenses. Although all of the data points do not have to agree, there will generally be a trend suggesting the amount of plus that should be prescribed. Cases 10.1 to 10.3 in Chapter 10 provide specific examples about determining the amount of added plus to prescribe.
When prescribing added plus lenses, a bifocal prescription is almost always preferable. With children younger than about 10 years, we recommend setting the segment height at about the lower pupil margin to ensure that the child reads through the segment. A flat-top 28-mm segment works well with young children. In older children and adults, the segment height can be set at the lower lid margin.
Added minus lenses should also be considered in certain cases. Added minus lenses have been proposed as a treatment of young children with intermittent exotropia. In a pilot randomized clinical trial, the Pediatric Eye Disease Investigator Group (PEDIG) found that overminus spectacles improved distance control at 8 weeks in children 3 to <7 years old with intermittent exotropia.39 In such cases, the lenses are used to reduce the angle of deviation using accommodative convergence to supplement fusional vergence. These lenses can be prescribed
as training lenses to be used only during active vision therapy or for general wear. When used as a training device, large amounts of minus can be prescribed. For a constant exotrope, it would not be unusual to prescribe 6 or 7 D of additional minus. To determine the prescription, the clinician would find the least amount of minus that allows the patient to fuse. The power of the lenses would gradually be reduced as therapy progresses and the patient’s ability to fuse improves.
as training lenses to be used only during active vision therapy or for general wear. When used as a training device, large amounts of minus can be prescribed. For a constant exotrope, it would not be unusual to prescribe 6 or 7 D of additional minus. To determine the prescription, the clinician would find the least amount of minus that allows the patient to fuse. The power of the lenses would gradually be reduced as therapy progresses and the patient’s ability to fuse improves.
Table 3.6 CONDITIONS RESPONDING FAVORABLY TO ADDED LENSES | ||||||||||
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Added minus lenses can also be prescribed for full-time wear. This would be done to reduce the percentage of time that an intermittent exotropia occurs or to provide more comfortable fusion in high exophoria. When prescribed for this purpose, smaller amounts of minus (i.e., 1.00 to 2.00 D) are used. In such cases, the AC/A ratio is not the critical factor in determining the amount of minus to prescribe. The objective of the added minus is to create a stimulus to convergence. Once this is accomplished, the patient is able to maintain fusion using fusional vergence.
Prism
The use of prism to treat binocular anomalies should be a consideration in all cases. Generally, there are five situations in which the use of prism may be helpful:
Horizontal relieving prism
Vertical relieving prism
Prism as an aid to begin vision therapy
Prism used when vision therapy is inappropriate or impractical
Prism used at the end of vision therapy.
HORIZONTAL RELIEVING PRISM
If a large lateral heterophoria or an intermittent strabismus is present, it may be helpful to prescribe prism to decrease the demand on fusional vergence. Prism is most often effective in cases of high tonic vergence or esophoria at distance along with a normal to low AC/A ratio. Prism can be prescribed as a temporary measure until a vision therapy program has been completed or as an attempt to eliminate the patient’s symptoms without vision therapy.
Although the use of prism to treat heterophoria has been recommended by numerous authors,40,41,42,43,44,45,46,47,48,49 there is surprisingly limited research support for its effectiveness. A study by Worrell et al47 investigated the effectiveness of prism prescribed based on Sheard’s criterion. They found that prism was only preferred by patients in cases of distance esophoria. For exophoria and esophoria at near, the authors did not find any preference for prism glasses. Payne et al48 did a similar study prescribing prism based on fixation disparity testing. They prescribed two pairs of glasses that were identical in every way except that one had prism and one had none. After wearing each pair of glasses for 2 weeks, the subjects were asked to choose the ones they preferred. All 10 subjects chose the glasses with the prism. In a study that investigated the effectiveness of base-in prism glasses for the treatment of CI, Stavis et al49 found that patients reported subjective improvement in asthenopic symptoms and headaches after 2 weeks of wear. However, the authors did not have a placebo control group. Thus, there is no way to know whether the reported improvement in symptoms was due to a placebo effect.
Until recently, all of the published research on the effectiveness of base-in prism for the treatment of CI has had fundamental design flaws, such as small sample size, lack of a control group, no randomization, and nonmasked outcome examinations. Scheiman et al9 completed a randomized, double-masked, placebo-controlled clinical trial of base-in prism reading glasses (based on Sheard’s criterion) for the treatment of symptomatic CI in children. They found that the base-in prism glasses were no more effective than placebo reading glasses. Neither treatment group demonstrated clinically significant changes in the near point of convergence or positive fusional convergence at near, although nearly half of the children assigned to each of the two treatment groups reported a statistically significant decrease in symptoms (albeit neither group achieved a decrease in symptoms to a level considered clinically asymptomatic). The authors attribute these improvements in symptoms to the placebo effect.
Teitelbaum, Pang, and Krall50 studied 29 symptomatic CI subjects aged 45 to 68 years. All subjects took the CISS V-15 and scored ≥16 points. Each subject was assigned two pairs of progressive addition glasses made by the same manufacturer in a randomized sequence, one with base-in prism and one with the same lens prescription but no prism (placebo). Subjects wore each pair of glasses for 3 weeks and completed the CISS at the end of the third week. Symptom level measured with CISS was the major outcome measure.
The authors found that the mean (standard deviation) CISS score was 30.21 (9.30) at baseline and decreased to 13.38 (9.44) with the base-in prism glasses, versus 23.62 (10.76) with the placebo glasses. There were significant differences between the baseline survey score and the score with the base-in prism glasses (p < 0.0001) and between the score with placebo glasses and the score with base-in prism glasses (p = 0.001).
They concluded that progressive addition glasses with base-in prism were effective in alleviating symptoms of presbyopes with symptomatic CI.
They concluded that progressive addition glasses with base-in prism were effective in alleviating symptoms of presbyopes with symptomatic CI.
In another randomized clinical trial with a placebo control, O’Leary and Evans51 studied the effectiveness of prism (based on the associated phoria) for improving reading rate and symptoms in patients with heterophoria. In 58 exophores, there was no significant improvement if the prism prescribed was between 0.5 and 2.0 Δ. In the group of exophores who required 2.5 to 3.0 Δ, there was a statistically significant increase in reading rate (3.2% faster) for those wearing prism glasses. However, it is doubtful whether this change is clinically relevant. For example, the mean reading rate with prism was 150.4 words per minute, and in the placebo glasses group it was 145.7 words per minute. There were no significant changes in symptoms in any of the groups. For patients with esophoria, there was no improvement in reading rate or symptoms with prism compared to the placebo lenses.
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