The treatment of convergence insufficiency (CI) by orthoptics/vision training has been well accepted by optometrists. Grisham1 reviewed the literature and reported a combined improved and cure rate of 93%. For those patients who are unable to avail themselves of this treatment because of time or financial constraints, the prescribing of base-in (BI) prism has served as an alternate form of management.2
The treatment of convergence disorders using prism was proposed over100 years ago by Percival3 who developed a treatment algorithm based on the heterophoria measurement and its compensating vergence. Sheard4 modified the algorithm using the same measurements and proposed that the fusional reserve must be twice the amount of the heterophoria. The amount of prism to be prescribed was further refined when the concept of fixation disparity was introduced. This method differed from the previous two in that the measurement of the deviation was defined in minutes of arc and the measurement was taken while the patient was binocular. Mallett5 proposed the neutralization of the fixation disparity with prism (associated phoria), whereas Sheedy and Saladin6 proposed a prism correction that would place the patient demand on the flat portion of the forced vergence fixation disparity curve. Regardless of the method chosen, there have been very few clinical studies to assess the efficacy of BI prism in the treatment of CI and most have focused on a pediatric population.7–9
Stavis et al.7 concluded that BI prism improved reading ability and comfort in a group of children between the ages of 8 and 18 with “mini-CI,” which was defined as having an exophoria at near of at least 4Δ greater than the exophoria at distance. Lie and Opheim8 also found a decrease in subjective complaints in a group of heterophoric subjects corrected with prism. Their patient population was overwhelmingly pediatric in the age range of 10 to 18 years. Scheiman et al.,9 in a randomized double-blind study of CI found no difference in CI symptoms between subjects wearing BI reading glasses and placebo reading glasses. Once again, the study was confined to children whose ages ranged from 9 to 18 years. Finally, Worrell et al.10 compared the responses of 43 patients with heterophoria, who did not meet Sheard criterion, by having them compare spectacles with and without prism. The amount of prism prescribed allowed the patients to satisfy Sheard criterion. There was no significant difference in CI symptoms when wearing prism vs. no prism in the 24 patients with CI. Five of the 24 patients in the study were presbyopic and all but one of the five preferred the prism prescription.
The purpose of this study was to investigate whether or not prescribing BI prism was successful in alleviating symptoms in presbyopes with CI. We used fixation disparity and specifically the associated phoria to determine the amount of prism to be prescribed. Previous studies11,12 have found this measurement to be superior to heterophoria in the correlation of symptoms in patients with convergence disorders.
Twenty-nine subjects from a private optometric practice in South Dakota were recruited for the study. The study protocol (see Appendix A, available online at http://links.lww.com/A663) and informed consent forms were approved by the Illinois College of Optometry Institutional Review Board and followed the tenets of the Declaration of Helsinki. Informed consent was obtained from all subjects in the study.
Potential subjects in the practice whose age was ≥45 years were asked to complete the CI Symptom Survey-V15 (CISS-V15), which was developed and validated by the Convergence Insufficiency Treatment Trial Study Group and consists of 15 questions which are scored from 0 to 4 based on the severity of the symptoms.13 (see Appendix B, available online at http://links.lww.com/A664) The survey was self administered. A comprehensive eye examination including refraction was performed for the patients whose CISS-V15 score was higher than 16 to select the subjects who met the inclusion criteria. To be qualified for the study, patients had to have an exophoria at near of at least 4Δ greater than at distance. The detailed inclusion and exclusion criteria for the study are listed in Table 1. The diagnostic data for the 29 subjects in the study were collected by one of the authors (JK) and included:
- Dissociated heterophorias at distance and near by cover test.
- Dissociated heterophorias at distance and near by the Modified Thorington method.
- Associated phoria at near tested with the Bernell binocular refraction slide.
- The break and recovery value of the near point of convergence using accommodative target.
- Positive fusional vergence at near measured with Risley prisms.
Patients were assigned two pairs of progressive addition lenses (PAL) fabricated by Signet Armorlite with an updated lens prescription, in a randomized sequence. Prism was prescribed in only one pair of glasses with other pair serving as a placebo. The study had a double-blind design as neither the examiner nor subject was aware of the glasses assignment. The amount of prism prescribed was based on the associated phoria and was always within 0.75Δ of the associated phoria value. The amount of prism was split equally between the two eyes. The treatment PAL was a novel design, not commercially available, that confines the prism to the near portion only. This lens uses the design platform of the company's commercially available PAL with the exception of BI prism. The amount of prism that can be incorporated in the near portion of this PAL is dependent on base curve and add power and can range from 0.375 to 0.75Δ per lens. The prism power is gradually introduced along the corridor of the PAL and reaches its maximum power at the point of maximum add power. Because of the limitations on the magnitude of prism that could be incorporated in this design, additional prism was added to both the distance and near prescriptions if needed. If additional prism was needed, an associated phoria at distance was measured through the tentative prescription. To minimize symptoms unrelated to the CI, an ortho fixation disparity at distance was necessary for the subjects to continue in the study. Subjects wore one pair of PAL for 3 weeks and completed the CISS-V15 at the end of the 3rd week. They were then given the other pair of PAL to wear for 3 weeks after which the survey was completed again. Therefore, a baseline, placebo, and treatment symptom score was tabulated. The CISS-V15 score was the primary outcome measure. An analysis of variance was performed to make comparisons within the CI symptom scores of the baseline, placebo, and treatment groups. Statistical sample size calculation was performed using the pilot data (the first 18 subjects) to determine the number of subjects required in the study to detect differences in CI symptom scores. A sample size of 21 would be required to give 80% power at the 0.05 level, and 28 subjects are needed to give the 90% power. Hence, a sample size of 29 subjects was determined to be adequate for this study.
The baseline characteristics of the subjects are displayed in Table 2. The characteristics of the clinical measurements are listed in Table 3. Nine patients had prism confined to the near portion only, whereas the remaining 20 patients had some BI prism in both the distance and near portions. The mean total prism prescribed for all 29 patients was 1.97 ± 1.29Δ, whereas the mean prism at distance was1.15 ± 1.26Δ. The CI symptom scores for the three groups are displayed in Table 4. The mean (standard deviation) CI Symptom Score for the baseline group was 30.21 (9.30), for the placebo condition was 23.62 (10.76), and for the treatment (prism) condition was 13.38 (9.44). An analysis of variance showed a statistically significant difference in the CI Symptom Score between the treatment group and both the baseline group (p < 0.0001) and the placebo group (p = 0.001). There was also a statistically significant difference between the baseline and placebo groups (p = 0.044). Twenty-five of 29 subjects reported a lower CI symptom score with the prism glasses. Of the four subjects who had lower symptom scores with the placebo glasses, three complained of distance vision problems with the prism glasses.
The results of our study support the efficacy of BI prism in the management of CI in the presbyopic population. The statistically significant difference between the baseline CI symptom score and the placebo is not surprising given the often powerful effect of a placebo treatment. Interestingly, the CI Symptom Score for the treatment group probably overrepresents subject symptoms and therefore reduces the treatment effect of the prism glasses. This is due to the fact that the prism at distance is capable of inducing some asthenopia even though the subjects demonstrated no fixation disparity through the prism glasses.
The use of fixation disparity/associated phoria to determine eligibility and magnitude of prism in this study was selected instead of Sheard criterion, which uses the heterophoria and its compensating vergence. Although the work of Sheedy and Saladin6,14 has demonstrated that heterophoria and Sheard criterion were important discriminators of asthenopic symptoms, all of their subjects were optometry students (non-presbyopes). In addition, Sheedy and Saladin15 found in a previous study that, although there were significant differences in the heterophorias and vergences between the presbyopic and non-presbyopic groups, there were no differences between the asthenopic symptoms in each group. They suggested that the heterophoria and vergence measurements in presbyopes do not give a true picture of the binocular status of the patient. Yekta et al.11 also addressed this issue and reported that the fixation disparity and associated phoria are both better predictors of nearpoint symptoms than the heterophoria in both presbyopic and non-presbyopic populations. This was confirmed by Jenkins et al.12 who concluded that a 2Δ associated phoria is the minimum prism to best differentiate symptomatic from asymptomatic patients. However, there were many symptomatic patients with an associated phoria of 1Δ in their study. Therefore, on the basis of the previous studies, we used the associated phoria as one of our study's inclusion criteria and to determine the amount of prism prescribed for our subjects.
Our study results regarding use of BI prism in the treatment of CI are not in agreement with the study by Scheiman et al.,9 which evaluated children ages 9 to <18 years in contrast to our presbyopic population and used Sheard criteria instead of the associated phoria used in our study. Another possible explanation for this difference is the issue of the impact of accommodation on the source of symptoms. Marran et al.16 found accommodation as the primary cause of nearpoint symptoms in a study of 299 elementary school children. Whereas Scheiman et al.9 excluded subjects with accommodative insufficiency, their exclusion criteria of having an amplitude of accommodation <4 D tested with the push-up method in a pediatric population, could still include subjects with mild to moderate accommodative insufficiency. Marran et al.16 used a more liberal threshold for diagnosis and included subjects with an amplitude of accommodation 2 D below Hofstetter17 age-based norms. If some of the subjects in the Scheiman et al.9 study had a mild accommodative insufficiency, the effect of the prism on the CI would be minimized and a significant change in the CI Symptom Score would not be expected. Irrespective of the possible influence of accommodation, other attributes of an adult population, such as the type of near activities and the sophistication in responding to the questionnaire, could lead to the disparity in the results.
There are also limitations to the study as the amount of prism in the near portion of the PAL was limited by the lens design. As a result, additional amounts of prism were prescribed in both the distance and near portions to closely approximate the near associated phoria. As previously indicated, the prism at distance may have induced some asthenopia which could have led to higher CI symptom scores associated with the BI prism glasses. This would underestimate the efficacy of BI prism for management of CI but would not affect the ultimate conclusion. An additional observation from the study is the positive acceptance of small amounts of BI prism at distance by patients who wanted to wear a correction full time instead of separate pairs of distance and reading glasses. Although technology exists for incorporating small amounts of BI prism in only the reading portion of PAL, it is not commercially available at this time. If a currently available PAL is used, one can initially prescribe prism in the traditional manner (same prism power for distance and near) assuming the associated phoria is ortho at distance through the prism, and be optimistic that symptoms can be reduced in patients with CI.
Finally, the possible effects of prism adaptation were not controlled in this study. However, North and Henson18 found that subjects with binocular anomalies which result in nearpoint symptoms display reduced prism adaptation and therefore are unlikely to influence the results of our study. In fact Brautaset and Jennings19 specifically studied prism adaptation in patients with symptomatic CI and also found reduced prism adaptation at distance and near. Because associated phoria measurements were not taken after the glasses were worn, we cannot comment on the degree of prism adaptation that occurred and the assumption that fixation disparity is a sign of decompensation of the binocular vision process. Nonetheless, our double blind study design with a control group attempted to isolate the affect of the prism and therefore did not require subsequent associated phoria measurements to assess prism adaptation to substantiate the affect of the prism on the CI.
Our study found BI prism to be effective in alleviating symptoms of CI in presbyopes. More research is needed to determine the optimal prism to be prescribed in this condition and the adaptability of patients to BI prism prescribed at distance.
We thank Signet Armorlite who funded the study and provided the spectacle lenses and Jeff Krall, OD, and Kelly Frantz, OD, for their assistance in the preparation of the manuscript.
Portions of this paper were presented at the American Academy of Optometry annual meeting, October 24, 2007, Tampa, Florida.
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