Characterization of Bangerter filter effect in mild and moderate anisometropic amblyopia: predictive factors for the visual outcome

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1 Graefes Arch Clin Exp Ophthalmol (2011) 249: DOI /s x PEDIATRICS Characterization of Bangerter filter effect in mild and moderate anisometropic amblyopia: predictive factors for the visual outcome Carlos Laria & David P. Piñero & Jorge L. Alió Received: 12 July 2010 /Revised: 27 October 2010 /Accepted: 29 October 2010 /Published online: 19 November 2010 # Springer-Verlag 2010 Abstract Purpose To evaluate the outcome achieved by using Bangerter filters in cases of mild and moderate anisometropic amblyopia, and to define the predictive factors for the final visual outcome. Methods Prospective study comprising 62 children (age range 3 11 years) with unilateral amblyopia associated with myopic or hyperopic anisometropia. In all cases, the use of Bangerter filters were prescribed for amblyopia treatment due to the unsuccessful outcome with refractive correction. The prescribed filter always induced a visual acuity reduction in the non-amblyopic eye of 2 lines below the BSCVA of the amblyopic eye. A follow-up of 12 months was completed. This study has been supported in part by a grant from the Spanish Ministry of Health, Instituto Carlos III, Red Temática de Investigación Cooperativa en Salud Patología ocular del envejecimiento, calidad visual y calidad de vida, Subproyecto de Calidad Visual (RD07/ 0062) The authors have no financial interest in any of the issues contained in this article and have no proprietary or commercial interest in the development of marketing of materials used in this study or discussed in this article. C. Laria : D. P. Piñero (*) : J. L. Alió Vissum/Instituto Oftalmológico de Alicante, Avda de Denia s/n, Edificio Vissum, 03016, Alicante, Spain dpinero@vissum.com D. P. Piñero Departamento de Óptica, Farmacología y Anatomía, Universidad de Alicante, Alicante, Spain J. L. Alió Division of Ophthalmology, Universidad Miguel Hernández, Alicante, Spain Results A statistically significant improvement in visual acuity was observed at 3 months in both groups, amblyopic and non-amblyopic eyes (p<0.01). At 6, 9 and 12 months, additional small but statistically significant improvements in visual acuity were detected only in the amblyopic group (p 0.02). Filter density had to be changed during the follow-up in a total of 22 eyes (35.5%). A significant correlation of the visual acuity with the baseline interocular visual acuity difference was found at 3 months (r=0.44, p< 0.01), but not at the end of the follow-up (r=0.12, p=0.36). Multiple linear regression analysis revealed that the final visual outcome was significantly correlated with the baseline visual acuity, sphere and the difference in visual acuity between eyes (R 2 : 0.42, p<0.01). Conclusions Bangerter filters are useful for treating mild or moderate amblyopia due to anisometropia, but ocular dominance inversion should be maintained during treatment for obtaining an optimized outcome. Keywords Amblyopia. Bangerter filter Introduction The term amblyopia derives from the Greek language (ambly αμβlυσ = dull and ops ωy = vision) [1], and it has the meaning of dimness or dullness of vision. It is defined as a reduced best-corrected visual acuity with no pathological or organic associated factors [2, 3]. Several alternatives have been proposed for the treatment of this condition, such as the correction of the refractive error [4 6], occlusion or patching therapy [7 14], atropine penalization [7, 10, 11, 14 17], optical penalization [16 18], or the use of the Bangerter filters or foils [19 21]. However, it should be considered that some of these alternatives present signifi-

2 760 Graefes Arch Clin Exp Ophthalmol (2011) 249: cant limitations, such as non-compliance with patching [22, 23] or side-effects with the use of atropine for penalization [24 26]. The use of the Bangerter filters or foils in amblyopia were first described in 1953 [19 21]. This type of filter induces a graded reduction of the image quality in the nonamblyopic eye (light transmittance reduction), which forces the use of the amblyopic eye. These blur filters are available in several graded densities that are classified according to the theoretical level of visual acuity that could be achieved by a normal eye with each of them: from 20/20 to 20/200. The advantages of the use of these foils over the occlusion therapy are the maintenance of the binocular visual function when filters of low to moderate density are used, the lower cosmetic impact, the presumed higher patient compliance because of reduced social stigma, and in general the better tolerance of the treatment by the patient [27]. Several previous studies have confirmed the efficacy of this treatment, and it has even been compared with occlusion therapy [27, 28]. However, the real effect of these filters, the criteria for using them, and the predictive factors for the success in the treatment of mild or moderate anisometropic amblyopia have not been well-defined. The aim of the current study was to evaluate the recovery achieved by using the Bangerter filters in cases of mild or moderate anisometropic amblyopia, and to define the predictive factors for the final visual outcome. Material and methods Patients This prospective study comprised 62 children with unilateral amblyopia associated with myopic or hyperopic anisometropia. All these cases of amblyopia were treated by prescribing the use of Bangerter filters (Trusetal occlusion, Trusetal Verbandstoffwerk Gmbh, Schloss Holte Stukenbrock, Germany). A total of 35 patients (56.5%) were male, and 27 were female (43.5%). The amblyopic eye was the right eye in 53.2% of cases (33), and it was the left eye in 46.8% of cases (29). Inclusion criteria for the study were presence of mild or moderate unilateral amblyopia associated to refractive anisometropia, 11-year-old or younger children (no previous evidence of efficacy with Bangerter filter for children over 11 years old), best spectacle-corrected visual acuity (BSCVA) of 0.1 LogMAR (20/25) or worse in the amblyopic eye, at least a difference of 1 LogMAR line (0.1 LogMAR) of visual acuity among both eyes, and unsuccessful outcome with refractive correction (no improvement of the visual acuity of the amblyopic eye). Anisometropia was defined as the presence of differences in spherical equivalent between the two eyes of each patient larger than or equal to 0.5 D and/or differences in astigmatism higher than or equal to 1.50 D [8]. Exclusion criteria were presence of active ocular disease, amblyopia due to strabismus or deprivation, severe amblyopia (LogMAR BSCVA of 0.70 or worse, 20/100 or worse), and patients not compliant with the use of refractive correction. The local ethics committee approved the study. The study followed the tenets of the Declaration of Helsinki. The parent or guardian of each child provided written informed consent. Follow-up protocol Table 1 summarizes the protocol followed for the follow-up of patients included in the current study. Initial visit: refractive study and prescription of corrective lenses Two drops of cyclopentolate 1% (Colircusi cicloplejico, Alcon Cusí, Barcelona, Spain) were instilled at a 5- minutes interval in all cases. Subjective manifest refraction was evaluated approximately 50 minutes after instillation of drops, using the objective refraction obtained by retinoscopy as the starting point for refraction. Myopia or myopic astigmatism was corrected fully. Hyperopia >3.00 D of SE was either fully corrected or symmetrically undercorrected by no more than D in both eyes. Hyperopia 3.00 D was corrected at investigator discretion [2, 3]. Patient s parents or guardians were informed about the necessity for continuous use of the refractive correction. In addition to refraction, other tests were also performed to evaluate the sensoriomotor conditions and to rule out the presence of any type of strabismus: visual acuity, cover test, fixation status by means of visuoscopy, stereopsis by means of the TNO stereotest, distance vision vectography, and the four-prism diopter test. Furthermore, the anterior segment was evaluated by means of the slit lamp, and the fundus by means of indirect ophthalmoscopy, with the aim of ruling out other amblyogenic factors (Table 1). First control visit: control of the efficacy of the refractive correction The first control was performed 3 months after the prescription of the refraction. At this visit, the BSCVA was evaluated with Snellen charts in order to confirm the efficacy of the refractive correction. Parents were also asked about the compliance of the children with the treatment.

3 Graefes Arch Clin Exp Ophthalmol (2011) 249: Table 1 Examination and follow-up protocol in the current study CLINICAL TESTS First visit 3-month visit 6-month visit Follow-up visits (monthly over 1 year) Best spectacle-corrected visual X X X X acuity (BSCVA) Cover test X X X Stereopsis X X 6 months only Four-diopters prism test X X 6 months only Cycloplegic refraction X X 6 months only Slit lamp examination X X X X Fundus examination X X 6 months only Density filter suitability X X Second control visit: inclusion visit BSCVA as well as sensoriomotor conditions were evaluated at this visit. In addition, a cycloplegic refractive exam was also done in order to confirm the stability of refraction. Patients were prescribed with Bangerter filters if the BSCVA of the amblyopic eye did not improve in spite of using the refractive correction, and if refraction was unchanged (zero change in sphere and cylinder). The type of Bangerter filter to use in the non-amblyopic eye was selected according to the testing performed with the Bangerter filter bar (Trusetal Verbandstoffwerk Gmbh, Schloss Holte Stukenbrock, Germany). In all cases, the filter selected had to induce a reduction of the visual acuity of the non-amblyopic eye of 2 lines below the BSCVA of the amblyopic eye. Different density filters for achieving this aim were necessary in each specific case. A wide range of densities were available, using a specific numeric notation for designating each level of filtration (0.1, 0.2, 0.3, 0.4, 0.6, and 0.8). Specifically, these numbers represented the theoretical visual acuity that should be induced in a healthy eye with this filter. Only the density filter was changed during the follow-up if an improvement in BSCVA was observed in the amblyopic eye, with the aim of maintaining the 2 lines of difference in visual acuity among eyes. If an improvement was not observed in the amblyopic eye, the same density filter was used, provided that the non-amblyopic eye was maintaining the same level of visual acuity. The filters were placed on the internal surface of the glass lens of the non-amblyopic eye, and they had to be used continuously during the whole study, except in those cases where the total recovery was achieved before finishing the 12-month follow-up. The filter density was re-evaluated at each monthly visit over the 12-month follow-up, and it was changed if necessary (visual acuity experienced a progressive improvement) always following the same criterion: the filter selected had to induce a reduction of the visual acuity of the non-amblyopic eye of 2 lines below the BSCVA of the amblyopic eye. Rest of follow-up visits: evaluation of the efficacy of the Bangerter filters Patients were evaluated monthly over a 12-month followup since inclusion in the study. BSCVA was evaluated at all visits in order to assess the efficacy of the treatment (Table 1). Furthermore, a cycloplegic refractive evaluation was performed 6 months after beginning the treatment with the filters, in order to confirm the stability of the refractive status. The primary outcome measure was the monocular BSCVA of the amblyopic eye, as well as the interocular difference in visual acuity. The monocular BSCVA of the non-amblyopic eye was considered as a secondary outcome measure. Visual acuity measurements At each visit, visual acuity was measured in each eye by an experienced and trained examiner (CL) using the same testing protocol. Snellen charts at 5 meters with an average illumination of around 2237 lux on the surface of the chart were used. All subjects were asked to read slowly (at a rate not faster than about one letter per second) in order to achieve the best identification of each letter and to not proceed until they had given a definite response. When the subject said that he or she could not read a letter, he or she was encouraged to continue. Examination was stopped when the subject was unable to read at least three letters of a line of optotypes. When this occurred, the decimal visual acuity associated with the previous line read was considered as the visual acuity of the patient, adding a value of 0.02 to this decimal value for each letter read in the last Snellen line (i.e., if the patient only read two letters of the Snellen line corresponding to a value of 0.5 of visual acuity, the final visual acuity of the patient would be 0.44). If the subject changed a response aloud before he or she had read aloud the next letter during the visual acuity testing, then the change was accepted. However, if the subject changed a

4 762 Graefes Arch Clin Exp Ophthalmol (2011) 249: response after beginning to read the next letter, the change was not accepted. Children in the 2- to 3-year age range were often tested with isolated optotypes, but final acuities at cessation of treatment were all tested with full-line letter optotypes. Statistical analysis SPSS statistics software package version 15.0 for Windows (SPSS, Chicago, IL, USA) was used for statistical analysis. Normality of all data samples was initially checked by means of Kolmogorov Smirnov test. When parametric analysis was possible, the Student's t-test for paired data was performed for visual acuity comparison between baseline and posterior examinations or consecutive visits. On the contrary, when parametric analysis was not possible, the Wilcoxon rank sum test was applied, to assess the significance of differences in visual acuity between consecutive visits of the follow-up, using in all cases the same level of significance (p<0.05). Correlation between visual outcomes and other clinical data were also investigated (Pearson or Spearman correlation coefficients, depending if normality condition could be assumed or not). Furthermore, a multiple regression analysis was performed using the backward elimination method, with the aim of obtaining a mathematical expression relating the final visual outcomes to different baseline parameters and treatment features. Model assumptions were evaluated by analyzing residuals, the normality of unstandardized residuals (homoscedasticity), and the Cook s distance in order to detect influential points or outliers. In addition, the lack of correlation between errors and multicollinearity was assessed by means of the Durbin Watson test and the calculation of the collinearity tolerance and the variance inflation factor (VIF). For an accurate statistical analysis of the visual acuity outcomes, the decimal values were transformed into the logmar scale, calculating the minus logarithm of the decimal visual acuity [29]. It should be considered that each step of 0.1 LogMAR units is defined as a line with this specific scale. Table 2 Comparative table showing the baseline conditions of amblyopic and non-amblyopic eyes prior to the treatment with the Bangerter filters. The corresponding p-values for the comparison between groups are shown for each parameter evaluated Mean (SD) range Amblyopic eye atropine (five eyes, 8.1%), but with a limited outcome. For this reason, the use of the Bangerter filters was thought to be a potential solution in such cases. The mean difference in LogMAR BSCVA among amblyopic and fellow eyes was 0.13 (range, 0.10 to 0.50, SD 0.06). General visual outcomes Non-amblyopic eye P-value (statistical test) Sphere (D) (4.16) (2.66) 0.01 (Mann Whitney) to to Cylinder (D) 1.59 (1.27) 1.02 (1.14) <0.01 (Mann Whitney) 7.00 to to 0.00 LogMAR BSCVA 0.16 (0.08) 0.03 (0.04) <0.01 (Mann Whitney) 0.10 to to 0.15 Abbreviations: SD, standard deviation; D, diopters; BSCVA, best spectacle-corrected visual acuity Figure 1 shows the mean change in visual acuity for amblyopic and non-amblyopic eyes during the follow-up. A statistically significant improvement in visual acuity was observed at 3 months after the beginning of the treatment in both groups, amblyopic and non-amblyopic eyes (Wilcoxon test, both p<0.01). At 6, 9 and 12 months, additional small but statistically significant improvements in visual acuity Results A total of 62 children with ages ranging between 3 and 11 years (mean: 6.13 years, standard deviation SD: 1.82) were included. Table 2 summarizes the baseline visual and refractive conditions in the group of amblyopic and nonamblyopic eyes. As expected, statistically significant differences were found (Mann Whitney test, p 0.01). Some cases included in the study had been treated previously with occlusion therapy (16 eyes, 28.5%) or penalization with Fig. 1 Change in the best spectacle-corrected visual acuity during the follow-up in both groups: amblyopic eyes (green line) and nonamblyopic eyes (orange line). Snellen equivalents for LogMAR visual acuity: 0.1 LogMAR=20/16, 0.0 LogMAR=20/20, 0.1 LogMAR= 20/25, 0.2 LogMAR=20/30, 0.3 LogMAR=20/40, 0.4 LogMAR=20/ 50

5 Graefes Arch Clin Exp Ophthalmol (2011) 249: were detected in the group of amblyopic eyes (Wilcoxon test; 3-6 months, p<0.01; 6-9 months, p<0.01; 9-12 months, p=0.02). However, the visual acuity remained unchanged in the group of non-amblyopic eyes during the rest of the follow-up (Wilcoxon test; 3-6 months, p=0.09; 6-9 months, p=0.07; 9-12 months, p=0.68). When amblyopic and fellow eyes were compared, statistically significant differences in the visual acuity outcome were detected at all time points of the follow-up (Mann Whitney test, all p<0.01), with the best vision in the group of nonamblyopic eyes. At baseline, the interocular difference in BSCVA was larger than 0.20 in 46.77% of eyes (29 eyes), whereas at the end of the follow-up this difference was below 0.05 in all cases (mean 0.01, SD 0.02) (Fig. 2). Visual outcomes according to the changes in the filter density The mean filter density used at the baseline was 0.5 (SD 0.12) (median 0.6). A statistically significant correlation although weak was found between the baseline filter densities and the visual acuity at 3 (r= 0.36, p<0.01), 6 (r= 0.36, p<0.01) and 9 months (r=-0.29, p=0.02). However, this correlation became very weak at the end of the follow-up (12 months, r=-0.19, p=0.15). The filter density had to be changed during the follow-up in a total of 22 eyes (35.5%). In all cases, the filter density was diminished. No significant differences were present at baseline and at the end of the follow-up between eyes with filter change and those with no filter change during the followup (Mann Whitney; baseline, p=0.42; 12 months, p=0.45). No significant differences in the initial filter density used were found either between eyes with and without filter density changes during the follow-up (Mann Whitney test, p=0.08). There were cases with more than a single change in filter density: 17 eyes with one change (27.4%), three eyes with two changes (4.8%) and two eyes with three changes (3.2%). No correlation was found between the number of filter density changes and the final visual acuity (r= 0.09, p=0.49). However, a very weak but statistically significant correlation was found among the number of filter density changes and the baseline BSCVA (r=0.29, p=0.02). Visual outcomes according to the treatment duration The use of the filter was discontinued during the first 12 months in a total of 13 eyes (21%). Specifically, the group of eyes with an early discontinuation presented a significantly better visual acuity at the baseline [Mann Whitney test; no discontinuation, mean 0.17 ( 20/30), SD 0.08; discontinuation, mean 0.13 ( 20/25), SD 0.09; p= Fig. 2 Scattergrams showing the relationship between the baseline difference in LogMAR BSCVA among eyes and the BSCVA at 3 (a) and 12 months (b) after the beginning of the treatment. The adjusting line to the data obtained by means of the least-squares fit is shown in the three graphs: Difference LogMAR BSCVA among eyes= month LogMAR BSCVA (R 2 : 0.21). Difference LogMAR BSCVA among eyes= month LogMAR BSCVA (R 2 : 0.06). Abbreviations: BSCVA, best spectacle-corrected visual acuity. Snellen equivalents for LogMAR visual acuity: 0.1 LogMAR=20/ 16, 0.0 LogMAR=20/20, 0.1 LogMAR=20/25, 0.2 LogMAR=20/30, 0.3 LogMAR=20/40, 0.4 LogMAR=20/ ]. At the end of the follow-up, a significantly better visual acuity was still found in the group of eyes with treatment discontinuation (Mann Whitney test; no discontinuation, mean 0.03 ( 20/20), SD 0.03; discontinuation, mean 0.01 ( 20/20), SD 0.02; p=0.02), although the difference was of small magnitude. A weak but statistically significant correlation was found between the final visual outcome and the treatment duration in those eyes discontinuing the use of the filters during the 12-month follow-up (r= 0.31, p=0.02).

6 764 Graefes Arch Clin Exp Ophthalmol (2011) 249: Visual outcomes according to the refractive status and age No significant correlations were found between the visual outcomes and the refractive sphere (3 months, r=-0.12, p=0.35; 6 months, r= 0.09, p=0.48; 9 months, r= 0.08, p=0.55; 12 months, r=0.18, p=0.16) or cylinder (3 months, r=0.09, p=0.49; 6 months, r=0.10, p=0.46; 9months,r=0.08, p=0.55; 12 months, r=0.23, p=0.08). With regard to age, no significant difference were found either (3 months, r= 0.20, p=0.12; 6 months, r= 0.12, p=0.34; 9 months, r= 0.11, p=0.40; 12 months, r= 0.09, p=0.49). At the beginning of the follow-up, a significant correlation of the visual acuity with the baseline visual acuity difference among eyes was present (3 months, r= 0.44, p<0.01), but it became significantly weaker at the end of the follow-up (12 months, r=0.12, p=0.36). Predicting model for the visual outcome As several factors seemed to be implicated on the effect of Bangerter filters, a multiple regression analysis was performed in order to find the appropriate mathematical expression relating all the influencing factors on the final visual outcome. This multiple regression analysis revealed that the final visual acuity outcome (12 months) was significantly correlated with the baseline visual acuity, sphere and the difference in visual acuity between eyes (p<0.01). For this relation a predicting model with predictability (R 2 )of 0.42 and adjusted R 2 of 0.39 was found (Durbin Watson statistic=1.87): BSCVA f ¼ 0:010 0:002 Sphere 0 þ 0:389 BSCVA 0 0:322 DifBSCVA 0 where BSCVA f is the 12-month best spectacle-corrected visual acuity, and Sphere 0,BSCVA 0 and DifBSCVA 0,the sphere, best spectacle-corrected visual acuity and the difference in visual acuity among eyes at baseline, respectively. Homoscedasticity of the model was confirmed by the normality of the unstandardized residuals distribution (p=0.20) and the absence of influential points or outliers (mean Cook s distance, mean 0.04, SD 0.18). The lack of multicollinearity and the independence of the residuals were also confirmed (tolerances between 0.25 and 0.95; VIFs between 1.06 and 3.95). With this model, 41.94% of unstandardized residuals were lower than 0.1 and 70.97% lower than or equal to 0.2. Discussion The use of Bangerter filters has been proved to be a useful procedure to reverse mild and moderate amblyopia [27, 28]. Agervi and colleagues [28] found a more rapid visual acuity recovery with these filters than with spectacles alone in eyes with anisometropic amblyopia. However, the Pediatric Eye Disease Investigator Group [27] found an average difference in visual acuity between Bangerter filters and patching of less than half a line. In any case, this group concluded that the impact of treatment with respect to adverse effects, compliance and social stigma on the child and family was lower with the Bangerter filters, and that these filters were a reasonable option to consider for initial treatment [27]. These authors also concluded that cases prescribed with Bangerter filters and not improving during the study were probably due to poor compliance [27]. In the current study, we have performed an analysis of cases prescribed with Bangerter filters in our pediatric ophthalmologic unit for the treatment of mild and moderate anisometropic amblyopia. The aim was to evaluate the recovery achieved by using these filters in this type of amblyopia and to define the predictive factors for the final visual outcome. These filters were only adapted in mild and moderate amblyopic cases in our clinical practice. It should be considered that if this type of penalization is applied in cases of severe amblyopia, there is a high probability of non-compliance with treatment, because the visual reduction will be very significant and the patients will refuse to wear the optical correction with filters. The continuous use of Bangerter filters in this study induced a visual acuity improvement at 3 months after beginning the treatment, not only in the amblyopic eye, but also in the non-amblyopic eye. This confirms the low risk of amblyopia development in the healthy eye, which can occur with other modality treatments [30], such as occlusion therapy. These positive findings were confirmed over the rest of the follow-up, with a progressive significant improvement of the visual acuity in the amblyopic eye and maintenance of visual acuity in the non-amblyopic eye. This progressive visual improvement with the Bangerter filters during the first year of treatment has been also found in previous studies [27, 28]. The visual improvement in the non-amblyopic eye was of small magnitude (around half of a line), although statistically significant. Possibly, this improvement could be the consequence of the normal evolution of visual acuity in children at these ages, which is not stopped with the use of these filters. Other authors [27, 31 33] have also found improvements in the visual acuity of the fellow eye in cases of amblyopia treated by means of different options.. The filter density was customized depending on the specific characteristics of each case, following the criteria defined by Odell et al. [34], which were to achieve a reduction of visual acuity in the non-amblyopic eye capable of inverting the ocular dominance. A correlation between filter density and visual acuity was found in the first half of

7 Graefes Arch Clin Exp Ophthalmol (2011) 249: the follow-up (3 6 months), indicating that the lower the density, the better the visual acuity in the amblyopic eye was. This finding supports the idea of a larger initial efficacy with those filters inducing a more relevant visual loss of the non-amblyopic eye. However, this correlation disappeared at the end of the follow-up, probably due to the several density filter changes performed during all followup. This suggests that comprehensive monitoring of these patients is mandatory in order to evaluate periodically whether the prescribed filters are appropriate in each phase of the recovery process. Density filter changes are required in some cases in order to optimize the final outcome. No significant differences in the final visual outcomes were observed between eyes with and without density filter changes. In addition, no correlation was found between the number of filter density changes and the final visual acuity. Therefore, it seems clear that if a progressive control and customization of the filter density is performed, the final visual outcome is independent of the filter density because the important thing is to achieve the inversion of dominance. Some modifications in the filter density were required to maintain this dominance inversion during treatment in some specific cases. We used a specific criterion of filter selection, which consisted of inducing a reduction of the visual acuity of the non-amblyopic eye of 2 lines below the BSCVA of the amblyopic eye. It is unknown if a better or worse effect could be achieved by inducing a visual loss of the non-amblyopic eye larger than 2 lines. An additional finding of the current study was the relationship between the outcome achieved and the treatment duration. It was observed that eyes with an early discontinuation of the treatment (before finishing the 12- month follow-up) had a significantly better visual acuity at baseline. Therefore, as expected, less affected amblyopic eyes showed better visual prognosis with treatment. In addition, a weak but statistically significant inverse correlation was found between treatment duration and the final visual acuity achieved. This meant that the longer the treatment was, the better the final visual outcome. Therefore, treatment duration seems to be another relevant factor for the final outcome achieved with this filter treatment. No correlation was found between age or refraction and the final visual outcome. It should be considered that patient ages were in all cases within the critical period of recovery, and therefore its influence had to be minimal [6]. With regard to the severity of amblyopia, it was significantly correlated with the visual outcome achieved at the beginning of the follow-up. The larger the baseline difference in visual acuity between eyes, the worse the final visual acuity in the amblyopic eye was. It seemed clear that the severity of amblyopia was a crucial factor for this type of treatment. However, this correlation was not observed at the end of the follow-up. Therefore, if appropriate case monitoring is performed and filter density changes are performed adequately, the severity factor has a less relevant influence. In any case, it should be considered that we only have included cases with mild and moderate amblyopia, because we have observed a lack of efficacy and compliance with treatment in severe amblyopia. It is also important to remark that no losses of stereoacuity with the use of the Bangerter filters were detected by means of the TNO test in any case of the current case series. However, the exact numerical values of stereoacuity were not recorded and analyzed. This is a limitation of the current study. The evolution of stereocuity with the use of Bangerter filters for the treatment of anisometropic amblyopia should be addressed in future prospective studies. Finally, we have attempted to define a predicting linear model for the final visual outcome, considering the baseline conditions of the anisometropic amblyopia. A consistent linear model was found, although with a limited predictability. Therefore, it should be used with care for clinical purposes. The goodness-of-fit of this specific model was confirmed by means of the appropriate statistical analysis. This model revealed that the final visual outcome could be predicted from the baseline sphere, the best-corrected visual acuity and the relative degree of amblyopia (difference in visual acuity among eyes). According to this model, a better visual outcome could be expected in eyes with positive sphere (less negative sphere), better BSCVA at baseline and low interocular difference in BSCVA at baseline (less negative difference). The visual acuity level at the beginning of the treatment has been also confirmed by other authors as a predictive factor for the visual outcome with other modalities of amblyopia treatment [35 37]. In conclusion, the use of Bangerter filters is a useful procedure for treating mild or moderate amblyopia due to anisometropia, with no risk of developing amblyopia in the non-amblyopic eye. It is necessary to make a continuous adjustment of the filter density until the best visual outcome has been achieved, in order to maintain the inversion of dominance (non-amblyopic eye with 2 lines of visual acuity below the amblyopic eye) during treatment. A careful and exhaustive follow-up is required for obtaining an optimized outcome in every case treated with this option. References 1. Loudon E, Simons HJ (2005) The history of the treatment of amblyopia. Strabismus 13: Ciuffreda KJ, Levi D, Selenow A (1991) Amblyopia: basic and clinical aspects. Chapter 1: history, definitions, classifications and prevalence. Butterworth-Heinemann, Boston, pp 1-42

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