Effects of different types of refractive errors on bilateral amblyopia

J Clin Exp Invest www.jceionline.org Vol 3, No 4, December 2012 1 Develi State Hospital, Kayseri, Turkey 2 Ankara Atatürk Education and Research Hospital, Ophthalmology Department, Ankara, Turkey 3 Yıldırım Beyazıt University, Department of Ophthalmology, Ankara, Turkey Correspondence: Mücella Arıkan Yorgun, Develi Hatice Muammer Kocatürk Devlet Hastanesi DeveliKayseri, Türkiye Email: mcllarkn@yahoo.com Received: 29.05.2012, Accepted: 10.11.2012 Copyright © JCEI / Journal of Clinical and Experimental Investigations 2012, All rights reserved JCEI / 2012; 3 (4): 46771 Journal of Clinical and Experimental Investigations doi: 10.5799/ahinjs.01.2012.04.0204


INTRODUCTION
2][3] It can be easily diagnosed in patients with high refractive errors.In clinical practice amblyopic patients with different types of refractive errors can be observed.There are few limited reports comparing response of patients with bilateral amlyopia with different types of refractive errors. 3We aimed to evaluate the amount of binocular visual acuity improvement of these patients with different refractive errors after treatment of amblyopia by optical correction.

MATERIALS AND METHODS
The study enrolled patients with bilateral refractive amblyopia excluding those with >1.5 D anisoametropia (spherical equivalents), myopic or hypermetropic spherical equivalents more than 8 D, ocular albinism, ocular disease or history of ocular surgery, a neurological disease or syndrome.The research followed the tenets of the Declaration of Helsinki, with local ethical committee approval and the full, informed consent of parents and assent from the children.Each subject had complete routine ophthalmological examination.Bilateral amblyopia was defined as 6/10 to 6/60 best corrected visual acuity (BCVA) OU on the Snellen chart or the tumbling "E" chart.Best corrected visual acuity was measured after the refraction by using trial frames at both first and the final examinations.The test distance was 6 meters and the tester knowing the child's refractive error.The best corrected visual acuity value was accepted as the line where at least 4 letters from 5 can be identified by the patient.The refractive errors of all patients were measured after topical cyclopentolate 1% (dropped three times with 5 minutes interval) application 45 minutes after the last drop with the retinoscopy.The sphere was corrected partially 1-1.5 D below the cycloplegic hyperopic correction.and the cylinder was given in full.Each 26 patient was followed with only optical correction.The best level of visual acuity reached was confirmed with pinhole disc in each patient.
Stereopsis was tested with refractive correction by the TNO test before dilating the pupil in the first visit and the final visit with prescribed glasses.
The patients were classified according to the level of spherical equivalent (0-4 D and >4 D of hypermetropia), the level of astigmatism (below and above 2D in positive cylinder) and type of composed refractive error [<4 D of hypermetropia and < 2 D of astigmatism (group I), > 4 D of hypermetropia and < 2 D of astigmatism (group II), and < 4 D hypermetropia and > 2 D of astigmatism (group III)].Initial and final binocular best corrected visual acuities (BCVA) of the patients between groups were compared.
Visual acuity data for patients were converted to a common logarithm of the minimum angle of resolution (logMAR) scale for statistical analysis.
The mean of best corrected binocular BCVA levels of patients at admission and at the last control visits were compared by the Mann Whitney-U test.The patient groups classified according to refractive errors were compared by the Kruskal-Wallis test and in case of significance pairwise comparisons were done by the Mann Whitney U test.The stereopsis ratios of groups at the final visit were compared by the chi-square test.The p values below 0.05 were considered significant.

RESULTS
Thirty nine patients with bilateral refractive amblyopia with a mean follow up of 2.49±1.46years (min: 6 months-max: 6 years) were enrolled in this study.The mean age of patients was 7.29±2.27years (min: 4-max:12).The number of patients in the classified groups according to the amounts of spherical equivalents, astigmatism and composed spherical equivalents and astigmatism are shown in Table 1, Table 2 and Table 3 respectively.The age when the visual defect was detected (p=0.16) and the duration of treatment (p=0.48) were not significantly different between the defined refractive groups (<4 D, 4-7 D, >7 D).Similarly the age when the visual defect was detected (p=0.16) and the duration of treatment (p=0.85) were not significantly different between patient groups defined according to the composed spherical equivalent and astigmatism.The mean final and initial binocular BCVA values of patients with different levels of spherical equivalents are summarized in Figure 1.The initial binocular BCVA levels were significantly lower in patients with > 4 D of hypermetropia (p=0.028),but their final binocular BCVA levels after treatment with glasses were not statistically different compared to those with < 4 D of hypermetropia (p=0.235).For analysis of composed spherical and astigmatic refractive errors three groups [<4 D of hypermetropia and < 2 D of astigmatism (group I), > 4 D of hypermetropia and < 2 D of astigmatism (group II), and < 4 D hypermetropia and > 2 D of astigma-tism (group III)] were in sufficient numbers to make statistical analysis.The mean initial and final binocular BCVA levels of the groups with composed hypermetropia and astigmatism are summarized in Figure 3.The initial binocular BCVA levels were significantly different between groups (p=0.02).Subgroup analysis by Mann Whitney U test revealed that group I had significantly higher mean initial binocular BCVA as compared to group II (p=0.015).The mean final binocular BCVA was not significantly different between groups I-III (p=0.35)After treatment by correction of refractive error with glasses, the visual acuities and ratios of stereopsis were increased significantly in all of the patients at the final visit (p= 0.001 for each).The levels of initial and final binocular BCVA and stereopsis of patients are observed in Table 4 and Table 5 respectively.

DISCUSSION
4][5] In case of anisometropia the greatest amount of improvement in visual acuity is reported to be observed in myopes and the least in simple hyperopes. 3,7However effects of different types of refractive errors in case of bilateral amblyopia remains to be defined.We aimed to evaluate visual acuity levels of the patients with bilateral amblyopia and different refractive properties.
Increased magnitude of the hyperopia has been shown to have the greatest influence on the visual acuity outcome both at initial correction of refractive error and 1 year or longer after correction in a younger patient population (mean age 3.97 years) with 5 D or more of isometropic hyperopia. 7e found a significant difference in visual acuity between patients with lower (<4D) and higher levels (>4D) of hypermetropia before treatment with optical correction, but after treatment there was no significant difference between them.Similarly the difference in initial binocular BCVA between group I and group II reflects the difference in patients with high (> 4 D) and low (<4D) levels of hypermetropia.In this patient group with a relatively higher mean age (7.04±2.30,range 3-13) compared to other studies 3,[7][8] (5 years 1 month -5.5 years) this good response in this patient group probably suggests extension of plasticity period in case of high hypermetropia.
Children with significant bilateral hypermetropia are reported to have greater binocular acuity improvement than those with significant bilateral astigmatism; however the cumulative probability of reaching 20/25 or better binocular acuity over one year is reported to be similar. 5We found no significant difference in final BCVA levels between the group with high hypermetropia (group II) and that with high astigmatism (group III).
In the analysis of astigmatism, the final mean BCVA levels were significantly lower in patients with 4-6D of astigmatism compared to the patients with 2-4 D of astigmatism (p=0.001).] Duration of correction and the age of first correction also are reported to influence the visual acuity outcome, 5,[9][10][11][12] in our study the age when the visual defect was detected (p=0.16) and the duration of treatment (p=0.48) were not significantly different between the defined refractive groups (<4 D, 4-7 D, >7 D).Similarly the age when the visual defect was detected (p=0.16) and the duration of treatment (p=0.85) were not significantly different between patient groups defined according to the composed spherical equivalent and astigmatism.
Obviously there are some limitations to our study.First the number of patients in some groups (patients with high hypermetropia and high astigmatism) were not enough for statistical analysis.Therefore the results do not include all refractive error groups.
The 9 patients among those patients with less than 4 D of hypermetropia had also less than 2 D of cylindrical refractive errors.The cause of amlyopia in these cases remains to be answered.The refrac-tive status of the patients at presentation may not represent their previous refractive errors.This is an important possibility and can only be confirmed by future prospective cohort studies involving patients at lower ages.Underlying inability to accommodate normally affecting emmetropization in hypermetropic subjects maybe another factor for development of amblyopia as suggested. 8The unknown natural history of amblyopia remains to be resolved.
Finally we think that lower levels of hypermetropia should also be evaluated cautiously as bilateral refractive amblyopia.The age at diagnosis and duration of treatment and the initial and final BCVA levels after treatment with optical correction seem not to be significantly different among patients with different types of refractive errors and also different levels hypermetropic errors.However the stereopsis ratios are lower with higher refractive errors.But larger cohort studies maybe needed for confirmation of these findings.

Figure 1 .
Figure 1.The mean final and initial binocular best corrected visual acuity (BCVA) values of patients with different levels of spherical equivalents.

Figure 2 .
Figure 2. The mean initial and final binocular best corrected visual acuity (BCVA) levels of patients classified according to the amount of astigmatism in positive cylinder form.

Figure 3 .
Figure 3.The mean initial and final binocular best corrected visual acuity (BCVA) levels of the groups with composed hypermetropia and astigmatism.

Table 1 .
The frequency and percent of levels of spherical equivalents [in diopters(D)] observed in patients

Table 2 .
The frequency and percent of levels of astigmatism [in positive cylinder and in diopters (D)] observed in patients

Table 3 .
The number of patients classified according to composed spherical equivalents and astigmatism in diopters (D)

Table 4 .
The initial and final best corrected visual acuity (BCVA) levels of patients in LogMar units

Table 5 .
The initial and final stereopsis values in patientsOn admission After treatment