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  5. FP1421 : Determinants of Improvement in Visual Acuity Following Patching for Anisometropic Amblyopia

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Dr. Antony Arokiadass B, B18727, Dr. C A Nelson Jesudasen

Purpose

To assess predictive factors for success to occlusion therapy in anisometropic amblyopia(AA)

Methods

This prospective interventional study was done in patients with AA, aged ≤ 25 years and a minimum 1 month use of refractive correction. Visual functions were measured at baseline. Part time patching was prescribed. Review was at the end of 6 and 12 months. Differences in baseline visual functions between those whose acuity improved at 1 yr and those who did not were analysed.

Results

Of 20 evaluable patients, 16 had > 10% acuity improvement, at the end of 1 yr. Younger age, lower Anisometropia, higher contrast and good stereopsis were associated with improvement in acuity while colour vision and accommodative parameters were not predictive of success.

Conclusion

In this study age and visual functions ( contrast, stereopsis , quantum of anisometropia) were found to be predictive of patching therapy success.

Keywords: Amblyopia, anisometropic amblyopia, patching

  1. Introduction

Amblyopia is one of the most common causes of preventable visual impairment in children. Anisometropic amblyopia is caused by a difference in the refractive error between the two eyes. Prescription of refractive correction, in the form of glasses or contact lenses, and patching of the sound eye, form the mainstay of treatment. Visual functions, such as contrast sensitivity, stereopsis and accommodation,  are known to be affected in amblyopia. But, the role of various clinical functions as predictive factors for success in treatment is less well-known. This study aimed to assess the predictive factors for success to occlusion therapy.

  1. Methods

2.1 This was a prospective, interventional study performed at a tertiary eye care institute after approval from the Institutional Ethics Committee. Patients with anisometropic amblyopia, aged less than 25 years, capable of understanding and cooperating for the test, were included in the study. Individuals  with other forms of amblyopia, those who gave a  previous history of amblyopia treatment or of ocular surgery, and those presenting with  manifest strabismus or  any other anterior or posterior segment pathology , were not considered for inclusion in the study.  Individuals who had been enrolled but were then non- compliant to patching or lost to follow- up, were excluded from the study. Subjects whose visual acuity improved with spectacle correction alone or who were unwilling to undergo the tests were also excluded.

2.2 All subjects underwent cycloplegic refraction and were given spectacle correction at the first visit. They were reviewed with spectacles after one month. All visual function tests were performed uniocularly with spectacle correction in place. The tests performed included Visual acuity assessment for distance and near ( LogMAR chart), contrast sensitivity assessment ( LEA low contrast acuity chart), Binocular single vision (BSV) for distance using Worth four dot test ( WFDT),  assessment of accommodative parameters such as  Near point of Accommodation (NPA) and  Accommodative amplitude (AA)( calculated using RAF rule) and Accommodative facility (AF) (tested with +2.00/-2.00 flippers), colour vision testing ( online Farnsworth D 15 colour arrangement test ) and stereopsis (tested using TNO Cards).

2.3 After recording the baseline visual functions, the subjects were advised part- time patching with a large spectacle patch.  They were also advised to perform near work, such as reading, painting, drawing or video game -playing while wearing the patch.

2.4 At the review visits (at 6 months and 1 year) change in visual acuity status, if any, was documented with LogMAR charts, while compliance was assessed by history.

2.5 Percentage improvement in visual acuity was calculated as =

Percentage improvement =   [(V aei – Vae f) / ( Vaei – Vafe)] * 100

Where Vaei is visual acuity in the affected eye at baseline, Vaef is visual acuity in the affected eye at the final visit and Vafe is visual acuity of the fellow eye at the end of 1 year.[1]

Those with at least a 10% improvement in acuity were considered to have improved.

2.6 The demographic characters and baseline visual functions in those with acuity improvement versus those without acuity improvement were compared to determine at the possible predictive factors for successful outcomes of patching therapy. The statistical significance of differences were assessed by appropriate statistical tests.

  1. Results

Thirty patients were enrolled in the study. Among these,  visual acuity of one subject improved with refractive correction alone. Nine individuals were excluded either because they were lost to follow up or did not follow the patching regimen at all.  Among the 20 evaluable patients 11 were females and nine males. There was improvement in visual acuity in 16 (80%) of the 20 evaluable patients while four (20%) did not show improvement in percentage visual acuity.

3.1 The mean age of the patient and mean values of refractive error indices in the affected eye, such as  quantum of refractive error and quantum of anisometropia, were compared between patients with improvement in visual acuity versus those without improvement in visual acuity (Table  1). All these three factors were found to be significantly lower  in patients who showed improvement compared to those who did not  show any improvement(Table 1).

3.2 Contrast sensitivity values, measured as number of letters read at 2m using the affected eye, was found to be better at baseline in patients who had improved visual acuity compared to those who did not show improved visual acuity(Table 1).

Table 1: Mean age and mean values of refractive error status in relation to improvement in final visual acuity following patching in patients with anisometropic amblyopia

Parameter assessed Mean value in patients/eye with improved visual acuity (n=16) Mean value in patients/eye without improved visual acuity(n=4) Statistical significance of difference [Independent ‘t’ test     ( d.f. = 18)]
Mean Age 9.75 ± 3.60 years 14.75 ± 2.25 years ‘t’ = 3.49, p = 0.009
Mean quantum of refractive error  2.72 ± 1.71 D 4.62 ± 1.26 D  ‘t’ = 2.48, p = 0.047
Mean quantum of anisometropia 1.74 ± 0.90 D 4.43 ± 1.14 D ‘t’ = 4.37, p = 0.012
 Mean value of contrast sensitivity

 ( at 2 m)

 15.6 ± 7.9 letters  6.7 ± 5 letters ‘t’ = 2.25,p = 0.037

Abbreviation: d.f = degree of freedom

3.3 A comparison of outcomes in relation to gender distribution and presence or absence of Binocular Single Vision (BSV) was made (Table 2). These factors were not found to differ significantly between subjects who improved and those who did not.

Table 2 : Gender distribution and binocular single vision status in relation to improvement in final visual acuity following patching in patients with anisometropic amblyopia

Parameter assessed Patients who improved ( n = 16) Patients who DID NOT improve ( n = 4) Statistical significance of difference (Chi square test

[ d.f = 1]
Gender distribution

( Male: Female)

 7:9 2:2  χ 2= 0.051, p = 0.822

“ not significant”
BSV

( Present: Absent)

 11:5 1: 3 χ 2= 2.55  p = 0.110

“ not significant”

Abbreviations: BSV= binocular single vision; d.f.= degree of freedom

3.4 Accommodation parameters and colour vision indices were not found to be significantly different between those who improved and those who did not. (Table 3)

Table 3: Accommodative parameters and confusion angle ( colour vision index) in relation to improvement in final visual acuity following patching in patients with anisometropic amblyopia

Parameter assessed Mean value in patients/eye with improved visual acuity (n=16)  Mean value in patients/eye without improved visual acuity(n=4) Statistical significance of difference (Independent ‘t’ test     [ d.f. = 18])
Near point of Accommodation 12.16 ± 3.56 cm 21.00 ± 13.31 cm ‘t’ = 1.24, p= 0.249
Accommodative amplitude 10.58 ± 3.05 D 6.50 ± 5.00 D ‘t’ = -1.32, p = 0.266
Accommodative facility 8.75 ± 4.57 cycles/min 9.75 ± 8.01 cycles/ min ‘t’ = 0.16, p =0.878
Confusion angle             ( Colour vision) 17.79 ± 62.27 deg – 6.60 ± 72.77 deg ‘t’ = – 0.90,p = 0.376

Abbreviation: d.f.= degree of freedom

3.5 When a comparison was made between enrolled patients of grade of stereopsis at baseline in relation to final visual outcomes (Table 4), patients with better grades of stereopsis at baseline were found to have a better mean percentage improvement in best corrected visual acuity (BCVA).

Table 4: Grade of stereopsis at baseline in relation to mean  percentage  improvement in final best corrected visual acuity following patching in patients with anisometropic amblyopia

GRADES OF STEREOPSIS AT BASELINE No. of patients Mean percentage improvement in best corrected visual acuity Statistical significance of difference (Kruskal wallis test)
No 8 25.83%  ‘H’ =3, (d.f. = 3)

P = 0.039

‘ significant’
Gross 4 15.58%
Fair (> 60 arc sec) 7 57.69%
Good (≤ 60 arc sec) 1 100%

Abbreviation: d.f.= degree of freedom

3.6. A comparison was made between the enrolled patients of the interacuity difference at baseline in relation to mean percentage improvement in visual acuity following   patching (Table 5). It was found that the denser the amblyopia, the poorer was the improvement in final visual acuity (Table 5).

Table 5: Grade of amblyopia at baseline in relation to mean percentage improvement in final best corrected visual acuity following patching in patients with anisometropic amblyopia

GRADE OF AMBLYOPIA

 (INTER ACUITY DIFFERENCE)

(median value 0.45)

 No. of patients Mean percentage improvement in `VA Statistical significance of difference (Mann Whitney U test*)
Low ( ≤ 0.45) 10 54.4 ± 33.5%  U = 24,

P = 0.04
High (>0.45) 10 22.9 ± 21.8 %

Abbreviation: BCVA= final best corrected visual acuity; ANOVA= analysis of variance; d.f.= degree of freedom * – non parametric test used since the values were not normally distributed in the subgroups

4.Discussion      

Amblyopia is a neuro- developmental disorder of the visual cortex, in which the best corrected visual acuity is decreased due to abnormal visual functions in infancy or early childhood during the sensitive period of visual development. Apart from visual acuity, many other visual functions, such as contrast sensitivity, accommodative functions, stereopsis and colour vision are affected in the amblyopic eyes. [2,3,4]

Defining the treatment outcome in amblyopia requires consideration of the initial depth of amblyopia, the improvement attained in the visual acuity and the residual amblyopia, defined as the difference in the visual acuities of the fellow eye and the affected eye at the end of the treatment, as brought out lucidly by Stewart et al [1] in the form of a formula. This formula was used in the current  study to evaluate the final outcomes.  We have inferred that those with at least 10% improvement in percentage visual acuity actually had significant improvement. This was considered to be approximately equivalent to a one line improvement from the least acuity possible at 4 m at baseline in LogMAR chart.

Compliance to patching is a major confounding factor across many studies hampering the comparability of treatment outcomes. In the current  study, those with less than 50 percentage of prescribed patching and those who did not attend the final follow up at 1 yr were excluded from the analysis of the data obtained. This has also been followed by Kirandi et al [5] in their study on treatment outcomes of anisometropic amblyopia. In the current study, the history obtained from the parents regarding adherence to the patching regimen had to be relied on to assess the compliance to patching in the absence of special metered dose patches, which have been used in certain landmark studies.[6]

Identification of predictive factors for success or failure of conventional treatment for amblyopia, may enable the practitioner to schedule cases prone for failure for more aggressive measures of treatment, more frequent follow up, early consideration of supportive measures and counsel the patient/ parents with regard to their expectations about the treatment outcomes.  Various studies have pointed to the dose of occlusion, the initial severity of the amblyopia, binocular vision, fixation of the amblyopic eye, and the age of the subject at the start of treatment as significant factors predicting the success of patching therapy for unilateral amblyopia.[7].

Woodruff et al [8], in one of the earliest papers on the factors affecting outcomes of amblyopia therapy , asserted that  pure anisometropic amblyopia, better initial visual acuity  and good compliance to patching were associated with better final visual outcomes. Age at the onset of patching was not found to be a significant factor in his study.  Age was again not found to be a significant factor by Cobb et al [9], but as explained by the author, it could be because there were more children with strabismus in the younger age group being diagnosed early; the presence of strabismus was found to confound the relationship between age and visual outcomes. A more recent metaanalysis of four randomised control trials found that amblyopia responded better to patching in children below the age of seven years.[10] In our study, the subjects who had visual improvement after patching were significantly younger compared to those whose visual acuities did not improve (Table 1). Similar results were also reported with Wallace et al. from the Pediatric Investigator Group ( PEDIG)[11], where  younger age at enrolment was found to be associated with better improvement in the vision of the affected eye.

Better baseline acuity of the amblyopic eye was found to be an important predictor of amblyopia resolution in a few studies. [11], [12],[13]  We found that the visual acuity improvement in the amblyopic eye in subjects with mild degrees of amblyopia was significantly better than with those who had moderate or severe degrees of amblyopia(Table 5) . In our study the depth of amblyopia was arrived at by calculating the difference in the visual acuity between the affected and the fellow eye.

While most of the studies on unilateral amblyopia include strabismic and anisometropic amblyopia, they are entirely different subsets of amblyopia with regard to the pathophysiology, clinical features and prognosis. There are a very few studies analysing patients with amblyopia purely due to Anisometropia. In one such retrospective review, Kirandi et al [5] found that among children aged 7 – 9 years with anisometropic amblyopia, the spherical equivalent of the affected eye was the most influential risk factor for treatment failure and recurrence. Although our study included subjects with a different age distribution, the quantum of refractive error in the affected eye, measured as spherical equivalent was significantly lower in patients who had improvement in the visual acuity after patching compared to those who did not improve following patching (Table 1). Hussein et al. [12] found age more than 6 years, presence of significant astigmatism, poor compliance to treatment and poor initial visual acuity to be factors associated with lower chances of success after patching in patients with ansiometropic amblyopia.

Caputo et al [14] found presence of good stereopsis at the baseline to be associated with better visual outcomes following patching in treatment of anisometropic amblyopia. In our study too, although presence or absence of BSV for distance using WFDT, was not significantly associated with either outcome following patching(Table 2), subjects with stereopsis better than ‘gross’ stereopsis , had significantly better outcomes compared to those who had just gross stereopsis or none (Table 4).

To our knowledge, there has been just a single previous study which investigated the role of visual functions, such as visual acuity, accommodation, contrast sensi­tivity, fixation, mesopic visual acuity, and stereopsis as determinants of visual improvement in amblyo­pia treatment; [13] initial visual acuity and accommodation were found to be strong determinants of response while stereopsis and mesopic visual acuity were found to have some value as determinants[13]. In the current study, accommodative parameters, such as accommodative amplitude, near point of accommodation and accommodative facility in the affected eye were not significantly associated with visual outcomes(Table 3). This different conclusion drawn by our study could be due to a number of reasons. In the study by Singh and Agrawal (2013) [13], etiologically diverse causes of amblyopia, and both unilateral and bilateral amblyopia had been included; moreover, the accommodative effort in the affected eye was calculated by improvement in near visual acuity by addition of + lenses. In contrast, in the current study, techniques such as NPA, AA and AF were used to study the accommodative status of the amblyopic eye.

Limitations of the current  study include  the small number of participants and non-utilisation of  more robust statistical tests, such as multiple regression, to assess the role of various factors, which might be interrelated as prognostic factors for amblyopia.

In conclusion, in patients aged 25 years or younger suffering from anisometropic amblyopia and undergoing patching, a younger age, a lower  quantum of refractive error, a lower degree of anisometropia, a milder degree of amblyopia, better levels of contrast sensitivity and better grade of stereopsis were found to be associated with better visual outcomes.

Bibliography

  1. Stewart CE, Moseley MJ, Fielder AR, Defining and measuring treatment outcome in unilateral amblyopia. British Journal of Ophthalmology 2003;87:1229-1231
  2. Levi DM, Knill DC, Barvelier D, Stereopsis and amblyopia: A minireview. Vision Res 2015; 114:17 – 30
  3. Abrahamsson M, Sjostand J, The contrast sensitivity and acuity relationship in strabismic and anisometropic amblyopia. Br J Ophthalmol, 1988; 72: 44- 49.
  4. Singman E, Matta N, Tian J et al, Association between accommodative amplitudes and amblyopia. Strabismus 2013; 21: 137-9.
  5. Kirandi EU, Akar S, Gokyigit B, Onmez FEA, Oto S. Risk factors for treatment failure and recurrence of anisometropic amblyopia. Int Ophthalmol. 2017 Aug;37(4):835–42.
  6. Wallace MP, Stewart CE, Moseley MJ, Stephens DA, Fielder AR, Monitored Occlusion Treatment Amblyopia Study (MOTAS) Cooperatives, et al. Compliance with occlusion therapy for childhood amblyopia. Invest Ophthalmol Vis Sci. 2013 Sep 17;54(9):6158–66.
  7. Stewart CE, Fielder AR, Stephens DA, Moseley MJ. Treatment of unilateral amblyopia: factors influencing visual outcome. Invest Ophthalmol Vis Sci. 2005 Sep;46(9):3152–60.
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  9. Cobb CJ, Russell K, Cox A, MacEwen CJ. Factors influencing visual outcome in anisometropic amblyopes. Br J Ophthalmol. 2002 Nov 1;86(11):1278–81.
  10. Holmes JM, Lazar EL, Melia BM, Astle WF, Dagi LR, Donahue SP, et al. Effect of age on response to amblyopia treatment in children. Arch Ophthalmol Chic Ill 1960. 2011 Nov;129(11):1451–7.
  11. Wallace DK, Lazar EL, Crouch ER, Hoover DL, Kraker RT, Tamkins SM, et al. Time course and predictors of amblyopia improvement with 2 hours of daily patching. JAMA Ophthalmol. 2015 May;133(5):606–9.
  12. Hussein MAW, Coats DK, Muthialu A, Cohen E, Paysse EA. Risk factors for treatment failure of anisometropic amblyopia. J AAPOS Off Publ Am Assoc Pediatr Ophthalmol Strabismus. 2004 Oct;8(5):429–34.
  13. Singh V, Agrawal S. Visual functions in amblyopia as determinants of response to treatment. J Pediatr Ophthalmol Strabismus. 2013 Dec;50(6):348–54.
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