INTRODUCTION

Amblyopia is a unilateral or, less commonly, bilateral reduction in corrected visual acuity in the absence of visible organic abnormalities and is due to misdi- rected, blurred, or absent retinal images during devel- opment of the visual system (1). It is the second lead- ing cause of bilateral visual impairment in children after refractive errors, and has been reported as the leading cause of unilateral visual impairment in pedi- atric patients (2,3).

Anisometropia, constant unilateral strabismus, bilat- eral isoametropia, amblyogenic unilateral or bilateral astigmatism and ocular media opacities are causes of Amblyopia (4).

The overall prevalence of amblyopia varies between

1.6to 3.6% for preschool and school population and from 3.25% to 5.3% in clinical population in different regions of the world (5).

Amblyopia, being unilateral, commonly even severe cases may not be detected by parents or care givers

(6). And failure to detect and treat amblyopia at young age, when the prognosis for successful treatment is best, leads to permanent visual impairment, adverse effects on school performance, poor fine motor skills, weak social interactions, and self –image (7). An am- blyopic individual is at a significantly higher risk of becoming blind compared to an individual with nor- mal visual acuity and individuals with childhood- onset unilateral amblyopia have a greater lifetime risk of eventual bilateral visual impairment and age-related macular degeneration (8).

There are very few studies focused on amblyopia from sub-Saharan Africa countries. The preva- lence of amblyopia among school children in Gha- na and Nigeria were 9.9%, and 0.3% respectively. And anisometropia was the major amblyogenic factor in these studies (9, 10).

Understanding the prevalence, burden and pattern of Amblyopia is important for adequate healthcare planning in an effort to establish a routine school eye screening. In Ethiopia there is paucity of pub- lished data on prevalence and pattern of amblyo- pia. Hence, this study was conducted to estimate the prevalence and determine the causes of ambly- opia among children aged 7-15 years in Wolliso town, Southwest Ethiopia.

SUBJECTS AND METHODS

This school based cross- sectional study was car- ried out in May 2018 in Wolliso town, Southwest Ethiopia. For this research purpose public schools were defined as schools supported by either public or government funds and whereas private schools were defined as schools run and supported by pri- vate individuals or a corporation.

The department of Ophthalmology of Addis Ababa University’s Institutional Review Boards gave approval for the research and informed written consent was obtained from parents and/or legally authorized representatives of the study partici- pants.

Requisition letters were sent to all the selected schools seeking permission from the respective school heads. All study procedures adhered to the principles out- lined in the Declaration of Helsinki for human subject research. Those children with visual impairment were referred to the nearby hospital and managed accord- ingly.

Using Leslie-Kish formula a sample size of 1226 was calculated (11) and 4 schools (2 publics and 2 private) were selected using a random cluster sampling meth- od. A multistage random sampling technique was used in recruiting the students, aged range from 7-15 years, in each grade level/class using the class registers as the sampling frame.

Pre –survey trainings were conducted to the research team to familiarize themselves with the standard oper- ating procedures involved in the study. The first step of the study was screening of the students within the school compound. A large room inside the school premises was selected for the screening program. Monocular distance visual acuity was tested using a logarithm of minimum angle of resolution (logMAR) chart. Step 2 was conducted at the nearby hospital. The parents/guardians of students whose Best Correct- ed Visual Acuity (BCVA) worse than or equal to 6/12 (or ≤ 6/12) in at least one eye, in absence of any or- ganic lesion according to Amblyopia Treatment Stud- ies (12), were requested to bring their children to the nearby, Saint Lukas hospital, for further evaluation. In the outpatient department, a detailed history about past and present ocular problems and treatments was obtained from the parents by the principal investigator of the study. Distance visual acuity was re-checked using the Snellen’s chart. Assessment of ocular align- ment, ocular motility and associated deviation, and slit lamp examination for the assessment of any anterior segment pathology was performed. A detailed fundus examination was done with 90D lens to rule out any posterior segment pathology.

Cycloplegic refraction by streak retinoscope was per- formed after pupillary dilation using 1% cyclopento- late eye drops.

Operational DefinitionFor the purpose of this study , amblyopia was defined as BCVA < 6/12 the affected eye without any underlying structural abnormality of the visual pathway, a 2-line difference between the two eyes, and the presence of an amblyogenic factor. The severity of amblyopia was further graded into mild (BCVA 6/12-6/18), moderate (BCVA 6/18–6/

36)and severe (BCVA <6/36) ( 12). Standard defini- tions for various types of amblyopia were used for diagnosis (12). Bilateral amblyopia was defined as best VA in both eyes ≤ 6/12.

Anisometropic amblyopia includes patients who had amblyopia in the presence of anisometropia that is 1.5 D or greater in spherical equivalent, or a 1.5 D or greater difference in astigmatism between the eyes in the absence of any measurable heterotropia at distance or near.

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Strabismic amblyopia included that due to conflict- ing visual inputs between the eyes due to squint. Combined amblyopia includes either patients with a heterotropia at distance or near along with aniso- metropia of 1.5D or more in spherical equivalent or a 1.5 D or more difference in astigmatism in any meridian between the eyes. (5, 12)

Data generated were collected using a structured data collection form, and statistical analysis was done using SPSS 21.0 version (SAS Institute, Cary, NC 2010) software. Statistical association between categorical variables was computed using Fisher’s exact test and Pearson chi-square (χ2 ) test. P < 0. 05 was considered statistically signifi- cant.

RESULTS

In total, we screened 1,226 children of which 63 (5.14 %; 95% CI: 3.9%-6.4%) were found to have amblyopia. No student was reported to have been treated for amblyopia previously. The mean age of children with amblyopia was 10.45+ 2.09 years with age range between 7 to 15 years. Almost half amblyopic children 30 (47.6 %) were between 10- 12 years old (Table 1).

TABLES AND FIGURES

Table 1: Age at presentation and gender distri- bution of amblyopia among school children at Wolliso town, Southwest Ethiopia, May 2018.

Thirty-nine of the subjects, 61.9% were female. There was no significant difference in amblyopia prevalence between males and females (P = 0.367).

Fifty-two (82.5%) subjects with amblyopia were from public schools (Table 1). There was statistical association between students from public school and amblyopia. (p=0.003).

From 63 children identified as amblyopic, 42(66.67 %) had bilateral and 21 (33.33 %) had unilateral am- blyopia. Hence a total number of 105 amblyopic eyes of 63 children were studied. Among unilateral ambly- opic children 9 had in the right and 12 had in the left eye as shown in table 2.

Table 2: Laterality and causes of amblyopia among school children at Wolliso town, Southwest Ethiopia, May 2018.

FIGURE 1: Distribution of amblyogenic factors with gender among school children , May 2018, Wolliso Town, Southwest Ethiopia.

Anisometropic amblyopia and isometropic amblyopia were the commonest types of amblyopia accounting 31 (49.2%) and 23(36.5%) of the subjects respectively

(Fig 1). For both anisometropic and isometropic am- blyopia, myopia was the commonest type of refractive error, contributing 43 (80%) of the subjects.

From seven sensory derivational amblyopia five had unilateral, one had bilateral corneal opacity and one had ptosis. Both Strabismic cases were exotropia.

As far as the severity of amblyopia is concerned, mild amblyopia was seen in 39(37.14 %), moderate ambly- opia in 22(20.95%), and severe in 44(41.90%). Rela- tively severe amblyopia was higher than mild and

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moderate as shown in table 3. Majority of severe amblyopia patients had sensory derivational am- blyopia. Severity of amblyopia was statistically associated with derivational amblyopia (p= 0.013). All the types of amblyopia were significantly more common in the public school students (P = 0.016).

DISCUSSION:

Amblyopia is the most common cause of monocu- lar vision loss in children with an estimated preva- lence of 1.6 to 3.6% (1). Early diagnosis and treat- ment of amblyopia result in better outcomes (13). In this population-based study, we reported the prevalence and pattern of amblyopia among school children aged 7–15 years at Wolliso town, South- west Ethiopia.

Prevalence of amblyopia varies due to different age-group of studied populations and different factors prevailing in that region, like literacy rate, frequency of visual screening programmes and geographical factors. Accordingly, the prevalence of amblyopia worldwide varies. In a multi-ethnic pediatric eye disease study (MEPEDS) conducted on African-Americans and Hispanics, amblyopia was detected in 2.6% of Hispanic/Latino children and 1.5% of African-American children (14). A study from Iran (2010) reported the prevalence of amblyopia was 2.32 in boys and 2.26% in girls (8). The criteria for diagnosis of amblyopia are almost the same in these studies. Prevalence of amblyopia among African countries varies from 0.3 to 9.1% (9, 10, 15).

The sampled population, study design (clinical/ population based), the criteria used to define am- blyopia and location could account for the differ- ence in these studies. With similar study design with ours, a cross-sectional study in Indian school children aged between 5 and 15 years showed that the prevalence of amblyopia was 1.1% (16). The results of these studies is very low compared to our study. The higher prevalence of amblyopia in the Ethiopian studies might be due to poor awareness amongst general population on importance of visu- al assessment and lack of regular school screening programs nation wide and limited knowledge on the conditions by guardians.

We found no significant difference in the preva- lence of amblyopia between different age groups. These finding is comparable with many other stud- ies in children and adults, where these studies have shown no increase or decrease in the prevalence of amblyopia with age (17,18). Most studies, like ours, have reported that the difference in the preva- lence of amblyopia is not significant for sex and the difference mostly results from sampling errors or differences in the response rate and participation of women and men in screening programs (6,19).

Our findings showed a higher prevalence of amblyo- pia in females than males which is also in line with studies from other countries (20, 21). Gender biases in eye health service delivery might be a reason for such difference in female students.

Two third of the cases in this study had bilateral am- blyopia, which is quite different from other studies made by Fu et al (5), (66.7%) and Chia et al. (3) (69.7%) . Bilateral amblyopia is predominant because isometropic amblyopia is common causes of amblyo- pia in our study.

The results of our study showed that the prevalence of amblyopia was 4 times higher in public school partici- pants as compared with those with private school chil- dren. It is obvious parents with better socioeconomic status send their children to private schools. And high- er socioeconomic status in parents/ guardians have a direct positive effect on use of effective health care services, leading to a decrease in the prevalence of visual disorders, including amblyopia (22).

The main cause of amblyopia varies between studies, depending on how the amblyopia is defined and the characteristics of the study sample. The type of am- blyopia seen in different aspect countries also varies. Our findings showed nearly half, 49.2%, of the ambly- opic cases had anisometropic amblyopia and 36.5% had isometropic amblyopia. Hence, in this study the amblyopia caused by the refractive error was 85.7 %, which was comparable with a report from china , (85.2%), (18), and India (86.9%) (23). In the present study, the prevalence of strabismus was 3.17%, simi- lar to other studies (24, 25). It is possible that Strabis- mic amblyopia is detected early due to the obvious deviation of eyes and therefore can be managed in a timely manner compared to other forms of amblyopia which may go undetected for a long time.

And besides lack of a school screening programs can be the reason for higher number of refractive error as a cause for amblyopia.

Limitation

Our study has some limitations. Small-angle strabis- mus and intermittent strabismus may have been missed given the nature of both conditions.

REFERENCES

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This could have underestimated the prevalence of strabismus. The other limitation of the study is that we used a Snellen chart to measure visual acuity rather than an Early Treatment Diabetic Retinopa- thy Study chart, so the impact of ‘‘the crowding effect’’ could not be measured. Consequently, some children with mild amblyopia may have been missed, while others with more severe amblyopia may have been misclassified as having moderate amblyopia.

Conclusions

The result of this study showed that the prevalence of amblyopia among school children in Wolliso town, Southwest Ethiopia was higher than other Sub Saharan African countries. The lack of a regu- lar vision screening program in the study area could be considered as the main causes for late diagnosis of amblyopia. Therefore, a regular school based vision screening initiatives program is recommended.

Competing interest

The authors declare that this manuscript was ap- proved in its form and that no competing interest exists.

ACKNOWLEDGEMENT

This study was supported by a grant from CBM. The study sponsor had no involvement in the study design; in the collection, analysis and interpreta- tion of data; in the writing of the report ; and in the decision to submit the paper for publication. We are indebted to the kindness of all the staffs who were unduly supportive in all ways throughout the study period. We are also grateful to the school directors and teachers who facilitated the data col- lection within the respective schools.