Table 1. Characteristics of the study population

 
                  Table 2. Frequency of refractive errors and ocular diseases in the study population

 
Table 3. Relationship between variables studied and refractive errors in study subjects

 

difference between refractive errors and eye disease was 33.3%, which was statistically significant (P=0.031). The findings showed that the prevalence of refractive errors and eye problems was higher in girls, children under the age of 11 years, and preschool students than in the rest of the cases. Nonetheless, the differences observed between the variables studied were not statistically significant (Table3).
 
Discussion
Based on the findings of this study, almost 80% of the children attending exceptional schools had one of the refractive errors (i.e., myopia, hypermetropia, and astigmatism). Studies have shown that children with intellectual disabilities attending special schools have a higher prevalence of refractive errors than their normal peers.
Any kind of disability can affect the development and learning of children. This impact on the learning, progress, and quality of life of the child will be much greater in case of the presence
of several physical and mental disabilities simultaneously. Whenever there is a disturbance in one of the child's senses, the child tries to use other sensations to compensate. Among other things, sight is one of the important senses in learning, academic achievement, and other life skills. Children with mental or physical disabilities may not receive attention for the correction of refractive errors and eye diseases that can be easily treated. Meanwhile, these disabilities can affect their lives and make it miserable.
Consistent with our findings, in a study performed in 2008 in Turkey, 77% of children with mental disabilities and approximately 40% of healthy children were reported to suffer from refractive errors and eye diseases [5].In a study carried out by Joshi et al. in 2013 in India, more than half of the children studying in special schools had refractive errors [9]. In line with our study, Karadag et al., evaluating 180 patients with intellectual disabilities, found refractive errors as the main ocular disorders observed in 56 out of 166 patients [7]. In another study performed by Akinci et al. (2009) on children with Down's syndrome, nearly 98% of the children had refractive errors.
Refractive errors are even more prevalent in adults with mental disabilities. In a study conducted in 2003 in the Netherlands, it was found that 60% of adults with mental disabilities had refractive errors, with hyperopia and astigmatism having the highest incidence [10]. Van Istardael et al. [11], investigating the data extracted from patient's records, reported a high percentage of eye problems in the medical history of adults with mental disabilities. To the best of our knowledge, limited studies have investigated this issue in Iran. In a study performed by Yekta et al. [12] in 2015, refractive errors were more prevalent in children with learning disabilities than in their healthy peers, while these defects can be treated with simple measures, such as wearing glasses and refractive surgery. In the present study, approximately 60% of children had astigmatism in one or two eyes, which is higher than the rate reported by Yekta et al. This difference might be due to the difference in the study population. Therefore, it is required to perform more comprehensive studies in this domain.
In general, children with intellectual disabilities studying in exceptional schools are more likely to have refractive errors. Accordingly, they are exposed to the risk of poor vision or blindness, as well as lower quality of life, poor academic achievement, and high cost of care. These children (i.e., those with refractive errors and eye diseases) will suffer more in their daily activities. Consequently, they should be subjected to more accurate ophthalmic and optometric examinations.
Our results revealed that nearly one per five children with intellectual disabilities suffers from amblyopia or eye laziness. In most countries, including Iran, amblyopia can lead to important social issues, which plays an important role in reducing vision. The visual pathway in the brain evolves from birth to about 10 years of age. The most important factor driving the development of this pathway is the formation of a clear image of objects on the retina. Regarding this, any factor that prevents this trend, including refractive errors, can lead to amblyopia.
Multiple studies have been conducted about amblyopia in many countries around the world. In addition, numerous statistics have been published on the prevalence of amblyopia and its causes and types. According to a study published in the United States on the prevalence of amblyopia, nearly 2% of Americans suffer from amblyopia [13]. With regard to the types and causes of amblyopia, the most common cause of amblyopia is refractive error, while in other countries, the refractive errors and strabismus tend to be relatively effective or even strabismus is more important. Various intermediary factors, such as the low socioeconomic status of the parents, lack of awareness of the need for using glasses or other methods of treatment, and lack of appropriate and standard tools for detecting refractive errors can be considered in this regard.
Therefore, the high prevalence of amblyopia in the present study can be due to the high incidence of refractive errors, including astigmatism, since approximately 70% of the children in the study had at least one eye with this disorder. Based on the findings of this study, out of 78 patients with refractive errors, 18 (23.1%) cases had an ocular disease, including ophthalmic amblyopia. On the other hand, kappa's concurrency coefficient showed that ocular diseases, including amblyopia, were seen in about 30% of children with refractive errors.
One of the major causes of amblyopia is refractive amblyopia or anisometropia. In this regard, the eye with a lower rate of refractive error makes the image clearer and is usually preferred over the other eye with a high rate of refractive error, thereby resulting in amblyopia in the weaker eye. In a study performed by Almeder et al. in the United States in 1990, the incidence of significant anisometropia (greater than or equal to 2 diopters) in children under 9 years of age who had refractive errors was reported as 5%, which was lower than the rate obtained for the population of the present study.
In the present study, eye diseases, such as refractive errors, were more prevalent in girls, children under the age of 11 years, and pre-primary schoolers. The probable cause of this finding could be a greater proportion of girls, early diagnosis of the disease, and the diagnosis and correction of the older cases of refractive errors at an earlier age.
 
Conclusions
Based on the findings of the present study, the prevalence of refractive errors and ocular diseases was high among children with intellectual disability. Furthermore, girls and children with a lower age were at a higher risk of such diseases. Therefore, it is required to give special attention to the screening and periodic examinations of these children.
 
Acknowledgments
The authors extend their gratitude to the Hamadan University of Medical Sciences for their material provision and spiritual support, as well as the officials of the exceptional schools for helping us with this research.
 
Conflicts of Interest
The authors declare no conflicts of interest.
 
References