Vision and hearing are by far the most important human senses that help us to take in the world around us. When one of these two senses suffers a serious defect, the other sense rises in importance [1]. Defects in the hearing system cause a disturbance in one's ability to communicate with others, resulting in a failure in communication and social interactions. Hearing impairment is sorted into two major categories: hearing loss and deafness. According to the World Health Organization, a deaf individual is described as a person who is unable to process auditory information with or without the use of hearing aids, and a hearing-impaired person is someone capable of auditory processing with the use of hearing aids [2-5].
As detailed by researchers, there is no single cause for hearing loss; rather, a wide range of issues are at play, including congenital and hereditary diseases, infections, and traumas, affecting different parts of the ear and its mechanisms. It has been observed that 10%-15% of children are rejected in screening programs; therefore, it is of utmost importance to recognize the causes of hearing loss, the types of damage, the degrees of these damages, and the subsequent impact on the child's performance. It is also necessary to be fully aware of other factors associated with hearing loss, such as race, age, gender, and socioeconomic characteristics, and assess their effects on hearing loss in the final assessment.
Hearing loss is a hidden disability in children, especially in neonates and toddlers, and considering that human hearing is complete at 12 months, undetected and untreated hearing loss in this population leads to speech and language delays, as well as emotional, social, and academic problems. Like adults, children experience different degrees of hearing loss, each leading to specific problems for the child [6]. Hearing-impaired and deaf individuals rely on their visual senses more than healthy people; therefore, even low degrees of refractive error, if uncorrected, can reduce visual performance and present people with serious problems [7]. In addition, previous studies that assessed visual disorders among the deaf and hearing impaired demonstrated that the prevalence of refractive errors, amblyopia, strabismus, and low vision among deaf and hearing-impaired children is higher in comparison with their hearing peers [3,4,7].
The prevalence of vision problems among normal-hearing people is 17%-30%, while it is 44%-%65 among hard of hearing and deaf people [7,8]. The higher number of vision disorders in deaf and hearing-impaired people compared to normal-hearing people can be attributed to the fact that the retina and cochlea originate from a single layer in the sixth and seventh weeks of the embryonic period. They may be affected by genetic and environmental factors, such as lack of oxygen, toxic agents, and viruses [1, 4, 5, 7]. Rogers observed that the prevalence of vision problems rises with increased severity of hearing loss. The noteworthy issue was the relatively high prevalence of vision disorders in hearing loss associated with congenital rubella syndrome [8].
The studies conducted on deaf-blind children indicated the following points: 1. A high prevalence of neurological disabilities, including neuromuscular disorders, is observed in congenitally deaf-blind children; 2. Severe hearing impairment is more common than severe visual impairment in deaf-blind children; 3. Many deaf-blind children have normal peripheral hearing without normal behavioral responses to sound; and 4. The combination of auditory and visual conflicts, along with neurological disabilities, have profound effects on language and cognitive abilities, as well as the overall development of the child [6].
As mentioned earlier, deaf and hearing-impaired children rely on the sense of sight to understand the world around them and develop effective communication skills, such as learning sign language and lip reading. Due to the higher prevalence of vision disorders in these children, it is necessary that all of them undergo a comprehensive eye exam immediately after the diagnosis of deafness so that their vision disorders can be diagnosed and resolved. Early diagnosis and correction of vision disorders among deaf and hearing impaired children, the correction of correctable problems, such as eyeglass prescriptions in refractive errors, and treatment of curable problems, such as cataract surgery, increase visual performance exert a positive effect on the process of learning, education, and communication. Binocular vision dysfunction can be prevented and treated if diagnosed in childhood [2,5,7].
Although the prevalence of vision disorders in deaf and hearing-impaired students has been proven, the prevalence and types of these disorders are different in various regions across the globe. Despite the importance of early detection of vision disorders in deaf and hearing-impaired students, few studies have been conducted in this regard in our country. Consequently, it is of utmost importance to conduct studies on vision disorders in these students, considering the impact of genetic and environmental factors, as well as people's lifestyle, on the prevalence and type of these disorders.
In light of the aforementioned issues, the present study aimed to assess the type and prevalence of refractive errors, strabismus, amblyopia, and low vision in deaf and hearing-impaired students in Kermanshah in the academic year 2018-2019.