Prevalence of visual impairment among adults was 16.8% (95% CI: 13.5%, 20.2%). The result of visual impairment in this study is higher than other studies done in South Sudan (11.8%) (12), Cape Town South Africa(7.2%) (13), Sokoto state of Nigeria(11%) (14), Atakunmosa, South Western Nigeria (7.4%) (15), Bangladesh(9.3%) (16), Malaysia (9.2%) (17), South Korea(4.3%) (18), East Delhi district of India (11.4%) (19), Mahabubanagar district of India (8.4%) (20), Iran (1.39%) (21) and Botucato, Brazil (7.4%)(22).
The studies in South Sudan, Sokoto and Atakunmosa, Nigerian state and Cape Town, South Africa were done by better eye presenting visual acuity which means they considered bilateral visual impairment only. If one eye was visually impaired and the other was not impaired, they considered as no vision impairment which under estimate the magnitude of visual impairment compared to the present study which is done by considering either eye’s visual acuity. The reason for magnitude of visual impairment in Cape Town, South Africa was lower than the present study might be due to socio-economic and the access of eye care service difference.
The possible difference between this study and other studies done in Brazil, Bangladesh, Mahabubanagar district of Indian and Malaysian might be due to the definition of visual impairment and population difference. These studies had used the better eye presenting visual acuity to define visual impairment unlike the present study which uses the presenting visual acuity of either eyes (1). Use of better eye visual acuity underestimate the magnitude of visual impairment because of missing monocular visual impairment. The Iran and Korean studies were based on best corrected better eye visual acuity. This might under estimate the burden of visual impairment, since measuring visual acuity after correction didn’t include the reduced vision due to different cases.
The prevalence of visual impairment in this study is lower than studies done in upper Egypt (38.8%) (23), Cocoa farmers of Ghana(22.7%) (24), Saudi(23.5%) (25) and rural areas of Coastal Karantaka, India (25.7%) (9).
The possible discrepancy between the studies in Upper Egypt and Cocoa farmers of Ghana compared to this study, might be due to study population difference. They studied on rural and more aged (≥ 40 years) group of the populations. This might overestimate the result because of including the more affected population groups but the present study is done on urban adult’s aged ≥18 years.
The study in Saudi had used 6/9 as a lowest cut of point of visual acuity to define visual impairment (25) but in this study the lowest cut of point of visual acuity for visual impairment is 6/18 (1). This might be the possible reason that visual impairment was higher in Saudi compared to this study. The majority (>40%) of the participants in rural areas of Coastal Karantaka, Indian study were above 50 years who were in the more risk age groups compared to this study in which most of the study participants (56.5%) were young adult age groups. Hence, this population difference might overestimate the burden compared to the present study.
The prevalence of visual impairment in this study is in line with the studies done in China (17.17%) (6), Andhra Pradesh state of India(14.3%) (26) and Southern Mexico(14.1%) (27). This might be due to studying the same ages of the populations (≥18 years), use of presenting visual acuity and similar cut of point (VA<6/18) for defining visual impairment.
In this study, illiteracy is positively associates with visual impairment which was supported by other studies done in china (6), rural area of Karntaka India (8), Cape Town, South Africa (13), Southern Mexico (27). The possible reason for increasing visual impairment in illiterates might be due to their poor health related behaviors (28).
Age from 40–64 years and greater than 64 years are positively associated with visual impairment which was supported by studies done in Singapore(29), South Africa (13), China((6)), South Korea (18) Western Cameroon (30), Southern Mexico (27), and Nigeria (15).The possible reason for increased visual impairment in old age might be due to increasing age related eye diseases and degenerations (31).
History of trauma to the eye had the odds of 4 times more likely to have visual impairment compared to those adults who have no history of eye trauma in this study. The possible reason might be due to deterioration of the eye structure, functional loss and exposure to ocular infections.
The odds of visual impairment in adults with family history of eye problems is nearly 7 times higher in reference to adults with no family history of eye problems in this study. This might be due to inheritance of genetic factors.
The odds of visual impairment in adults within >5 family size is nearly 4 times more likely to have visual impairment compared to those adults within <2 family size. This might be due to difficulty to cater for the provision of food, health service use, education and low standard of leaving for the siblings in such large families (32).