Prevalence of visual impairment among adults aged 18 years in this study was 16.8% (95% CI: 13.5%, 20.2%) which is higher than other studies done in South Sudan (11.8%) [10], Cape Town South Africa (7.2%) [16], Sokoto state of Nigeria (11%) [17], Atakunmosa, South Western Nigeria (7.4%) [18], Bangladesh (9.3%) [19], Malaysia (9.2%) [20], South Korea (4.3%) [21], East Delhi district of India (11.4%) [6], Mahabubanagar district of India (8.4%) [22], Iran (1.39%) [23] and Botucato, Brazil (7.4%) [24].
The studies in South Sudan, Sokoto and Atakunmosa, Nigerian state, Brazil, Bangladesh, Mahabubanagar district of Indian and Malaysian were done by better eye presenting visual acuity which means they considered bilateral VI only. If one eye was visually impaired and the other was not impaired, they considered as no VI which under estimate the magnitude of VI compared to the present study which considered the visual acuity of either eye. The lower prevalence of VI in Cape Town South Africa might be caused by differences in socioeconomic variables and access of eye care services. The Iran and Korean studies were based on best corrected better eye visual acuity which might under estimate the burden of VI.
The prevalence of visual impairment in this study is lower than the studies reported by Upper Egypt (38.8%) [9], Cocoa farmers of Ghana (22.7%) [8], Saudi (23.5%) [25] and rural areas of Coastal Karantaka, India (25.7%) [7].
The possible discrepancy between the studies in Upper Egypt and cocoa farmers of Ghana compared to this study might be due to study area and population difference in which they studied on rural and aged ≥ 40 years populations. The study in Saudi had used 6/9 as a lowest cut of point of visual acuity to define VI [25] unlike in the current study that used the lowest cut of point of visual acuity for VI was 6/18 [1] which may be the possible reason for the discrepancies.
The prevalence of visual impairment in this study is in line with the studies done in China (17.17%) [26], Andhra Pradesh state of India (14.3%) [27] and Southern Mexico (14.1%) [28]. This may 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 VI.
In this study, illiteracy is positively associates with visual impairment which was similar that found by other studies done in China [26], rural area of Karntaka India [29], Cape Town, South Africa [16], Southern Mexico [28]. The possible reason for this trend of VI may be poor health related behaviors in illiterates [30].
Age ≥ 40-64 years are positively associated with VI which was supported by studies done in Singapore [31], South Africa [16], China [26], South Korea [21] Western Cameroon [32], Southern Mexico [28], and Nigeria [18] that may be related to an increased prevalence of age related eye diseases and degenerations in these age groups [33].
History of trauma to the eye had 4 times more likely to have VI which can be explained due to deterioration of the eye structure, functional loss and exposure to ocular infections following trauma.
Visual impairment in adults with family history of eye problems is nearly 7 times higher than no family history which may be due to inheritance of genetic factors.
In adults within >5 family size is nearly 4 times more likely to have VI compared to those adults within <2 family size which can be explained 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 [34].
Since community based study on the presenting visual acuity was not conducted in the study area or other similar areas which had related population characteristics and methodology of the current study, 50% proportion of sample size determination might overestimated the prevalence. In addition this study might have an inheritance limitations of the cross-sectional study design and information bias due to the tools that used to collect ocular trauma history, family ocular history and cigarette smoking.