Study area and period
We conducted school based cross sectional study on school based children in Lideta Sub-city, Addis Ababa, Ethiopia from April to May 2019. Addis Ababa is the capital city of Ethiopia with an area of 540 square kilometers and 3.38 million inhabitants. The structure of power organization of the city includes the city government, 10 sub-cities, namely Arada, Addis Ketema, Kirkos, Lideta, Gulele, Kolfe-Keranyo, Nifas silk Lafto, Bole and Akaki Kaliti . Number of children enrolled in primary schools in Addis Ababa for the 2018/2019 academic year was 45, 8661. In Lideta sub-city, there are 18 governments owned and 11 private owned schools. There are a total of 16,172 school children enrolled in the regular primary schools for 2019 academic year. Out of these 10,640 school children are enrolled in government primary schools and the remaining 5,532 were enrolled in private, public and church schools .
In this study, school child was considered a student whose age is between 7 - 18 years and who was attending school from grades 1 up to 8 in government schools. Similarly, presenting visual acuity was considered the visual acuity of the child without correction with glasses and impaired vision was considered if the visual acuity of a child <6/12 in the better eye. Vision impairment (VI) refers to reduction of vision resulting in a lower than normal VA i.e. VA < 6/12 but better or equal to 6/18 in the better eye as measured using Snellen’s chart. Low Vision (LV) refers to a more severe form of visual reduction i.e. VA worse than 6/18 and better than or equal to 3/60 as measured using the Snellen’s chart.
Study variables and measurements
The major outcome variable of interest was prevalence of visual impairment (VI). Independent variables included; age of child, sex of child, family history of spectacle use, guardian occupation, parental marital status, parental education, family income, duration of TV watching, distance of TV watching, duration of mobile phone exposure, outdoor playing, service availability and visit to eye care facility.
Visual Acuity is considered normal and is called 6/6 vision, the United States Customary System (USCS) equivalent of which is 20/20 vision. Vision of 6/12 and corresponds to a lower VA than 6/6. Accordingly VA expressed with a larger denominator represents lower vision .The most popular VA testing chart is Snellen’s VA chart. It is used in routine VA testing in eye care units. The other VA testing tool named as Log-MAR chart is mainly used for research purposes [3, 2]. Visual acuity can be expressed in four notations namely, the foot notation, metric notation, decimal notation and Log MAR notations . Only the metric notation which expresses normal vision as 6/6 was used in this study.
Sample size determination and sampling technique
Total sample size of 816 was determined by applying a formula for single population proportion with 95 % confidence interval, prevalence of visual impairment at 9.5% from previous study (25), 3% (α = 0.03 margin of error), design effect of 2 and by considering 10% non-response rate. In this study two-stage cluster sampling method was used. Lideta sub city was the primary sampling unit (PSU). Primary schools in Lideta sub-city were selected using proportional allocation to size (PPS) method which became secondary sampling units (SSU). From 29 primary schools in Lideta sub-City, all the 18 government primary schools in Lideta sub-city were considered as clusters. The total number of children attending at government primary schools was 10,640. Among these, 6 (30%) primary schools were selected using Probability Sampling to Size (PPS) method and were included in the study.
The first school was selected according to a random start (RS) = 28 which was generated using Excel command. Accordingly the first school to be included in the study is “Meskerem 1”. The consecutive clusters (schools) were identified using the formula to calculate the random number that fall in the cumulative sum corresponding to a school. Accordingly, “Omedla”, “Karamara”, “Alem Maya”, “G. Hayelom Araya” and “Tesfa Kokeb” schools were selected and included in the study. The process of identification of the schools included in the study using PPS is indicated in the Annexes.
The total number of children in 6 selected schools was 5,148. The calculated sample size which is 816 was allocated to each school proportional to the number of school children in each specific school. Further, it was planned to select study units SRS method by creating one sampling frame for each school and generate random numbers using Excel. This was not possible as the preparation of sampling frame for each school on Excel was found to be time consuming and data collection had to take place before students were dismissed to study for final exams. Instead of SRS, systematic random sampling method was found to be time saving and was used to select the study units by making use of the class attendance list as a sampling frame. Accordingly, the sample allocated to each school was further proportionally allocated to each class in that specific school. Sampling interval (SI) was calculated for each class in the school and study units were selected until the allocated sample size was fulfilled. School children enrolled in selected schools who are under the age of 18 years and the parents/ guardians of whom have given their consent to participate were included into the study.
Data collection tool, procedure and quality control
Visual acuity testing was conducted by trained nurses who are staff members in an eye department of Minilik II Hospital. Since it was not possible to get LogMar projectors and rooms in schools with standard lighting conditions, VA testing was conducted using standard 3 meter Snellen’s VA charts outside classrooms under the ambient lighting conditions. By convention, the right eye was tested first while the left eye was occluded. To avoid learning effect the left eye was tested using a chart with different optotypes which have similar size and sequence. Eyes were tested separately and together and the findings were recorded on the recording form. The findings of VA testing were verified by optometrist and therefore VA testing was done two times for children with no visual impairment (VI) and three times for children with VI. Any child with a VA < 6/12 was referred to the optometrist who repeated VA testing to confirm findings of trained nurses. For children who were confirmed to have VA <6/12, the optometrist did both objective and subjective refractiondoing measurements to identify visual problem that can be corrected with glasses). Children whose vision did not improve with glasses were referred to an ophthalmologist who did further eye examination to establish the cause of VI. The parents/guardians of all school children who received eye exams were interviewed by trained nurses using a structured questionnaire. Standard Snellen 3 meter VA charts were used in the possible appropriate environment. Measurements were done by trained nurses and were repeated by an optometrist. Measurements were checked for the possible existence of variation. If variations existed, the source of variation were identified and corrected. The principal investigator supervised the VA measurement process. Structured questionnaires were prepared in English and were translated to Amharic and then translated back to English to check and confirm its consistency. Staff nurses who are currently working in an eye department were selected and were given training on study objective, sources of bias, ethical issues and interview techniques. The questionnaire was pre-tested in one primary school in Lideta sub-city which was not included in the study. By taking 10% of the total sample, a total of 81 children were selected by systematic random sampling for VA testing and examination. Subsequently, the parents/guardians of the examined children were interviewed. Based on the findings of the pre-testing, all the necessary adjustments in the process of data collection and revision of the questionnaires were made. These revised questionnaires were administered to parents/guardians of children who have undergone VA testing and eye examination in the selected children in the 6 primary schools. The trained staff nurses administered the questionnaire by using face-to face interview technique and the process of questionnaire administration was supervised by the principal investigator. Each questionnaire was also checked for completeness, missed values and inconsistency of responses were manually cleaned up. Explorative analysis was done to explore the data for missed values, outliers and inconsistent values. Identified errors were corrected accordingly
Data management and analysis
Data were entered using Epi-data and analyzed using SPSS version 20 statistical software. Frequency tables, graphs and descriptive summaries were used to describe the data. Based on the objective of the study, the association between stated dependent and independent variables were analyzed using bivariate logistic regression and Chi-Square tests after checking fulfillment of assumptions. Variables with P values ≤ 0.25 at bivariate regression analysis level were recruited for multivariate logistic regression. Finally, the strength of association between outcome and predictor variables was assessed at P Value ≤ 0.05 using adjusted odds ratio (AORs) with corresponding 95% CIs.