Prevalence of refractive errors among school children in gondar town, northwest ethiopia.

PURPOSE: Many children with poor vision due to refractive error remain undiagnosed and perform poorly in school. The situation is worse in the Sub-Saharan Africa, including Ethiopia, and current information is lacking. The objective of this study is to determine the prevalence of refractive error among children enrolled in elementary schools in Gondar town, Ethiopia.
MATERIALS AND METHODS: This was a cross-sectional study of 1852 students in 8 elementary schools. Subjects were selected by multistage random sampling. The study parameters were visual acuity (VA) evaluation and ocular examination. VA was measured by staff optometrists with the Snellen E-chart while students with subnormal vision were examined using pinhole, retinoscopy evaluation and subjective refraction by ophthalmologists.
RESULTS: The study cohort was comprised of 45.8% males and 54.2% females from 8 randomly selected elementary schools with a response rate of 93%. Refractive errors in either eye were present in 174 (9.4%) children. Of these, myopia was diagnosed in 55 (31.6%) children in the right and left eyes followed by hyperopia in 46 (26.4%) and 39 (22.4%) in the right and left eyes respectively. Low myopia was the most common refractive error in 61 (49.2%) and 68 (50%) children for the right and left eyes respectively.
CONCLUSIONS: Refractive error among children is a common problem in Gondar town and needs to be assessed at every health evaluation of school children for timely treatment.

INTRODUCTION

Children with severe learning and behavioural disabilities only are often brought to eye care personnel for evaluation.1 There are no national preschool/school screening services for visual disorders in Ethiopia.2

In the Global Initiative, Vision 2020, for the elimination of avoidable blindness, refractive error has been emphasized and prevalence of refractive error worldwide has been reported.34 Refractive error is the second (33%) leading cause of low vision in Ethiopia next to cataract.56

Studies have reported a prevalence of refractive error between 1% to 11%.6–10 Screening is recommended in areas where prevalence exceeds 2%.11 Visual impairment due to refractive errors is defined as visual acuity less than 6/18 in the better eye.12 In developed countries the prevalence of refractive error has been reported at 8.2% in Baltimore, 10.4% in Kazuhiro, Japan and 18.2% in Santa Monica, USA.13–15

Blindness due to refractive error usually manifests at an early age and the number of blind-person-years due to refractive error in developing countries is approximately twice as high as cataract related blindness.16–18 Personal and socio-cultural factors contribute to an inability to visit eye care professionals leading to diverse consequences.19–21

The level of refractive error in Gondar, Ethiopia has not been previously published. This study determined the prevalence of refractive errors and classified the frequency of the various types of refractive errors in relation to sex and age.

MATERIALS AND METHODS

A descriptive, cross-sectional study was performed on a cohort of elementary school children in Gondar town, northern Ethiopia to screen for refractive error. The study locale, North Gondar zone is situated 730 km from the capital, Addis Ababa. The population of Gondar town exceeds 300,000 and it is well known for its historic and tourist attractions. It has got 20 kebeles (administrative units) and 52 elementary schools hosting approximately 45,952 students from grades 1-8. There is a referral University hospital which provides different specialty eye care services and training of mid-level eye care workers such as optometrists, ophthalmic officers and cataract surgeons.

The source population for the study included all elementary school children in Gondar town. Children attending elementary schools of Gondar town who were enrolled for day time education were included in the study. School children enrolled for evening classes and those sick during study period were excluded from the study. The sample size was calculated using the formula for estimation of a single proportion

Where; n = sample size; Z = value of z statistic at 95% confidence level = 1.96; P = proportion of children with refractive error = 11.8%; q = 1–p = 88.1%; W = maximum allowable error = 2% and a design effect of 2 for multistage sampling and the final sample size was 1,997students.

A multistage sampling technique using two stage of the sampling process was used. Initially, 8 schools out of 52 were selected using computer generated random numbers after a census list for all the elementary schools in Gondar town was obtained from the city education office. The number of students for each school was assigned according to the proportion to size of students in the respective schools. A systematic random sampling was finally used to identify study subjects from each of the selected grades using a sampling fraction of 5 from a continuous list in the school from grades 1-8.

The Research and Publications Committee of the University of Gondar provided ethical clearance for this study. The purpose of the research was explained and submitted to the Education department of North Gondar Zone and permission granted. Each participating school was sent an official letter from the University explaining the purpose of the study. Further explanations and discussions were conducted with the headmaster/headmistress of each participating school. Consent was obtained from each student and class supervisor after they were fully informed about the research. Students found to have refractive errors during the survey were sent to the optometry unit of Gondar University Hospital and treated for their refractive error.

Personal details of each subject were recorded in standard format and then the child underwent an ophthalmic examination, and all findings were registered. Visual acuity was measured in the school compounds (under the tree shade/well lighted class rooms) using the Snellen E chart. Detailed ocular evaluations, by an ophthalmologist were performed for those with subnormal vision.

The subnormal vision, was defined as uncorrected visual acuity of <6/9 in the worse eye. Refractive error was considered: a) when there was a subnormal vision with a non-plano power on retinoscopy, in the absence of anterior or posterior segment abnormalities; b) if the visual acuity improved with pinhole testing and; c) in subjects with latent hyperopia i.e. visual acuity of 6/6 with a +1.75 D lens.

Only children with visual acuity less than 6/6 in at least one eye underwent evaluation by retinoscopy. Objective refraction was performed with retinoscopy by an optometrist, half an hour after instillation of 1% cyclopentolate eye drops. Myopia was defined as refractive error ≤-0.5 D, hyperopia as refractive error ≥+1.5 D, and astigmatism as refractive error >0.5 D.

Data were collected by practicing optometrists and ophthalmologists experienced in the study measurements and interview techniques. Data quality was insured via training of data collectors, supervision and cross-checking, by an ophthalmologist on 5% of the students. Data were entered into a computerized database using SPSS version 15.0 (IBM Corp., Armonk, NY, USA).

RESULTS

Socio-demographic characteristics of participants

A total of 1852 students (45.8% males and 54.2% females) from 8 randomly selected elementary schools were included in the study with a response rate of 93%. The mean (± standard deviation) age of participants was 12 (±2.8) (range, 4 years to 24 years). The age range for the majority of the cohort was 10 years to 14 years [Table 1].

Table 1

Sociodemographic characteristics of students attending elementary schools in Gondar, Northwest Ethiopia, March 2010

Visual acuity with and without pinhole correction

Among 1852 students, a majority, (91.9% for right eye and 92.3% for left eye), of students had normal visual acuity. Thirty-six (1.94%) of students had visual acuity of 6/36 and lower in their right eye and 27 (1.46%) of the students in the left eye. The rest of students had visual acuity ranging between 6/9 to 6/24 [Table 2].

Table 2

Visual acuity of the right and left eyes without Pinhole Correction among elementary school students in Gondar, Northwest Ethiopia March 2010

The visual acuity was repeated using a pinhole correction for 150 students for their right eye and 142 students for the left eye. Approximately a third (34.70%) of students had normal visual acuity for the right eye while 41.55% students had normal visual acuity for the left eye. There were 17 (11.30%) and 15 (10.56%) of students with a visual acuity of 6/36 or lower for their right and left eyes respectively [Table 3].

Table 3

Visual acuity of the right and left eyes with pinhole correction among elementary school students in Gondar, Northwest Ethiopia, March 2010

Frequency of refractive errors in relation to age and sex

Ninety school children have bilateral refractive error resulting a prevalence of refractive errors in both eyes of 4.9%. There was a greater preponderance of refractive error in females and in the 10 years to 14 years age group compared to males and other age groups respectively [Table 4].

Table 4

Refractive error in both eyes by age and sex among elementary school students in Gondar, Northwest Ethiopia, March 2010

Refractive error in each eye in relation to gender and age

All forms of refractive errors were more common among females than males. Myopia in either eye was very common among females. Twenty seven of 42 students with myopia were females. Among males, myopia was the common refractive error. The 10 years to 14 years h age group had the highest rate of refractive errors. Hyperopia was common in the age group between 5 years to 9 years. However, for 10 to14 year olds, myopia was more prevalent; the same was true for the 15 years to 19 years age group [Table 5].

Table 5

Types of refractive errors in relation to age and sex among elementary school students in Gondar, Northwest Ethiopia March 2010

Degree of refractive error

Retinoscopy of children with refractive errors indicated almost half, (61 (49.2%)) had low myopia in the right eye. High myopia was also found in the right eyes of 15 (12.1%) children. Low hyperopia was the most common form of hyperopia found in 41 (33.1%) of children. Moderate and high hyperopia were very low at 0.81% of children each.

On examination of the left eye, among the 136 children found with refractive error, 68 (50%) of children had low myopia, 13 (9.6%) had moderate myopia and 6 (4.4%) had high myopia. Thirty six (26.5%) had low hyperopia of the left eye while moderate and high hyperopia were 0.74% and 1.48% respectively [Figure 1].

Figure 1

Degree of refractive error of the right and left eyes under retinoscope examination among school children in northwest ethiopia, may 2010

DISCUSSION

Among the total 1852 students, 174 [9.4%] had abnormal vision for any reason. This is lower than a study in central Ethiopia.7 The difference between studies may be due to the fact that the previous study assessed any ocular abnormality some of which were not limited to visual impairment alone. This however is higher than the findings of a Tanzanian study.8 The 20 years to 24 years age group was not included in all the tables of refractive error and abnormal vision as there were no cases.

The overall prevalence of refractive error among all students assessed was 9.0%. This is comparable to a study done in Uganda.9 The prevalence of refractive errors was 3% for myopia [about 31.6% of students with refractive error] in either the right or left eye, hyperopia was observed in 46 (2.5%) and 39 (2.11%) in either eye respectively. This figure is much lower than previous studies. According to some studies the overall prevalence of refractive errors for children was 9.2% for myopia, 12.8% for hyperopia, and 28.4% for astigmatism.6

The level of refractive error in both eyes in this study is very high at 2.3% for myopia, 1.3% hyperopia and astigmatism indicating that students are affected mainly by visual problems [Table 4]. Females were more affected by all types of refractive errors than their male counterparts; this could be attributed to the recruitment of larger number of females in the study.

Refractive error among children is a common problem and needs to be assessed at every health evaluation of school children for early intervention. Refractive errors were common among female study participants. It is vital to establish regular screening programs at schools for refractive errors. Health institutions should be linked to the schools to help correct refractive errors.

In this study myopia particularly low myopia is more common in the study population. This is because myopia is more common in younger ages and significant number of children will improve with age. Alternately, hyperopia was lower compared myopia as the former increases with increasing age.