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 Table of Contents  
ORIGINAL ARTICLE
Year : 2016  |  Volume : 23  |  Issue : 4  |  Page : 196-201

Prevalence and pattern of amblyopia among primary school pupils in Kosofe town, Lagos state, Nigeria


1 Ancilla Catholic Hospital Eye Centre, Lagos, Nigeria
2 Department of Ophthalmology, Guinness Eye Centre, Lagos University Teaching Hospital/College of Medicine, University of Lagos, Lagos, Nigeria

Date of Web Publication20-Dec-2016

Correspondence Address:
Segun Joseph Ikuomenisan
Ancilla Catholic Hospital Eye Centre, Lagos
Nigeria
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1117-1936.196261

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  Abstract 

Objective: The objective of this study was to determine the prevalence and pattern of amblyopia in primary school pupils in Kosofe town of Lagos State with a view to providing baseline data that may facilitate the establishment of routine school eye screening programs in primary schools in Lagos State.
Materials and Methods: A descriptive, cross-sectional study was conducted among pupils from public and private primary schools in Kosofe town. The pupils underwent visual acuity assessment with the HOTV charts, detailed ocular examinations and cycloplegic refractions. Data obtained were analysed using the Statistical Package for Social Sciences version 20.0.
Results: A total of 1702 pupils participated in the study, out of which 899 (52.8%) were males. The age range was 4-16 years with 1630 (95.8%) pupils within the ages of 4 and 10. Amblyopia was detected in 24 (1.4%) of the screened pupils (95% confidence interval = 0.610-0.914). There was no significant sex predilection (P = 0.367) or age trend (P = 0.158) in this study although amblyopia occurred more in boys (54.2%). Refractive amblyopia (58.4%) was the most common type of amblyopia out of which anisometropic amblyopia accounted for 78.6%. Unilateral amblyopia was observed in 87.5% of the amblyopic pupils, and the left eye was more commonly affected than the right eye. All the types of amblyopia were more common within the age group of 4-10 years and in public school children (P = 0.039 and P = 0.015, respectively).
Conclusion: The prevalence of amblyopia in this study was low and it falls within the general prevalence range for amblyopia in Nigeria.

Keywords: Amblyopia, pattern, prevalence, private, public, pupils


How to cite this article:
Ikuomenisan SJ, Musa KO, Aribaba OT, Onakoya AO. Prevalence and pattern of amblyopia among primary school pupils in Kosofe town, Lagos state, Nigeria. Niger Postgrad Med J 2016;23:196-201

How to cite this URL:
Ikuomenisan SJ, Musa KO, Aribaba OT, Onakoya AO. Prevalence and pattern of amblyopia among primary school pupils in Kosofe town, Lagos state, Nigeria. Niger Postgrad Med J [serial online] 2016 [cited 2022 Jan 27];23:196-201. Available from: https://www.npmj.org/text.asp?2016/23/4/196/196261


  Introduction Top


Amblyopia is a visual disorder that is characterised by a reduction in the best corrected visual acuity (BCVA) in an eye with no organic pathology. [1],[2] Clinically, amblyopia is defined as a difference in the BCVA of two or more lines (or >1 log unit) in the absence of an organic lesion. [2] Amblyopia develops early in life during the critical periods of visual development.

Visual acuity (VA) in amblyopia is usually better when reading single optotypes than when reading in a row of letters (crowding phenomenon). [1],[2] Other monocular visual functions (asides VA) that are also affected include grating acuity, vernier acuity and contrast sensitivity. [2],[3],[4] Amblyopic eyes can also have defective accommodation and can exhibit ocular motor deficits such as unsteady fixation and inaccurate tracking. [4] Children with amblyopia may also present with learning difficulty and may sustain recurrent injuries (bumping into objects on the side of the amblyopic eye). Reducing the amblyopia rate can decrease the severity of bilateral vision loss because persons with amblyopia are at an increased risk of injury to the healthy eye and suffer loss of function when this occurs. [3],[4]

Amblyopia is a cause of lifelong, avoidable visual impairment if not detected and managed at an early age. [4] Children with amblyopia may not be aware, nor complain of defective vision. Thus, the need for screening programmes to aid early detection and treatment of this condition. Children in the school age group are easily accessible (through school eye health programmes) to interventions that will help improve their vision. Most studies on amblyopia in Africa [5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15] looked only at the prevalence of the disease. Examining the pattern of presentation and distribution of this condition may yield results that will guide our approach to its screening and early detection. This study aims to determine the prevalence and pattern of amblyopia in primary school pupils in Kosofe town with a view to providing baseline data that may facilitate support for the establishment of routine school eye screening in primary schools in Lagos State.


  Subjects and Methods Top


This study was carried out in 41 primary schools (21 public and 20 private) in Kosofe township over a 9-month period (April 2014-December 2014). The Lagos University Teaching Hospital Health Research and Ethics Committee (protocol number: ADM/DCST/HREC/1645, approval dates: 09-01-2014-09-01-2015) and the Lagos State Universal Basic Education Board gave approval to conduct this study. Written consents from the schools' head teachers and parents of selected pupils were obtained before screening their wards. The research protocol adhered to the tenets of the Declaration of Helsinki for research involving human beings.

A minimum sample size of 1631 was calculated using Leslie-Kish's formula. [16] The standard normal deviate was set at 2.33, the previous prevalence of amblyopia in the population was taken as 3.1% [10] while the desired level of precision was set at 1%. The minimum sample size was further increased to 1794 after allowance for 10% attrition. A multistage random sampling technique was used in recruiting the pupils. In stage one, the number of pupils required for the study in each group (i.e., public primary schools and private government approved primary schools) was calculated based on proportional allocation. In stage 2, 14 primary schools (i.e., 1 public and one private government approved school from each of the seven wards that makeup Kosofe local government area [LGA]) were selected by simple random sampling technique (balloting process) using a numbered list of school names obtained from the Local Government Education Authority and the State Universal Basic Education Board. In stage three, the number of pupils required for the study in each school was determined based on proportional allocation using the list of schools and students' population. In stage 4, the 14 selected primary schools (seven public and seven private government approved primary schools) were stratified into six grade levels (classes 1-6), and the numbers of pupils required for the study in each grade level/class were determined by proportional allocation using each school's class register. Finally, a systematic random sampling technique was used to determine the participants in each grade level/class using the class registers as the sampling frame.

Pre-survey trainings were conducted to enable the research team familiarize themselves with the standard operating procedures involved in the study. Pilot studies were conducted in two schools (one government and one private) to validate the data collection forms and to minimise inter-observer variations. Monocular VA tests (unaided, with pin-hole and with glasses if available) were done with the HOTV chart from a distance of 3 m. Pupils who had unaided VA of <6/9 (logarithm of the minimum angle of resolution [logMAR] 0.18) in at least one eye were subsequently refracted. Ocular alignment was assessed using the Hirschberg light reflex, cover tests and prism cover-uncover tests. Cover tests were performed using fixation targets at both distance (6 m) and near (30 cm). The presence of ocular misalignment, its characteristics, types and degrees were also recorded. The pupils underwent full ocular examination, and any pathology involving the anterior and posterior ocular segments were documented.

Cycloplegic refractions were performed 30 min after the instillation of three drops of cyclopentolate 1% into the inferior fornix at 5-min intervals with a table-mounted autorefractor (Topcon 80,00 ® , Japan) whenever possible, or streak retinoscope (WelchAllyn RF18240) when autorefraction was not possible. Five consecutive autorefractor readings were obtained from each pupil, all of which had to be within 0.25 diopter sphere (DS) of each other. Spherical equivalents were thereafter calculated. The cycloplegic refractions were carried out at the close of school to limit the effects of cycloplegia on academic activities. Post-cycloplegic refractions were carried out 3 days later to determine the BCVA of the pupils. The BCVA and the types of refractive errors were recorded for each pupil. Children with refractive errors were given prescriptions for their spectacle corrections while those with other ocular morbidities were treated and referred appropriately.

Unilateral amblyopia was defined as a ≥2-line difference in BCVA between the two eyes when the VA was <6/9 (logMAR 0.18) in the worse eye, and with amblyogenic factors such as past or present strabismus, anisometropia and past or present obstruction of the visual axis. [17],[18],[19] Anisometropia was defined as ≥1.00 DS difference in hyperopia, ≥3.00 DS difference in myopia, or ≥1.50 DS difference in astigmatism. Bilateral amblyopia was defined as BCVA in both eyes <6/12 (logMAR 0.3) in the presence of amblyogenic factors such as hyperopia ≥4.00 DS, myopia ≥−6.00 DS, or astigmatism ≥2.50 DS, or past or present obstruction of the visual axis. [17],[18],[19]

The data obtained were cleaned and analysed using the Statistical Package for Social Sciences version 20.0 (IBM Corp., Armonk, NY, USA). Descriptive statistics such as frequencies, mean and standard deviations were determined. Exact binomial 95% confidence interval (CI) was calculated for the prevalence estimate with Poisson distribution. Categorical variables were compared by Chi-square test. P <<i> 0.05 was considered statistically significant.


  Results Top


There were 1794 pupils enrolled into this study, but only 1702 completed the examination, representing a response rate of 94.9%. There were 899 males (52.8%) and 803 females (47.2%) students, representing a male to female ratio of 1.1:1. The age range was 4-16 years, with a mean age of 7.5 ± 1.6 years. There were 1142 (67.1%) participants from public schools while 560 (32.9%) were from private schools.

Of the 1702 pupils examined, 24 (1.41%) met the criteria for amblyopia diagnosis (95% CI = 0.610-0.914) [Table 1]. There was no significant difference in amblyopia prevalence between boys and girls (P = 0.367). There was also no significant age trend evident in the study (P = 0.158). Although amblyopia was more frequent (4.17%) among pupils aged 11-16 compared to 1.29% of pupils aged 4-10 all the types of amblyopia were significantly more common in children within the age group of 4-10 years (P = 0.039) as shown in [Figure 1]. No pupil was reported to have been treated for amblyopia previously.
Figure 1: Distribution of amblyopia by age groups in the screened amblyopic children

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Table 1: Prevalence of amblyopia by age and sex


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The probable aetiology of amblyopia was attributed to refractive errors in 14 (58.4%) out of the 24 pupils with amblyopia and anisometropia was found to be the more common type of refractive amblyopia accounting for 11 (78.6%) out of 14 [Table 2]. Out of the 11 anisometropic amblyopic pupils, 7 (63.6%) had astigmatism, 3 (27.3%) had hypermetropia and 1 (9.1%) had myopia. Astigmatism and hypermetropia were significantly more prevalent in anisometropic amblyopic pupils (P < 0.001).
Table 2: Types of amblyopia


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Unilateral amblyopia was observed in 21 (87.5%) of the amblyopic students with the left eye being more commonly affected than the right eye [Figure 2]. Anisometropic and sensory deprivation amblyopia occurred more in the left eye while strabismic amblyopia occurred more in the right eye of the affected children. These results were found to be statistically significant (P = 0.03 for the trend). Although more male pupils had anisometropic and strabismic amblyopia, most cases of sensory deprivation amblyopia and all the cases of isoametropic amblyopia were diagnosed in female students [Figure 3]. These results were not statistically significant (P = 2.012). All the types of amblyopia were significantly more common in the public school pupils (P = 0.015) [Figure 4]. Out of the 24 amblyopic pupils recorded in this study, 16 (66.7%) were in public schools while 8 (33.3%) were in private schools.
Figure 2: Types of amblyopia in the screened amblyopic children. RE: Right eye, LE: Left eye

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Figure 3: Distribution of amblyopia by gender in the screened amblyopic children

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Figure 4: Distribution of amblyopia by type of school in the screened amblyopic children

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  Discussion Top


The prevalence of amblyopia in this study was 1.4%, and it falls within the general prevalence range for amblyopia in Nigeria 0.1%-3.1%. [5],[6],[7],[8],[9],[10],[11] The prevalence rate from this study compares with that found by Onyekwe et al. [11] who noted a prevalence of 1.1% while studying visual impairment amongst school children and adolescent in Jos, Nigeria. Ajaiyeoba et al. [7] in Ilesa (0.3%), Akpe [5] in Benin (0.23%), Megbelayin [6] in Calabar (0.3%), Nkanga and Dolin [9] in Enugu (0.1%) and Ayanniyi et al. [8] in Ilorin (0.4%), all noted prevalence rates lower than this study. On the other hand, Adegbehingbe et al., [10] while screening a group of high school adolescents in Ile-Ife, found a higher prevalence rate (3.1%) than the rate found in this study. The marked difference and variation between the prevalence rate from this study and other Nigerian studies could be attributable to the different working definitions for amblyopia used, the different VA criteria used in screening these children, the difference in population size screened, the difference in the age groups screened, and the region of the country where these children were screened. Ethnic differences could also account for the disparity in these Nigerian studies. The influence of race and tribe on refractive errors has been established [15],[19] but there is paucity of reports of similar effects on the prevalence of amblyopia. The prevalence rates from studies within and outside Africa varies greatly when compared to the result of this study. [12],[13],[14],[15],[17],[18],[19],[20],[21] Similar reasons to the ones stated earlier may be the cause of the disparity in the prevalence rate. In all, this study revealed a relatively low prevalence of amblyopia in the study population which is in keeping with other results from various part of Nigeria. [5],[6],[7],[8],[9],[10],[11]

Most (87.5%) of the children with amblyopia were within the age group of 4-10 years and all the types of amblyopia were more common in this age group. This may be because over 90% of the children in the study population are within this age group. Despite having most of the amblyopic types within the 4-10 years age group, this study found no significant age trend evident in the study population. Some studies [17],[18],[19] also found that the prevalence of amblyopia did not vary with age of the children but Yekta et al. [22] found that the prevalence of amblyopia reduces significantly with age in Iranian school children. The prevalence of amblyopia in this study was also slightly higher in males than in females (though not statistically significant). Just over half (54.2%) of the cases of amblyopia are seen in males. This finding was supported by studies that found a higher prevalence of amblyopia in males. [5],[6],[10],[12],[23] The gender disparity quadrupled in those with anisometropic amblyopia (37.5% of cases in males against 8.3% in females) and sensory deprivation amblyopia (16.7% of cases in males against 4.2% in females). Other studies [17],[18],[19],[22],[23] found no significant variation for amblyopia by gender.

Anisometropic amblyopia was the most common type of amblyopia in this study, accounting for about 45% (45.8%; 11 out of 24) of the cases. This was followed by strabismic amblyopia and sensory deprivation amblyopia, each accounting for around 20% (20.8%) of the cases found. This result was similar to the findings by Akpe [5] in Benin City where over half (60%; 3 out of 5) of the cases of amblyopia were anisometropic, while strabismic amblyopia and isoametropic amblyopia accounted for 20% each. No case of sensory deprivation amblyopia was noted in the study by Akpe. [5] Megbelayin [6] found that in Calabar, all 4 (100%) children with amblyopia in the selected schools had anisometropic amblyopia. Although no percentage was given, Adegbehingbe et al. [10] noted that most children with amblyopia had anisometropic amblyopia. In contrast to the findings of this study, Ayanniyi et al. [8] while examining primary school children in Ilorin found both strabismic amblyopia (40%) and isoametropic amblyopia (40%) to be more common than anisometropic amblyopia (20%).

The types of amblyopia seen in other African and non-African countries also varies. Noche et al. [12] showed anisometropic amblyopia (43%) to be the most common type of amblyopia in Yaoundé, Cameroon. Yassur et al. [13] and Wedner et al. [14] both showed that strabismic amblyopia was the most common type of amblyopia accounting for 72.2% and 66.7% in their respective studies. The Multi-ethnic Paediatric Eye Disease Study in Los Angeles, [17] the Strabismus, Amblyopia, and Refractive Error in Young Singaporean Children Study, [19] and the Sydney Paediatric Eye Disease Study, [20] all showed anisometropic amblyopia as more common than other types of amblyopia in their respective studies (57%, 42% and 53%, respectively). In the same vein, Yekta et al. [22] and Ashok et al. [23] both noted anisometropic amblyopia as more common than other types of amblyopia (35.3% and 41.7%). The Baltimore Paediatric Eye Disease Study [18] showed that anisometropic amblyopia and strabismic amblyopia were equally common (31.6% each). The preponderance of anisometropic amblyopia may be because obvious structural abnormality is not often associated with anisometropic amblyopia compared to strabismic and sensory deprivation amblyopia.

This study found unilateral amblyopia (87.5%) more common than bilateral amblyopia (12.5%). This finding compares with the studies within Nigeria. Akpe, [5] Megbelayin, [6] and Ayanniyi et al. [8] all found unilateral amblyopia to be more common (80%, 100% and 60%, respectively). Other African studies also showed unilateral amblyopia to be more common than bilateral amblyopia. [14],[15] Studies outside Africa also confirmed unilateral amblyopia as the most common type of amblyopia [17],[18],[19],[20],[21],[22],[23] In contrast, Noche et al. [12] found that children in Yaoundé, Cameroon had more of bilateral amblyopia (60.7%) in a hospital-based study. This study also found unilateral amblyopia involving the left eye more frequently than the right. This may not be unrelated to the fact that most children are right handed and as such the right eye may be more dominant. Anisometropic and sensory deprivation amblyopia occurred more in the left eye whereas strabismic amblyopia predominates in the right eye.

Over 60% (66.6%) of the cases of amblyopia were recorded in the public school children in this study. Furthermore, all the types of amblyopia were more common in public school children than in private school children. This compares with the study by Akpe [5] in Benin City, with 62% of amblyopia occurring in public school children. It is thought that children in private schools have higher socioeconomic status and have access to eye health care more readily than their counterparts in public schools. Thus, the prevalence of amblyopia may be reduced in them.

There are some limitations to this study. First, not all the children identified and enrolled into the study were examined. This could have biased the findings either towards a lower or higher prevalence of amblyopia. Furthermore, some of the children who do not have VA deficits at the time of examination may still be at risk of developing amblyopia. Finally, findings from this study may not be applicable to pre-school and secondary school students.


  Conclusion Top


This study has shown that the prevalence of amblyopia among primary school children in Kosofe LGA is low and it falls within the general prevalence range for amblyopia in the country. Refractive amblyopia was the most common type of amblyopia accounting for almost 60% of the cases. Amblyopia affected slightly more males and over 60% of the amblyopic children were in public primary schools. Identification of children at risk of developing amblyopia, regular eye screening at birth, school entry and at least every 2 years for each child is recommended. Furthermore, there should be a public enlightenment of parents on refractive errors and the need for early correction of these errors. This can be achieved through mass media and health talks during routine antenatal and eye clinic visits. Children and their parents could by these media enlightenments be educated about signs and symptoms of refractive errors and consequences of untreated cases. Furthermore, school health programs should be established where teachers and other ancillary staff are trained to assess VA of their students. Identified students with subnormal vision can be referred to centres with refractive services. Finally, eye health education and promotion should be incorporated into the school education curricula. This will help orientate the children towards safe eye care practices at an early age.

Acknowledgement

Special thanks to the children, parents and teachers of Kosofe local government area of Lagos, Nigeria.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2]


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