Background
Strabismus, also called squint, is a condition of misalignment of the visual axes of the eyes, and was first known as eye misalignment since the Hippocratic era [
1]. It can adversely affect not only binocular single vision, like simultaneous perception, fusion and stereopsis, but also cosmetic impairment, which can cause significant psychosocial consequences [
2,
3] Strabismus is a common contributing factor to amblyopia in children, which will persist into adulthood if not treated in time [
4]. Hence, early detection and treatment should be considered for strabismus and amblyopia, especially in preschool children, to maximize binocular potential and to improve outcomes [
5,
6].
An accurate epidemiology of disease will help in evaluating the screening results, improving effective treatment strategies and guiding the allocation of medical resources [
7]. However, although the prevalence of strabismus in school-based studies have been estimated in recent years, the prevalence of various studies differs. Screening for eye diseases in school-aged children has largely been done in large cities, with very few information available in remote areas, like Tibet at a high-altitude plateau area. Impaired vision in early childhood can have a profound impact on a child’s development. Screening in childhood has become an important part of the children’s eye health program and should be done early [
8,
9].
Tibet is a region in Asia covering much of the Tibetan Plateau, with an average elevation of 5000 m. The atmospheric conditions are hypobaric, with thin air, high numbers of sunshine hours and strong ultraviolet radiation [
10]. To our knowledge, although some epidemiological surveys of common eye diseases have been carried out in recent years, they lacked longitudinal investigation on eye diseases of school-age children in the Tibetan plateau areas [
11]. The purpose of this survey was to assess the prevalence and progressive nature of strabismus in native Tibetan children, to determine its associated risk factors and to evaluate the demand for eye care service in Lhasa Prefecture, Tibet Autonomous Region, China.
Discussion
To the best of our knowledge, the LCES is the first comprehensive school-based cohort study in the high-altitude plateau Lhasa, Tibet, China and provide baseline data of the Tibetan Minority for epidemiology of Grade one students eye disorders. We found the prevalence of strabismus was 3.66% with an exotropia: esotropia ratio was 5.38:1, and participants with esotropia were more likely to affect stereopsis, with most being hyperopic.
The overall prevalence of strabismus in our study was higher than previous reports from China and most of other studies around the world. We summarized the previous prevalence of Chinese pediatric strabismus study in Table
4 [
3,
7,
16‐
23]. The prevalence of strabismus in our study was lower than the 5.65 and 5.8% in different 3–6-year-old children from the Nanjing Pediatric study [
7,
18]; Nanjing is a metropolis in eastern China. Compared with the prevalence from other countries, the prevalence of LCES was higher than the 3.3% in Caucasian and 2.1% in African American children in the Baltimore Pediatric Eye Disease Study [
24], 2.4% in Hispanic/Latino and 2.5% African-American children in the Multi-Ethnic Pediatric Eye Disease Study [
25], 1.3% in Japan [
26], 2.4% in Turkey [
27], and 0.8% in the Strabismus, Amblyopia and Refractive Error in Singaporean Children (STARS) [
28].
Table 4
Prevalence of strabismus in previous Chinese studies
Zhu H | 2019 | Yunnan | 1656 | 7–8 | 1.50 | 10.6:1 |
1394 | 13–14 | 2.44 | |
Pan CW | 2017 | Yunnan | 3122 | 6–8 | 2.47 |
4305 | 9–11 | 3.69 | 9.4:1 |
1836 | 12–14 | 4.96 | |
Pan CW | 2016 | Nanjing | 5831 | 3–6 | 5.8 | / |
Chen X | 2016 | Nanjing | 5884 | 3–6 | 5.65 | 6:1 |
Fu J | 2014 | Anyang | 3112 | 7–8 | 2.73 | 6:1 |
2362 | 12–13 | 5.02 | 51:1 |
Pi LH | 2012 | Chongqing | 3079 | 6–15 | 0.29 | 1.7:1 |
Lu P | 2008 | Qinghai | 1084 | 6–14 | 2.49 | 5.7:1 |
He M | 2007 | Yangxi | 2454 | 13–17 | 1.63 | 3.5:1 |
He M | 2004 | Guangzhou | 4364 | 5–15 | 1.9 | 4:1 |
Zhao J | 2000 | Shunyi | 5884 | 5–15 | 2.8 | / |
The higher prevalence of strabismus in LCES may relate to several reasons. Firstly, among the 1856 participants, 94.93% were native Tibetan Minority. The potential influence of ethnic, environmental, lifestyle and genetic susceptibility differences [
16] is still unclear. Qian reported that the prevalence of ocular disease differences in Tibet may relate to more exposure to sunlight and easier schooling [
29] Whether Tibetan children have a higher prevalence of strabismus requires further investigation. Secondly, different age ranges across the different studies may play an important role [
3,
16]. The mean age of our voluntary students was 6.83 ± 0.46 years, and the age range was narrower than other studies, although, older than the two Nanjing pediatric studies [
7,
18]. Finally, we recruited four professional strabismus and pediatric ophthalmologists to examine the participants. They have professional experience in checking strabismus, with a likely greater detection rate of less obvious strabismus and mildly impaired visual acuity.
The ratio of exotropia: esotropia ratio in LCES was 5.38:1, which was lower than many studies [
3,
7,
16‐
23]. The reason for this probably relates to the status of refraction; the distribution of the cycloplegic SE of esotropia in LCES tend to be hyperopia, which was consistent with the finding that exotropia was associated with astigmatism, myopia and low to moderate hyperopia, while esotropia was associated with hyperopia in a dose-response manner [
16,
30,
31]. Qian also found that Tibetan adolescents had a lower prevalence of refractive errors than the plains (central China) area except for hyperopia, and also suggested that the rate of hyperopia in Tibet is higher [
29]. As we show here, in the present LCES study the prevalence of hyperopia was higher at 6.91%. However, the prevalence of hyperopia was 2.21% in ACES. Comparing the significant hyperopic prevalence difference between LCES and ACES, this probably explains the discrepancy in the ratio of exotropia: esotropia between the two studies.
Strabismic amblyopia typically influences the dominance of the fixating eye and chronically reduced responsiveness to input by the non-fixating eye [
8]. The mean BCVA of the strabismus and non-strabismus eyes showed students with strabismus had worse BCVA. We also found that participants with stereopsis impairment had bigger deviation angles and esotropia was more likely to affect stereopsis. The VA in strabismic amblyopia can be improved after successful treatment in younger ages and stereopsis can be restored after surgical alignment in many strabismus cases [
32,
33] The earlier that clinically significant refractive error and strabismus are detected and treated, the greater the likelihood of preventing or reducing amblyopia.8 However, most children with strabismus and amblyopia in LCES were not diagnosed earlier, let alone treated.
Some studies found higher prevalence of strabismus in older students [
3,
16]. Our study did not find that for students in LCES from Grade 1. However, the age span was not as large as others. The effect of some regional factors like oxygen saturation and ethnicity characters were not statistically significant likewise, which is in agreement with the previous study [
16]. Dietary (fruits and vegetables) intake did not influence the prevalence of strabismus, either. Tilting one’s head when writing was a risk factor for the prevalence of strabismus. In a forward multiple logistic regression for strabismus, writing posture was statistically significant, reflecting a higher prevalence of strabismus with habits like tilting one’s head when writing. Bao and co-authors reported that the near heterophoria state can affect near vision posture, while head tilt angle and ocular gaze angle had a potential relationship with strabismus [
34]. However, it is possible that head tilt may have developed because of strabismus, and can be accepted as a sign of misalignment. Hence, although we consider that tilting one’s head when writing may be a risk factor for the prevalence of strabismus, it requires further research to verify or refute this observation.
There are some limitations to the present study. Some risk factors might be potentially inaccurate due to the self-reported questionnaires from the students’ parents, even though the questionnaires used in the LCES were calibrated for cultural differences. In addition, strabismus history and treatment were not always remembered by their parents clearly, which could lead to a mis-estimate for prevalence of strabismus.
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