Axial length is one of the most valuable parameters extending with emmetropization. The annual ocular axial elongation of children in lower grade (age 6 to 9) was between 0.21 mm [11] and 0.70 mm [12]. In children aged 6 to 12, the annual axial elongation was 0.36 mm [13]. Apart from genetic factors, activities such as outdoor durations, indoor studying and near work [14] had been shown to affect refractive state as well.
Light was essential for ocular growth, and its coorelations with outdoor activites, sleep duration and myopia were widely investigated. Lieberman et al [15] first hypothesized that natural outdoor illumination and artificial indoor lighting might suppress melatonin secretion, interfering with sleep. Abbott et al[16] also found that prolonged outdoor duration for young adults would raise the secretion of melatonin in the morning, associating with sleep disturbance and daytime fatigue. The interact between sleep and myopia could also found its molecular mechanism in regulating circadian rhythms. High intensity of light exposure might inhibit myopia by stimulating the ipRGCs (intrinsically photosensitive retinal ganglion cells) which have synoptic connections with dopaminergic amacrine cells[17]. Dopamine then modulates melanopsin mRNA as to modify retinal circadian rhythms[18]. Besides, prolonged indoor light exposure did not exhibit any effect on myopia progression [19]. Kearney et al[20] found that in myopic young adults, the concentration of melatonin was higher than that in non-myopes. Ayaki et al[21] discovered that children with high myopia were more inclined to have sleep problems. Liu et al[22] further claimed that it was the late bedtime that took precedence over sleep duration as a predictive factor toward myopia progression. It could be concluded that melatonin is more abundant in myopes, but its effects on sleep might vary during different age periods. Insufficiency or excess of light exposure meant elaborate modifications of ocular growth based on several mechanisms.
Excessive “screen time” was significantly correlated with sleep deprivation in preschooler[23], school-aged children[24], adolescents [25] or young adults [26].There were many studies ascribing myopia development to electronic devices usage or TV watching [27][28]. Using electronic devices or watching TV meant potent risk of eye overuse in near-distance, increasing accommodative spasm, or even lead to acute acquired comitant esotropia[29] in rare cases. We found that average axial length only correlated with TV duration and electronic devices usage in junior and senior high school students, and an average of 1 to 3 hours of outdoor duration per day was associated with a shorter axial length in primary school students. Guo et al[30] found that shorter time spent on outdoor activities and more time studying indoors were significantly correlated with a longer axial length in higher grade children (grade 4) instead of in lower grade children (grade 1). It could be inferred that ocular growth could be accelerated more prominently in elder children, and the outdoor durations might not be as effective a protective factor toward ocular length as other therapies such as orthokeratology[31][32] or atropine[33]. Apart from social factors, maternal educational degree and parental myopic background that implied genetic factors were also of importance for myopia or axial length prediction[34][35][36].
We noticed that only age was associated with ALD in primary school students, and other social as well as life habits were irrelevant in all the students. The harmfulness of anisometropia was well described in previous studies as for anisometropia and refractive error were the main amblyogenic factors toward children older than 3-year-old in China[37]. Kulp et al[38] found that in children aged 3 to 5, higher level of hyperopia was the risk factor for amblyopia and strabismus, which was also the case for studies conducted by Pascual et al[39]. The difference[40] in axial length between emmetropic eyes, myopic or hyperopic eyes were 0.80 mm and 0.44 mm, respectively. According to Patel et al[5], there was an average ALD of 1.57 mm(average 0.32 to 3.16 mm) in in children aged 7 to 8 who had anisometropic amblyopia(defined as the difference of spherical equivalent refraction > 3D). Hansen et al[41] also found that in amblyopic eyes(the difference of spherical equivalent refraction > 2D or axial length difference ≥ 1 mm), the mean axial length was 0.6 to 1 mm shorter than their counterparts. Limited by the cross-sectional nature of this study, we could not provide a precise conclusion on whether children with shorter or longer AL were more likely to have axial anisometropia, but the risk factors of anisometropia in different age groups should be underlined.
To date, researches that focused on axial anisometropia were listed in Table 4.
Table 4
Reports on refractive/axial anisometropia during childhood and adolescence
No.
|
Author, year
|
Age
|
Number
|
Highlights
|
1
|
Abrahamsson et al[42],1990
|
1 year until 4 years
|
310
|
Anisometropia was probably in a decline from infancy or was variable during emmetropization.
|
2
|
Tong et al[9],2006
|
7–9 years
|
1979
|
Anisometropia was correlated with bilateral axial length difference and was more prominent in myopic anisometropia
|
3
|
Chia et al[43],2009
|
9 years
|
543
|
No significant difference in spherical equivalent refraction and axial length between dominant eyes and nondominant eyes
|
4
|
Deng et al[44],2012
|
6-month
5 years, 12–15 years
|
1827
|
1.The prevalence of anisometropia increases in children aged 12 to 15
2.Anisometropia was more prominent in myopes and hyperopes.
|
5
|
Donoghue[6],2013
|
6–7 years
12–13 years
|
1050
|
1.Anisometropia was more common in children aged 12 to 13 with hyperopia ≥ + 2DS
2.Anisometropic eyes had greater axial length asymmetry than non-anisometropic eyes
|
6
|
Deng et al[45],2014
|
9.29 ± 1.30
(at baseline)
|
358(at baseline)
|
1.In children who had more axial elongation over 13 years of observation, axial differences between both eyes (aniso-AL) were also more prominent.
2.The amount of anisometropia at commencement did not affect myopia progression.
|
7
|
Hu[10],2016
|
10.0 ± 3.3 years
(4–18 years)
|
6025
|
1.Refractive anisometropia was associated with longer axial length and larger interocular difference in axial length
2.Myopic anisometropia was correlated with paternal education and more time indoors while hyperopic anisometropia did not correlate with eye care habits.
|
8
|
Palamar et al[46],2016
|
11.09 ± 5.27
(4 to 33 months)
|
42
|
Axial length and mean keratometry were the leading causes of hyperopic anisometropia and rendered a total of 2.82D/mm and 2.14 D/D of refractive difference.
|
9
|
Hansen et al[41],2019
|
11.7 ± 0.4 years
(10.5–12.8 years)
|
1335
|
Six children that were axial anisometropia (intraocular difference in axial lengths ≥ 1 mm) were all amblyopic.
|
10
|
Bach et al[47],2019
|
30.62 ± 18.04 months
(3 months to 7 years)
|
165
|
1.Mean AL: 21.37 ± 1.03 mm
2.The steepest increase of AL was present in 10 months of age
3.No statistical difference was found in axial length between both eyes.
|