Understanding Myopia: Prevalence, Risk Factors, and Classification 

13
Apr

Understanding Myopia: Prevalence, Risk Factors, and Classification 

According to the International Myopia Institute (IMI), myopia is defined both qualitatively and quantitatively. Qualitatively, myopia is a refractive error where light rays entering the eye focus in front of the retina rather than directly on it when ocular accommodation is relaxed. This condition typically results from an elongated eyeball but can also be caused by an excessively curved cornea or a lens with increased optical power. Quantitatively, myopia is characterized by a spherical equivalent refractive error of ≤ -0.50 D under relaxed accommodation conditions1. The global prevalence of myopia is rising at an alarming rate, with projections estimating that nearly 50% of the world’s population could be affected by 2050, and approximately 10% experiencing high myopia2.

In the Eastern Mediterranean Region (EMR), myopia prevalence is also of significant concern. A recent systematic review and meta-analysis reported that the overall pooled prevalence of myopia among school-age children in the EMR was 5.23%, with higher rates found among females (4.90%) compared to males (3.94%). The prevalence was also notably higher among older children aged 11–17 years (7.50%) compared to younger children aged 5–10 years (3.90%). These findings highlight the growing burden of myopia, particularly in older children and adolescents 3. Early intervention to slow myopia progression is vital to prevent long-term vision impairment in this region. This growing prevalence highlights the importance of early detection and intervention to prevent the progression of myopia and its associated risks.

Etiology/ Myopia Risk Factors:

Myopia development is influenced by both genetic and environmental factors, with their interaction playing a crucial role in its onset and progression.

 

  1. Genetic Factors:Genetic predisposition has been supported by familial and genome-wide association studies. Children with myopic parents have a significantly higher likelihood of developing myopia. Having two myopic parents increases the risk by 5.07 times, while having one myopic parent raises the risk by 2.08 times. However, it is important to consider that shared environmental conditions within families may also contribute to these trends 4.

 

  1. Environmental Factors:The rapid increase in myopia prevalence, particularly in certain regions, suggests that genetic factors alone cannot explain the trend, as genetic changes occur over long periods. Studies indicate that populations of the same ethnic background show varying prevalence rates depending on their environmental exposure 4.
  2. Education and Near Work:
    Higher education levels are correlated with increased myopia prevalence across different populations. Regions with less educational advancement tend to have lower myopia rates. Near work is believed to contribute to this correlation 4. A three-year cohort study involving 522 non-myopic high school students found that engaging in near work for 28 or more hours per week significantly increased the risk of myopia. Conversely, maintaining a reading or writing distance of at least 30 cm was associated with a reduced incidence of myopia 5. Similar findings from the Sydney Myopia Study indicate that prolonged reading sessions (>30 minutes) and close reading distances (<30 cm) increase the likelihood of myopia development 6& 7. While numerous studies support the link between near work and myopia progression, some have found no association. It is hypothesized that excessive near work induces accommodative lag, triggering ocular growth 5, 8. However, longitudinal research has yet to establish a definitive connection between accommodative lag and myopia progression4.
  3. Reduced time outdoors:

Evidence suggests that outdoor exposure plays a crucial role in myopia prevention. Increased time spent outdoors has been linked to a reduced risk of myopia onset. A study found that incorporating a daily 40-minute outdoor session into the school curriculum significantly reduced myopia incidence over three years 9. Some clinical trials have reported that spending 40 to 80 minutes outdoors daily decreases myopia incidence in children aged 6 to 114. However, while outdoor activity appears effective in preventing myopia onset, its impact on slowing progression remains inconclusive. A 23-year follow-up study suggested that engaging in outdoor activities for more than three hours per day may slow myopic progression. Thus, further studies are needed to support the possible inhibition of myopia progression due to outdoor activity 4.

  1. Quality of Light:

Light exposure quality significantly influences emmetropization. Both animal and human studies highlight the role of light intensity and wavelength composition in ocular growth regulation 10-12. Bright light (e.g., 15,000 lux) has been associated with myopia resistance, whereas dim lighting (e.g., 500 lux) is linked to myopia progression13&14.

Research indicates that light chromaticity plays a crucial role in regulating eye growth. Animal and human studies have shown that short-wavelength blue light may help inhibit myopia progression, whereas long-wavelength red light has been associated with increased ocular growth and axial elongation 15-20.

Contrastingly, recent studies in China have explored the use of low-level red light as a promising myopia control method for children. Findings indicate that administering red-light therapy significantly reduces myopia progression and axial elongation compared to conventional vision correction 21&22

These insights suggest that both blue and red light therapy hold potential as interventions for slowing myopia progression in children. Further research is essential to optimize treatment protocols and understand the long-term effects of light-based myopia control.

  1. Digital Screen Time:
    The role of digital screen exposure in myopia development remains debated. Some studies suggest that prolonged screen use increases the risk of myopia by four to eight times in Indian schoolchildren aged 5 to 15. Additionally, an association between screen time and higher myopia prevalence has been noted in Caucasian children. However, other studies have found no significant correlation. Given the inconsistent findings, further research is required to determine the precise relationship between digital device use and myopia progression 4.

Myopia Classification:

According to IMI, myopia can be classified as follows1:

  1.     Axial myopia: A myopic refractive state primarily resulting from a greater than normal axial length.
  2.     Refractive myopia: A myopic refractive state that can be attributed to changes in the structure or location of the image-forming structures of the eye, i.e., the cornea and lens.
  3.     Secondary myopia: A myopic refractive state for which a single, specific cause (e.g., drug, corneal disease, or systemic clinical syndrome) can be identified as not a recognized population risk factor for myopia development.

IMI Classification of myopia levels 1:

  1.     Low myopia: A condition in which the spherical equivalent refractive error of an eye is ≤ – 0.50 and > – 6.00 D when ocular accommodation is relaxed.
  2.     High myopia: A condition in which the spherical equivalent refractive error of an eye is ≤ -6.00 D when ocular accommodation is relaxed.
  3.     Pre-myopia: A refractive state of an eye of ≤ + 0.75 D and > – 0.50 D in children where a combination of baseline refraction, age, and other quantifiable risk factors provides a sufficient likelihood of the future development of myopia to merit preventative interventions.

 

 

References 

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  2.     Holden, B. A., Fricke, T. R., Wilson, D. A., Jong, M., Naidoo, K. S., Sankaridurg, P., Wong, T. Y., Naduvilath, T. J., & Resnikoff, S. (2016). Global Prevalence of Myopia and High Myopia and Temporal Trends from 2000 through 2050. Ophthalmology, 123(5), 1036–1042. https://doi.org/10.1016/j.ophtha.2016.01.006
  3. Alrasheed, H. S., & Alghamdi, W. (2024). Systematic review and meta-analysis of the prevalence of myopia among school-age children in the Eastern Mediterranean Region. East Mediterr Health J, 30(4), 312–322.https://doi.org/10.26719/2024.30.4.312
  4.     Martínez-Albert, N., Bueno-Gimeno, I., & Gené-Sampedro, A. (2023). Risk Factors for Myopia: A Review. Journal of Clinical Medicine, 12(18), Article 18. https://doi.org/10.3390/jcm12186062
  5.     Qi, L., Yao, L., Wang, X., Shi, J., Liu, Y., Wu, T., & Zou, Z. (2019). Risk Factors for Incident Myopia among Teenaged Students of the Experimental Class of the Air Force in China. Journal of Ophthalmology, 2019. https://doi.org/10.1155/2019/3096152
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  7.     Lin, Z., Vasudevan, B., Mao, G. Y., Ciuffreda, K. J., Jhanji, V., Li, X. X., Zhou, H. J., Wang, N. L., & Liang, Y. B. (2016). The influence of near work on myopic refractive change in urban students in Beijing: A three-year follow-up report. Graefe’s Archive for Clinical and Experimental Ophthalmology, 254(11), 2247–2255. https://doi.org/10.1007/s00417-016-3440-9
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  9.     He, M., Xiang, F., Zeng, Y., Mai, J., Chen, Q., Zhang, J., Smith, W., Rose, K., & Morgan, I. G. (2015). Effect of Time Spent Outdoors at School on the Development of Myopia Among Children in China: A Randomized Clinical Trial. JAMA, 314(11), 1142–1148. https://doi.org/10.1001/jama.2015.10803
  10.     Norton, T. T., & Siegwart, J. T. (2013). Light Levels, Refractive Development, and Myopia – a Speculative Review. Experimental Eye Research, 114, 48–57. https://doi.org/10.1016/j.exer.2013.05.004
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  18. Rucker, F. (2019). Monochromatic and white light and the regulation of eye growth. Experimental Eye Research, 184, 172–182.https://doi.org/10.1016/j.exer.2019.04.020
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  21. Jiang, Y., Zhu, Z., Tan, X., Kong, X., Zhong, H., Zhang, J., Xiong, R., Yuan, Y., Zeng, J., Morgan, I. G., & He, M. (2022). Effect of Repeated Low-Level Red-Light Therapy for Myopia Control in Children. Ophthalmology, 129(5), 509–519.https://doi.org/10.1016/j.ophtha.2021.11.023
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