Identifying risks is a critical step in managing and preventing systemic and ocular disease, regardless of our method of clinical practice.
Proactively identifying critical risk factors allows for the development of strategies to reduce future risk, implement early treatments, and target those at higher risk.
This is why, before treating the progression of myopia, it is important to first understand and identify the risk factors that influence its development.
Myopia risk factors
Genetics
The likelihood of developing myopia increases with the number of myopic parents, and having very myopic (-6.00 diopters or higher) parents further increases the risk. While genetics have a part, it's important to note that genetics only account for 10 to 35% of an individual’s refractive error.1
What matters more is how these genes interact with environmental factors that contribute to the development and progression of myopia.
Ethnicity
To date, the majority of clinical investigations have focused on Asian populations to better understand myopia onset and treatment success.
While Asian people have the highest risk and rate of progression of any ethnicity, it is nevertheless critical that we monitor all individuals—including Caucasian, Black, and Hispanic/Latino patients—with the same criteria, since environmental factors are contributing to the rise in myopia rates across all ethnicities.
Environment
Influences from the environment can be further subdivided into time spent outdoors and on near work.
- Outdoor Time: Sunlight exposure can help prevent the development of myopia and continues to be supported in literature across the globe.2-6
- Spending 1 hour every day outside has been shown to postpone the onset of myopia in children; thus, we must emphasize its value in all children who have little to no daily outdoor exposure.6
- Near Work: The increase in near work in children additionally influences the onset and progression of myopia in school-aged children. Working at distances less than 20cm for more than 45 minutes may have a significant impact on myopia development.7
- It is critical that we educate parents on the importance of setting the appropriate Harmon distance for their child and taking periodic breaks.
- Forget the 20/20/20 Rule: Despite the catchy name, taking a 20-second break every 20 minutes has no effect on reducing near-demand stress.
- We must convey the significance of our newer understanding that a 10-minute break is recommended for every 60 minutes of near work.8
Binocular vision dysfunction
Assessment of binocularity and binocular vision is a key area of the examination that is often overlooked, but it is crucial to evaluate these during an assessment of myopia risk and its management. Binocular vision dysfunction results in defocus of the retinal image in the peripheral and central retina, stimulating and inducing axial elongation.9
Indicators of binocular vision dysfunction that we must be aware of include:
- Reduced accommodative amplitudes
- Higher accommodative lag
- Higher accommodative convergence to accommodation (AC/A) ratio
- High exophoria or esophoria at near
Furthermore, if not adequately evaluated and managed, binocular vision dysfunction can impact the efficacy of myopia management therapies.
Age
While age alone is not a predictor of myopia risk, it should be considered alongside refractive error and other known risk factors when determining the possibility of developing myopia.
The COMET study helped to highlight one of the most important reasons to intervene as early as possible when the risk of developing myopia is high—children diagnosed with myopia before the age of 10 are twice as likely to progress as children diagnosed at the age of 11 or later.10
Axial length
Measuring axial length with an optical biometer is considered the gold standard for assessing myopia risk and treatment efficacy. Because the risk of myopia associated with ocular pathology is significantly related to elongation of the eye, axial length must be assessed during examination.
Once collected, age-expected axial length values can then be referenced on the Tideman growth chart (Figure 1) to determine a child’s percentile for myopia risk relative to their gender and age.11
Additionally, axial length must continue to be monitored over time to establish a rate of growth. Normal emmetropic growth is estimated at 0.2mm per year; therefore, anything greater than 0.2mm per year is abnormal and increases the risk for myopia onset.11
Figure 1 highlights axial length growth charts (for men [left] and women [right]) showing axial length versus age and the associated risk of developing myopia and high myopia as an adult.11
Figure 1: Courtesy of Tideman et al.
Refractive error
Myopia is not just a diagnosis of refractive error. As optometrists, it’s important to be aware that there are different types of myopia. If we don’t understand that refractive myopia is different from axial myopia, then we might make the mistake of trying to treat all of these cases the same when only one of the three will respond to treatment.
Myopia may be classified as axial, refractive, or secondary:
- Axial Myopia: The development of myopia due to axial length elongation.
- Refractive Myopia: The development of myopia through increased refractive properties of the cornea and lens in the presence of normal axial length.
- Secondary Myopia: The development of myopia secondary to corneal pathology, such as keratoconus, or congenital and genetic mutations, such as Marfan or Stickler syndrome.
For the purposes of myopia management, we must concentrate on axial myopia because it offers the greatest risk for ocular pathology, and its impact on visual impairment can be reduced with treatment. Importantly, secondary myopia is a failure of emmetropization caused by a combination of variables that do not respond to myopia management.
Lastly, while myopia is no longer considered a simple diagnosis of refraction, it is clinically diagnosed at a refractive error of -0.50D or greater. A strong refractive error predictor of developing myopia is the presence of a lower-than-expected amount of hyperopia at a given age.
Therefore, we must be diligent in monitoring for pre-myopia, Table 1 highlights the refractive error criteria for pre-myopic children.12
| Age | Refractive Error |
|---|---|
| 0 to 6 years | ≤ +0.75D |
| 7 to 8 years | ≤ +0.50D |
| 9 to 10 years | ≤ +0.25D |
| 11 years | Emmetropia |
Table 1: Courtesy of Wolffsohn et al.
Conclusion
While there is no standardized questionnaire that quantifies all myopia risk factors, it is critical that we collect data on factors that may influence the onset of myopia in our patients.
We can best accomplish this by reviewing the child's case history, inquiring about time spent outdoors and on near work, and conducting a thorough examination.
We must understand all relevant risk factors so that we can implement lifestyle changes and treatment as soon as possible in order to reduce the impact that axial elongation can have on the patient's long-term ocular health.
