Peripheral hyperopic defocus and the cause of myopia progression

Several myopia control treatments are designed to address peripheral hyperopic defocus. Learn more about what it is and how these treatments work to prevent myopia from worsening.

In recent years, there has been an alarming increase of people who have myopia or nearsightedness. Studies have shown that almost all teenagers and young adults in high-prevalence countries like China, Taiwan, or South Korea are nearsighted, and around half of the young adults in the United States and Europe have the same problem. 

It is estimated that by the end of this decade, 2.5 billion people will be affected, which is why it is important to find ways to stop this growing issue.

Myopia is characterized by an elongation of the eye. This causes light entering the eye to focus in front of the retina instead of directly on it, resulting in blurred distant vision. 

While several theories are proposed to explain why it occurs – in addition to its’ genetics or not enough outdoor light, one of the most important theories that a number of myopia control treatments are based on is the peripheral defocus theory. 

What is the peripheral defocus theory?

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The peripheral defocus theory suggests that in myopic eyes, the light rays enter the eye and fall behind the retina, instead of on the retina, in the periphery area. 

This leads to biochemical signals that tell our body to correct the issue by elongating the eye so the light ray can fall on or in front of the retina. 

And the more the eye tries to correct the peripheral defocus by growing the eye longer, the worse myopia becomes. 

In a few animal experiments, researchers were able to demonstrate the theory. They found that large changes in peripheral defocus could lead to eye growth in chicks and monkeys within a few hours. 

Furthermore, they also found that any level of defocus in these animals led to eye growth of the same length, which suggested myopia of any level would lead to an elongation of the eye.

Treatments that address peripheral defocus

Several glasses and contact lenses are designed to help control myopia, stopping its progression. And their effects can be explained by peripheral defocus.

1. Myopia control glasses

There are several myopia-control glasses available outside of the US. Regardless of the specific technology, the lenses used on the glasses provide clear central vision while having areas on the lens designed to create myopic defocus across the retina.

On the other hand, progressive lenses – commonly used for presbyopia – were prescribed by some eye doctors before the availability of the specifically designed myopia control glasses. 

These progressive lenses can also introduce myopic defocus due to the bifocal addition in the bottom of the glasses. And they were found to have an effect slowing down myopia progression.

2. Soft contact lenses

Soft multifocal myopia-control contact lenses are also designed to change how lights focus. In addition to having an area that helps the light focus on the retina centrally to correct the refractive errors, the area around is designed to refocus the light to the front of the peripheral retina, changing from peripheral hyperopic defocus to myopic defocus.

Interestingly some contact lenses that were not originally designed for myopia control could also introduce myopic defocus due to their multifocal design, similar to the progressive lens discussed earlier. 

As a result, these contact lenses were also prescribed by eye doctors when myopia-control contact lenses were not available or didn’t have the appropriate prescription power.

3. Orthokeratology (Ortho-K)

Orthokeratology (Ortho-K) is a non-surgical myopia control treatment that involves wearing specially-designed hard contact lenses overnight. It results in a temporary reshaping of the cornea, which produces an effect similar to wearing contact lenses.

This leads to the benefits of not needing to wear glasses or contact lenses during the day while still retaining vision correction and myopic defocus effects. And unlike progressive glasses, these effects are consistent regardless of where the person looks. 

Final thoughts

As mentioned in the beginning, the abnormal elongation of eyes in myopia progression is multi-factorial.

Studies on myopia-control glasses, contact lenses, or ortho-K showed good effectiveness in preventing myopia progression in children. However, the fact that there was still myopia progression in these studies demonstrated that there were also other factors contributing to the myopia's worsening.

For a child who may not respond well to a myopia treatment that works on peripheral defocus, consider switching or combining another intervention not based on the same principle. 

For myopia control to succeed, there are many factors at play. But with a good understanding, you can keep myopia at bay. 

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