What's Happening to Our Vision? Peripheral Defocus and Myopia Progression7:00 AM
We're exploring the most talked about theory of why near sighted people keep getting more and more near sighted throughout childhood today -- Peripheral Defocus theory. If you want to read about other theories, check out the first post in this series discussing the Nearpoint Stress model. Keep in mind that none of these theories purports to stand alone -- we know that genetics plays a large role in why we become near sighted, but because the rates of myopia are increasing so rapidly worldwide (the US alone has seen an increase of myopia of 66% since the 1970s), there has to be environmental factors at work too.
Let's start from the true basics of what's going on in the eye, because if you try to read scientific journal articles about peripheral defocus it can get complicated really quickly. The key to understanding this theory is to have a god idea of how light focuses on the retinal photoreceptors in the back of the eye. Light enters through the pupil, is refracted by the lens, and is focused onto the retina to send a clear image to the brain. In emmetropes (people with "perfect" eyes or no prescription) light from a distant object will fall perfectly onto the retina without any need for the eye muscles to work and reshape the lens to change the eye's focus. In hyperopes (farsighted people), light would actually focus behind the retina, but the eye muscles move to reshape the lens to bring light onto the retina and into clear focus. That's why farsighted people tend to see very well in the distance unless they have a really high or unequal prescription -- their eye muscles are strong enough to bring things into focus and clear the world around them without any additional help. This muscle and lens action to bring light into focus on the retina is called accommodation. In myopes (near sighted people), light focuses in front of the retina. Accommodation can't shift this image backwards, so near sighted people can't see clearly at a distance no matter what they do.
|Diagramatic of Biofinity Multifocal soft contact lens design -- note the concentric rings of progressive addition around the more peripheral aspect of the lens! via|
|An orthokeratology lens via|