We've all seen the startling numbers -- society as a whole is getting more and more myopic with each generation at a speed that can't be explained by genetics alone. About 1/3 of the US population is near sighted, an increase of 66% since the 1970s, and in Asia the number of school children that are near sighted may be as high as 90%. But with the World Health Organization now discussing a true myopia epidemic, scientists and researchers are struggling to understand in a very short time why our vision as a population is changing so rapidly. The real truth is that we can only theorize what is causing our eyes to change -- studies on chicks and monkeys about how the eyes develop in different environments give us some insight, but to understand the human eye we often have to rely on correlational study data that can prove cause, only relationships. The result is a number of theories for why myopia occurs and progresses:
Nearpoint Stress Model
The most studied and discussed theory behind why children develop myopia is that it is a result of visual stress from near work. Our eyes are evolutionarily made to excel at distance vision. Images from about 20 feet away would have perfect resolution to details as small as 8.87 mm in size in a "perfect" or normal eye. But today's world finds us even as very young children spending less time looking across the horizon, and more time looking at objects much closer. Think about a book, cell phone, tablet, computer, television. All of these items are much closer than what our eyes are theoretically best designed to see. In order to see things clearly up close, our eyes use the focusing system (which includes the eye muscles and the lens inside the eye, also called accommodation) to bring focus closer in. Our eyes are designed to be able to change this focus back and forth between distance and near until we age to the point that the system no longer works -- this is called presbyopia and it's why people can't see as well to read after age 40. But in a young child, the focusing system is flexible and should be able to transition back and forth to see both up close and far away. The near point stress model of myopia theorizes that if too much time is spent up close, or if for some reason up close work is more difficult than it should be, the eye will begin to change to reduce the amount of stress it has to overcome to focus clearly at near. When the eyes design themselves for easier, clearer reading vision that means they no longer focus light clearly far away -- this is myopia.
Here's the body chemistry behind the model: our body responds to stress by activating the sympathetic nervous system. You'll often hear this referred to as fight or flight. When our body is stressed by environment or emotion or fatigue, or just about anything difficult or stressful, the sympathetic nervous system takes over. This is the system that increases our heart rate, speeds up breathing, slows down digestion, and makes us more alert. With the eyes, an activation of the sympathetic nervous system means that your pupils dilate so that you can take in more light and be more aware of your surroundings. In order for your pupil to dilate, your ciliary muscle (the muscle around the iris that also controls our focusing system!) relaxes. As a side effect of this muscle relaxation, your focusing system relaxes. Your body has entered an alert mode that was meant to help it see threats from far away --but that means you've just diminished the action of those eye muscles that let you focus on things up close. So the theory here is that when you get stressed (you are learning to read and it's a hard word, you've been reading too long, you're tired), your body enters sympathetic/fight or flight stress mode and your body now makes it even harder for you to see what you are reading! The result of all this stress? Your body has two choices: Fight (which can lead to common binocular vision issues like convergence disorders, accommodative spasm, and ultimately even the prescription changes that result in myopia) or Flight (which is what some children do -- they avoid reading, and may even get incorrectly diagnosed with ADD).
Let's say your body decides to fight -- what's going on in the eyes when they are fighting to stay focused up close? When we are trying to focus on a near image but our accommodative system isn't able to handle the work to bring the image into focus, our eyes turn to the convergence system. Convergence is the system that brings the eyes together to focus on the same point in space. When we look at something up close, our eye muscles turn in (convergence), and shift focus closer to the object (accommodation). If you don't have enough accommodation to see clearly, then you draw more effort from the convergence system. If you are over-converging to make up for insufficient accommodation, then your eyes have what's called an esophoric posture. The problem? If you are overfocusing on a target, that means your eyes are much more likely to see double since they aren't positioned equally to where you are looking. How do the eyes try to prevent from seeing double? They try to pull in more accommodation. This back and forth balance between accommodation and convergence as they work to keep vision clear and single is called the AC/A ratio and one of the foundations of binocular vision system performance. The theory of near point stress implies that accommodation and convergence are constantly out of balance in the patient at risk, and the eye adjusts its shape in order to relieve some of this constant pull and tug by becoming myopic. Image can't focus clearly on the retina? Well let's make that eyeball longer so that when I look up close the image falls on the retina without any effort from this beat up focusing system. But if lengthening the eye is the body's way to solve the problem, why does the myopia keep getting worse? The reason is that the balance of accommodation and convergence is still messed up, so the eye has to keep lengthening to keep things in check.
In this theory, vision therapy may play a role in myopia control. If we can find early children at risk -- children that have accommodation or vergence issues that are just starting to express themselves, then we can help the child develop their focusing system skills to handle the amount of near work required to get through the day at school and home. If your doctor has talked to you about your child needing vision therapy or reading glasses or both, they may have mentioned something to the extent of "wearing glasses now may help keep them from needing glasses later." If so, they are citing this near point stress concept of myopia progression.
Theoretically if nearpoint stress was the sole cause of myopia development, then wouldn't wearing bifocals that reduce the accommodative demand for near work prevent myopia progression? Unfortunately this doesn't hold true in clinical research. Studies where children were prescribed single vision glasses compared to children prescribed progressive glasses (with a reading power in the bottom to reduce near accommodative demand) report no clinically significant reduction in how quickly myopia progressed. Undercorrecting myopia has also been disproven as a possible means to slow myopia creep. In fact, undercorrecting a child has been shown to make myopia progress faster than fully correcting the child's prescription! So if near point stress isn't the whole story, what else is causing myopia to develop in our children? In our next post we talk the other two theories of myopia development: sunlight exposure and peripheral defocus theory.
Nearpoint Stress Model
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| Kids are drawn to visual stimuli like lights and colors, and they tend to like to get very close! With the near point stress model of optometry, too much time or strain on the eyes this close may eventually result in us becoming myopic! It gives credence to listening to mom and dad about not sitting so close to that TV. via |
Here's the body chemistry behind the model: our body responds to stress by activating the sympathetic nervous system. You'll often hear this referred to as fight or flight. When our body is stressed by environment or emotion or fatigue, or just about anything difficult or stressful, the sympathetic nervous system takes over. This is the system that increases our heart rate, speeds up breathing, slows down digestion, and makes us more alert. With the eyes, an activation of the sympathetic nervous system means that your pupils dilate so that you can take in more light and be more aware of your surroundings. In order for your pupil to dilate, your ciliary muscle (the muscle around the iris that also controls our focusing system!) relaxes. As a side effect of this muscle relaxation, your focusing system relaxes. Your body has entered an alert mode that was meant to help it see threats from far away --but that means you've just diminished the action of those eye muscles that let you focus on things up close. So the theory here is that when you get stressed (you are learning to read and it's a hard word, you've been reading too long, you're tired), your body enters sympathetic/fight or flight stress mode and your body now makes it even harder for you to see what you are reading! The result of all this stress? Your body has two choices: Fight (which can lead to common binocular vision issues like convergence disorders, accommodative spasm, and ultimately even the prescription changes that result in myopia) or Flight (which is what some children do -- they avoid reading, and may even get incorrectly diagnosed with ADD).
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| Need a refresher? Accommodation is what each eye does to bring the image into focus clearly on the retina. Convergence is the movement of the eyes together to point both eyes at the right place in space for clear focus on the image. These two systems work in conjunction to keep vision clear and single. via |
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Theoretically if nearpoint stress was the sole cause of myopia development, then wouldn't wearing bifocals that reduce the accommodative demand for near work prevent myopia progression? Unfortunately this doesn't hold true in clinical research. Studies where children were prescribed single vision glasses compared to children prescribed progressive glasses (with a reading power in the bottom to reduce near accommodative demand) report no clinically significant reduction in how quickly myopia progressed. Undercorrecting myopia has also been disproven as a possible means to slow myopia creep. In fact, undercorrecting a child has been shown to make myopia progress faster than fully correcting the child's prescription! So if near point stress isn't the whole story, what else is causing myopia to develop in our children? In our next post we talk the other two theories of myopia development: sunlight exposure and peripheral defocus theory.
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