What's Happening To Our Vision? A Working Theory of Myopia Progression Part 17:00 AM
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
|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).
|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|
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.