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Refractive Error

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The detection of nearsightedness (myopia), farsightedness (hyperopia) and/or astigmatism is found by refracting the eye.  Refractive status is influenced by vision development, adaptation to environmental stress, and hereditary factors, and most often is the result of your eyeball shape or the curve of the cornea (front clear part of the eye).  It affects how clearly you see or how hard you work to see clearly.

Remember that eye health can influence refractive testing. A thorough health check of the eye inside and out must be done to rule out pathology.

Signs/Symptoms of Problems with Refractive Status or Visual Acuity:

  • Blurred Vision
  • Headaches
  • Frequent eye rubbing
  • Frequent red eyes

Hyperopia - Farsightedness

Farsighted (hyperopic) individuals have more trouble seeing up close than in the distance, usually because the light will only come to a point "behind" the fovea. In order to make the image clear, a hyperopic person will use their focusing system even out in the distance in order to make objects clear. This allows them to move the image onto the fovea. People that don't have any farsightedness do not use any focus power to see in the distance clearly.

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Example: Let say a child has a refractive error of +2.00D but does not wear glasses. This means they use 2 units of focusing power in order to see clearly in the distance. However when you need to shift your focus to up close, like when reading a book (and for example sake, the book is held at about 16 inches), they must focus 2.5 units (Diopters) of focusing power that the book requires everyone to use, but additionally focus the 2 units of focusing power they already had to use in order to compensate for their refractive error in the distance. That is 4.5 units altogether, which puts a bigger strain on the visual system and can make it more difficult to work up close for activities such as reading with ease and comfort.

NOTE: Diopters (D) are the power units used by your eyecare professionals to measure your eyes. It's a hard concept to grasp (almost like a calorie!), but if you are a math person, the definition of a diopter is 1/meter.

What does this mean?

It means that kiddos with hyperopia work harder than kids without it. Now, they may be able to handle it for short periods of time (even long enough to do a quick acuity test up close!), but it takes a lot of energy and attention and they fatigue fast.

Children with vision related learning disabilities statistically have more hyperopia than those that are not. These children are not usually found on a traditional school vision screening, which only checks for distance visual acuity. Even if near visual acuity was checked, many would still pass since they only have to make it clear for such a short time.

FACT: 33% of kids from age 6 months to 6 years are hyperopic by +1.50D or greater.

Screening Tip: At the same distance that you test for distance visual acuity, if the child is able to see 20/30 or better, place a +1.50 lens (which you can get over the counter at any drugstore) over the eye(s). If the person can still view below 20/30 on the distance chart, they fail the test.


Myopia - Nearsightedness

Nearsighted (myopic) individuals have trouble seeing clearly out in the far distance because the light comes to a point before it hits the fovea in the back of the eye. The light will only hit the fovea on objects that are closer. Without glasses, most individuals that are nearsighted exert less energy up close because their eye is preset to see clearly at near distances.

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Example: If you have a child with a refractive error of -2.00D without wearing glasses, this means their point of focus is at about 50 cm (Again for the math people, 1meter/2D=50cm). This means they can see clearly up to this point, but beyond it is blurry. If that child is holding their book at 40 cm, which takes 2.50D to read, they only have 0.50D to crank in to see the book clearly.

What does this mean? Myopia is a great way to adapt to nearpoint stress, as far as the visual system goes. Kids that are nearsighted usually don't have to work as hard to see up close without their glasses unless they are so nearsighted that their clearest point is even closer than a comfortable working distance for reading.

FACT: 9.4% of kids from age 6 months to 6 years are myopic by -0.50D or greater. By age 6 to 18 years, myopia soars to 20.2% prevalency.

It has been pretty well established in the literature that the amount of nearsightedness increases, not only the prevalency, but the amount measured increases the longer a student remains in school. Nearsightedness is also more common in industrial nations because of the degree of nearwork required in our society.


Astigmatism

Astigmatism is a vision problem that causes objects to appear distorted and blurry and will affect visual acuity at all distances. It can be found in combination with hyperopia and myopia. Many individuals have a small amount of astigmatism that does not interfere with their ability to see 20/20.

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Astigmatism is mostly caused by an "oval" or oblong shaped cornea (the front clear cap of the eye) (though irregularly shaped lenses in the insight of the eye can also contribute). Instead of having one point of focus, as in nearsightedness or farsightedness, light will be greatly focused at two points, causing all points to be blurry or distorted.


Nearpoint Stress

Near point stress is literally stress on the visual system due to near work. Our visual system is ideally set for use at greater distances (like in the past when people had to hunt and gather and watch for bad things in the bushes) and not really designed to do a huge amount of detailed, near work. The need to look at near targets for long periods of time did not start happening until industrialism.

dIn today's world most of us spend a great deal of time working at close range, children and adults alike. Think about your average day; how much time do you spend reading, writing or working on a computer?

Add to this time many hobbies like playing an instrument where you must read music, sewing, tye flies for fishing, pleasure reading, computer games, internet surfing, handheld gaming and a host of others.

Increasing numbers of people are spending greater amounts of time working up close. All this time spent at near without allowing our eyes to relax the way they were meant to, often translates into overworking the visual system.

Nearpoint stress may display itself as generalized eye fatigue, headaches, transient blurred vision (distance or near), or poor ability to concentrate on tasks up close.

Often there are no consistent physical signs of near point stress, since people compensate in a variety of ways, so it is often overlooked.

An example of what is happening during times of near point stress might look something like this: Clench your fists and hold them there for several minutes then release your grip. How easy or difficult was it to straighten your fingers? Could you smoothly and quickly extend your fingers? The muscles in your focusing system are often "clenched" in this position for hours on end building in a kind of charlie horse or muscle spasm, but without the intense pain. In your eyes, we call this an accommodative spasm.

161When we finally give our eyes a break, they have a difficult time relaxing their focus. Our perception is that our distance vision is blurry.

Another theory is that your body (including your eyes) constantly reacts to its environment. If you are in a state of stress, you will either learn a method to adapt to it (fight), or you will avoid it (flight).

Myopes tend to elicit the fight response and hyperopes tend to elicit the flight response.

When the visual system gets overtaxed, people will either avoid near tasks or your body finds a way to adapt to the near work. One way it can adapt is to build in plus power (almost like natural reading glasses) to make it easier for you to see up close. This is great!...up close. When you look far away again, things tend to be blurry.

Instead of nearsightedness being only a distance problem, it can actually start out as a near problem!

The good news is if a doctor detects this shift while it is happening, they may be able to prevent nearsightedness with techniques such as stress relieving lenses. You wear lenses to make it easier for you to do things up close (not necessarily clearer up close) to keep your system in better balance so when you take them off, the distance stays clear. If the adaptation gets deeply embedded, however, it can become permanent.

Some people's visual system cannot handle these types of lenses. A complete eye examination by a behavioral optometrist will let you know if you or your child can benefit from stress relieving/performance lenses.

For information on finding a behavioral optometrist in your area, click here.

TIP: Some people that do experience nearpoint stress will continue to increase their prescription as they become more myopic by continuing to wear their glasses up close, putting them back into that stressful state. If your prescription is -2.75 or less, try not wearing your glasses for prolonged near work.

For more information on prescribing lenses and philosophies of care, click here.

Remember, not everyone can prevent myopia with stress relieving lenses alone.

There are many, many factors that can effect your eyes changing, including things like age, environment, nutrition and heredity. Ask your doctor about vision therapy, CRT contact lenses and other techniques that can arrest or prevent myopia.

For more information on risk factors, click here.

There is evidence that environmental factors play a much stronger role in vision than heredity.

There was a famous study done by Francis Young of Eskimo populations in Alaska. Even though older generations were farsighted, the first generation of children required to go to school had a 65% incidence of nearsightedness attributed to all the nearwork that was never part of their culture before.

What does this mean to you?

Practice good visual hygiene to minimize the effects of near point stress.

If you must sit and do a lot of nearwork, every 15 minutes give yourself 15-20 seconds to look in the distance, out the window or across the classroom or office at an object far away. It should ideally be a distance greater than 20 feet (looking across the cubicle doesn't count). Make sure the object is as clear as you can make it during these 15-20 seconds before resuming your work.

A good way to remind yourself when you are reading is to use a bookmark. Place the bookmark 5-10 pages in front of the place you are reading. When you get to the bookmark, stop and take your 15-20 second break, move the bookmark ahead again and then resume reading.

Children and adults alike should practice these techniques, teachers can call for "eye stretching" breaks during class time and parents can do the same when their children are studying or engaged in other near point activities.

Remember, the best thing to minimize near point stress is having an efficient and flexible visual system.

Do you know if you or your child has a good working visual system? Maybe it is time to find out. Find a behavioral optometrist in your area by clicking here or for more information contact the Parents Active in Vision Awareness (PAVE) group.

   

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