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Why does my child’s glasses prescription change so frequently?
- September 17th, 2010
Why does my child’s glasses prescription change so frequently?
To best answer this question, I first need to explain a little about nearsightedness, farsightedness, astigmatism, and a little about how our eyes see. The eye gathers light from the objects in our environment. First, the light passes through our cornea- the clear front surface of our eye. The pupil then dilates (gets bigger) or contracts (gets smaller) to allow enough light to enter through our eye. After the light passes through the pupil, it then passes through another clear structure called the lens. Finally, the light comes to a sharp focus on our retina. Our retina then sends signals to our brain through our optic nerve to interpret the focused light as an image of the object we’re looking at. The cornea and the lens are responsible for focusing the light directly on to the retina. If the power of the cornea and lens are too strong or too weak, the light does not come to a sharp focus on the retina resulting in blurry vision.
Myopia is the technical name for nearsightedness. It means that you have difficulty seeing things far away. In a nearsighted eye, the power of the cornea and lens are too strong. The sharp focus occurs in front of the retina. In other words, the eye is too long for its focusing power.
Hyperopia is the technical name for farsightedness. It means that the focusing power of the cornea and lens are too weak. The light is focused behind the retina—in effect, the eye is too small for its focusing power. In order to make the image clear the eye has to accommodate, or focus more. Children have a tremendous ability to accommodate their eyes, so they can often still see clearly if they are mildly farsighted. However, if the amount of farsightedness is too great then they will have a blurred image. It is this ability to accommodate our eyes that allows us to see clearly up close. Unfortunately, as we age we lose our ability to accommodate and have trouble seeing at near distances. This is why bifocals or reading glasses help—they do that focusing for us.
Astigmatism occurs when the cornea of the eye is shaped more like an egg than like a ball. This causes the light coming into the eye to focus in two places instead of one, resulting in a distorted image.
Glasses change where the light focuses. It makes the light come to one sharp focus directly on the retina. Glasses that correct for hyperopia have a “plus” power effectively increasing the power of the eye to make the light focus on the retina. Because a myopic eye is focusing the light too much, the glasses decrease that power and have a “minus” power.
If you think about a hyperopic eye being too short for where the light focuses and a nearsighted eye being too long for where the light focuses, you can better understand why children’s prescriptions change so frequently. Just as a child’s pants become too short and shoes become too small because of their growth, the eye changes as well often making the eye longer. This is why the hyperopic prescription can improve over time and the myopic prescription increases over time. Because children grow at such a rapid rate as compared to adults, their prescriptions change much more frequently.
Michelle L. Anderson, OD
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