When you wear corrective lenses in order to achieve clear vision, you have what’s called a refractive error. Refraction is a physics term that refers to how light is bent as it passes through a lens. When your eye doesn’t do a good job of bending and focusing light onto your retina-and thus allowing you to see-then there’s an error in the eye’s “refraction ability.” (Refraction also describes the technique doctors use to determine what your glasses prescription should be.) Refractive errors for the eye are myopia, hyperopia, astigmatism, and presbyopia.
Myopia means “nearsightedness.” A nearsighted eye can focus better on close objects than on distant objects. Nut this is a relative term, meaning that the degrees of nearsightedness vary considerably. Without corrective lenses, someone who is extremely nearsighted really only sees clearly at too close a distance to be functional, while people who are only mildly nearsighted can generally see well enough to perform most of their daily tasks.
The myopic eye is actually an eye that has too much focusing power. There are three possible reasons for this excess power: either the cornea has too much curvature, or the eye is too long from front to back, or the lens within the eye is focusing excessively. In all of these instances, light is focused in front of the retina (in other words, it undershoots its mark). Your prescription is designed to have the opposite effect, so that your corrective lens offsets the eye’s high power and allows the light to focus directly on the retina. Eyeglass prescriptions written for someone who’s nearsighted have a minus sign in front of the lens power. This indicates that the lens of the eyeglasses is effectively taking away the excess power of the eye. A nearsighted eye is not weak; instead, it’s too strong. According to the
According to the American Academy of Ophthalmology, “The link between myopia and glaucoma has been explored for nearly a century. A number of studies, including the Blue Mountains Eye Study and the Beijing Eye Study, have found that the risk of glaucoma appears to increase in persons with high myopia”. Glaucoma can be easily treated with a performed trabeculectomy by a skilled and experienced ophthalmologist. Seek out an eye doctor immediately, especially if you’ve experienced a sudden loss of vision.
Hyperopia is the technical term for farsightedness. As you might expect, a hyperopic eye is the opposite of a myopic eye in that it doesn’t have enough power to focus light precisely on the retina. Either the cornea or the lens doesn’t have enough curvature, or the eye is too short for light to be focused appropriately. The term accommodation refers to the way the eye muscle and lens work together to focus on something nearby-a book, for example, or a makeup mirror. Farsighted people use this same accommodative system to compensate for their lack of focusing power , in order to see at a distance.
When farsighted people try to read, however, they must focus for their hyperopia as well as for the reading distance-an effort that requires significantly more eye muscle power. For farsighted people, then, vision is relatively clearer at a long distance than up close, because it takes less muscular effort. For someone who is extremely farsighted, the world really is blurry at all distances. The hyperopic prescription is the opposite of the myopic prescription: there’s a plus sign before the lens power, indicating that the eyeglass or contact lens is adding more power to the eye.
Astigmatism may be one of the most misunderstood and misused terms in our field, we’ve heard patients use it to describe everything from lid twitches to floaters. Actually, astigmatism is a refractive problem in which an eye doesn’t focus light evenly.
A short lesson in physics may help illustrate this. Imagine that you have a tiny light bulb, the size of a pinpoint. This light sends rays out equally in all directions like a basketball-then this light, when focused onto a flat surface such as a wall, forms a circle, just like the round spot of light you create when you shine a flashlight at a wall.
A lens that has astigmatism isn’t curved evenly in all directions. Instead of a basketball, it’s like a football. In an eye that has astigmatism, usually the corneal curvature is greatest in one direction and least in the opposite direction. A football has less curvature across the ball than around it. Both directions-across the ball and around the ball-have some curvature, but it’s not evenly distributed, as it is on the basketball.
IF you focus a pinpoint of light through a football-shaped lens, the light gets focused more around the lens than across the lens, and it exits as a stretched-out circle oval. Because the eye’s variations in curvature are much more delicate, you really can’t see this football shape. Bu this subtle difference in curvature tends to blur images that we see, so that the world looks stretched out, just like that contorted circle of light.
If you have astigmatism, a good way to demonstrate this distortion is for you to look at the tail lights on a car in front of you at night without your glasses (while someone else is driving, of course). The red tail lights will send off streamers in the same direction as your astigmatism. These streamers will diminish when you put your glasses back on, but they won’t fully go away, because of some scattering of the light at the edges of your glasses.
In eyeglasses, an astigmatic prescription is designed to create the opposite of the eye’s curvature, so that light is focused evenly onto the retina. In an astigmatic prescription, the power is referred to as a cylinder power. The axis is specified to tell the optician at what angle to direct the cylinder power (either horizontally, vertically, or somewhere in between.
The word presbyopia is derived from Latin and translates literally-through rather unflatteringly to those of us who develop it-as “old eyes.” The term describes the phenomenon that eventually befalls everyone: the loss of our ability to focus up close. Presbyopia generally starts anywhere from age thirty-five to fifty. You may first notice it when you’re trying to read the label on a bottle of aspirin or the fine print of a magazine ad.
This is what happens: Remember the accommodative system, the muscle and lens system within the eye that enables us to focus on near objects? The ciliary muscles, part of this system, sit just behind the iris. When these muscles contract, they cause the lens to bulge forward and change the focus of the eye, so that we can see something up close.
Now, remember how the lens in our eye is always growing? Well, by the time we reach our mid-thirties, this constant thickening has taken its toll; the lens has lost the flexibility needed to adjust its shape. Consequently, we can’t accommodate-in other words. we can’t see close up-nearly as well as we used to. Over time, the closest point at which the eye can focus moves farther away-which is why one day we start holding the newspaper at arm’s length to read it. But our arms are only so long, and eventually they’re just not long enough to let us read without the help of corrective lenses.
Because presbyopia is part of the normal growth and development process of the lens, it isn’t really what we think of as an “age-related degenerative change” to our body. That is, the ciliary muscles aren’t weakening, and no tissue is breaking down. This ongoing evaluation of the lens is simply an inevitable part of aging, something we can’t change or control. There are no exercises or treatments that can stave off presbyopia, or even slow it down.
How will presbyopia affect you? It depends. If you’re mildly to moderately nearsighted, presbyopia means you can read comfortably without your glasses, because your myopia does the job of focusing for a near object in place of your ciliary muscle and lens system. Because you can read more easily without glasses, presbyopia might not even trouble you until your late forties or early fifties. If you’re farsighted, problems with presbyopia will likely occur much sooner; your ciliary muscles already have extra work just compensating for the hyperopia as well as focusing on objects up close. People with astigmatism often compensate by bringing reading material closer, which makes it relatively larger and easier to see. When presbyopia begins, you probably won’t be able to compensate as well for your astigmatism, and you might need a presbyopic prescription sooner.
The presbyopic prescription is described as the lens addition or the add, it’s usually the same in both eyes. That is, your distance prescription (if you need one) balances out your vision, so that you see as well as you can with either eye by itself and with both eye together. Because both eyes receive the same message from the brain to accommodate for up-close viewing, once your distance vision is balanced, you’ll probably need an even amount of lens power added to both eyes to help you see to read. You can think of the distance prescription as the basic foundation for an eyeglass correction. The reading prescription is added on to this to give the eyes the extra power they need to see up close.