Nearsightedness, Farsightedness & Astigmatism What's the Difference
Focus in on a common vision problem
This month I'd like to spend a little time explaining the most common vision problems experienced by all people, including those of us with diabetes. When a person's vision is blurry, much more often than not it is due to some focusing problem (also known as "refractive error") caused by the optical components of the eyes and, fortunately, we can almost always correct such optical imperfections with glasses, contact lenses and/or refractive procedures (e.g. LASIK eye surgery, surgically implanted corrective lenses, and non-surgical corneal molding). But what exactly are we correcting?
To understand different kinds of refractive errors, it will help to know what happens with an optically "normal" eye. Light rays emitted from or reflected off of objects in our environment are precisely focused ("refracted," or bent) by the cornea (clear windshield at the front of the eye) and internal lens (located within the eye behind the colored iris) onto the light sensitive retina at the back of the eye to form a clear image that our brains can interpret (see figure 1). If the eyeball is not exactly the correct length in relation to the curvature of the cornea and lens, the image formed on the retina will be defocused, resulting in blurred vision (see figures 2, 3, & 4).
Nearsightedness (myopia) occurs when the cornea or internal lens is too curved, and/or when the eyeball itself grows too long. This causes light entering the eye from distant objects to be bent too much and focused in front of the light sensitive retina, causing blurred distance vision (the distance at which objects are blurry depends on the severity of the nearsightedness). This optical imperfection can be corrected with use of lenses that refocus light precisely on the retina or by changing (flattening) the curvature of the cornea to do the same thing.
Farsightedness (hyperopia) occurs when the cornea or internal lens is not curved enough, and/or when the eyeball has not grown long enough. This prevents light entering the eye from being bent enough to focus on the retina without using internal focusing muscles that flex the elastic internal lens to increase its curvature. Small degrees of farsightedness usually have little impact on vision until people are around age 40, when the internal lens loses its elasticity (see my previous article on "The Fine Print") and both distant and especially close objects become blurry. Higher amounts of farsightedness are more difficult to compensate for, and make it very difficult to achieve clear vision at any distance, but especially when trying to read at close range; this is one reason why farsightedness is associated with difficulty learning to read and poor school performance. This optical problem is corrected with lenses or by changing the shape (increasing the curvature) of the cornea.
Astigmatism occurs when either the cornea or the lens are out of round, analogous to the shape of an egg, the back of a spoon, or a football. This means that their curvatures change like hours on a clock dial with various directions, resulting in some light rays being focused on the retina while others are not and causing objects at all distances to be less than perfectly focused. The overwhelming majority of all humans have at least some small degree of astigmatism, but when the amount increases, symptoms of blur (most often, confusing similarly shaped letters like Bs and Es, or Os, Cs, Ds and Gs), eyestrain, and headache result. Astigmatism is corrected with lenses that compensate for the eye's changing curvature, by making the cornea more round (with rigid contact lenses, or various laser or surgical techniques), or (rarely) by removing an astigmatic internal lens. Astigmatism often occurs in combination with either nearsightedness or farsightedness.
It is important to know that large changes in blood sugar levels can quickly change the optical components of the eye, particularly the internal lens, resulting in often dramatic changes in refractive error and a person's prescription over a matter of weeks, days, and sometimes hours. This is why eye doctors often detect previously undiagnosed cases of diabetes, and why they should always ask patients about their blood glucose control before recommending treatment. It's also important to remember, I think, that blurred vision in a person with diabetes does not necessarily mean that he or she has eye disease; the reason the vast majority of patients with blurred vision and diabetes can't see well is the same reason other people can't see well – uncorrected refractive error. The only way to make this critical distinction, however, is with regular and thorough dilated eye examinations.
For more information on diabetic eye disease, consult Dr. Chous' book Diabetic Eye Disease: Lessons From a Diabetic Eye Doctor, Fairwood Press, Seattle, 2003.
Read more about Dr. Chous here.
Visit Dr. Chous' website here.
NOTE: The information is not intended to be a replacement or substitute for consultation with a qualified medical professional or for professional medical advice related to diabetes or another medical condition. Please contact your physician or medical professional with any questions and concerns about your medical condition.
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