Why Does My Eye Prescription Change?
High blood glucose can get trapped behind lens.
By A.Paul Chous, MA, OD, FAAO
Quite frequently, diabetes will be the reason for a contact lens or eye prescription change as a patient's blood glucose levels rise and fall, sometimes over a matter of weeks, days, or even hours. In fact, both poor blood sugar control and undiagnosed diabetes are notorious for causing this phenomenon.
Elevated blood glucose enters the eye via the aqueous humor, the clear liquid nourishing the internal eye (see Figure 1). Aqueous humor is essentially ultra-filtered blood and contains glucose in direct proportion to levels in our bloodstream. Once the aqueous humor becomes laden with glucose, it enters the eye's internal lens where it is converted into sorbitol, a form of modified glucose (known as a sugar alcohol) that becomes trapped within and draws fluid into the lens, much like a wrung out sponge that is placed into a bucket of water. This, in turn, causes lens swelling that can significantly and more or less rapidly alter a person's prescription: A person who is nearsighted his whole life may become farsighted; a farsighted person may become nearsighted; a patient with no prescription may suddenly find that she cannot see well without glasses. Which ‘direction' the prescription goes depends on what anatomic part of the lens becomes swollen. Blurry vision results almost immediately, but is often correctable with an eye prescription change.
If this process goes on for too long, it may disorganize lens proteins in such a way as to result in opacity, what is commonly referred to as a cataract. The lens consists of water, collagen fibers that are precisely arranged in a way that renders the entire lens transparent, and specific lens proteins that maintain this precise arrangement of collagen fibers. When sorbitol disrupts these lens proteins, thereby ‘deranging' (disorganizing) the lens fibers, a so-called ‘diabetic cataract' results (see Figure 2)
The most important thing for you and your eye doctor to remember about such fluctuations in your prescription is that they are a sign of poorly controlled blood sugar levels. Getting a new pair of glasses or contact lenses when you are chronically hyperglycemic usually means that the prescription will not work well when your blood sugars are normal – a recipe for disaster. Bringing your blood glucoses into a more normal range invariably stabilizes such a dramatic eye prescription change and will help prevent cataract formation. My advice is to make sure your hemoglobin A-1-c (HbA1c) is under 7% and that your at-home blood glucose readings fluctuate by less than 100 mg/dl within any given day before getting a new lens prescription. As an extra precaution, it's also a good idea to test your blood glucose during or immediately before your appointment with the optometrist or ophthalmologist. Finally, make sure that your eye doctor examines the inside of your eyes through dilated pupils to check for diabetes-related eye diseases at least once a year.
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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