Have you ever wondered why your vision changes from year-to-year or month-to-month or even day-to-day? The term “vision” includes a wide spectrum of visual skills, and for this article, we will concentrate on two inter-related aspects – eyesight and prescription. The references for this information are listed at the end of the article for your review.
According to research studies and clinical observation, the following can cause temporary or longer-lasting changes in a person’s visual acuity and/or prescription. Not everyone who experiences one or more of the following will experience a vision change, but vision changes have been observed in enough of these situations to be aware of its potential effects:
- Diurnal changes in corneal curvature
- Menstrual cycle
- Stress hormones
- Dry Eyes
- Increasing axial length
- Muscle spasms
The curve of the cornea changes during the day. Clinical studies show that a person’s cornea can change as much 0.83 diopters. This would effectively mean that how you see out of the same pair of glasses could go from 20/20 to approximately 20/40 just from normal daily changes in your corneal shape. To help you better understand how this works, think about LASIK. This refractive surgery procedure changes your prescription by permanently altering the shape of your cornea through lasers. If you flatten the cornea, then you would change the prescription a certain way. Alternatively, if you steepen the cornea, you would change the prescription another way. Similarly, normal changes in corneal shape during the day can alter your prescription and how you see.
Changes in the cornea have also been observed during the menstrual cycle. Clinical studies reveal that the curvature of the cornea steepens in the early phase while flattening during ovulation. The thickness of the cornea changes too during the menstrual cycle. The cornea thickens at ovulation and then thins out afterwards. Changes in curvature, as discussed earlier, and changes in thickness will affect your prescription and acuity. This is why some people may notice variations in acuity and prescription on a monthly basis even though they are wearing the same prescription.
Stress can affect many aspects of a person’s health including his/her vision. In response to stress, your body releases many different hormones including cortisols, catecholamines, vasopressin, gonadotropins and potentially more. A number of these hormones will affect the function and physiology of your eyes and visual system. That discussion would be too detailed for this forum, but this is what it effectively means: If you’ve been stressed out all day and then visit the doctor, your prescription may be different once your stress has subsided and vice versa.
Changes in blood sugar levels can cause changes in vision and prescription. Sugar can be trapped in the crystalline lens of the eye thereby causing it to swell. This swelling will change a person’s prescription. Alternatively, when a person’s sugar levels decrease, the crystalline lens will release the trapped sugars. The prescription again will change because the shape of the crystalline lens has changed. So, if you have diabetes, you can expect to occasionally experience changes in visual acuity and prescription as your blood sugar levels fluctuate.
Cataracts alter how light passes through the front of your eye to the back. This pathway change will affect your prescription in different ways depending on what type of cataract(s) that you have.
Medications can change a person’s prescription in one of three ways: 1) spasm of the muscle located towards the front of the eye called the ciliary muscle, 2) retention of fluid in the crystalline lens or 3) swelling of the ciliary processes towards the front of the eye. The list of medications that can cause changes to your prescription and visual acuity is too lengthy to list in this forum.
Dry eyes can cause changes in prescription and/or fluctuations in visual acuity because of this condition’s effect on the front surface of the eye. The cornea plays an important role in focusing light to the proper place in the back of the eye. As a matter of fact, the cornea accounts for 65-75 percent of the eye’s prescription. Consequently, changes to the cornea due to dry eye can have a profound effect on visual acuity and/or prescription.
Changes in prescription seen in pregnancy can be temporary or long-lasting. The hormones that are released during pregnancy can change corneal thickness, corneal curvature, how the crystalline lens transmits light to the back of the eye and how the front eye muscles focus light. Some women experience changes in myopia (nearsightedness), hyperopia (farsightedness) or astigmatism. There has even been a published case in which one woman experienced an increase in myopia during the first trimester only to experience a swing to hyperopia in her second and third trimester.
The length of the eyeball is known as axial length. Increasing axial length is a common reason for changes in prescription seen in children up to the age of twenty-two years. Clinical studies seem to indicate that multiple factors working in concert result in changes to axial length rather than just one component being the culprit. Every one millimeter change equates to a three diopter change in prescription.
Spasms of the ciliary muscle will change the shape of the crystalline lens. This change in shape will result in changes in prescription and visual acuity. These spasms can be temporary or longer-lasting in different individuals. Different medications as well as inefficient focusing visual systems can cause these spasms of the ciliary muscle. This can cause transient changes in how you see through your glasses or contacts, or it can cause your prescription to change from one doctor visit to another.
For more information, please contact Dr. Edward Fong and the Sugar Land, TX eye doctors and optometrists at Bright Eye Care & Vision Development.
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