I am often asked “why did my prescription change” from last year? There are many, many reasons for this, but most of the time it is one of a few things that can cause the prescription to change. I’m going to spend a few pages describing first the optics of the eye and how changes in each structure in the eye can cause a change in Rx.
Patients who are nearsighted will have the image of distant objects come to focus in front of the retina. We use eyeglasses, contact lenses or refocusing techniques (surgical and non-surgical) to move the focal point back to the retina. The image below shows where images come to focus in a nearsighted eye.
When a patient is farsighted, images are focused such that they would come to focus behind the eye. While the light obviously doesn’t pass through the eye, the virtual image is focused that way. As with nearsightedness, we use glasses, contact lenses and other techniques to move the focal point “forwards” so that it comes to focus on the retina.
When a patient has “no Rx” for distance vision, all of the light that enters the eye is focused on the retina, resulting in a clear image. Very few patients truly have “perfect” focus of this image, but we humans are pretty tolerant and can put up with and/or not be bothered by a small amount of defocus. The picture below shows an eye with the light focusing on the retina.
You may notice that first surface that light entering the eye comes in contact with is the cornea. The cornea bends light and starts the focusing process. Changes in the cornea can cause the Rx to change. Surgical techniques such as LASIK either increase or decrease the degree of corneal curvature. Non-surgical techniques such as Vision Shaping Treatment (VST) do the same thing, working to reduce nearsightedness and astigmatism and creating a clear focus on the retina.
Some patients have a very curved corneal surface, this usually results in myopia. Other patients have a very flat cornea, these patients are usually farsighted. Patients who wear contact lenses may experience a warpage or more regular but unintended change in corneal curvature. While frustrating, these undesired changes in corneal curvature can usually be reversed.
It doesn’t take much of a change in curvature to result in a noticeable change in vision, .1mm is enough for a very noticeable change in vision. As we move through life, our cornea usually maintains a consistent curvature, but can undergo changes which will result in a change in prescription.
So, the first answer to the “Big Question” is changes in the corneal curvature, either natural, intended or unintended.