Why Did My Prescription Change – Part 3

If the cornea and lens don’t change, but the eye becomes longer, a patient will become nearsighted since the image that previously focused on the retina will now come to focus in the same place, but now in front of the retina.

 

 

 

 

 

 

 

This is the most common cause of nearsightedness in younger patients.  During periods of long bone growth, the eye will grow longer also.  As with the cornea, a very small change in the length of the eye will result in a noticeable shift into nearsightedness.  This is quite common in boys and girls, starting around age 10 and ending around age 16 for girls and 20 for boys (like we’ve been told, we males mature later than the ladies do and your eyes are no exception).

Until the patient is done growing, it is normal for the amount of nearsightedness to increase every six to twelve months.  There have been several studies showing that Vision Shaping Treatment may slow down the development of nearsightedness, but the jury is still out to some degree on this cause and effect relationship.

Dr. Warren

www.warreneyecarecenter.com

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Why Did My Prescription Change – Part 2

The Lens

The lens is located just behind the iris (the colored portion of the eye) and can be seen through the pupil.  When we change focus from distant to near objects, it is the lens that changes to create the change in focus.

Unlike the cornea which stays pretty consistent throughout our adult lives, the lens continues to grow in size.  A cataract is a loss of clarity in the lens and happens to all of us that live long enough.

As we age, our ability to change the shape and focal power of the lens diminishes (the exact cause of this is still somewhat debated, but believe this 45 year old optometrist, it happens!).

As the lens grows the focal power of the lens will change slightly, but it takes a much larger change in the lens size to create a noticeable change in prescription.  These type of changes don’t usually happen until later in life (after 60 year or age or older) and they typically cause a change in prescription toward nearsightedness or a reduction in farsightedness.  This type of change typically accompanies the development of cataracts which can ultimately result in poor vision and require a surgical removal of the eyes natural lens and replacing its optical power with an ocular implant.

Another cause of a change in the lenses optical power is Diabetes.  In Diabetes, the body’s fluid levels can become quite out of balance, resulting in swelling or shrinking of the lens.  This can result in a rather large swing in refractive power, up to 5.00 Diopters of change in a period of a few days.  Once the blood sugar levels are normalized, vision will usually stabilize, but the final prescription may end up being quite a bit different than before the blood sugar levels were altered.

So answer #2 the “Big Question” is changes in the optical power of the eyes natural lens.

Why Did My Prescription Change – Part 1

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.

Dr. Warren

www.warreneyecarecenter.com

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