Does Cross Linking Improve Vision?
Frequently, by the time keratoconus is detected, there has already been a significant decline in vision. And worse more, it can be challenging to correct this vision with glasses and contact lenses. Fortunately there is a procedure called corneal cross linking which can prevent the keratoconus from getting worse. But can it improve the vision as well?
Vision can actually improve after corneal cross linking. Corneal cross linking can cause the cornea to become flatter. This can reduce the prescription of the eye, reduce distortions in the cornea and improve vision with and without glasses.
But while having better vision is great, it actually isn’t the REAL purpose of cross linking. The purpose of corneal cross linking is to prevent worsening over time. But let’s see how it also comes with an additional benefit of improving vision.
Let’s Remember The Primary Purpose
Corneal cross linking is performed for corneas that have become weak. This includes conditions such as keratoconus as well as abnormal weakening to the cornea after a lasik procedure (called post-lasik ectasia).
As the cornea becomes weak, it changes shape. Instead of having a dome shape, a cone shape starts to form. This cone shape increases the prescription and astigmatism of the eye. And when it becomes severe, this irregular cone shape also distorts vision causing loss of vision.
Corneal cross linking is designed to prevent this cone shape from getting worse. It actually isn’t designed to improve vision.
But a “side effect” of cross linking the cornea is that vision can actually improve in many cases. While it won’t reverse ALL the changes, it still can have some noticeable effects.
How Does Corneal Cross Linking Change The Cornea
What happens when you bind the structural framework of the cornea together? Well, the cornea gets stronger. That’s how cross linking works.
But what also happens?
Here’s an experiment:
Take a sheet of paper. With this sheet of paper, you can bend it very easy (without putting any creases into the paper).
Now take a thick piece of cardboard. Without putting any creases in the cardboard, try to bend it. Because of how much stronger the cardboard is than the sheet of paper, it will remain much flatter. It is much more resistant to bending.
If we were somehow able to turn the sheet of paper into cardboard, it would resist bending a lot more and remain flatter.
This is what happens in the cornea. By making the cornea stronger, it resists bending a lot more. This will make the cornea flatter.
Numbers don’t lie
The cornea is measured via optical measurements called diopters. A diopter simply measures the ability of something to bend light. Your eye glasses prescription is also measured in diopters.
A steeper cornea will bend light more and have a higher diopter measurement. The vast majority of normal corneas have values between 40 and 46.
With keratoconus, the cornea becomes much steeper and can get above 50 (and even much higher in severe cases).
On average, this number goes down after cross linking (because the cornea becomes flatter). Much of this happens within the first year, but over the long term, the cornea can become flatter by approximately 1 to 2 diopters. This can change / improve your vision.
Change In Vision
Because the cornea affects the prescription of the eye, this can change the prescription of the eye.
When the cornea becomes steeper over time, your prescription becomes more nearsighted. So in reverse, by making the cornea flatter, the prescription becomes less nearsighted. And on average, this can reduce the prescription by around 1.00 diopter.
Not everyone is average, however, and some can experience an even greater change. Approximately 30% of people may experience a much greater change of 2.00 diopters or more.
Vision Without Glasses
It makes sense that if the prescription of the eye gets lower, the vision without glasses or contact lenses (known as uncorrected vision) gets better as well. And this is true!
When looking at an eye chart without any correction in front of the eyes (such as glasses), on average, one can read anywhere between 1 to 3 smaller lines than they could before the procedure. By reducing the prescription of the eye, vision gets better. Note: once again these are averages; some people may notice a big improvement while others notice very little improvement.
But while great, reducing the prescription and improving vision without glasses may not necessarily be important in real life - since most people are going to continue to still wear glasses or contact lenses. But what about that vision? Also known as corrected vision.
Best Corrected Vision
Arguably more important is vision with glasses or contact lenses (since if you have a significant prescription, you aren’t going around very often without glasses). This is called the best corrected vision.
The best corrected vision can become blurry because of the irregular cone shape with keratoconus or ectasia. This irregular shape creates distortions called higher order aberrations which blur vision. These distortions can’t be corrected with glasses and normal soft contact lenses; as such, the best corrected vision becomes blurry.
For some keratoconus patients, this requires specialty contact lenses such as rigid glass permeable lenses (RGPs) or scleral lenses in order to fix. These lenses essentially mask these distortions to sharpen vision beyond what glasses and traditional contact lenses can provide.
After corneal cross linking, the flattening effect on the cornea can actually reduce the size of these distortions. This results in a reduction of the higher order aberrations blurring vision.
Because of this, your best corrected vision can improve after corneal cross linking! Close to half of all patients can notice this improvement. While it doesn’t improve as much as the uncorrected vision, on average corneal cross linking results in an improvement in a little over 1 line of smaller print on the eye chart of best corrected vision.
Can Vision Get Worse?
While overall corneal cross linking will improve vision, there are a few situations in which one may notice their vision getting slightly worse while healing from the procedure.
- Change in prescription. As mentioned before, the prescription of the eye actually can become less nearsighted after corneal cross linking. While this can be a good thing and can improve vision without glasses, this can be annoying since your glasses or contact lens prescription doesn’t change automatically. As your prescription changes, this can make it more difficult to see through your current pair of correction. And while the natural lens inside our eye can compensate for some of the changes, it is still possible to notice some more blurred vision.
- Haze after corneal cross linking. Following cross linking, the cornea has to go into recovery mode. During this stage, the cornea will develop some degree of haze. This haze increases over the course of the first 1 to 3 months following the procedure. While in the vast majority of cases, this case won’t have any major effect on vision, it is possible that this haze will cause things to be slightly more blurry.
- Progression despite treatment. While the success rate of corneal cross linking is high, it isn’t 100%. If the keratoconus or ectasia continues to get worse after the procedure, this can cause your vision to continue to change and get worse as well. See also How Good Is The Corneal Cross Linking Success Rate?
Beyond just stopping the progression of keratoconus, there can also be significant vision benefits to corneal cross linking. By flattening the cornea, corneal cross linking can change the prescription of the eye and reduce distortions or higher order aberrations. This can improve both the vision without glasses or contact lenses as well as the best corrected vision.
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