July 17, 2022 | Keratoconus

The Significant Cross Linking Side Effects

By Barrett Eubanks, M.D.

The Significant Cross Linking Side Effects

Life would be great if no procedure had any side effects at all. But let’s come back to reality. Pretty much all procedures have some side effects (some much more than others). Corneal cross linking to treat keratoconus is no exception.

There are two main side effects from having corneal cross linking. One can expect some haze to develop within the cornea. Fortunately, this has very limited impact on vision. More impactful is that cross linking can change the prescription of the eye. This can make you need to update your glasses prescription.

So yes, corneal cross linking has side effects. However, overall in the big picture, these side effects are very minimal compared to the benefits of halting the progression of keratoconus or weakening of the cornea.


UV light works wonders for corneal cross linking - it is really the reason why the procedure exists. The UV light reacts with a special riboflavin eye drop. This reaction forms free radicals and leads to the development of strong bonds within the cornea. This strengthens the cornea and prevents the keratoconus from getting worse.

But you didn't think that we could blast the cornea with UV light without causing some extra side effects, did ya? A direct result of the UV light is the development of cornea haze.

The cornea is full of collagen fibers perfectly spaced and aligned to be transparent. This allows light to easily pass through. That’s very cool!

After corneal cross linking, these collagen fibers undergo some changes. One, they become cross linked together - bonded by the UV light (which is what we want), but in addition, the spacing of these fibers changes. This disrupts the perfect transparency of the cornea; light can’t pass through as easily as before - causing some haze.

But beyond just the change in the architecture of the cornea, the cornea has to undergo healing after being irradiated with UV light. UV light causes support cells in the cornea, called keratocytes, to die off. These cells are replaced by new ones in the months following cross linking. Until these new cells become situated, they are slightly less transparent than the old ones. Again, this leads to some extra haze.

Typical course

Gradually over the first month after corneal cross linking, this haze builds up. But between the first and third months, this haze plateaus out. We’ve reached the peak of haze during these months. Beyond the three month mark, this haze gradually disappears. By the end of the first year, it is almost completely gone from the eye.

The haze after corneal cross linking is temporary and peaks between the first and third months.

Those with steeper corneas or thinner corneas before cross linking (ie those with severe keratoconus) can be at a higher risk of developing haze after corneal cross linking. But those with severe keratoconus are also at risk of developing haze and scarring just from their keratoconus as it gets worse.

Fortunately, while this haze happens to almost everyone, it generally doesn't really matter. Yes, this haze does make the cornea a little bit less transparent. Light entering the eye will hit this haze and scatter a little bit. But despite that, it doesn’t have a tremendous effect on vision.

Change In Prescription

But here’s something that may have an effect on your vision:

A side effect of corneal cross linking is that the prescription that you need in your glasses or contact lenses to correct your vision may likely change.

When you were first diagnosed with keratoconus, you probably experienced a prescription that was continuing to get worse and worse. As the keratoconus progressed, you started to need a stronger prescription and astigmatism correction.

As keratoconus gets worse, the cornea changes shape into a cone shape and becomes steeper. And the cornea is responsible for about two-thirds of the focusing power of the eye. So as the cornea becomes steeper, light bends out of focus and you become more nearsighted. As the keratoconus gets worse, the cone shape creates more astigmatism in your vision.

Cross linking strengthens the cornea. And by doing so, it actually partially reverses the keratoconus process for some people. Now, don’t expect the keratoconus to go away; cross linking can’t fully reverse everything. But it can lead to some improvement.

A weak cornea bulges out and becomes steep. By making the cornea stronger, it bulges out less. This causes the cornea to become less steep. As a result, your prescription will change. How much are we talking about? Well, on average, one may experience an improvement in 1.00 diopter of prescription (ie improving from -4.00 to -3.00).

While having less prescription is good for improving overall vision, it can cause the short-term side effect of your glasses or contact lenses not working as well as before.

So what do you do? You get an updated pair of glasses or contact lenses.

But another problem…

The cornea still continues to change.

One or two months out after cross linking, you may still not have a stable prescription. Despite getting an updated pair of glasses or contact lenses, the prescription may still continue to change. Those new glasses you got may gradually work less and less over time (however, this continuing change is typically a small degree).

Even when you get to the one year mark, things may still continue to change. The cornea can continue to change a few YEARS out from corneal cross linking. But fortunately most of this change happens within the first year.

While overall, this is a good thing since your vision gets better as the prescription improves, its an annoying side effect in the short term since it makes it more challenging to get a proper glasses prescription.


Corneal cross linking often leads to the development of haze within the cornea. This haze gradually fades over time. Of bigger importance is that corneal cross linking may change the prescription of the eye and require frequent updates to glasses prescriptions over time.

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