Improving Vision After CXL: PRK
There are several ways to improve your eyesight after cross-linking, and these include: special contact lenses, intrastromal rings and wavefront-guided PRK.
Importantly, none of these approaches slow or stop the progression of keratoconus – only cross-slinking can achieve this. Contact lenses, intrastromal rings and the excimer laser are more a means of improving visual quality.
The Goal of Laser Surgery
The goal of keratoconus laser surgery is to achieve better vision and visual acuity – in combination with lenses. “Vision” and “Visual acuity” do not mean the same thing. Visual acuity is purely a description of how small a letter patients can see on a vision test chart (it doesn’t matter if the letter is blurred or the patient sees three of them, the smallest sized letter the patient can read defines their score. “Visual acuity” says nothing about the quality of their vision, and the laser surgery is intended to reduce the irregularities in the cornea that cause these visual disturbances and make special contact lenses fit and work better.
Improvement of picture quality
There are two images below: the image on the left gives an example of good picture quality, and the arrow points to the 80% visual acuity line. The image on the right gives an example of poor visual quality, and again, the arrow points to the 80% visual acuity line. You can probably read both lines now, and the patient’s visual acuity would be 80% in both cases. However, you can see for yourself that the picture quality is completely different.
And this is the typical case of keratoconus: even if a certain visual acuity can be achieved, the quality can still be extremely poor. This is where the laser can help.
Same visual acuity, different image quality.
What can I expect?
In most cases, there is an improvement in visual acuity and image quality. Predicting outcomes in individual cases can be difficult – this type of laser is not a “simple” laser for treating regular refractive errors or astigmatism, but is instead a complex restorative procedure. Complications are very rare, but can include, for example, post-procedure infections.
Prerequisite: stable cornea
In a first step, a cross-linking is done in preparation. After healing, eyesight is unchanged compared to the condition before cross-linking. Six months after this surgery, we can then assess whether the cornea can be lasered or whether it should wait.
Laser and CXL – simultaneously or in sequentially?
We do the lasering only after cross-linking. The reason for this is that even very safe and successful cross-linking does not always work 100%: in 3 to 7% of cases, keratoconus progresses even after maximal cross-linking. In other cases, which are also very rare (0.5%), the cornea may flatten off strongly after cross-linking. Again, one is glad if not lasered at the same time.
Rather, it should be waited with the lasers until the cornea are stable.
Just a few years ago, the basic prerequisite for a precise lasering of a cornea after cross-linking was missing: it was not known exactly how much corneal tissue after a CXL is removed by an excimer laser pulse. Is it just as much tissue as in a cornea without cross-linking? Or 20% less? Or 40% more?
Not knowing how high the ablation rate is means an inaccurate result.
In a large study, our group has determined the ablation rate in corneas after CXL. The work was published in 2014 in the American Journal of Refractive Surgery. Since then, we know that in a cornea with CXL, 12% less tissue is removed per excimer laser pulse than in a cornea without CXL. This knowledge enables us to significantly increase the accuracy of the treatment.
In addition to the increase in visual acuity, it is above all the increase in image quality that represents a benefit for our patients. Unlike visual acuity, there is still no simple measure for the term “image quality” in ophthalmology.
Therefore, the increase in visual acuity is just one of the elements that improves. It is also important that an increase of more than 50% visual acuity can improve the three-dimensional vision (depth perception).
41-year-old man, 4 months after laser surgery
Increase vision with glasses from 30% to 60%.
35ähriger Mann, 1 Jahr nach Laserung
Anstieg der Sehkraft mit Brille von 20% auf 60%.
41ähriger Mann, 3 Monate nach Laserung
Anstieg der Sehkraft mit Brille von 60% auf 80%.
33ährige Frau, 2 Jahre nach Laserung
Anstieg der Sehkraft mit Brille von 50% auf 80%.
51ährige Frau, 2 Jahre nach Laserung
Anstieg der Sehkraft mit Brille von 50% auf 125%.
40ährige Frau, 1 Jahr nach Laserung
Anstieg der Sehkraft mit Brille von 40% auf 50%.
28ähriger Mann, 3 Monate nach Laserung
Anstieg der Sehkraft mit Brille von 40% auf 60%.