THERAPEUTIC LASERS OF THE CORNEA
By Dr Theunis Botha
28 March 2024
Introduction
Recent advances in excimer laser technology provide us with the most accurate technique for “cutting” of the human cornea and therefore changing the shape of the cornea. We often use the excimer laser for refractive purposes, but there are some less performed techniques for treating irregular corneal surfaces. Consider that a smooth glass surface will give pristine transmission of light with a definitive focus point; compared to sandblasted glass which blocks the transmission of light. In the same way, any irregularity of the cornea within the visual axis will have an impact on the transmission of light.
Options in Therapeutic Lasers
Before even looking at the actual laser, consider that we have laser application options onto the cornea. We can either go through the epithelial (TransPTK or Transepithelial Phototherapeutic Keratectomy profiles), as well as directly onto the stroma (traditional PTK or Phototherapeutic Keratectomy) after epithelial removal. Both techniques have their merits and indications. Both techniques change shape on a “global” corneal scale. We can also laser onto a masking fluid, the so called WetPTK. With this technique, energy is applied to the masking fluid that has been added to the stromal bed, and the energy is dispersed laterally instead of straight down into the stroma. Small “localized” vertical ridges are smoothed with this technique.
Any depth can be planned, from 1µm upward. It is not only the depth that can be manipulated, but also the profile shape which we laser. Profiles include:
Uniform. With this profile, a single depth is chosen across the entire profile, with a specific ablation zone size. Certain lasers can also position this specific profile where you would like onto the cornea. And again, certain lasers can change the shape to a circle, ring, or an oval shape of any orientation.
Corneal Wavefront Optimized. This profile is based on the mathematical expression of the wavefront measurement of the cornea.
Topoguided. This profile is based on the topographical expression of the front surface of the cornea.
Ocular Wavefront Optimized. This profile is based on the wavefront measurement of the whole eye.
Refractive. Traditional refractive profiles can also be used. Note that with therapeutics one can sometimes see very interesting refractive shifts that cannot be wholly calculated by the uninitiated prior to the laser.
Therapeutic laser techniques then combine the different profiles with the different corneal applications. The technique chosen is completely dependent on the pathology. For example, if the epithelium follows the “Reinstein’s rules of epithelial modulation”, as it often does in say RK (radial keratotomy) eyes, then a TransPTK technique is to be preferred because we have a natural occurring masking agent to work with that will give us a constant and scientific based improvement in surface regulation. If the epithelial modulation rules are not followed, for example in a case of basement membrane dystrophy, then one cannot use the TransPTK technique because one will induce stromal irregularity. A PTK approach is then to be preferred.
I almost always perform the WetPTK after any therapeutic laser, to smooth the bed as much as possible. This has two functions: one, to improve the stromal irregularity as much as possible, but two, to give the epithelium a smooth pathway to close quicker. This means, I do two laser runs on the same eye at the same setting. The so called “Dry Run”, where I apply the calculated profile that will give me global reduction in stromal irregularity. And the “Wet Run” that will smooth the stromal bed of small and localized irregularities. I plan the Dry Run on the eye (within the planning file) that I am operating on, and the Wet Run on the contralateral eye (within the same planning file) so that I do not have to switch to a new planning file in between the two runs on the same eye.
Examples of pathologies to perform Therapeutic Lasers on.
1. Irregularities
As mentioned, we can have global irregularities, or local irregularities. Or even local irregularities combined with global irregularities.
Case 1. The case to follow is an example of this scenario: local irregularities on global irregularities. This young dentist had LASIK performed, he then developed ectasia. Crosslinking was performed, after which he developed an infective corneal ulcer. The combination of these pathologies gave him a typical ulcer type cornea, on top of an ectatic cornea, with micro-folds of the LASIK flap. A simple TransPTK with a WetPTK was performed, and his vision improved from BCVA 0.3 to BCVA 1.5 immediately post-operative.
Figure 1. Microfold visible within Bowman's laser; seen with magnification of an OCT image.
Figure 2. Topographical changes before (above) and after (below) therapeutic laser.
Case 2. Radial Keratotomies (RK). Below is an impressive improvement of an example of corneal regularization after a single therapeutic laser. Note that this amount of improvement is spectacular, we normally see much less of a topographical change, but still usually RK eyes can correct to 1.0 after a therapeutic laser.
Figure 3. Topographical changes before (left) and after (right) therapeutic laser of an RK eye.
2. Opacities or Scars
Below is an example of a contact lens related corneal ulcer.
Figure 4. OCT sections before (above) and after (below) therapeutic laser.
Figure 5. Topographical changes before (left) and after (right) therapeutic laser of the above-mentioned ulcer case.
3. Recurrent Corneal Erosions.
PTK of the stromal bed, albeit 8µm, resolves recurrent corneal erosions in up to 95% of cases.
4. Corneal Dystrophies
Superficial corneal dystrophies can be lasered. Depending on the type, they sometimes recur. Below is a case of a Thiel Behnke Corneal Dystrophy. The epithelium was irregular in thickness and did not follow the modulation rules, therefore a simple 8µm PTK followed by a very short WetPTK was done on both eyes. BCVA improved from 0.4 to 1.5 within 5minutes of completion of the therapeutic laser. Note that prior to surgery, there is no visible opacity of the cornea (below and left), but the patient could only get to BCVA 0.4. This is a perfect example where micro-irregularities on the stromal surface (the hyaline deposits depicted in the magnified photo on the right below) is causing major drop in BCVA.
Figure 6. Progressive magnification from left to right of a case of Thiel Behnke Corneal Dystrophy.
Figure 7. OCT section before (top) and after (below) therapeutic laser of a case of Thiel Behnke Corneal Dystrophy.
5. Corneal Degenerations
There are a few techniques to remove calcific band keratopathy from the surface of the eye. Below is a case of an eye with silicon-oil in situ that a WetPTK as the main procedure was used to clear the calcifications.
Figure 8. Photos of an example of calcific band keratopathy removal. Before (left) and after (right).
6. Apical Syndrome.
This entity was originally described in corneas that had undergone hyperopic LASIK, and then developed epithelium breakdown at the apex of the ablation zone. We can think of this problem in the following way: snow does not stay on top of a steep rocky mountain, but rather builds up in the flat valley below. In the same way, epithelium struggles to maintain its conformity on a “sharp apex” of the stroma. We do not often see apical syndrome after large hyperopic ablations, purely because these size ablations are not often done. But, we often get recurrent corneal erosions after pathologies such as herpetic corneal ulcers for example. One must be mindful in the event of a recurrent erosion in such an eye, that it might not be the virus causing the breakdown of the epithelium, but rather a subset of apical syndrome. Below is an example of such a case. BCVA before therapeutic laser was CF (count fingers), and after 0.9.
Figure 9. The stromal apex is clearly visible in the above section. The patient developed recurrent corneal erosions for 25years on this apex. Below shows a smooth surface after therapeutic laser.
7. Keratoconus and Ectasias
There are now a few options in techniques when considering lasers in these cases. It falls outside of the scope of this article to discuss all of these developments. But, I will mention a single important point here. Considering LASIK ectasias; remember that the flap does not contribute much to the tensile strength of the cornea, only about 12%. This means, that one could potentially quite safely remove some of the irregularity of a post-LASIK ectasia with a TransPTK approach. Always consider adding a WetPTK to these cases, as most of them will have microfolds on the stromal flap.
Conclusion
Corneal surface irregularity is more detrimental to vision than corneal opacity. Consider the options mentioned on how one could potentially smooth that irregularity or opacity, with a fast, easy, safe and accurate procedure: the therapeutic corneal laser.