Topography-guided PRK, with or without the addition of corneal crosslinking, can improve visual acuity and quality in patients with keratoconus.
There are many conditions that can cause irregular corneal surfaces, one of which is keratoconus. Surgeons looking for new approaches to treat keratoconus and improve patients’ vision should consider topography-guided treatment in combination with crosslinking. This is especially, but not exclusively, effective in the early stages of the condition, where crosslinking might be used to halt its progression before the corneal refractive procedure is applied.
It should be noted that treating patients with keratoconus using laser-vision correction is an off-label procedure that should be approached with caution and only after taking into consideration many factors, discussed as follows.
If a patient is older than 40 years, I do not primarily offer crosslinking because nature itself performs crosslinking once a person reaches a certain age.1,2 Such patients can undergo photorefractive keratectomy (PRK) or topography-guided PRK without crosslinking to achieve stable results without any progression of keratoconus if they are followed up for 12 months with stable topography.1
The refraction will not be fully altered down to zero because the regularity must come before the refraction. In keratoconus patients the refraction is not usually the main problem.
If the cornea is too thin or too steep, for example with a reading of 65 K, topography-guided treatment for the keratoconus procedure is not effective. To be a suitable candidate, a patient must have a calculated corneal thickness of at least 400 µm at the thinnest point after the topography-guided treatment.
For example, if a patient has a corneal thickness of 470 µm and receives the treatment for 70 µm, that leaves another 400µm of residual tissue following the laser treatment. I then feel comfortable with the procedure, safe in the knowledge the patient will be okay postoperatively.
Stage of keratoconus
Typically, the early stages of keratoconus are in younger patients and the condition is progressive until the age of 30.2 For patients aged between 25 and 30 years, I do a combination topography-guided session, which means I perform a topography-guided treatment followed by crosslinking immediately afterwards.
Patients aged over 40 years can start to experience a decrease in vision because they no longer tolerate contact lenses. In these patients, I am happy to perform topography-guided treatment without crosslinking. However, all these patients must show up for regular follow-up every 6 months and I would perform crosslinking in cases of progression.
A high number of patients with keratoconus also suffer from atopic disease and do not tolerate rigid gas-permeable contact lenses.
Laser vision correction is not an ideal approach for treating keratoconus but might provide an alternative to corneal transplantation. Vision in patients with corneal transplants is limited to irregular astigmatism and they have a lifetime risk for corneal graft rejection in cases of penetrating transplantations.
The main reason I screen patients for topography-guided treatment to treat keratoconus is because they are unhappy with their glasses or do not tolerate their contact lenses. First, I consider the patient’s age. If the patient is young and does not achieve good vision with glasses, I recommend topography-guided treatment in combination with crosslinking.
If the patient is 40 years or older, then I consider corneal thickness, corneal steepening and how happy or unhappy the patient is with his or her visual acuity. If the visual acuity is better than 20/30, I do nothing.
If the visual acuity is less than 20/40, then I move forward with the procedure. The K reading must be 60 D or less and the corneal thickness must be at least 450 µm.
An adult refugee from Iran presented to me with keratoconus. He was referred to me by chance because I am of Iranian origin and can speak Farsi.
He had undergone crosslinking to stop the progression of keratoconus, in Iran, at the age of 32. His preoperative visual acuity was only 20/100 (Figure 1). His vision could not be increased with refraction and he was unable to tolerate contact lenses.
He shared that he could not read and was having trouble learning the German language because the courses he was undertaking were in German and he was unable to see anything. He was the first patient I treated with topography-guided PRK because he had been crosslinked 4 years earlier.
Three months postoperatively, his vision had increased from 20/100 to 20/25 unaided. The difference map on the Pentacam (Oculus) was identical to the calculated ablation pattern (Figure 1). Four months later, I performed the same procedure on his second eye.
For surgeons who are new to topography-guided treatment, I recommend starting with the ‘topo smooth’ software of the CRS-Master (Carl Zeiss Meditec). This has the advantage that you do not need to perform any calculations. It is just ‘one click’.
If the patient is highly myopic, one can just reduce the sphere by 1 to 2 D but should not add astigmatism to the refraction because this can result in a refractive surprise. The optical zone should not be too big because the laser increases the treatment zone up to 9 mm including the transition zone. The optical zone should be 6 to no more than 6.5 mm, depending on how much tissue ablation is needed.
I also use a treatment-planning device (OCT Zeiss Cirrus 5000, Carl Zeiss Meditec) for epithelial mapping to assess how thick the corneal epithelium is. I can receive a calculation for the ablation and add the tissue.
This means if the patient has an epithelial thickness of 45 µm, I can add 45 µm in the calculation. Then I can perform a trans topography-guided PRK with the Mel 90 Excimer laser in one step, which makes it all easier.
All topography-guided PRKs in patients with keratoconus must be performed as transepithelial procedures. One should also never forget to apply 0.02% mitomycin to the treated area for 60–80 seconds to inhibit the development of corneal haze.
Topography-guided PRK can be a reasonable procedure, with or without the addition of corneal crosslinking, to increase visual acuity and quality. The procedure is easy to calculate and to perform.
Navid Ardjomand, MD
Prof. Navid Ardjomand is a consultant ophthalmic surgeon specialising in cornea, cataract and refractive surgery, and an associate professor at the Medical University Graz. He is a consultant for Carl Zeiss Meditec.