This article is more than 5 years old. Information may no longer be current.
Novel cross-linking application for low myopia shows promise
The technique might offer an option for the many low myopes who want to be spectacle free but do not want surgery.
Click Here to Manage Email Alerts
Photorefractive intrastromal cross-linking, or PiXL, may be a promising option for patients with low-grade myopia, according to one surgeon.
“It fills in the gap between emmetropia and the amount of refractive error that we would start treating with laser or other more invasive procedures. There are many low myopes who would like to get rid of spectacles but are not considered good candidates for LASIK,” Anders Behndig, MD, PhD, told Ocular Surgery News.
This new application of corneal cross-linking, developed by Avedro, is performed with the epithelium on under topical anesthesia. Isotonic riboflavin is applied every 90 seconds for 10 minutes, and a circular 4-mm central zone of the cornea is irradiated with pulsed energy, 30 mW/cm2 for 16 minutes 40 seconds, resulting in a total ultraviolet A dose of 15 J/cm2. The idea is to use cross-linking to increase the stiffness and stability of the central part of the cornea, making the periphery bulge out slightly and obtaining an overall flattening effect.
Low-grade myopia study
Behndig discussed the preliminary data of an ongoing study of 39 eyes of 20 volunteers with low-grade myopia ranging between –0.5 D and –2.5 D and astigmatism less than 0.75 D.
“Our inclusion criteria are quite narrow, which makes sense because this method does not have the potential to treat higher grades of myopia in its current form,” he said.
He found plenty of volunteers for this study because Umeå, a university city in Sweden, is home to a lot of students, many with low myopia.
“We started out doing this treatment in ordinary room air but then moved to using high oxygen (94%) through an oxygen mask with the idea of increasing the effect of the treatment,” he said.
Oxygen, together with riboflavin and UV light, is known to be one of the key players in the cross-linking process. In this study, a higher oxygen level was related to significantly better results.
“There is a lot of discussion on the absorption of riboflavin in epi-on CXL, but it turns out that when you increase the oxygen, you increase the effect of the treatment,” Behndig said.
Of the 39 eyes included so far in the study, 12 were treated in room air and 27 in high oxygen. While the average correction obtained in the whole group was only 0.25 D, the high oxygen group achieved an average decrease of 1.25 D of spherical equivalent, with no sign of regression at 3 months.
“Myopia even decreases a little bit further between 1 and 3 months. The mean keratotomy value was also lower in the oxygen group, in correlation with the reduction of myopia. Endothelial cell count did not change significantly, showing that this method is safe for the endothelium,” Behndig said.
The majority of subjects treated with the high oxygen protocol are spectacle-free today, he said.
A downside of PiXL is the long time the procedure requires, about 1.5 hours, including the preparation of the patient.
“Of course this is a study design and we are doing measurements that may not be necessary in a clinical setting,” he said.
Another important downside is the variability of effects, even between the two eyes of the same patient.
“We have no clear explanation for this. It might be because the oxygen concentration varies with the head positioning, or it could be due to the individual response of the cornea, which explains the variations between individuals but not the difference between the right and left eye of the same patient,” Behndig said.
Further studies are needed to clarify this point, but Behndig is optimistic about the future of the PiXL method.
“It requires fine-tuning to increase the efficacy and reduce the variability. However, it is a promising new option for patients with low-grade myopia who want to be free of spectacles but don’t want surgery, a large population that had no options so far,” he said. – by Michela Cimberle
- For more information:
- Anders Behndig, MD, PhD, can be reached at the Department of Clinical Science, Ophthalmology, Umeå University Hospital, SE-901 85 Umeå, Sweden; email: anders.behndig@ophthal.umu.se.
Disclosure: Behndig reports he received funding from Avedro for his research but has no personal interests to disclose.