Wavefront-guided PRK after CXL for keratoconus

July 14, 2016

Article

Technique targets the main cause of vision degradation and a large clinical series finds positive results.

Alexandria, Egypt-Wavefront-guided photorefractive keratectomy (WFG PRK) performed using a proprietary high-definition aberrometer (iDesign, Abbott Medical Optics) and excimer laser system (VISX STAR S4 IR, Abbott Medical Optics) appears to be a safe and effective option for improving vision in eyes with keratoconus that achieve stability after corneal collagen crosslinking (CXL), according to the experience of Mohamed Shafik, MD, PhD.

Results from analyses of data collected in a consecutive series of 34 eyes of 25 patients followed for up to 12 months post-WFG PRK were reported in a paper published online in Cornea on May 17, 2016. The outcomes show statistically significant improvements in uncorrected and corrected distance visual acuity (UCDVA and CDVA) corresponding to statistically significant reductions in mean manifest sphere and cylinder. Statistically significant improvements were also achieved in some topographic corneal irregularity indices and in higher order aberrations (HOA), including total HOA, primary coma, and trefoil.

No eyes lost CDVA or developed significant haze-the surgical protocol included mitomycin-C 0.02% applied to the ablated tissue for 20 seconds-and there was no evidence of keratoconus progression after WFG PRK.

Dr Shafik noted that the high-definition aberrometer has made WFG PRK feasible in eyes with keratoconus because the device can capture an accurate wavefront in highly aberrated corneas and thereby allows a reliable ablation profile. And, he told Ophthalmology Times Europe that he considers the WGF ablation superior to a topography-guided approach for this indication. In addition, he believes that there is an advantage for performing surface ablation sequentially rather than simultaneously with CXL. Now, further studies are needed to validate the theories underlying these ideas, said Dr Shafik, Professor of Ophthalmology, University of Alexandria, and medical director, Horus Vision Center, Alexandria, Egypt.

“Excimer laser surgery to correct the refractive error in eyes with keratoconus treated by CXL has been controversial because of concerns that it would further compromise biomechanical stability. However, surface ablation has less effect on corneal biomechanics than LASIK, and the eyes in my series have maintained good stability so far,” he stated.

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“In addition, the functional and refractive outcomes achieved with WFG PRK in eyes with keratoconus stabilised by CXL compare favourably with those reported in the literature for topography-guided PRK performed in combination with CXL or as a second procedure. Nevertheless, longer follow-up in more eyes is needed to confirm the safety of WFG PRK after CXL and comparative studies are needed to evaluate outcomes using different ablation protocols.”

All eyes in the consecutive series had grade I or II keratoconus (Amsler-Krumeich classification) and had undergone CXL at least 1 year earlier. In addition, they had to meet the following criteria: <0.5 D variation on subjective refraction at three consecutive monthly visits; <0.75 D increase in the cone apex keratometry in the previous 6 months; manifest spherical equivalent refraction ≤6 D; logMAR CDVA 0.5 or better; clear cornea within the pupillary area; and thinnest pachymetry ≥400 microns. Presence of dry eye syndrome or history of herpetic eye disease, autoimmune disease, or active anterior and posterior segment pathologies are considered contraindications to performing WFG PRK after CXL.

Improvements

Mean preoperative central corneal thickness (CCT) was close to 500 microns and minimum corneal thickness (MCT) was about 475 microns. The mean intended central ablation depth was 54.5 microns, the mean change in CCT was 53.7 microns, and the mean change in MCT was 53.7 microns. CCT remained stable during follow-up between 1 and 12 months.

Dr Shafik observed that UDVA, CDVA, sphere, cylinder, and keratometry values (K1, K2, and Kmax) had already improved significantly by one month after the WFG PRK procedure and continued to improve as follow-up continued.

At the last available visit, logMAR UDVA averaged 0.14 and was 0.3 or better (Snellen equivalent 20/40 or better) in 97% of eyes.

“Prior to WFG PRK, only about 75% of eyes had distance BCVA of 20/40 or better. At one year, almost half of the eyes gained at least two lines in distance BCVA and mean logMAR BCVA was 0.05, Dr Shafik said.

“The reduction in corneal irregularity and HOAs probably explains the improvement in BCVA.

At 12 months, SE was ±0.50 D in almost two-thirds of eyes and ±1.00 D in more than three-fourths of the cohort. Whereas cylinder magnitude was ±0.50 D in only 11.8% of eyes prior to PRK, almost half of the eyes had <0.5 D of cylinder at 12 months after the procedure.

Rationale for sequential WFG PRK

Dr Shafik said he performed PRK after CXL rather than at the same session considering evidence suggesting ablation and refractive outcomes are less predictable in diseased corneas compared with healthy eyes.

“The explanation may relate to the potential for tissue-related differences in laser ablation rates. In addition, manifest refraction measurements may be less accurate in eyes with keratoconus due to an interaction between second order and higher order aberrations, and compensatory epithelial remodeling after surface ablation in eyes with an irregular cornea may also play a role,” Dr Shafik said.

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“In order to maximise the accuracy of our surgical planning, we measure the refraction a minimum of three times and check for agreement in the values, and in general, the predictability of the refractive outcomes achieved in my series is among the best reported to date.”

He listed several reasons why a WFG ablation profile provide is more optimal for correcting refractive error and improving visual quality in eyes with keratoconus compared with a topography-guided technique. First, the WFG ablation addresses the HOAs that underlie degradation of image quality in these complex corneas.

“In other words, it offers a better cause and effect relationship than a topography-guided profile,” Dr Shafik said.

Another advantage

In addition, the WFG procedure uses actual data from the central zone of the corneal that is mainly responsible for vision degradation in eyes with keratoconus.
“Topographers do not read the central cornea and so the topography-guided central ablation profile is derived from an extrapolation of data. HOAs in the central cornea, primarily coma, are the main cause of vision degradation in eyes with keratoconus, and the high-definition aberrometer obtains exact measurements to calculate a proper and effective ablation profile,” he explained.

As another advantage, the WFG procedure allows for precise placement of the laser spots because beam delivery is guided by iris registration that compensates for both cyclotorsion and pupil centroid shift due to supine position and different light conditions under the aberrometer and the laser.

“Accuracy of the ablation is critical for addressing corneal surface regularity in eyes with keratoconus. Misplacement of spots that hit troughs rather than peaks in the surface can worsen irregularity. Only just recently have topography-guided procedures begun to adjust for cyclotorsion, and they still cannot compensate for pupil centroid shift,” Dr Shafik said.

The planning software for the WFG procedure also uniquely allows the surgeon to adjust the sphere and cylinder to correct the refractive error and target emmetropia.

“The topography-guided profile is more crude in this regard as it only targets corneal surface regularisation,” Dr Shafik said.

Dr Shafik has no relevant financial interests to disclose.