FP600 : Photorefractive Intrastromal Crosslinking (PIXL) for Low Myopic Refractive Correction.

Dr. Urvija Choudhary, C17816, Dr.Pooja Khamar, Dr. Rohit Shetty

AUTHORS:

Urvija Choudhary, Rohit Shetty, KrishnaPoojita Vunnava, Pooja Khamar

INTRODUCTION: Corneal collagen crosslinking (CXL) is a widely used treatment to halt the disease progression in Keratoconus.¹ First described by Spoerl et al², it acts by increasing the biomechanical strength of the cornea through induction of new covalent bonds in the stromal matrix.³ Its use in diseases other than keratoconus like post-refractive surgery ectasia, keratitis, corneal ulcers and corneal edema in bullous keratopathy are also described.⁴ It is also used sequentially with other procedures like intrastromal ring segments and photo refractive keratectomy.⁵ With the standard Dresden protocol, the corneal surface is uniformly irradiated in central 9 mm zone. Multiple studies have reported significant refractive changes after this procedure. However, it is difficult to regulate or control the post-treatment refractive improvement with standard CXL. Introduction of computer simulations of asymmetric UVA irradiation centralised on the ectatic keratoconus showed that such a treatment pattern induced more pronounced flattening.⁶ Also higher levels of CXL energy can be safely delivered which opens up individualised treatment plan based on corneal topography, with local augmentation of the treatment effect in the most ectatic zone.⁷ Photorefractive intrastromal crossliniking (PiXL) is a novel application of CXL to correct mild refractive error without the need for tissue ablation through zonal application of ultraviolet-A (UVA) light, facilitated by the development of a customizable differential UV delivery system by Avedro . PiXL is a minimally invasive, non- surgical procedure, which uses a pulsed zonal application of ultraviolet- A (UVA) light. Specific patterns and intensities of UVA are used for irradiation depending on refractive error. This is different from conventional CXL that utilizes broad beam UVA light.

METHOD: 5 cases of low myopia ranging from -0.5 to -1.5D and with normal corneal topography were enrolled.

Inclusion criteria:

1. Be at least 18 years of age, male or female, of any race.

1. Provide written informed consent.
2. Willingness and ability to follow all postoperative instructions.
3. Having myopia with intended spherical correction of -1.00 to -1.
4.  Contact Lens Wearers Only: Removal of contact lenses for 1 week (soft CL) or 2 weeks (rigid gas permeable CL) prior to the screening refraction.

Exclusion criteria:

1. Endothelial cell count that is less than 2500 cells/mm²
2. Eyes which are aphakic;
3. Eyes which are pseudophakia
4. Pregnancy or lactation
5. Patients with nystagmus or any other condition that would prevent a steady gaze during the treatment or other diagnostic tests;
6. Patients with a current condition that, in the investigator’s opinion, would interfere with or prolong epithelial healing;
7. A history of the insertion of Intacs in the eye to be treated or previous corneal surgery other than cataract extraction with PCIOL implantation;

Preoperatively, uncorrected and best corrected distance visual acuity (UCVA and BCVA) were noted and cycloplegic refraction was done for all cases. All cases underwent Intraocular pressure measurement, complete slit lamp examination, specular microscopy, corneal topography and anterior segment ocular coherence tomography (ASOCT). Intraoperatively, Paracel solution (Avedro Inc., Waltham, Mass., USA), hypotonic 0.25% riboflavin solution with 0.02% benzalconium chloride is applied for 4 minutes with two drops every minute, followed by VibeX Xtra (Avedro Inc), 0.25% isotonic solution for 6 minutes, with one drop every 30 seconds. The first step loosens the epithelial channel and the second ensures riboflavin penetration. Total soak time is 10 minutes in this procedure followed by UV radiation. This was followed very high-fluence collagen cross-linking by novel device (KXL II) in a predetermined circular pattern at 30mW/cm²  UVA irradiance in 4mm zone, pulsed (1:1) with a total energy dose upto 15J/cm² with oxygen. Postoperatively, UCVA, BCVA, manifest refraction, slit lamp examination, corneal topography, ASOCT and specular microscopy were evaluated over 6 months.

RESULTS: 5 eyes of myopia ranging from -0.5 to -1.5D sphere (Mean: -1.15 ± 0.25D sphere) treated with transepithelial PiXL showed mean correction in myopia of -0.25 ± 0.12 D sphere over a period of 6 months. Improvement in UDVA by more than 2 lines was achieved at 6 months in all cases and all had UDVA of 6/9 or better. None of them lost preoperative best corrected vision. Refractive stability was achieved by 1 month and all were stable till 6 months. Corneal haze was noted in any of the cases. There was no significant change in endothelial cell counts and none of the patients reported any signs of infection after the surgery.

DISCUSSION: There was no loss of preoperative best corrected visual acuity and all patients achieved spectacle independence by 1 month of treatment. There was no loss of endothelial cells on specular microscopy over 6 months and no significant haze proving to have a good safety profile. The technique is easier to perform and calms fears of post procedure ectasias as seen in other ablation procedures. However, there are a few limitations with our study, it has small sample size and needs longer follow-ups to study corneal changes. We also need to study the difference between the transpeithelial and epithelium off procedures. Further more understanding of different energy settings and zones of treatment is required for developing nomograms.

CONCLUSION: PIXL technique is safe and effective for correction of low myopia and has the advantage of essentially no loss of corneal tissue to ablation and its related complications. 

REFERENCES:

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2. Spoerl E, Huhle M, Seiler T. Induction of cross-links in corneal tissue. Exp Eye Res. 1998 Jan;66(1):97-103.
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