EyeWorld Asia-Pacific September 2017 Issue

High oxygen enhances epi-on PiXL efficacy by Stefanie Petrou Binder, MD EyeWorld Contributing Writer Epi-on PiXL treatment of low-grade myopia. The treatment zone is 4 mm and the total energy delivered 15 J/cm 2 . Note oxygen mask, which increases the oxygen level to above 90% to increase the treatment effect. Source: Anders Behndig, MD continued on page 50 EWAP REFRACTIVE 49 September 2017 Preliminary data shows corrections of more than 1.0 D in cases of low-grade myopia using 4 mm epi-on PiXL under high oxygen conditions P hotorefractive intrastro- mal crosslinking (PiXL, Avedro, Waltham, Mas- sachusetts) is a procedure that offers a non-surgical option for the correction of low grade myo- pia in patients reliant on glasses or contact lenses who are either ineligible for laser vision correction or uncomfortable with the idea of invasive eye surgery, or for post-cat- aract patients with a small degree of residual, uncorrected myopia, which in one study was shown to be ±1.0 D in roughly 25% of post- cataract patients. 1 What is PiXL? PiXL is a simple two-step proce- dure involving the application of riboflavin (vitamin B2) eye drops, a naturally occurring non-toxic pho- tosensitizer, onto the cornea with subsequent ultraviolet A (UVA) light irradiation of the riboflavin-saturat- ed cornea, which strengthens and reshapes the cornea by crosslinking corneal collagen fibers. Efforts to clarify the advantages of PiXL treatments performed with the epithelium on or off are under- way that take into consideration potential limiting factors such as epithelial riboflavin absorption, ox- ygen diffusion, and incoming light absorption by the epithelial cells. Riboflavin needs to be absorbed through the epithelium to be ef- fective, evoking questions regard- ing the best type of riboflavin to use and the duration of riboflavin exposure on the cornea. One study revealed that the effects of PiXL in eyes with keratoconus that received a prolonged preoperative riboflavin application of 2 hours achieved similar depth of effect in the stroma and less pronounced confocal microscopic changes as eyes treated with standard crosslinking with epithelial debridement. 2 As oxygen availability plays an important role in crosslinking, altering the effects on diffusion through the epithelial layer by the removal of the epitheli- um should be carefully considered. High oxygen versus room air The preliminary data from an ongoing Swedish study showed bet- ter myopia reducing effects using high oxygen compared to room air with epi-on PiXL, according to a presentation from the 21st Winter Meeting of the European Society of Cataract and Refractive Surgeons (ESCRS), in Maastricht, Holland. “This is something quite new and exciting, as I see it,” said Anders Behndig, MD , Department of Clinical Sciences/Ophthalmol- ogy, Umea University Hospital, Sweden, who presented the study. “We achieved approximately 1.25 D of myopic correction with this technique so far, 3 months after the treatment, with no signs of regres- sion, meaning that the myopic correction may even increase some- what more over time.” Dr. Behndig’s investigation involved healthy volunteers over 18 years of age with a stable pre- scription for at least 2 years, who had mean low grade myopia with a spherical equivalent (SEQ) of –1.5 ± 0.5 D (range: –0.5 to –2.6 D). He and his team performed epi-on PiXL treatments in 39 eyes of 20 patients using topical anesthesia and isotonic riboflavin, which was applied every 90 seconds for a 10-minute duration. The eyes were irradiated in the central 4 mm zone of the cornea using pulsed crosslinking (CXL) with 30 mW/cm 2 for a mean 16.4 minutes at 15 J/cm 2 . The study incorporated two groups: The first group included 12 eyes of six patients that were treated under room air (20% O 2 ) conditions, and the second group included 27 eyes of 14 patients that underwent the same treatment under high oxygen (94.3±1.3% O 2 ) conditions, using an oxygen mask. The primary outcome measure was logMAR uncorrected visual acuity (UCVA), measured preoperatively and at 1 month and 3 months post- operatively. The preoperative mean UCVA was 0.6 logMAR in the room air group and 0.65 logMAR in the high oxygen group. Postoperative UCVA was significantly different between the two groups at the 1-month follow-up (P=0.031) as well at the 3-month follow-up (P=0.017), with the high oxygen group’s UCVA under 0.1 logMAR and the room air group’s UCVA at 0.4 logMAR, at 3 months. The mean preoperative SEQ was –1.25 D in the room air group and –1.6 D in the high oxygen group. In the room air group, a slight in- crease in SEQ was seen at 1 month of +0.33±0.15 D, whereas a more pronounced effect was seen in the high oxygen group of +1.18±0.48 D (P<0.001). The preoperative K mean was 43.0 D in the high oxygen group and 43.5 D in the room air group. The high oxygen group had a reduction in K mean related to the reduction in the SEQ. At 1 month after surgery, the K mean in the high oxygen group was under 42.5 D and almost 43.5 D in the room air group (P=0.022). The endothelial cell count did not change significantly in either group, indicating that the method is safe for the endothelium. Dr.