Corneal Refractive Therapy: A LASIK Enhancement Alternative
BY SAL MUSUMECI, OD, FAAO
The FDA approved Paragon CRT (corneal refractive therapy) for corneal reshaping in June 2002. It's an effective way to temporarily reduce myopia up to 6.00D with our without up to 1.75D diopters of astigmatism.
While Paragon Vision Sciences doesn't currently have an FDA indication for post-surgical applications, in my practice and in my own eyes, I have found the Paragon CRT system is an excellent nonsurgical, postLASIK enhancement option to reduce myopia in a regressed or undercorrected eye. As a LASIK patient who has undergone bilateral enhancements and is now experiencing a second regression, I'd like to practice what I preach.
My LASIK Experience
As a spectacle/contact lens wearer since age 10, I decided to become an early adapter and have LASIK surgery in January 1997 -- just before my 38th birthday. Preoperatively, I had a cycloplegic refraction of OD 5.00 sphere and OS 6.00 sphere, achieving acuities of 20/20. Motilities, confrontation fields and pupils were all within normal limits. My pupillary size was 7.5mm OU. Slit lamp and fundus exam were unremarkable.
Antonio Prado, MD, performed my surgery, using the VISX S2 excimer laser and the procedures were uneventful. Our postoperative refractive goal was plano OU. Uncorrected acuities post-op day one were 20/25 OD and 20/30 OS. At week one, visual acuities were 20/20 OD and 20/25 OS. The postoperative flap and corneal observation were unremarkable. At about six weeks postoperatively, my visual acuity at distance had decreased to OD .75 and OS 1.00, giving unaided acuities of 20/40 and 20/50.
At six months, I decided to undergo a bilateral surgical enhancement. The procedures were again uneventful and produced uncorrected acuities of 20/20 OD and OS. This level of acuity lasted for approximately four-and-a-half years.
Within the last year, I've noticed another regression in my vision. We still don't completely understand the exact mechanism of regression, but we believe it may be related to postoperative epithelial hyperplasia or changes in keratocyte density. Whatever the cause, I was no longer 20/happy.
Reviewing the Options
Given the fact that my post-LASIK keratometry readings were less than 36.00D, a second surgical enhancement was not indicated based on our practice protocols. My remaining options were as follows: Part-time spectacle wear, soft contact lenses or Paragon's CRT. Spectacles were fine, except that I was playing the trombone with the reading material, as I have finally reached that famous milestone that I've been telling patients about all these years: Presbyopia.
I know some of you are saying, "Just have a pair of progressives made." I did consider that, but because we offer surgical and non-surgical options to reduce dependency on glasses, this wasn't the best PR option. I then tried silicone hydrogels as my next option and addressed the presbyopic issue by wearing a lens on my dominant eye only. Because of their increased rigidity over conventional hydrogels, I couldn't achieve stable vision (the lens would excessively vault my central cornea and flex with blinking).
So as a practitioner who proactively promotes CRT, I couldn't think of a more powerful way to endorse the procedure than to become a CRT patient myself.
CRT on a Post-LASIK Eye
Again, CRT isn't FDA approved for overnight corneal reshaping on a post-surgical eye. Nevertheless, it's certainly acceptable to offer this option off-label.
My original experience with CRT came to light as an, "Oh, by the way ...."
I had the pleasure of seeing a patient for a routine eye exam in November 2002. She had LASIK performed by one of our competitors four years previously. Uncorrected acuities were 20/40 OD and 20/50 OS. Manifest refraction was OD 0.75 0.50 x 170 and OS 0.750.75 x 45 giving corrected acuities of 20/20 and 20/25, respectively. Corneal topography showed well-centered ablations with flat Ks of 43.75 OD and 43.87 OS.
Because she was 52 years old at the time, we decided on reshaping her dominant (right) eye only. I based my lens of first choice on Paragon's standard slide rule nomogram, which called for an 8.0mm base curve, 525 return zone depth and 33 landing zone angle. This produced a well-centered lens with good edge lift. The treatment zone was adequate, but a change to a .500 return zone depth (to reduce overall sagittal depth) still allowed the lens to center with a larger treatment zone and we dispensed a 8.0/500/33 CRT lens. One day follow-up exam displayed a well-positioned lens giving an uncorrected acuity of 20/20. At her one-month follow up, the lens was superiorly displaced. I achieved centration with an 8.0/550/33, which gave uncorrected acuities of 20/20 that she maintains today.
Because of this initial experience with using CRT for post-LASIK enhancement, I've fit two other patients. One is a physician who has regressed to 0.50 in his dominant eye. His uncorrected acuity is 20/25+, so the risk of surgical enhancement outweighs the benefit; however, the lack of clarity is bothersome to him.
The other patient is a 38-year-old female who had bilateral LASIK surgery in 2000 and has now has regressed to 0.50 OD and OS. She declined surgical enhancement but was interested in corneal reshaping because I had fit one of her co-workers and she was intrigued by the nonsurgical procedure. Both were successfully fit and enjoy uncorrected 20/20 vision during all waking hours.
Sharing Success Secrets
I have discovered the following pearls along the way:
- Use the flattest base curve in your Paragon CRT Diagnostic Set to determine the treatment curve. (From the standard set, place an 8.8mm base curve and over-refract to a plano to +0.50.)
- Use preoperative Ks to determine the suggested landing zone angle (LZA). For example, pre-op Ks of 40D to 42D indicate a 31 LZA, 42D to 44D indicate a 32 LZA and 44D to 46D indicate a 33 LZA
- Start with a return zone depth that's shallow enough to provide a 4mm+ treatment zone but will allow the lens to center
- Follow fitting guidelines developed by Paragon
My CRT Experience
How did I fit my own CRT lens? I looked for the flattest base curve in my set and based the LZA on my preoperative Ks of 41.50D. The closest lens that I had was a 8.8/500/32. After placing this initial diagnostic lens on my eye, I instilled some fluorescein and positioned myself behind the slit lamp. I then used a handheld mirror and the cobalt blue light of the biomicroscope to evaluate the fluorescein pattern. The lens centered well with a 4mm+ treatment zone and adequate edge lift. Fluorescein pattern was the classic bulls-eye. That evening, I slept in the lens without any adverse reaction. The following morning, my uncorrected acuity was a crisp 20/20 and corneal topography displayed a well-centered lens in the closed eye position. I have continued to wear the same lens with crisp, clear, consistent vision and have adapted to monovision quite well.
The CRT Alternative
Fitting a CRT lens on a post-LASIK patient who has had myopic regression is an effective alternative to surgical enhance-
ment. Myopic regression after LASIK occurs in about five percent to 20 percent of patients undergoing the procedure. Although surgical enhancement is common, risks do exist, including diffuse lamellar keratitis and epithelial ingrowth. Some patients prefer to not undergo surgical enhancement because of these risks. Other patients are just more critical about their 20/25 vision and surgical enhancement risks outweigh the benefits. Paragon CRT provides a safe and effective alternative.
As you can see, corneal reshaping isn't just for those practitioners who belong to a secret society and who have the secret codes to this correction option. Paragon's CRT Fitting System makes it as easy as 1, 2, 3.
Dr. Musumeci is in practice in Tampa, FL at Prado Vision and LASIK Center. He specializes in pre- and post-operative refractive care as well as in CRT.