Refractive surgery extends beyond PRK, LASIK
Most people think of “refractive surgery” as simply LASIK. While it is true that LASIK is the most commonly performed refractive surgery today, it is far from the only tool in the comprehensive refractive surgeon’s toolbox.
We think of refractive surgery in the context of the three milestones of human visual development that we will all encounter if we live long enough, each of which serves as a threshold (and sometimes a cutoff) for particular refractive surgery options.
The first milestone of human vision development is ocular maturity, when the eyes stop growing, and refraction essentially stabilizes. Most kids reach ocular maturity by age 18 or 20, and refractive surgery before this point is rare.
Between ocular maturity and the second milestone, presbyopia, we can choose from several procedures, including LASIK, PRK, small-incision lenticule extraction (SMILE), phakic IOLs and even corneal cross-linking. Once a patient becomes presbyopic, we add corneal inlays and refractive IOLs to the list of options.
Finally, with development of cataract (the third vision milestone), we typically consider IOLs as the leading refractive option; inlays are no longer considered; and other corneal procedures, including astigmatic incisions, may still be performed to enhance IOL outcomes.
Unfortunately, every day we see patients who have been told by a doctor somewhere along the way that they cannot have refractive surgery because of a variety of myths that are no longer true. Those myths include things like having thin corneas or astigmatism or because they are presbyopic or will become presbyopic someday. Each of those conditions is a barrier for some types of refractive surgery, but none of them rules out all forms of successful modern refractive surgery.
LASIK involves excimer laser reshaping of the cornea under a femtosecond laser flap to correct myopia, hyperopia and astigmatism. In addition to conventional ablations, wavefront-guided and topography-guided procedures are available. Vision recovers rapidly, with minimal discomfort. According to a recent review by Sandoval and colleagues, 99.5% of eyes are 20/40 or better uncorrected, more than 90% are 20/20 or better, and 98.8% of patients are satisfied with the results.
This procedure is ideal for pre-presbyopic patients with healthy corneas and sufficient corneal depth for the flap and ablation, presbyopic patients with a monovision target or in combination with other treatments. Range in clinical practice covers approximately +6.00 D to -9.00 D with up to 6.00 D of astigmatism, depending on the particular laser used.
PRK is similar to LASIK, but the excimer laser treatment is performed on the surface without a flap. The long-term results and treatment range are similar, but patients may experience more short-term discomfort and longer visual recovery.
This procedure is ideal for patients whose corneas are too thin or who have corneal scarring from contact lenses or otherwise, precluding successful creation of a flap.
SMILE is a recently approved procedure in which the VisuMax femtosecond laser (Zeiss) is used to create a lenticule inside the intact cornea. The lenticule is then removed through an incision to reshape the cornea. Thus far, in the U.S., SMILE can be used to treat myopes (-1.00 D to -9.00 D) without astigmatism (<0.5 D). A healthy cornea is a requirement just like it is with LASIK and PRK.
SMILE is ideal for candidates for LASIK or PRK. However, SMILE may have advantages for patients who struggle with dry eye or contact lens intolerance because it bypasses the corneal nerve plexus.
Phakic IOLs are implanted just in front of or behind the iris, with the crystalline lens remaining in place. In the U.S., the Visian ICL (Staar) and the Verisyse lens (Johnson & Johnson Vision) are available to treat myopia beginning at -3.00 D or -5.00 D, all the way up to -20.00 D. Toric versions are not available, but the procedure can be combined with astigmatic incisions.
Phakic IOL surgery carries different risks than corneal procedures given that it is intraocular. Recent studies have shown the risk of cataract formation to be less than 2% with contemporary phakic IOLs (Parkhurst). After an initial increase in endothelial cell (EC) loss right after surgery, similar to what occurs after cataract surgery, the rate of EC loss has been shown to level off to normal, age-related levels. Visual results are as good as or better than those of LASIK, especially for higher myopes.
This procedure is ideal for patients with moderate to high myopia. It may also be a good choice for patients with thinner corneas or suspicion of keratoconus in whom we do not want to perform a corneal procedure. Patients must have adequate anterior chamber depth for proper vaulting of the phakic IOL over the crystalline lens.
Cross-linking strengthens the corneal collagen fibers by applying UV light to a cornea that has been soaked in a riboflavin solution. Although corneal cross-linking is primarily used to treat ectasia and keratoconus, it is also being investigated as a tool to treat low myopia due to the hyperopic shift observed over time.
This procedure is ideal for low myopes, especially those with suspicious topography.
Corneal inlays are designed to be implanted in a corneal pocket or under a flap in the nondominant eye. They do not result in a “trade” of any significant distance vision to accomplish restoration of near. There are currently two inlays available in the U.S., the Kamra inlay (AcuFocus) and the Raindrop Near Vision Inlay (Revision Optics), with a third in clinical trials. Each works differently.
The Kamra relies on small-aperture optics to improve near vision, the Raindrop creates a change in elevation of the anterior cornea, and the investigational Microlens (Presbia) contains add power zones around a plano center. Inlays frequently need to be combined with laser vision correction in one or both eyes to achieve the desired results. Given the longer experience with Kamra inlays, there is already significant published data on combining it with LASIK or PRK; combination surgery parameters still need to be worked out for other types of inlays.
Corneal inlays are ideal for presbyopes with clear crystalline lenses, adequate corneal thickness and a healthy ocular surface. The Kamra inlay is best suited for patients whose nondominant eye is slightly myopic (-0.75 D to -1.00 D), while the Raindrop is best for mild hyperopes (+0.50 D to +0.75 D). The nonimplanted eye should be plano, which can be accomplished with any of the other procedures if the patient is not naturally emmetropic.
Refractive lens exchange
IOLs are now being commonly implanted in a refractive lens exchange (RLE). Options range from traditional multifocal IOLs, which provide strong near vision, to more recent low-add multifocal IOLs that provide improved near and intermediate vision, accommodating IOLs and an extended-depth-of-focus (EDOF) lens (Tecnis Symfony, Johnson & Johnson Vision) that provides a range of near to far vision. Both of the newer types of lenses (low-add and EDOF) have a reduced incidence of night vision problems compared to early-generation multifocal IOLs. Toric options are also now available for patients with astigmatism.
RLE is ideal for patients with cataract and good visual potential as well as hyperopes and older presbyopes.
Primary eye care providers who stay abreast of progress in refractive surgery will be best positioned to respond to patient inquiries and comanage their patients with surgeons who can address multiple points along the spectrum of refractive surgery procedures.
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- Igras E,et al. J Refractive Surgery. 2016;32(6):379-384.
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- Sanders DR, et al. J Refract Surg. 2002;18(6):673-682.
- Sandoval HP, et al. J Cataract Refract Surg. 2016;42:1224-1234; doi.org/10.1016/j.jcrs.2016.07.012.
- Tomita M, et al. J Cataract Refract Surg. 2015;41:152-161.
- For more information:
- Gregory D. Parkhurst, MD, FACS, is the physician-CEO of Parkhurst NuVision in San Antonio, Texas, and president of the Refractive Surgery Alliance. He is adjunct assistant clinical professor at the Rosenberg School of Optometry. He can be reached at GParkhurst@sanantonio-lasik.com.
Disclosure: Parkhurst is a consultant to Alcon Labs, Carl Zeiss Meditec, Johnson & Johnson Vision, ReVision Optics and Staar Surgical.