August 2020, Volume XXXIV, Number 5
New options for patients
urgical correction of refractive error (myopia, hyperopia, astigmatism, and presbyopia) continues to evolve and advance. Expansions in both corneal and lens-based surgical technologies provide new refractive surgery options for more patients, including some who may not have been surgical candidates in the past. In addition, the technology for evaluating patients’ eyes for consideration of a refractive procedure has advanced to allow physicians more information to counsel patients on their candidacy for refractive surgery and to identify the most appropriate procedure.
The “SMILE” procedure
Small incision lenticule extraction (SMILE) is the latest advancement in corneal refractive surgery. Since its approval by the FDA in September 2016, there have been more than two million SMILE procedures performed across the world. As with all laser-based corneal refractive procedures, SMILE reshapes the cornea to correct refractive error. SMILE is currently indicated for the correction of 1 to 10 diopters of myopia and 0.75 to 3.00 diopters of astigmatism.
The entire SMILE procedure takes 10–15 minutes, with about 30 seconds of laser treatment time. The patient’s eye is anesthetized with topical anesthetic, then a small 6mm incision is made on the superior cornea with the Zeiss VisuMax femtosecond laser. A contact lens-shaped lenticule is then made with the femtosecond laser in the stromal layer of the cornea 120 microns below the surface of the cornea.
The ability to map and measure the eye continues to become more sophisticated.
Subsequent removal of this lenticule changes the shape of the cornea to produce the desired myopic and astigmatic correction. The small incision, as well as tissue removal from deeper in the cornea, results in less impact on the corneal nerve plexus and a resultant lower risk of post-operative dry eye than with larger incision sizes. Patients often have the SMILE procedure done on both eyes the same day, as vision typically recovers to a functional level by the next day. These patients are on a post-operative course of antibiotic and steroid eye drops and are typically able to return to work the next day.
The best candidates for SMILE are patients with refractive error within the FDA-approved range of myopia and astigmatism, have corneal shape determined appropriate for refractive surgery by the refractive surgeon, and have the appropriate corneal thickness. The procedure is contraindicated for patients under the age of 18 and for those with visually significant cataract, uncontrolled glaucoma or external disease, and hyperopic refractive error.
Surgical correction of refractive error is not limited to laser vision correction. The Visian ICL (Implantable Collamer Lens) has been FDA-approved since 2005. The indication for the lens is for vision correction of 3 to 15 diopters of myopia and less than 2.5 diopters of astigmatism, and reduction of 15 to 20 diopters of myopia. In September 2018, the Toric ICL was approved by the FDA, expanding the ability to treat patients with higher degrees of myopia and astigmatism. This lens is indicated for the correction and reduction of the same degree of myopia, as well as for patients with up to 4 diopters of astigmatism. As of April 2019, more than one million ICL procedures had been done globally.
This procedure takes approximately 15 minutes in a sterile operating environment. A small incision is made in the cornea and the ICL is then implanted in the space behind the iris and in front of the lens (ciliary sulcus). The procedure can be done unilaterally or bilaterally. After the procedure, the patient is on a course of antibiotic and anti-inflammatory topical eye drops. Vision typically recovers within the first few days after the procedure. The ICL can be removed if necessary for any reason.
The best candidates for this procedure are patients between 21–45 years old who have an anterior chamber depth (distance between the corneal endothelium and anterior lens surface) of 3.0mm or greater, and have a stable refractive history. The procedure is not recommended for patients under the age of 21 or for those who have a shallow anterior chamber or a low corneal endothelial cell density. Safety has not been established in patients with other active ocular disease that would, in general, limit a patient’s candidacy.
Intraocular lenses (IOLs) are implanted at the time of lens replacement surgery (cataract or refractive). Presbyopic IOLs are available in various versions and designs, including accommodating, multifocal, and extended depth of focus. The benefit of these types of lenses for patients is an expanded range of focus after lens replacement surgery. This expanded range of focus allows the patient more freedom from glasses than traditional monofocal (single point of focus) lenses. The most recent FDA approval of a presbyopic IOL was in August 2019 for a trifocal IOL called PanOptix from Alcon. The PanOptix lens has a ringed design to the lens, providing patients with three zones of focus—distance, intermediate, and near—compared to previous multifocal designs with two zones of focus. The addition of the third focal point aims to provide patients with even less dependence on glasses than its predecessor lenses. Other trifocal and presbyopic IOL designs are currently in clinical trials.
The best candidates for presbyopic IOLs are patients with corneal shape appropriate for refractive surgery and have generally good ocular health. Other ocular conditions that may limit the visual potential of a patient may limit candidacy for a presbyopic IOL.
Mapping the eye
In addition to the advancements in surgical procedures for vision correction, the ability to map and measure the eye continues to become more sophisticated. More information on a patient’s ocular system allows the physician to appropriately advise patients seeking refractive surgery on candidacy as well as the most appropriate procedure for patients who are candidates. Corneal topography has long been used to measure the shape of the cornea. Keratoconus is a form of pathologic astigmatism where the cornea progressively becomes steeper and thinner, resulting in loss of best corrected visual acuity and, in some cases, the need for surgical intervention. The shape of the anterior surface and posterior surface of the cornea, as well as the relative curvature between these surfaces, can be an indication of keratoconus, and corneal topographers are now able to very precisely measure these relations.
The entire SMILE procedure takes 10–15 minutes.
The visual clarity through the visual system can also be objectively evaluated. The I Trace from Tracey technologies uses optical ray tracing and topography to show a visual depiction of the visual blur in an ocular system from the cornea versus internal structures of the eye, including the lens, vitreous, and macula. This is often used to determine the presence of a dysfunctional lens or subtle cataract or if the ocular surface is causing significant blur due to irregular astigmatism, dryness, or other causes.
HD Analyzer from Visiometrics uses an isolated light source from a laser beam that is reflected through the ocular medium twice. The size and shape of the reflected light source is analyzed and given a number on an ocular scatter index (OSI). A high OSI can be indicative of pathology within the optical system from media opacity, such as cataract, or significant dry eye or other pathology. Determining the clarity of the visual system often plays a role in the inclusion or exclusion of candidacy for certain refractive procedures. If significant lenticular opacity is present, the patient may be advised against corneal or ICL refractive surgery and an intraocular lens may be more appropriate. Significant ocular surface disease may need to be treated prior to consideration of any refractive surgery and may exclude a patient from candidacy for certain procedures.
Optical coherence tomography (OCT) is used to detect and monitor many ocular diseases. OCT uses light waves to take cross-section pictures of the structures of the eye. The thickness of the cornea is important in the evaluation for candidacy for refractive surgery. Too thin of a residual corneal stromal thickness after corneal refractive surgery is a risk for the development of ectasia, or pathologic astigmatism. If a cornea is very thin, the patient may not be a candidate for corneal refractive surgery. These patients may be better candidates for a lens-based procedure or continued use of glasses or contact lenses.
Corneal thickness has traditionally been measured with ultrasound to provide singular measurements of thickness of the cornea. OCT is now used to measure a more comprehensive thickness map of the entire cornea. This technology has the precision to separate the thickness of the epithelial layer of the cornea from the deeper layers of the cornea. This is an important advancement as the epithelial layer of the cornea thins over areas of ectasia. If the epithelium is thinned over an area of steepening on a topographic map of the cornea, it could be a sign of corneal ectasia. By contrast, an area of steepening which may be assumed to be pathology on a topographic map may be an artifact due to thickened epithelium. OCT technology is also able to take precise cross-section pictures of the retina and optic nerve to screen for any posterior segment pathology that may affect the vision potential or safety of a procedure for a patient.
Advancements in refractive surgery have allowed more patients access to surgical technology for vision correction, and the advancements in screening technology help physicians advise candidacy for surgical technology as well as the most appropriate procedure.
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