CORNEAL LASER REFRACTIVE SURGERY
The eye is an optical lens system. For the eye to see, light needs to focus onto the retina. When the optical axis is clear (no opacity obstructing the light to reach the retina), but the light does not focus onto the retina, a refractive error is present.
Types of Refractive Errors
Myopia / Nearsightedness. The focus of light falls in front of the retina. The patient can see well for near objects, but distance objects are blurred. Minus correction is needed to bring distance objects into focus, then accommodation brings near objects into focus.
Hyperopia / Farsightedness. The focus of light falls behind the retina. The patient can see neither far, nor near, and need to accommodate to bring objects into focus. Plus correction is needed to bring distance objects into focus. Accommodation (natural plus correction) often overcomes the issue in younger patients, but as one gets older the accommodation ability decreases and the eye needs progressively more and more plus correction to bring objects into focus.
Presbyopia. Near objects (compared with distance objects) always need more plus correction to be in focus. The normal aging process where the natural plus correction (accommodation) decreases is called presbyopia. This is the reason why most people need some form of correction after the age of 40.
Astigmatism. The eye system should be spherical like a soccer ball, but unfortunately the shape of the cornea (clear front part of the eye) tends towards asymmetry like a rugby ball. The two axes, steep and flat, has different points of light focus. These points can fall in front of, onto, or behind the retina in different combinations. But importantly, the two axes do not focus at the same point, causing blur at all distances as no object is focused entirely on the retina, no matter the distance from the eye.
Regular vs Irregular Astigmatism. When the two axes are 90˚ to each other (regular astigmatism), a spectacle or contact lens can correct for the difference in light bend. If the two axes are not 90˚ to each other (irregular astigmatism), then a spectacle or contact lens does not work well to correct the problem fully. In certain of these specific cases, a laser correction is the only possibly way to improve vision.
Types of Corneal Laser Refractive Surgical Procedures
LASIK. LASIK, or Laser In-situ Keratomileusis, is the most commonly done refractive procedure today. The effect is seen almost immediately, recovery time is minimal, side effects are minimal, risk factors are very low, and it is largely a painless procedure with excellent and consistent results. In this surgery, a thin corneal flap is cut with a motorized blade or another laser (Femto-LASIK / FLASIK). The flap is lifted, and the laser ablation is done on the exposed deeper corneal tissue. The flap is then closed. As the epithelium (outermost layer of the cornea) is not removed, the post-operative pain is minimal or negligible. In a very small number of cases, one can get flap complications. The tissue is cut deeper than other procedures, and therefore there is a 0.05% risk of ectasia (thinning with change in corneal shape) after this procedure. Interestingly, the LASIK procedure is the most studied and published procedure of all medical procedures.
PRK. Photorefractive Keratectomy is largely the same as LASIK, but here there is no flap cut before the laser surgery is applied. The epithelium is removed manually, or alcohol assisted, and then the laser surgery is done directly on the superficial tissue of the cornea. Because the epithelium is removed, there is considerable discomfort post-operatively. Recovery time is usually about a month until vision returns to the best possible amount. This procedure however is slightly safer than LASIK because of the lack of a flap. The depth at which tissue is altered in the cornea is less than in LASIK, making the risk for ectasia less. Haze is also a possible complication, but many studies prove that the modern techniques used has largely removed this complication.
Trans-PRK. Here the epithelium is removed with the laser, then the laser is applied to the superficial layers of the cornea exactly as in a PRK. The major difference with the Schwind Amaris laser is that because of the SmartSurfACE technology, the surface is smoother and therefore recovery is much quicker than with other systems. Recovery time is about a week, as a much smaller area of epithelium is removed. It is the most versatile procedure as all patients that qualify for any other procedure, will qualify for a Trans-PRK, but not vice-versa.
SMILE. The SMILE (SMall Incision Lenticule Extraction) is another relatively new procedure (about 10 years). This procedure is an excellent procedure that is largely equivalent to LASIK in that the deeper corneal tissue is removed to correct the error. The tissue is lasered with a different type of laser than the laser used for LASIK, and then removed through a small incision rather than making a large flap. Outcomes are comparable to the LASIK procedure. This procedure is very safe, recovery time is quick, but unfortunately it is more expensive technology than the other refractive procedures. It is limited in the fact that only myopia with regular astigmatism can be treated fully with this procedure meaning that not all eyes will qualify for a SMILE procedure. There are other technical drawbacks. However, this is still excellent technology and widely used and available.
Laser types in Refractive Surgery
Excimer Laser. The most used laser type for refractive work. Very precise, debatably more so than the Femtosecond laser. Proven technology with over 30 years of international exposure and experience. This laser is used for LASIK and PRK/Trans-PRK type procedures.
Femtosecond Laser. Currently, the only refractive procedure that this laser can perform is the SMILE procedure, making it a lot less versatile than the Excimer Laser.
Some points are up for debate between using the Femtosecond Laser rather than a microkeratome for flap creation during the Femto-LASIK / LASIK procedure: safety, time under suction, intra-ocular pressure under suction, suction loss rate, complication rate, cost, flap shape and thickness, etc. Neither has been successfully proven to be superior to the other, and both are acceptable to use in the modern era of refractive surgery.
Presbyopia Refractive Surgery
Certain techniques are utilized to give a non-accommodating patient both near and distance correction. In the young normal eye, the human lens moves. Unfortunately, there is currently no technology available to simulate this movement and remain clear. The pristine cornea has no aberrations, but with some of the techniques surgeons use aberrations to give extended depth of field. With other techniques, the combination of the two eyes are used to give both near and distance correction; i.e. one for near, the other for distance. These techniques can also be combined.
The dominant eye is used for distance, while the non-dominant eye is used for near. As the cornea is lasered for a single focus point, the visual quality per eye is excellent/normal. The drawback to this technique is that the two eyes do not focus together. Some patients tolerate this scenario very well in the fact that they do not even notice that the two eyes are different they just use the eyes at all focus distances together, while others do not tolerate it at all. A contact lens trial is used before the procedure by your optometrist to determine whether you will adapt to this scenario. For the patient that adapts to the fact that the two eyes are different, this is option number one as it achieves the best quality of vision with both eyes open.
The dominant eye is used for distance, while the non-dominant eye is used for intermediate to near. It is different to pure monovision in that the non-dominant eye is set for an intermediate distance, meaning very fine print is not seen, and a reader’s spectacle is still necessary for this. But bigger print (e.g. cellphone, menu in a restaurant, newspaper and dashboard in a car while driving) should be visible. Once again, because the cornea has very little aberrations, the quality of vision is excellent. Most patients can adapt to this scenario quite easily because the two eyes’ focus point is very close to each other and even overlap. A contact lens trial is not necessary. If one does not adapt well, then the procedure is easily reversible.
This technique lasers the cornea into such a shape that there is extended depth of focus. Corneal aberrations give the patient both distance and near vision in the same eye. Combining this technique with the combinations above (PresbyMax Hybrid), one can also decrease the aberrations and improve the quality of vision. The Schwind Amaris is the only laser with the formulae to perform the PresbyMax Hybrid procedure.
INDICATIONS AND COMPLICATIONS
Refractive surgery is always elective and voluntary. In most routine cases the error can be corrected with either spectacles or contact lenses. However, some patients’ refractive errors are of such a nature that neither spectacles nor contact lenses are tolerated or give good enough visual results. In these cases, surgery has a clear indication.
This procedure is seen as an aesthetic procedure by most medical aids in South Africa. Therefore, it is not routinely covered by them. However, there are instances where some funds can come from the medical aid’s savings account.
The modern refractive surgery procedure is extremely safe. It is seen as one of the safest procedures of any kind that can be done, with a combined risk of less than 1% irrespective of which exact procedure is used. Nevertheless, there are complications that can occur. Some of the more common complications are discussed below.
Dry eye symptoms. Up to 5% of patients have persistent dry eye symptoms. This usually resolves by 3-6 months and lubrication eye drops are usually more than enough in management.
Flap complications. When LASIK flaps are created, one has a flap of tissue that can tear, get a small buttonhole, move etc. This is still the most common serious complication and can be largely overcome with good surgical technique.
Haze. This used to be common with the PRK procedure, but with new techniques of MMC application, this has largely become an issue of the past.
Over- / under-correction. Modern ablation formulae are excellent irrespective of the laser used, but all eyes are not the same and this can still occur. However, most often it is negligible, and a quick routine top-up procedure is an option if desired.
Infection. As with all surgery, as soon as there is tissue manipulation, organisms can enter the tissue. The procedure is done under sterile conditions on the eye with sterile instruments and techniques making infection very rare. A wide range of antibiotics are available in management of this complication.
Epithelium defect. This usually just takes time to heal with conservative management and observation. Bandage contact lenses are usually placed for comfort when a defect is present.
Ectasia. Modern screening pre-procedure has made this feared complication to occur in less than 0.5% of cases. In any at-risk cornea, the PRK / Trans-PRK procedure would be preferred, making surgery even safer.
Diffuse Lamellar Keratitis. This develops 1-2 days post-procedure and most resolve by 5-8 days after initiation of appropriate therapy. It very seldom progresses to flap melting, but mostly in neglected cases. It can present with foreign body sensation, light sensitivity, pain and blurry vision.
Subconjunctival Hemorrhage. Patients can have a red eye induced by suction rings. There is no risk for visual loss and blood resolves spontaneously.
No preparation is needed. Patients can continue as normal before the procedure. No change in eating or medication routines.
The procedure is done under topical anesthesia. This means that only drops are used to numb the eye. No pain is felt during the procedure. As the patient needs to be awake to fixate on a light and give certain feedback, no general anesthesia is used. If a patient is very anxious, then tablets are given to relax the patient. The procedure is very quick and as a routine doing more than topical anesthesia is excessive.
Shields are placed on the eyes for a few hours, then removed and the patient can continue as before. Drops are prescribed and to be completed by the patient. The only constraint is no eye rubbing. With LASIK the eyes are reviewed the morning after the procedure. With the Trans-PRK procedure, the eyes are reviewed the morning after and then again at day 4 to remove the placed contact lenses. A routine follow-up is done at 3 months post procedure. Patients can however contact the practice at any time if there are issues.