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 ray of light to reach the retina), but the light waves do 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 distant objects are blurred. Minus correction is needed to bring distant objects into focus, and then accommodation brings close 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 required to bring distant objects into focus. Accommodation (natural plus correction) often overcomes the issue in younger patients. Still, 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 ageing process where the natural plus correction (accommodation) decreases is called presbyopia. Presbyopia 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 various combinations. But importantly, the two axes do not focus at the same location, 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 adequately. Some of these specific cases, a laser correction is the only possible way to improve vision.
Types of Refractive Surgical Procedures
LASIK. LASIK, or Laser In-situ Keratomileusis, is the most commonly done refractive procedure today. One can see the effect almost immediately, recovery time is minimal, side effects are minimal, risk factors are deficient, and it is mostly a painless procedure with excellent and consistent results. In comparison to PRK, the size of the error that can be corrected is less as the residual bed is affected by the flap thickness. Residual bed thickness is the reason that some thinner corneas prevents LASIK surgery from being done. In this surgery, a motorized blade or another laser (Femto-LASIK) cuts a thin corneal flap. After lifting the flap, laser application to deeper corneal tissues takes place. Closure of the flap completes the procedure. No removal of epithelium (outermost layer of the cornea) takes place, and hence post-operative pain is minimal or negligible. In a minimum number of cases, one can get flap complications. Deeper tissue cutting occurs than in other procedures, and therefore there is a 0.05% risk of ectasia (thinning with change in corneal shape) after this procedure.
PRK. Photorefractive Keratectomy is mostly the same as LASIK, but here there is no flap cut before the laser surgery is applied. Manual or alcohol assisted epithelium removal takes place, and then the laser surgery is done directly on the superficial tissue of the cornea. Because epithelial removal occurs, 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 of tissue altering 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 have mostly removed this complication.
Trans-PRK. Trans-PRK is a relatively new procedure offered by the Schwind Amaris. This laser platform is the only one with new SmartSurfACE technology. The epithelium and stromal refractive laser happens in a single step. The significant difference is that because the laser removes the epithelium, the surface is smoother, and therefore there are fewer aberrations. Recovery time is a few days, as a much smaller area of epithelial removal occurs. Some studies show that for significant errors (myopic errors more than 6 Diopters), the Trans-PRK supersedes the LASIK and PRK procedures in visual outcome. Some surgeons will disagree with this statement. However, this fact makes this procedure possibly the best option for more substantial refractive errors. It is the most versatile procedure as all patients that qualify for any other method, will be eligible for a Trans-PRK, but not vice-versa.
SMILE. The SMILE (SMall Incision Lenticule Extraction) is another relatively new procedure (about ten years). This procedure is an excellent procedure that is mostly equivalent to LASIK in that ablation of deeper corneal tissue takes place to correct the refractive error. This technique utilizes a different type of laser than used for LASIK. Without making a flap, removal of a slither of tissue takes place through a small incision. 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 systems. It is limited in the fact that only myopia with regular astigmatism treatments are possible with this procedure. Effectively, this means 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.
LASEK / Epi-LASEK / PTK. These are all variants of the PRK and LASIK type procedures. Some surgeons prefer specific types, but the differences are technical and not relevant to routine outcome of routine refractive cases.
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 performs 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 are proven to be superior, and both are acceptable to use today.
Presbyopia Refractive Surgery
Non-accommodating patients need both near and distance vision, therefore, utilization of specific techniques are necessary. In the young normal eye, the human lens moves. Unfortunately, there is currently no technology available to simulate this movement and remain transparent. The pristine cornea has no aberrations, but with some of the techniques, surgeons use aberrations to give extended depth of field. With other methods, the combination of the two eyes provide both near and distance correction; i.e. one for near, the other for distance. These techniques can also be combined.
Monovision. The dominant eye is used for distance, while the non-dominant eye for near. Lasering a single focus point into the cornea results in excellent visual quality per eye. 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. In contrast, 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 patients who adapt 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.
μ-Monovision. The dominant eye is used for distance, while the non-dominant eye for intermediate to near. It is different from pure monovision in that the non-dominant eye is set for an intermediate-range, meaning that fine print is too small, 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 minimal aberrations, the quality of vision is excellent. Most patients can adapt to this scenario quite quickly 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.
PresbyMax Hybrid. This technique lasers the cornea into such a shape that there is an 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 the image. 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 glasses 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. We have had full remuneration from certain funds.
The modern refractive surgery procedure is extremely safe. It is one of the most reliable operations of any kind that are possible, with a combined risk of less than 1% irrespective of which exact procedure performed. Nevertheless, complications can occur. Discussion of complications are below.
Dry eye symptoms. Up to 5% of patients have persistent dry eye symptoms. Dry eye symptoms usually resolve by 3-6 months, and lubrication eye drops are generally more than enough in management.
Flap complications. LASIK flap creation results in tissue that can tear, get a small buttonhole, move etc. Flap complications are still the most common severe complication and can be overcome mainly with proper surgical technique.
Haze. Previously haze was common, but with new techniques of MMC application, this has mostly 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 the management of this complication.
Epithelium defect. Epithelial defects usually just takes time to heal with conservative management and observation. Bandage contact lenses are typically 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. DLK 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 usual before the procedure. No change in eating or medication routines.
Topical anaesthesia numbs the eye for the procedure. There is no pain during the process. The patient needs to be awake to fixate on a light. If a patient is very anxious, then tablets are given to relax the patient. The procedure is rapid, and as a routine doing more than topical anaesthesia is excessive.
Placement of protective shields occur for a few hours, then removed, and then the patient can continue as before. The patient should complete the prescribed drops. The only constraint is no eye rubbing. With LASIK, the eyes are reviewed the morning after the procedure. With the use of the Trans-PRK system, the eyes are examined the morning after and then again at day four to remove the placed contact lenses. Routine follow-up is at three months post-procedure. Patients can, however, contact the practice at any time if there are issues.