LASER EYE SURGERY – SO FAR SO GOOD
Over half of the American population requires some degree of vision correction.[1] Eyeglasses and contact lenses remain the most common tools to restore proper eyesight but they are sometimes inconvenient. Not surprisingly, laser refractive surgery is becoming a popular option in vision correction for many people. Each year over a million Americans have a trained professional shine a high-powered laser in their eyes at a cost of $1500 to $2500 per eye.[1-3] The procedure is quick and almost painless, the recovery time is surprisingly short, and the results are nothing short of a miracle; however, nothing comes without a price. Laser eye surgery is relatively new and we are still in the process of determining the short- and long-term effects of this technology.
The three most common causes of vision loss are myopia (nearsightedness), hyperopia (farsightedness) and astigmatism. These conditions are caused by irregular formation of the cornea. The cornea is located on the surface of the eye and is responsible for focusing incoming light onto the retina at the back of the eye. If the cornea is too curved (myopia) light is focused in front of the retina. When this happens, nearby objects are in focus but distant objects appear blurred. A shallow curve in the cornea (hyperopia) causes the light to be focused beyond the retina, which makes it difficult to focus on objects close to the eye. If the cornea is not circular but is instead more oval-shaped, this is called astigmatism. It may be present alone or along with myopia or hyperopia. Astigmatism causes a distortion of the light as it passes through the cornea making it nearly impossible to focus at all.[2, 4-9]
Laser refractive surgery was made possible through the development of the excimer laser. This laser produces a beam of light that is capable of vaporizing thin layers of the cornea, effectively sculpting it into a more appropriate curvature for the eye. Slices measuring 0.25 microns thick (approximately 1/200th the thickness of a human hair) can be vaporized one layer at a time to create the proper curvature of the cornea without damaging the surrounding tissue.[3, 4, 9-12] A computer controls the length and number of pulses required to obtain the desired result. Correction of myopia requires flattening the curvature of the cornea by removing tissue from the center. Conversely, hyperopia is corrected by partially removing the outer edge of the cornea such that its curve is enhanced. Astigmatism is adjusted by removing tissue from the steepest areas of the cornea and making it circular again. The ultimate goal is to adjust the curvature of the cornea so that it more properly focuses light onto the retina. [3, 9]
The excimer laser first received FDA approval for use in the Photorefractive Keratectomy (PRK) procedure in 1995. It later received approval for use in the Laser Assisted In-Situ Keratomileusis (LASIK) procedure in 1998. [13] While both of these procedures use the excimer laser to reshape the cornea, they differ in how they manipulate the thin layer of protective cells that cover the cornea. In the PRK procedure, these cells are removed completely by the surgeon to expose the cornea before the laser is activated. The cells regenerate a few days after the surgery. [3, 6, 7, 9, 14, 15] The LASIK procedure uses a surgical tool called a microtome to create a partial slice of cells that is still connected to the eye by a thin piece of tissue. This ‘flap’ is held to the side while the laser sculpts the cornea; after the surgery it is carefully put back into place. Since the cells do not have to regenerate entirely the overall recovery time is reduced. [3, 6, 7, 9, 11, 15] The whole procedure takes approximately 30 minutes, of which the laser is only active for about 1 minute per eye.[9, 10]
A second system called the Holmium-Yttrium Aluminum Garnet laser console (YAG) was approved for use in the Laser Thermokeratoplasty (LTK) procedure in 2000. LTK is unique in that the laser does not remove tissue from the cornea. The laser creates heat spots in 8 points on the perimeter of the cornea and then repeats the pattern once more just a little further outwards for a total 16 spots. The whole process takes less than 4 seconds. The heat spots cause the cornea to shrink around the edges making it more steeply curved; the amount of shrinkage is directly related to the amount of heat created and the position of the spots. This treatment can only increase the corneal curvature and is therefore useful only in treating hyperopia. [16]
Not everyone is a good candidate for laser refractive surgery. Patients must be over 18 years of age and their eyes have to be in good health. There are a number of conditions that can be aggravated by laser treatment; for example it is not advisable for people with glaucoma or corneal scarring to undergo the treatment. These details are discussed on a pre-treatment consultation with the doctor. He/She will ensure that your vision is stable and that your eyes are fit to undergo the surgery. This involves making a map of the cornea to determine exactly what needs to be corrected and to ensure that the cornea is thick enough to endure the surgery. During the consultation the doctor will review the procedure and discuss the expectations of the patient and the risks involved. [2, 9, 12]
The goal is of laser surgery is 20/20 vision; however, the major determining factor is how the cornea heals after the treatment. Interestingly, statistics show that the most accurate predictions are made when moderate correction is required. When very mild or severe corrections are required the degree of predictability decreases and a second treatment is occasionally required to enhance the results. An improvement in visual quality is usually noticed a few weeks after treatment (the time it takes eyes to heal sufficiently). Over the course of a few months the cornea continues to heal and the quality of vision gradually improves. When it comes to obtaining perfect vision, the LASIK technique has a slightly better record than PRK, with studies showing that approximately 75% of patients achieve 20/20 vision and 95% at least 20/40. The minimum requirement to operate a vehicle in most regions is 20/40 and is considered acceptable by most people. Approximately 18% of patients undergo an additional ‘enhancement’ operation to obtain the results they desire. In LTK the goal is still 20/20 vision but it is more difficult to achieve because the initial adjustment relaxes significantly in the first few days post-treatment and continues to relax over the course of several years. To accommodate the relaxation of the cornea post-treatment a small degree of over correction is introduced during treatment. Due to the continual regression, the LTK procedure is considered non-permanent by the FDA even though 93.5% of patients see 20/20 or better 18 months after the operation. [1, 2, 7, 9, 10]
Complications that may arise after PRK and LASIK surgeries include: infection, scarring, dry eye, halos or starbursts of light, light sensitivity, reduction in night or fog vision, decrease in contrast sensitivity , decrease in best corrected vision (i.e., the patient cannot see as well after the surgery even with glasses/ contact lenses), irregular astigmatism and over- or under-correction. PRK operations are more prone to dry eye, scarring, halos and loss of contrast sensitivity, while LASIK surgeries have a tendency towards flap issues including infection and dislodging or improper placement of the flap. Fortunately, complications occur in less than one percent of operations and minor complications such as dry eye and halos usually resolve themselves in a few months as the cornea fully heals.[11] Over- or under-correction is the number one complaint among patients; however, when interviewing patients, these complaints were usually the result of expecting too much from the surgery. Occasionally the surgery can induce mild astigmatism due to suboptimal positioning of the laser; in this circumstance, a second surgery can be performed to enhance the visual quality. Very few patients experience infection-related difficulties which are usually due to improper sterilizing techniques in the clinic; overall, infections make up less than 1 percent of complications. Even though both techniques are very similar, patients are usually more satisfied with LASIK than PRK at 90 and 52 percent respectively. [9]
There are very few risks associated with LTK because no incision or tissue removal occurs. Again, the most common complaint is over- or under-correction. Additional treatment can increase the amount of correction but little can be done to reduce the degree of correction. LTK is designed for patients with low to moderate hyperopia so over correction is quite minor and usually regresses to acceptable levels shortly after the treatment. [15]
All three laser refractive surgery techniques appear to obtain satisfactory results with a very low number of complications. However, this does not mean that the operation is to be taken lightly. It is important to note that the FDA only approves the use of a laser for a particular application; they do not monitor the techniques used. Many of the complaints can be attributed to errors within individual clinics. Concern has been expressed over the “bargain basement” clinics that guarantee 20/20 vision for low, low prices. To make up the cost these clinics may cut corners which could involve using older equipment and sterilization techniques. [2, 17] These techniques are relatively new and improvements to the methods and equipment are being developed regularly.[18] Efforts are being made to reduce the amount of human error. The system is already largely computer driven and, with the introduction of eye tracking technology and wavefront-guided LASIK, it has become more so. The eye tracking technology allows the computer to check the position of the eye thousands of times per second and repositions the laser accordingly to ensure proper alignment during treatment. Wavefront-guided LASIK uses a second laser to help guide tissue removal and create a cleaner curve in the cornea. By keeping up-to-date, a clinic instills confidence that satisfactory results can be obtained while minimizing complications. [2, 9, 13, 18, 19]
With the outstanding success of laser refractive surgery millions of people are ‘going under the beam’ but they may be doing so with half the information they need. There is a lack of long-term studies focused on the performance and potential risks of refractive surgery. One study reported stability in PRK-treated eyes 12 years after treatment. [10] A more recent study however, has shown that the age degeneration of eyesight over 5 years was greater than normally expected in LASIK treated patients. While patients in this study still maintained vision that was better than their uncorrected state, this study suggests that the stability of the cornea may be compromised during the surgery. Stability of the cornea is of particular importance when the cornea is thin to begin with, possibly allowing for age-related degeneration to progress much more rapidly.[10] Genetic factors can contribute to this degeneration as well. One trait in particular, though rare (1 in 5000), can cause thinning of the cornea after surgery and premature vision loss that can only be corrected with transplants. One in five thousand sounds rare but when almost 2 million surgeries a year are performed the number of people suffering permanent vision loss due to this genetic variable alone is about 400. [4] It is important to remember that although PRK and LASIK are permanent, there are no guarantees that eye glasses will not be required in the future as age related degeneration occurs. [10]
Cataract and cancer formation after surgery are also theoretical concerns. Cataracts are a clouding of the natural lens of the eye that often occurs with age. [2] The laser is tuned so that very little damage occurs to the surrounding tissue but there is a potential for long-term complications. [3] Thus far, there are no studies that have investigated the relationship between laser refractive surgery and cataract formation. It is possible that insufficient time has passed since the first surgeries for an accurate study correlating cataract formation after laser refractive surgery. It may also be too early to correlate any potential increase in cancer in post-operative patients. The laser itself is deemed to be non-carcinogenic, but as our knowledge of factors contributing to cancer formation increases, this status may change.
As with most new techniques, improvements in refractive surgical methods and post operative treatment are still changing from year to year. [18] For example, the use of bandage soft contact lenses after LASIK surgery has been found to cause discomfort in a large percentage of patients and did not appear to contribute significantly to the overall healing process.[20] Small changes in methodology may seem minor but they suggest that further research is required to fully understand the nuances of refractive surgery.
Studies have shown that laser refractive surgery has an excellent track record of being safe and delivering the desired results. Consultation with a professional is essential to understanding the risks and the expected results of these procedures. Complications during and after these procedures are surprisingly rare and most are issues of minor discomfort that can be corrected with eye drops and further healing; the more serious complications such as over-correction and induced astigmatism can usually be corrected with enhancement surgery. One should keep in mind, however, that this data is based on surgeries performed within the last 5 years for LASIK and 12 years for PRK. The long-term effects of these procedures are still undetermined. Accelerated age-related degeneration may be a small price to pay for a mature patient but it may have a more pronounced effect on a 20-year old patient. As with any elective procedure, the patient must weigh the short term benefits versus the long term risks before undergoing laser refractive surgery.
References
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16. Aker, A.B.M.D.F.A.C.S.F.S.E.E. and D.C.M.D.F.A.C.S.F.S.E.E. Brown, Hyperion Laser Thermokeratoplasty for Hyperopia. [Article]. International Ophthalmology Clinics Summer, 2000. 40(3): p. 165-181.
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19. Wikipedia. Wavefront. 14 Nov 2005 [cited 14 Nov 2005].
20. Sekundo W, D.H., Meyer CH., Benefits and Side Effects of Bandage Soft Contact Lens Application after LASIK A Prospective Randomized Study. Ophthalmology, 2005. Epub Ahead of Print: p. 1-5.