by Edward R. Annis, M.D.
For millions of people not blessed with "normal , natural vision", we have cursed the seemingly ever increasing need for glasses or contact lenses to provide clear vision. For some, this process begins in early childhood and progresses throughout their life. For others, the onset may begin in their teens or 20's. An even larger group of people beginning in their 40's experience their arms "getting too short" for reading and find a need for reading lenses.
Most of the treatable etiologies for these visual problems are called refractive errors. We have heard the comparison of the eye to a camera. In a camera, the light rays are focused (or refracted) by the camera lens. The focusing of these light rays onto the camera film into a clear image allows us to see a clear and beautiful photograph when it is developed.
The eye has many focusing elements, but the main two are the cornea and the lens. The cornea is the clear window of the eye through which rays of light pass. They are bent and refracted until they reach the lens of the eye, where additional bending or refraction occurs. The combination of the refraction of the lens and the cornea is to create a clear image on the retina and is analogous to the refraction by the camera lens onto the film.
The retina is the inner layer of the eye which contains special types of nerves that pick up the light rays and send them via additional special ocular nerves to the brain where the image is perceived (developed).
For purposes of this discussion of refractive surgery, we will focus our mental processes on the cornea, since this is where the main realm of refractive surgery presently occurs.
If the image is focused in front of the retina, this condition is known as myopia or nearsightedness. Typically, nearsighted people have blurred vision at distances, but are able to see up close, depending on the severity of the nearsightedness.
If the images come to a focus behind the retina, this is known as hyperopia, or farsightedness. Farsightedness may have visual problems both at distance and near.
If the cornea is not round (like a basketball), but instead has unequal curves (like a football), then the rays of light will be focused at two different planes. This difference in the focus of the two planes is called astigmatism. An eye with astigmatism may have myopia or hyperopia as well.
As we mature, around the age of 42-45, we begin to loose the natural ability of accommodation, a process which allows us to focus clearly at near. This condition is called presbyopia. If we are normal for distance vision (emmetropia), then we will need reading glasses for near vision. If we are near-sighted, we can still see up close without our glasses, but when corrective lenses for distance are worn, we find it difficult to see at the near position. Therefore, either a bifocal is prescribed, or the myope removes the glasses for near vision. Far sighted people will need lenses to see both at distance and near, also.
Refractive problems are routinely improved by helping the eye to focus the light rays utilizing glasses or contact lenses. These techniques have proved to be safe and effective for many years. They have the disadvantage, however, of addressing the problem only while wearing the lenses. When the lenses are removed, vision is blurred because there has been no permanent correction, for the refractive error.
A procedure which has not gained wide acceptance is called orthokeratology. In orthokeratology, contact lenses are utilized to temporarily change the curvature of the cornea to allow improved vision for varying periods of time. The contacts are then removed and then reapplied as needed as the cornea attempts to return to its natural state.
Refractive surgical techniques which aim to permanently change the eye's focus by reshaping the cornea were first effectively introduced by Dr. Svyatoslav Fyodorov in Russia in 1974. Radial keratotomy was cautiously brought to the United States in 1979 and has been slowly gaining acceptance over the last 17 years.
In radial keratotomy, the surgeon makes deep, radial incisions with a microsurgical blade in the cornea for myopia and arcuate (or transverse) incisions near the periphery of the cornea for astigmatism. Although extremely effective and permanent, radial keratotomy did not gain total acceptance because of several factors. The accuracy of the results, though quite good, is not perfect. Enhancements, or touch-ups, are needed in approximately 20-35% of eyes. There is special difficulty in obtaining predicted results in patients with large degrees of myopia. Radial keratotomy, however, has remained an excellent treatment modality for lower degrees of nearsightedness. Astigmatic keratotomy has continued to improve and is an important method for astigmatic reduction. One to six arcuate (or transversely placed) incisions near the peripheral cornea have proved very effective in reducing mild to moderate astigmatism permanently. Complications and side effects have been exceedingly low, but again, perfect accuracy does not exist with astigmatic keratotomy.
Since radial keratotomy did not prove to be as safe and effective as always desired in reducing larger amounts of myopia, other surgical procedures have evolved. In the past decade, ALK (automated lamellar keratoplasty) has been to develop one technique of reducing large amounts of nearsightedness. ALK is a modification of the keratomelusis procedure that has been performed for over 15 years. A surgical microkeratome blade is utilized. Newer instrumentation and techniques are allowing ALK to evolve into a safer technique. Although ALK can correct very large amounts of myopia, it also has the difficulty of accurately predicting the amount of nearsightedness that can be reduced.
A number of surgical treatments for hyperopia (farsightedness) have been developed during the past 10 years, but most have proved to be ineffective, unsafe, or inaccurate. These have included hexagonal keratotomy, which has now been essentially totally abandoned. Hyperopic (farsightedness) ALK has had mixed results.
Attention has turned to higher technology instruments. For the past decade, many efficiently perform refractive surgery. After years of extensive studies, the United States Food and Drug Administration (FDA) has approved two excimer lasers to reduce myopia. The procedure is called photorefractive keratectomy (PRK). The excimer laser produces a very high intensity light which accurately microsculpts corneal tissue. The laser does not utilize any blades.
Although the excimer laser has only recently been approved for use in myopia in the United States, it has been used extensively in other countries for many years. Although primarily for the treatment of nearsightedness, the laser can also treat astigmatism and recent studies indicate the potential for the treatment of farsightedness.
The excimer laser has the advantage of being extremely accurate, especially those up to -7.00D of myopia. As with radial keratotomy, the accuracy of the excimer laser decreases with an increase in the myopia requiring treatment. However, the excimer laser appears to be more accurate than radial keratotomy, particularly in treatment of myopic errors greater than -3.00D. Initial studies have shown a very low degree of complications and side effects coupled with a high success rate.
Both in the U.S. and in foreign countries, surgeons are investigation a technique combining PRK and ALK, known as LASIK. In this procedure, the surgeon utilizes a microkeratome to create a hinged flap in the cornea. The exposed corneal tissue is treated with the excimer laser and the flap replace. This combined treatment has the advantage of faster healing and recovery time, and the ability to treat even greater amounts of myopia than can be safely treated with the excimer laser alone. Complications, although low, can be more significant than those usually encountered in PRK alone.
Other Forms of Treatment of Refractive Conditions
For patients with extreme high hyperopia and high myopia, present treatments listed above can reduce these errors significantly, but not as accurately or as safely as the presently FDA approved excimer laser achieved for myopia under -7.00D. In selected areas in the United States and overseas, several procedures are being investigated.
Clear lensectomy: This is removal of the clear, natural lens of the eye and its replacement by an intraocular lens. This procedure has been performed on millions of people for cataract surgery. Lensectomy is a very accurate procedure, but as with all types of intraocular surgery, complications can occur and although uncommon, can be quite serious.
Intrastromal corneal ring: In this procedure, a plastic band is placed within the cornea in a circular fashion. This provides a temporary flattening of the cornea, reducing myopia. Complications have been rare and its effect is still being studied investigatively.
Intraocular "contact" lens: This tiny lens is placed within the eye in front of the natural lens (phakic intraocular lenses). An implantable intraocular lens is placed within the anterior or posterior chamber of the eye with a power to compensate for the refractive error. Although encouraging results have been obtained, complications with present styles of lenses have proved to be higher than desired. Further modifications of these intraocular lenses is ongoing and the long-term potential looks promising.
The popularity of refractive surgical procedures has increased dramatically in the past five years, especially with development of the excimer laser. Results have been continually improving, while complications and side effects continue to be reduced with newer methods of treatment. The effectiveness of all treatments essentially is related to the amount of one's nearsightedness and astigmatism. The highest success rates and lowest complications and side effects occur in the mild to moderate degrees of refractive error. Although the greater majority of people are able to perform effectively without glasses, no procedure assures 100% success. A small number of patients may still require the use of glasses or contacts all the time following refractive surgery, while others may require only partial dependence upon these devices. While refractive surgery does offer many people an excellent alternative to their dependence upon glasses or contact lenses, it is not for everyone.
All treatments - glasses, contacts and refractive surgery - have their individual benefits and drawbacks. The best method of correcting your vision should be decided after a thorough examination and discussion with your eye surgeon. It is important that your eye surgeon be knowledgeable in all methods of refractive surgery.
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