The Study of Myopia and Photorefractive Keratectomy
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Myopia is defined as nearsightedness, which exists when the refractive elements of the eye (cornea and lens) place the image in front of the retina. The myopic condition is common in infants but generally levels off to normal vision as the infant ages (Vander & Gault, 1998). Myopia occurs in about 25% of the adult U.S. population. Many adults use corrective lenses or contacts to correct their myopic vision to 20/20 vision (Drexler et al., 1998). Many people find contacts or glasses hindering in their personal and/or professional lifestyle. For example, military pilots cannot wear glasses while flying and some firemen may find glasses too dangerous to wear during a rescue attempt. There is refractive surgery available to correct myopic eyes, like Photorefractive Keratectomy (PRK). Why do people have myopia, what can be done to correct myopia, and what are the results of corrective surgical procedures? These are a few questions that will be addressed and analyzed.
For an eye to focus correctly on an object, it must be placed in a certain position in front of the eye. The primary focal point is the point along the optical axis where an object can be placed for parallel rays to come from the lens. The secondary focal point is the point along the optical axis where in coming parallel rays are brought into focus. The primary focal point has the object's image at infinity, where as the secondary focal point has the object at infinity. For people who have myopic eyes, the secondary focal point is anterior to the retina in the vitreous. Thus, the object must be moved forward from infinity, in order to be focused on the retina. The far point is determined by the object's distance where light rays focus on the retina while the eye is not accommodating. The far point in the myopic eye is between the cornea and infinity. The near point is determined by which an object will be in focus on the retina when the eye is accommodating. Thus, moving an object closer will cause the perception of the object to blur. The measurement of these refractive errors are in standard units called diopters (D). A diopter is the reciprocal of a distance of the far point in meters (Vander & Gault, 1998). The myopic condition manipulates these variables in order to ultimately make a nearsighted individual.
Research shows that myopia is caused by refractive and axial conditions, yet the vast majority of research focuses on the latter condition. Refractive myopia is caused by too much refractive power and axial myopia is due to an elongated eye. It has been expressed that axial myopia (eye elongation) is more commonly studied than refractive myopia. Research seems to link eye elongation as a common factor among people who have myopia. It is proven that for every millimeter in elongation, there is an increase of three diopters in myopia (Vander & Gault, 1998). When an infant is born, the eye is naturally longer than a normal adult eye, and as the child ages the eye retracts to a short, oval shaped eye. Some adults' eyes never retract to a short oval shape, but the eyes reside in a long, elliptic shaped myopic eye. Research has shown that some individuals can actually induce this longer shape by eye accommodation to near work or by the amount of light that reaches the eye. These ideas have been researched and studied.
Drexler, Findl, Schmetterer, Hitzenberger and Fercher (1998) researched the idea that an important role in the development of axial myopia is the amount of near work and the accommodation associated with the progression of myopia. Drexler and his colleagues studied 23 pairs of human eyes, 11 were emmetropic while 12 were myopic. The far and near points were manipulated, and measurements were taken in order to determine if accommodation elongated the eye. He moved a target past an individual's far point, where the target could still be fixated upon without a blur. He estimated that eye elongation happens when one is trying to adjust and to adapt to visual stimuli that is close to one's eyes. This accommodative effort causes the eye's corneal thickness, anterior chamber depth and lens thickness to change shape to induce elongation. He found that all the eyes that were tested and investigated were found to change their shape and become more elongated. Drexler and colleagues also explained this phenomenon by the contractionary pull of the ciliary muscle. This muscle is adjacent to the sclera and is involved in forward pulling of the choroid. This pulling action can possibly cause a decrease of the circumference of the sclera which can cause an elongation of the axial length. Other researchers agree that axial myopia is caused by eye accommodation to near work.
Gwiazda, Bauer, Thorn and Held (1995) agree that there may be a causation between the amount of near work and the onset of a myopic eye condition. Not just near work causes the elongation, but that the blur that is not accommodated properly for causes eye elongation. Gwiazda and colleagues did a longitudinal study on 63 children ranging from 6 to 18 years of age. They measured accommodation and refractive error of the right eye only, where the left eye was covered with an occluder. The researchers used different lens thickness to test the accommodation efforts to a blur. The participants were periodically asked to read a row or column of letters while wearing the different lenses. The eye was shown to not properly accommodate to the blur itself. Not being able to accommodate to the blur was said to induce myopia. Besides accommodation efforts, other researches studied the idea that axial myopia was caused by the amount of light that reached the eyes.
Stone, Lin, Desai and Capehart (1995) agree that myopia is caused by eye elongation, but it is due to the amount of light that reaches the eye. Stone and colleagues studied myopia with an outbred and inbred group of chicks. Their theory emphasized that the amount of light the eye received causes an enlarged vitreous cavity. The greatest myopic condition occurred when medium amounts of light reached the eye. For Stone and colleagues, medium amounts of light were defined between 8 to 18 hours of light. Most axial length occurred during this 12 hour photoperiod, and the majority of length was in the vitreous chamber. When the amount of light (photoperiod) interacted with visual deprivation, eye elongation was also induced. The interaction showed that altering the proportion of light and dark affected ocular growth. When one eye was covered, the uncovered eye was more myopic than when both eyes where not covered. Gwiazda's (1995) research consisted of testing only the right eye, while the left eye was covered (described above). Unknowing to Gwiazda, her research could have been skewed according to Stone's findings of a more axial elongation due to an occlusion of one eye. Other research has found similar findings on exposure to a certain amount of light and visual deprivation in the eye to cause myopia.
Napper, Brennan, Barrington, Squires, Vessey and Vingrys (1995) found that occlusions over the eyes prohibit a natural amount of light to enter which will ultimately induce axial myopia. Napper and colleagues studied three separate batches of chicks. The experiments were to determine the minimum daily period of normal visual exposure to prevent myopia. Normal vision includes not looking through any types of occlusions. Occluders included transparent plastic domes and translucent adhesive film over the eyes. Napper and colleagues found that the chicks who had constant occluders to the eyes were more myopic than those chicks who had no occlusions. This could be similar to Gwiazda's (1995) findings based on blur-driven accommodation. Looking through occluders will not only limit the amount of light, but will also cause the line of vision to be blurry. Napper and colleagues found that short periods of normal vision without any occluders reduced myopia. They also found that the minimum amount of normal visual stimulation needed to reduce myopia included approximately two hours each day in a twelve hour light/dark cycle. Research on axial myopia has provided clues for surgical procedures, in order to shorten the length of the eye.
One surgical procedure that can correct the myopic eye is Photorefractive Keratectomy (PRK). This surgery involves using an excimer laser photoablation to remove tissue from the outer surface of the cornea for refractive purposes. The excimer 193-nm UV laser causes flattening of the central cornea through a photoablation process that removes central tissue. Also, the stromal tissue is removed to achieve the patient's best refractive effect. Research has shown that a certain ablation rate must occur to produce the best corrected vision (Huebscher, Genth and Seiler, 1996). Huebscher and colleagues researched 11 patients by doing PRK procedures to determine the ablation rate that is the most effective. They found that the best results to produce the best correction of the myopic eye was an ablation rate of .23 to .3 Êm per pulse of the laser. Any pulse rate less would result in undercorrection of the myopic eye. As with any surgery, PRK produces positive results as well as complications.
PRK results include positive and negative research reviews. After surgery, research has shown that 80% of myopia is corrected. Vander and Gault (1998) showed that there was an 80% correction of low to moderate myopic patients. The major advantage to PRK is that the cornea is not weakened by the laser, whereas it is weakened with other myopia surgical procedures, like LASIK or RK. PRK is a better option than the other procedures for people who do physical activities. Physical activities, such as jogging or heavy swimming, may further weaken the cornea (Vander & Gault, 1998). Complications are often a result of PRK. Pedersen, Li, Petroll, Cavanagh and Jester (1998) found that after six months of surgery the eye restored the corneal tissue to the natural myopic state. They found that starting two weeks after surgery that the stroma rethickened to 98% of its natural thickness. Some other problems discussed by Vander and Gault (1995) include a low to moderate stromal haze. This haze was shown to occur during the first 3 to 6 months after the surgery. Stromal haze was said to make patient's vision blurred and distorted. Although myopia is a common problem, it can be corrected with Photorefractive Keratectomy, which has positive and negative effects.
The current direction of research tends to be toward axial myopia and the correction of this eye condition. Research supports that axial myopia, elongation of the eye, is shown to be caused by the accommodation effort of the eye as well as the amount of light that enters the eye. Although axial myopia is studied more thoroughly, research may need to direct its attention to another form of myopia, refractive myopia. Few research reports acknowledged refractive myopia exists as a myopic condition. A common surgical procedure that is used for the correction of myopia is Photorefractive Keratectomy. Few research reports were found on PRK within the last five years. More attention should be directed to the PRK technique as well as the benefits and drawbacks of this laser procedure. Myopia is a common eye condition, and PRK is often used in correcting this axial elongation of the eye.
Drexler, W., Findl, O., Schmetterer, L., Hitzenberger, C. & Fercher, A. (1998). Eye Elongation during Accommodation in Humans: Differences between Emmetropes and Myopes. Investigative Opthalmology & Visual Science, 39, 2140-2147.
Gwiazda, J., Bauer, J., Thorn, F. & Held, R. (1995). A Dynamic Relationship between Myopia and Blur-driven Accommodation in School-aged Children. Vision Research, 35, 1299-1304.
Huebscher, H., Genth, U. & Seiler, T. (1996). Determination of Excimer Laser Ablation Rate of the Human Cornea Using In Vivo Scheimpflug Videography. Investigative Opthalmology & Visual Science, 37, 42-45.
Napper, G., Brennan, N., Barrington, M., Squires, M., Vessey, G. & Vingrys, A. (1995). The Duration of Normal Visual Exposure Necessary to Prevent Form Deprivation Myopia in Chicks. Vision Research, 35, 1337-1344.
Pedersen, T., Li, H., Petroll, W., Cavanagh, H. & Jester, J. (1998). Confocal Microscopic Characterization of Wound Repair after Photorefractive Keratectomy. Investigative Opthalmology & Visual Science, 39, 487-501.
Stone, R., Lin, T., Desai, D. & Capehart, C. (1995). Photoperiod, Early Post-natal Eye Growth, and Visual Deprivation. Vision Research, 35, 1195-1202.
Vander, J. & Gault, J. Opthalmology Secrets. Philadelphia: Hanley & Belfus, Inc.; 1998.