Myopia is defined as nearsightedness,which exists when the refractive elements of the eye (cornea andlens) place the image in front of the retina. The myopic condition iscommon in infants but generally levels off to normal vision as theinfant ages (Vander & Gault, 1998). Myopia occurs in about 25% ofthe adult U.S. population. Many adults use corrective lenses orcontacts to correct their myopic vision to 20/20 vision (Drexler etal., 1998). Many people find contacts or glasses hindering in theirpersonal and/or professional lifestyle. For example, military pilotscannot wear glasses while flying and some firemen may find glassestoo dangerous to wear during a rescue attempt. There is refractivesurgery available to correct myopic eyes, like PhotorefractiveKeratectomy (PRK). Why do people have myopia, what can be done tocorrect myopia, and what are the results of corrective surgicalprocedures? These are a few questions that will be addressed andanalyzed.
For an eye to focus correctly on an object, it must be placed in acertain position in front of the eye. Theprimary focal point is the point alongthe optical axis where an object can be placed for parallel rays tocome from the lens. The secondary focalpoint is the point along the optical axis where in comingparallel rays are brought into focus. The primary focal point has theobject's image at infinity, where as the secondary focal point hasthe object at infinity. For people who have myopic eyes, thesecondary focal point is anterior to the retina in the vitreous.Thus, the object must be moved forward from infinity, in order to befocused on the retina. The far point isdetermined by the object's distance where light rays focus on theretina while the eye is not accommodating. The far point in themyopic eye is between the cornea and infinity. Thenear point is determined by which anobject will be in focus on the retina when the eye is accommodating.Thus, moving an object closer will cause the perception of the objectto blur. The measurement of these refractive errors are in standardunits called diopters (D). A diopter isthe reciprocal of a distance of the far point in meters (Vander &Gault, 1998). The myopic condition manipulates these variables inorder to ultimately make a nearsighted individual.
Research shows that myopia is caused by refractive and axialconditions, yet the vast majority of research focuses on the lattercondition. Refractive myopia is causedby too much refractive power and axialmyopia is due to an elongatedeye. It has been expressed that axial myopia (eye elongation) is morecommonly studied than refractive myopia. Research seems to link eyeelongation as a common factor among people who have myopia. It isproven that for every millimeter in elongation, there is an increaseof three diopters in myopia (Vander & Gault, 1998). When aninfant 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 eyesreside in a long, elliptic shaped myopic eye. Research has shown thatsome individuals can actually induce thislonger shape by eye accommodation to near workor by theamount of light that reaches the eye. These ideas have beenresearched and studied.
Drexler, Findl, Schmetterer, Hitzenberger and Fercher (1998)researched the idea that an important role in the development ofaxial myopia is the amount of near work and theaccommodation associated with the progression of myopia.Drexler and his colleagues studied 23 pairs of human eyes, 11 wereemmetropic while 12 were myopic. The far and near points weremanipulated, and measurements were taken in order to determine ifaccommodation elongated the eye. He moved a target past anindividual's far point, where the target could still be fixated uponwithout a blur. He estimated that eye elongation happens when one istrying to adjust and to adapt to visual stimuli that is close toone's eyes. This accommodative effort causes the eye's cornealthickness, anterior chamber depth and lens thickness to change shapeto induce elongation. He found that all the eyes that were tested andinvestigated were found to change their shape and become moreelongated. Drexler and colleagues also explained this phenomenon bythe contractionary pull of the ciliary muscle. This muscle isadjacent to the sclera and is involved in forward pulling of thechoroid. This pulling action can possibly cause a decrease of thecircumference of the sclera which can cause an elongation of theaxial length. Other researchers agree that axial myopia is caused byeye accommodation to near work.
Gwiazda, Bauer, Thorn and Held (1995) agree that there may be acausation between the amount of nearwork and the onset of a myopic eye condition. Not just nearwork causes the elongation, but that the blur that is notaccommodated properly for causes eye elongation. Gwiazda andcolleagues did a longitudinal study on 63 children ranging from 6 to18 years of age. They measured accommodation and refractive error ofthe right eye only, where the left eye was covered with an occluder.The researchers used different lens thickness to test theaccommodation efforts to a blur. The participants were periodicallyasked to read a row or column of letters while wearing the differentlenses. The eye was shown to not properly accommodate to the bluritself. Not being able to accommodate to the blur was said to inducemyopia. Besides accommodation efforts, other researches studied theidea that axial myopia was caused by the amount of light that reachedthe eyes.
Stone, Lin, Desai and Capehart (1995) agree that myopia is causedby eye elongation, but it is due to the amount of light that reachesthe eye. Stone and colleagues studied myopia with an outbred andinbred group of chicks. Their theory emphasized that theamount of light the eye received causesan enlarged vitreous cavity. The greatest myopic condition occurredwhen medium amounts of light reached the eye. For Stone andcolleagues, medium amounts of light were defined between 8 to 18hours of light. Most axial length occurred during this 12 hourphotoperiod, and the majority of length was in the vitreous chamber.When the amount of light (photoperiod) interacted with visualdeprivation, eye elongation was also induced. The interaction showedthat altering the proportion of light and dark affected oculargrowth. When one eye was covered, the uncovered eye was more myopicthan when both eyes where not covered. Gwiazda's (1995) researchconsisted of testing only the right eye, while the left eye wascovered (described above). Unknowing to Gwiazda, her research couldhave been skewed according to Stone's findings of a more axialelongation due to an occlusion of one eye. Other research has foundsimilar findings on exposure to a certain amount of light and visualdeprivation in the eye to cause myopia.
Napper, Brennan, Barrington, Squires, Vessey and Vingrys (1995)found that occlusions over the eyes prohibit a naturalamount of light to enter which willultimately induce axial myopia. Napper and colleagues studied threeseparate batches of chicks. The experiments were to determine theminimum 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 adhesivefilm over the eyes. Napper and colleagues found that the chicks whohad constant occluders to the eyes were more myopic than those chickswho had no occlusions. This could be similar to Gwiazda's (1995)findings based on blur-driven accommodation. Looking throughoccluders will not only limit the amount of light, but will alsocause the line of vision to be blurry. Napper and colleagues foundthat short periods of normal vision without any occluders reducedmyopia. They also found that the minimum amount of normal visualstimulation needed to reduce myopia included approximately two hourseach day in a twelve hour light/dark cycle. Research on axial myopiahas provided clues for surgical procedures, in order to shorten thelength of the eye.
One surgical procedure that can correct the myopic eye isPhotorefractive Keratectomy (PRK). Thissurgery involves using an excimer laser photoablation to removetissue from the outer surface of the cornea for refractive purposes.The excimer 193-nm UV laser causes flattening of the central corneathrough a photoablation process that removes central tissue. Also,the stromal tissue is removed to achieve the patient's bestrefractive effect. Research has shown that a certain ablation ratemust occur to produce the best corrected vision (Huebscher, Genth andSeiler, 1996). Huebscher and colleagues researched 11 patients bydoing PRK procedures to determine the ablation rate that is the mosteffective. They found that the best results to produce the bestcorrection of the myopic eye was an ablation rate of .23 to .3Êm per pulse of the laser. Any pulse rate less would result inundercorrection of the myopic eye. As with any surgery, PRK producespositive results as well as complications.
PRK results include positive andnegative research reviews. After surgery, research has shownthat 80% of myopia is corrected. Vander and Gault (1998) showed thatthere was an 80% correction of low to moderate myopic patients. Themajor advantage to PRK is that the cornea is not weakened by thelaser, whereas it is weakened with other myopia surgical procedures,like LASIK or RK. PRK is a better option than the other proceduresfor people who do physical activities. Physical activities, such asjogging 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 aftersix months of surgery the eye restored the corneal tissue to thenatural myopic state. They found that starting two weeks aftersurgery that the stroma rethickened to 98% of its natural thickness.Some other problems discussed by Vander and Gault (1995) include alow to moderate stromal haze. This haze was shown to occur during thefirst 3 to 6 months after the surgery. Stromal haze was said to makepatient's vision blurred and distorted. Although myopia is a commonproblem, it can be corrected with Photorefractive Keratectomy, whichhas positive and negative effects.
The current direction of research tends to be toward axial myopiaand the correction of this eye condition. Research supports thataxial myopia, elongation of the eye, is shown to be caused by theaccommodation effort of the eye as well as the amount of light thatenters 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 refractivemyopia exists as a myopic condition. A common surgical procedure thatis 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 thebenefits and drawbacks of this laser procedure. Myopia is a commoneye condition, and PRK is often used in correcting this axialelongation 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 DynamicRelationship between Myopia and Blur-driven Accommodation inSchool-aged Children. Vision Research, 35, 1299-1304.
Huebscher, H., Genth, U. & Seiler, T. (1996). Determination ofExcimer Laser Ablation Rate of the Human Cornea Using In VivoScheimpflug Videography. Investigative Opthalmology & VisualScience, 37, 42-45.
Napper, G., Brennan, N., Barrington, M., Squires, M., Vessey, G.& Vingrys, A. (1995). The Duration of Normal Visual ExposureNecessary to Prevent Form Deprivation Myopia in Chicks. VisionResearch, 35, 1337-1344.
Pedersen, T., Li, H., Petroll, W., Cavanagh, H. & Jester, J.(1998). Confocal Microscopic Characterization of Wound Repair afterPhotorefractive Keratectomy. Investigative Opthalmology & VisualScience, 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.
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