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Introduction Myopia is the visual condition in which only nearby objects appear in focus, much like a camera permanently focused at a close distance. In a "normal" or emmetropic eye, distant objects are naturally in focus, like a camera focused at infinity. That is, the focal length of the optics of the eye (the cornea and lens) is appropriate to the length of the eye along the axis of these optical elements (the optical axis of the ever. The nearIv parallel rays of light from distant objects are bent just enough to come together to form an image on the retina the part of the eye that detects light. For a close object, the rays of light are diverging and a stronger lens is needed to bend them enough to form an image on the retina. This is done in the eye by increasing the optical power of the lens, an act called accommodation. In contrast, in a myopic eye, the essentially parallel rays from distant objects are bent too much to form an image on the retina; rather they would form an image in front of the retina. That is, the optical power of the relaxed eye is too great for its length. Myopia can result either because the optical power of the eye is abnormally great or because the eye is abnormally long. Since eyes differ greatly in length and optical power, however, one often cannot say that one parameter is abnormal. Instead, it makes more sense to say that, in myopic eyes, the focal length of the optics of the eye is too short for the physical length of the eye. In the opposite condition-farsightedness or hyperopia-the optical power of the lens is too weak for the length of the eye, with the result that even the most distant rays are not bent enough to focus an image on the retina, unless the eye accommodates. The measure traditionally used to express the degree of myopia or hyperopia (collec- tively called ametropia) is the power of the ophthalmic lens required to correct the optics of the eye so that parallel rays from distant objects fall on the fovea of the retina. A negative (concave) lens reduces the optical power of the eye; the more myopic the eye, the more powerful the negative lens required. The unit of measurement of lens power is the diopter (D.), which is the reciprocal of the focal length in meters. The more negative the value of the correcting lens, the greater the myopia. As an approximation, we can describe the degree of myopia as the difference between the focal length of the eye and its actual length (in reciprocal meters). ~, ~v v - er ~ THE WORKING GROUP,S TASK Between 15 and 25 percent of the adult population in the United States is unable to 3
4 see distant objects in sharp focus without spectacles or contact lenses. Almost all these individuals developed myopia after age 7, their myopia increasing in severity until the mid-teens. Typically, an individual's myopia remains relatively constant for the next 30 to 40 years. The evidence strongly favors a disproportionate elongation of the eyeball during growth as the optical basis for this juvenile-onset myopia. Numerous reports suggest, however, based primarily on the population of young adults engaged in college study, that a significant number of individuals may develop myopia or renew myopia progression as they approach their 20s. The prevalence of myopia onset in young adults is unknown, although it has been reported to occur in as many as 20 to 30 percent of young men who enter U.S. military academies. The impact of both juvenile and young adult myopia on the recruitment and reten- tion of Air Force Academy students is substantial and has enormous economic and social consequences. Qualifications for military pilots currently specify no refractive error greater than -0.25 diopter (D.) of myopia in any meridian although waivers may be granted for myopia up to -1.25 D. A high prevalence of myopia in the applicant pool necessarily places constraints on recruitment. Students who are either qualified at entry or have a waiver run the risk of losing their waivers for pilot qualification should they develop myopia greater than -1.25 D. by the time they complete four years of study. This poses a real problem for the U.S. Air Force, not to mention for students' career plans. Obviously, without change in these refractive error standards, enormous cost and human savings could be achieved by the selection of applicants whose refractive error changes could be predicted to be zero or minimal during their training and professional careers. Even if this predictive power were available for the progression of young-adult-onset myopia, however, the armed forces are still faced with the problem of the size of the appropriate pool that is available with respect to refractive error. Clearly the prevalence of myopia in the young adult population, if it assumed major proportions, would be a severely limiting factor in the number of visually qualified applicants for special training in the academies, unless the visual criteria were changed. Consequently, it is of considerable interest to establish the prevalence of myopia among college-age individuals who are eligible for training and to determine if the applicant pool is decreasing significantly because of greater prevalence. Such a study would require a determination of how race, sex, education, socioeconorn~c status, and other factors are related to myopia. Furthermore, there is a need to understand and document how subsequent changes in refractive error can be predicted for young adults. These concerns brought the U.S. Air Force School of Aerospace Medicine to the Com- mittee on Vision. Through it the Working Groun on Mv`,nia Pr~`rnl~n,.- war] Prmo~r"eoimn ~ ~ 0 ---Or ~^ Am, ~ ~ ~ ~ ~ MA-~ C~11~ ~ ~ AJAR ~1~11 1 ~ . ~ ~ was asked to focus its discussion on these questions, purposely avoiding other important issues-such as etiology, the influences of near work and nutrition, and the effects of treatment except as they bore on the central issues of the prevalence and progression of myopia. As a first step in our efforts, we conducted an extensive literature review, identifying more than 500 articles published since 1812 in U.S. and European journals on the subject of myopia. These articles, identified by the working group members, were subsequently pruned to about 150 articles concerning prevalence and progression. These articles were formally annotated in detail according to the following headings: type of study, years for which data were collected, population, criteria for myopia, specific rates, major findings, limitations on interpretability, and summary. These annotated references formed a rich resource for the preparation of our subsequent literature reviews. In addition to carrying out the literature search, the staff explored the availability of published research findings in
s Chinese, Japanese, and Russian, although the search of journals written in these languages was necessarily less exhaustive. INTERPRETING THE MYOPIA LITERATURE The working group conducted an extensive literature search to determine whether (1) there have been significant changes in the prevalence of myopia among young adults eligible for entry into U.S. military academies and (2) the nature of onset and progression of myopia among young adjusts. Certain problems arise, however, in interpreting this literature. Many authors do not make clear, for example, the characteristics of the study population, the measurement methods, or the criteria for myopia. Even when these factors are described, differences in defining them usually prevent direct comparative analysis of the studies. It is generally by interpretation of several studies which have some criteria and characteristics in common and by learning something of how specific factors influence comparisons, that we have gained certain insights into the prevalence and progression of myopia. Some of the difficulties we encountered in interpreting and comparing information from the published literature are described below. Sample Characteristics There are no studies of prevalence or progression of myopia among a completely random- ized~ population that report complete classification or measurement criteria. Most prevalence samples are made up of clinic patients or students. Prevalence outcomes are probably af- fected by ethnic differences, socioeconomic factors, and educational levels. Therefore only those studies that compare similar ethnic groups, similar with respect to age distribution, gender, and socioeconomic backgrounds, can provide useful information concerning changes in prevalence over time. A major problem in dc~cumentinF Fondler Hiff~r~n`~ in mvnni:` nr~val~n~" is that. they appear likely to be age dependent. The reporting of overall gender differences over a wide range of ages not only potentially obscures possible age-specific patterns, but also introduces the possibility of confounding by age distribution. Unfortunately, many studies are not large enough to allow stable estimates for age-specific gender comparisons, and statistical techniques that might allow for control of confounding or allow for aF~-F~nH~r interaction are not ~enerallv aDDlied ___ to to ~,., .^,~ _~ ._ lo, _, I_ a_ . ~~ ~ ~ v I. ___ ~_ ~a _ a _ ~ ~ ~ ¢ ~ ___ _ Samples in which progression is studied often change. For example, withdrawal from a sample may not be independent of the refractive error status of the subjects. Measurement Method Some authors estimate prevalence or onset and progression of myopia by measures of distance visual acuity employing refractive techniques. Nineteenth-century investigators often used an ophthalmoscope to estimate refractive error by determining the lens power necessary to bring the retina into focus. Subjective or retinoscopic refractions provide data for some of these earlier studies and for almost all studies during this century. More recently, automated refractors have provided the data for a growing number of studies. Cycloplegic refraction sometimes yields different results than noncycloplegic (i.e., drug- less) refraction. Maximum temporary ciliary paralysis results from repeated application of atropine (though this is rarely done). Other cycloplegic agents vary in their effectiveness with such factors as method of application and refractive error. There are also individual variations in response to cycloplegia related to ocular pigmentation.
6 The results of different examiners employing different testing methods make compar- isons of prevalence and progression difficult. In any event, the effect of measurement errors is greater the lesser the degree of reported myopia or myopic change. Classification Criteria A major problem in comparing data is due to classification criteria's not being reported, being significantly different across studies, or including very small degrees of refractive error. Prevalence studies ranged in classification of myopia from that greater than -2.00 D. to as little as -0.12 D. (Throughout the text, a negative sign indicates a change toward myopia, and a positive sign a change toward hyperopia.) Progression studies are also difficult to compare, especially when the change is less than -0.75 D. This is particularly true when different examiners, different instrumentation, or cycloplegic and noncycloplegic examinations are involved. In progression studies, a difference in refractive error from one period to another is the primary measure; therefore, reliability of the measure is critical and may be diluted when different examiners perform refractions at the two testing periods. Even when estimates of reliability are reported, often little or no attempt is made to incorporate them into interpretation of the data reflecting myopic shifts. For example, most studies legitimately can report a reliability of +0.25 D. for either cycloplegic or subjective refractive techniques. This means that a small number of subjects will show a shift of -0.75 D. or more simply due to measurement error. By comparison, a much larger group will show an actual shift of -0.25 D. or more. Despite this, many studies report shifts in myopia using -0.25 D. or less as the criterion for onset or progression of myopia. This would inflate the estimate of numbers demonstrating myopic shift, but probably not that of mean refractive changes in large samples. When classification criteria are known, the effect on prevalence data can be estimated. For example, we used a clinical population study (Hirsh, 1950) to derive the change in prevalence of myopia when the criterion for myopia is changed. If, instead of accepting myopia as -0.50 D. or more (used by many studies), we adopt a criterion of any minus refractive error, the prevalence level increases by a factor of 1.2. If the criterion now shifts from -1.00 D. or more to any refractive error, the prevalence increases by a factor of 1.5. Some studies report myopia prevalence in terms of a negative refractive error in either eye, while others report one eye only. Estimates of prevalence are greater when both eyes are reported. The presence of astigmatism also provides a potential complication in comparing data from different investigations. Many studies report myopia in terms of spherical equivalent, whereby the refractive error is an average of the refraction in the two major meridians. Others report only a spherical component of the refractive error. This component can be expressed as two numerically quite different amounts, depending on whether the cylindrical (astigmatic) component is expressed in terms of "plus" cylinder or "minus" cylinder correction. GUIDE TO THE REPORT This report is organized as follows: Chapter 2 is our analysis of the prevalence literature, Chapter 3 is our analysis of the progression literature, and Chapter 4 lists our conclusions and recommendations. Separate appendixes contain a description of the biological basis of myopia (Appendix A), a detailed review of the prevalence (Appendix B) and the progression (Appendix C) literature, a description of the etiology of myopia (Appendix D), and a
7 glossary of terms for those unfamiliar with work in this field (Appendix E). The report concludes with a complete bibliography. The detailed reviews of the prevalence and progression literature in Appendixes B and C differ from the analysis reported in Chapters 2 and 3 in that they focus on reviewing the literature, with minimal attempt to draw general conclusions. The appendixes provided the basis for the more selective analysis found in Chapters 2 and 3. For readers interested in a more comprehensive review of the literature, the appendixes provide an additional resource. CONCLUSION We are impressed with the diverse nature of myopia and the multiple factors that presumably contribute to it. Indeed, myopia is itself best thought of as the final consequence of variations in a number of biological, sometimes pathological factors influencing refraction in the human eye. Myopia may result, for example, from an abnormally steep cornea, a crystalline lens with greater than average thickness and power, or axial elongation of the eyeball. Alternatively, it may result from the mismatch of these optical components, even though each individual component itself is within the normal range of values. Finally, myopia onset in childhood may have a totally different basis than myopia onset in young adulthood. Understanding the growth of the separate dioptric components of the eye is important if significant progress is to occur in understanding the underlying causes. Even though we encountered limitations in interpreting the literature on progression and prevalence of myopia, it was nevertheless possible to formulate some important conclusions. These conclusions led us in turn to recommend several actions to be undertaken both by the military community and by clinical investigators to achieve a better understanding of the mechanisms of myopia onset and progression.
