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3 . Analysis of the Progression Literature The characteristics of myopic onset and progression between ages 7 and 16 are such that it can be considered epidemiologically as a distinct form of myopia and the most common. In addition, evidence is mounting that myopia that has its onset (or that increases after stabilizing in the teens) during young adulthood occurs in a significant portion of the young adult male population. Although it seems likely that females would be affected at approximately the same rate as males at this age if conditions were similar, males have been predominant in reported cases of adult myopia onset and progression. Much less information has been reported concerning females, although two studies with information on young adulthood changes in females were reviewed. One concluded that females are less likely to show myopic onset or increase after age 17, although it was noted that such findings could be biased by sex differences in culturally determined occupational patterns (Goss et al., 1985~. The other study reported equal incidence of loss of unaided distance visual acuity among male and female college students (Parnell, 1951~. Progression studies must involve longitudinal evaluations of change in refractive status during or prior to young adulthood. Some studies make comparisons between subgroups, but most report results of two refractions of a selected group of subjects taken at intervals of two years or more. In many studies, all subjects have been engaged in common working or academic environments during the period between examinations. JUVENILE MYOPIA Even though we are interested primarily in myopia that is characterized by onset during late adolescence or early adulthood, some review of the most common form, appearing between the ages of 7 and 16, may add to our understanding about myopia with later onset. Myopia that appears during childhood has been much studied since Cohn reported in 1867 its increasing prevalence with age. Cohn (1886) attributed the cause to intensive near- work demands in German gymnasia and implicated increased power of the crystalline lens. We now know that axial elongation is the mechanism primarily responsible (Baldwin et al., 1969; FIedelius, 1981b; Larsen, 1971d; Sorsby and Leary, 1970; Tokoro and Suzuki, 1968, 1969; Tokoro and Kabe, 1964, 19653. However, Cohn's findings of increased prevalence and progression during elementary and secondary school have been confirmed by studies in several countries throughout the intervening years (Baldwin, 1957; Banerjee, 1933; Blegvad, 1918; Bucklers, 1953; Conrad, 1874; FIedelius, 1981b; Goldschmidt, 1968; Hirsch, 23

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24 1952; KalogJera, 1979; Mantyjarvi, 1985b; Popov, 1931; Rosenberg and Goldschmidt, 1981; Saunders, 1986a; Sorsby and Leary, 1970). Many cross-sectional studies of refractive status provide means of subgroups at various ages. Most show that the average value of refractive status ranges from +1.00 D. to +2.00 D. (under cycloplegia) before age 7. At approximately age 7, the mean refractive status begins to shift toward myopia and continues to do so until the late teens. It then remains essentially constant up to about 40 years, at which time a slight shift toward hyperopia begins. Beyond age 65 a slight shift back toward myopia is evident, at least in part due to nuclear sclerosis of the crystalline lens in approximately 10 percent of the elderly population who develop senile cataracts. Similarly, the decrease in mean refractive status during the period beginning at age 7 is produced not by a general decrease in refractive status but by the onset of myopia and its progression among a portion of the population. By age 25, from 15 to 25 percent of samples from clinic or general population groups have been found to be myopic by one or both of the following criteria: visual acuity is less than 20/20 and improved to 20/20 by concave lenses, or an amount of myopia of -0.50 D. or greater is manifested by objective or subjective measuring techniques. Once myopia develops, it rarely decreases in magnitude prior to the onset of presbyopia. At approximately the same time as the development of presbyopia, decreases of small magnitude sometimes occur. The question of whether refractive findings obtained under cycloplegia can be compared with those determined without cycloplegia has been addressed. Myopes less than 40 years of age show significant differences less often than do hyperopes. The effect is to increase slightly the average positive values of refraction in cycloplegic studies. Myopes show less mean change than do hyperopes. Therefore, prevalence figures of myopia or mean refractive states of myopes probably are not affected significantly by use of any cycloplegia. An exception to this is when atropine cycloplegia is used to produce complete paralysis of accommodation; In these rare studies, greater caution is required in comparing cycloplegic to noncycloplegic results. The few longitudinal studies that have been conducted confirm the conclusion that from 15 to 25 percent of populations of children become 1 D. or more myopic between the ages of 7 and 13 rather than a smaller myopic shift occurring among a larger proportion. Many investigators have found that the earlier the onset of myopia, the higher the level at which it stabilizes (Bucklers, 1953; FIeischer, 1907; Norris, 1885; Rosenberg and Goldschmidt, 1981; Septon, 1984~. Goss and Winkler (1983) and others (Baldwin, 1957; Brown, 1938; Brown and Kronfeld, 1938; Bucklers, 1953; CalIan, 1875; Goss and Cox, 1985) have shown that myopic progression often slows during the early teens and typically stabilizes during the mid-teens. In a sample of 299 records of young clinic patients who were myopic, 75 percent of Goss and Winkler's male subjects showed no progression after age 17. Stabilization occurred a year earlier in the female sample, and 87 percent showed no progression after age 17. Saunders (1986b) performed cluster analysis on a longitudinal sample of young myopes and also found two distinct groups, but he found stabilization occurring over a wider age range: those who progressed after age 20 tended to continue to progress well into their 30s. Nonmyopic children who are at risk of becoming myopic have been identified by various investigators as those who exhibit emmetropia or a very mild degree of hyperopia. Hy- peropes above +1.50 D. rarely become myopic. Some investigators have found that they seldom become less hyperopic, and often become more severe hyperopes (Baldwin, 1957; Brown, 1936; Sorsby, 1933; Sorsby and Leary, 1970; Sourasky, 1928; Young, 1955~. Data from military academy studies involve very low numbers of severe hyperopes, but at least

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25 one study (O'Neal et al., 1987) suggests myopic shifts may occur in young adult students at least as often as among low hyperopes. Socioeconomic, cultural, vocational, ethnic, and personality differences have often been found between children who have become myopic and those who have not (Baldwin, 1981~. Many of these associations have been related to increased interest and activity in reading or other forms of near work or to greater success in these pursuits. While some association between juvenile-onset myopia and reading or academic achievement seems clear, there is little if any evidence to distinguish between cause and effect or whether these are commonly related to other variables. Additional risk or associated factors for juvenile-onset myopia have been reported by several authors. Both heredity and culture have been implicated by theorists. Ocular predictors include esophoria (Goss, 1986b; Roberts and Banford, 1967) and against-the- rule astigmatism (Baldwin, 1957; Fulton et al., 1982; Kronfeld, 1930; Hirsch, 1964b). Premature birth has been implicated in all degrees of myopia (Fletcher and Brandon, 1955; Lledo, 1976; Scharf et al., 1975; Yamamoto et al., 1979~. Higher prevalence of juvenile myopia has been found among those children who have suffered prior febrile disease (Hirsch, 1957; Sander, 1920~. Both dental caries (Goldstein et al., 1971; Hirsch and Levin, 1973; Keller, 1978) and malocclusion (Cattell, 1928; Manna and Mackiewicz, 1976) have been reported to be associated conditions. Incidence of myopia was found to increase among both children (Halasa and McLaren, 1964; McLaren, 1960) and adults (Livingston, 1946; Reed, 1947; Smith and Woodruff, 1951) suffering from severe malnutrition. This myopia may be reversible to some extent, suggesting a lenticular origin (McLaren, 1960~. Children of myopic parents are likely to become myopic (Hirsch and Ditmars, 1969; Keller, 1973~. It appears that there is a genetic factor. at least in the DredisDosition to become mvc,nic but neither , ~ , , ~,' ~, ~ its nature nor the degree of its influence has been established. It also appears that intensive near work sometimes produces a reversible form (accommodative or pseudomyopia) of low degree (Banerjee, 1933; Borghi and Rouse, 1985; Bothman, 1931; Conrad, 1874; Ebenholtz, 1986a; Hynes, 1956; Young et al., 1970~. It is not known whether this can cause axial elongation and, thereby, permanent myopia. YOUNG ADULT MYOPIA Few existing reports provide data specifically directed at studying refractive changes among general populations from ages 16 to 30. We cannot extrapolate from cross-sectional studies to determine whether the slight decrease in annual means of refraction from ages 16 to 30 results in continuing progress in myopia among a smaller portion of teenage myopes or whether new myopes appear in significant numbers after the mid-teens. Studies that were reviewed support the conclusion that a number (perhaps 20 to 40 percent) of those college-bound individuals who are hyperopes of low degree or erntnetropic before the late teens become myopes of low degree during this period or soon thereafter, and that a somewhat~greater proportion of myopes become slightly more myopic (perhaps as many as 60 percent). Because these changes are small in degree, they have minimal influence on measures of central tendency. This is particularly true when samples include a broad age range. In this way a phenomenon of considerable import to those who determine eligibility criteria for admission to military academies is masked. Groups of young males between ages 17 and 21 have been examined since 1876 to determine uncorrected visual acuity or refractive status prior to admission to college or military officer training, and again after a period of time. Studies of those ages 17 to 21 at entrance generally report that subjects exhibiting hyperopia of greater than +0.75 D. in the

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26 TABLE 8 Percentage of U.S. Naval Academy Entrants Who Became Myopic, 1956 Spcrical Equivalent Refraction at Entrance (Homatr 4.0%) ages 17-18 age 19 age 20 age 21 +1.00 D. and above 1.5 1.5 2 3 +0.62 D. to +1.00 D. 11 8 4 2 +0.50 D. or less 69 56 33 10 SOURCE: Hyncs (1956~. meridian of least hyperopia (perhaps as much as +1.25 D. if refraction is determined under cycloplegia) are unlikely to become myopic, even in those environments in which hyperopes of lower degree often become myopic, and in which myopes show progression. Myopes show myopic shifts approximately twice as often as low hyperopes, and mean magnitude of change is somewhat greater. More recently, young adult males from age 17 to the mid- to late 30s have been evaluated before beginning, and after exposure, to extensive near work in nonacademic settings. These are analyzed later again this chapter in the section on occupational studies. Some studies show that subjects who are low hyperopes at age 17 are more likely to show changes than those who are older (see Table 8~. Movement from low hyperopia into Tow myopia and myopic progression appears to be a more common phenomenon among young adults than traditionally assumed. The following analysis of myopia onset and progression among young male adults is based on a review of the pertinent literature. In several instances, data from the original study could be reordered with respect to refractive categories or age ranges, thus permitting comparisons. College Students Unquestionably, many young adult males who undertake college study or assume roles that involve extensive near work become less hyperopic or more myopic. For very low hyperopes, the change may be great enough to produce a low degree of myopia. While many myopes also become more myopic, the former group represents a unique concern to those organizations that require unaided distance visual acuity of 20/20 or better at the end of training periods as well as at the beginning. At present, both the U.S. Air Force Academy and the U.S. Naval Academy enforce these requirements for those who would become pilots and naval line officers. As noted earlier, U.S. military cadets have backgrounds similar in most respects to those who matriculate in U.S. colleges and universities (Houston, 1972~. The only exception known to be related to myopia onset or progression is that most cadets were in the top 10 percent of their high school class scholastically, while few of the colleges and universities surveyed were this selective. The prevalence of myopia among cadets in the West Point studies

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27 Occupational Studies Longitudinal studies of the effect of length of time spent in a targeted activity range from 1 to- 17 years' duration beginning with young adults. Cross-sectional studies sug- gest that greater mean rates of change occur during early periods because mean rates of change decrease after the early 20s. Data suggest that the total amount of myopia that develops among those who were emmetropes or low hyperopes before entering environments associated with risk seldom exceeds -1.00 D., and that when myopia progresses in these environments, the progression is seldom as much as -2.00 D. A few longitudinal studies involving small numbers show that individuals may change at a fairly constant rate over several years, even into their 30s (Diamond, 1957; Kent, 1963; Kinney et al., 1979; Provines et al. 1983; Riffenburgh, 1965~.

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28 TABLE 9 Hyperopic U.S. Air Force Pilots and Navigators Who Became Myopic, 1983 (percentage) Elapsed Time (years)Pilots Navigators 1-52.4 7.8 6-107.4 10.4 11-1510.4 22.6 16-2017.6 24.7 Total7.8 13.6 SOURCE: Prov~nes et al. (1983). Studies of incidence must be longitudinal to distinguish between nonmyopes who become myopic and myopes who progress. From this point only longitudinal studies are referenced in which all members of a group have been exposed to a common environment. The studies examined are those in which individuab are tested before and after prolonged exposure to heavy reading demand or to a restricted work environment. Studies that include determination of visual acuity and refractive change show a very high correlation between the two; reports that assess visual acuity deterioration can therefore be compared with those that measure only refractive change, as far as onset of myopia or its progression is concerned. Provines et al. (1983) compared myopic changes among U.S. Air Force pilots with those of navigators. Both were ages 20-25 when initial refractive data were collected; second records were obtained 1 to 20 years later. Table 9 indicates that a higher percentage of navigators became myopic at all elapsed time intervals after initial examination. Comparison of Provines' data can also be made with those of two U.S. Air Force Academy classes (Goodson, 1983; O'Neal et al., 1986, 1987~. Table 10 shows the data for O'Neal (1986~. Virtually none of those exhibiting hyperopia of +1.00 D. or greater at first encounter became myopic, although a high proportion shifted in the myopic direction. A significant number of those near emmetropia become -0.50 D. or more myopic. Others report myopia developing and progressing among individuals in this age range. For example, Diamond (1957) found that, over 5 to 17 years, 24 percent of the 67 pilots studied became myopic after age 21. No myopia occurred for clinically significant amounts of hyperopia. All who became myopic (-0.25 D. to -1.25 D. spherical equivalent) were emmetropes or very low hyperopes between ages 21 and 31. Kent (1963) found similar changes in a small sample of naval officers. Kinney et al. (1980) compared refractive changes of naval submarine crew members with those of National Guardsmen over a 3-5-year period. Subjects ranged in age from 18 to 41: 51 percent of the submariners showed unaided visual acuity loss versus 20 percent of the control group. Greene (1970) found that servicemen (all were college graduates) confined within ballistic missile sites tended to become myopic or increase in myopia. The mean change for those showing myopic shift over a four-year period was approximately -0.65 D.

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29 TABLE 10 Myopic Shift After Years in the U.S. Air Force Academy Class of 1985 Spherical Percentage Percentage Equivalent Total Myopic Shifting Percentage Refraction Number Mean Standard Shifting -.50 or Shifting At Entrance (Eyes) Change Deviation -.25 or more more -.75 or more +1.00 and above 35 -0.37 +0.51 65.7 37.1 20.0 +.25to+.87 336 -0.16 +0.34 45.8 19.6 73 +.12 to -.12 184 -0.21 +0.45 41.3 25.0 14.7 -.25 to -.87 192 -0.38 +0.48 68.2 43.8 22.6 -1.00 and above 247 -0.70 +0.70 77.S 64.0 48.4 SOURCE: Calculated from O'Neal et al. (1986~. U.S. Military Cadets Taken together, studies of cadets at U.S. military academies provide convincing evidence that about one in five young emmetropes and low hyperopes become low myopes in an intense academic environment and that more than half of all myopes, most of whose mvonia has stabilized under the same circumstances will show mvocia progression. ,, ~ That significant myopic shifts occur commonly among military cadets and consequently lead to visual disqualifications was first documented in the early 1940s. Hayden (1941) was the first to report myopic changes among midshipmen. His findings led him to recommend specific refractive error limits for entrance to the Naval Academy. From 1934 to 1940, unaided visual acuity dropped below 20/20 over the four-year training period for 21 percent of all graduates. On the basis of Peters's (1961) evaluation of visual acuity and myopia, we can assume that virtually all these changes in visual acuity were due to refractive shift into myopia. Hayden compared atropine cycloplegia at entrance examinations to homatropine cycloplegia: 65 percent of the 127 subjects found to be myopic under homatropine had been hyperopic under atropine when admitted. Almost half became visually disqualified within three years, while only 7 percent of those meeting the entrance standard of no myopia in any principal meridian under homatropine (4 percent) became visually disqualified during this period. Hayden reported that "the vast majority of candidates whose refraction was of the Plato type, or +0.25 D. of hypermetropia, on their entrance examinations and some who showed +0.50 D. of hypermetropia at this time and were therefore physically qualified for admission were found to be physically disqualified because of defective vision (less than 20/20 in each eye) within one or two years after admission to the Naval Academy." He reported that all had become myopic and recommended that at least +1.00 D. of hyperopia be required for admission to the Naval Academy. But how would such refractive constraints on admittees affect the eligible pool of candidates for the military acadeTn~es? On the basis of Betsch's (1929) large sample of adults, approximately 20 percent would remain eligible by this criterion. In the Betsch

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30 sample, less than 10 percent were -1.00 D. or more myopic. It is possible that hyperopes of this degree are underrepresented in this clinical sample, but Betsch and others have reported that there is no significant difference in prevalence at this and other refractive intervals between clinical and general Populations. For example. Bennett and Rabbetts ~ _ ~--or ~~~ ~ ~~~~~ ~ _ ~ ~ ~ ~ ~ . _ _ . ~ . ~ . . . . _ t1Yb4) found zu percent ot spectacle prescriptions, among those receiving free eye care services in England, were from +1.12 to +2.00 D. (this is the range for hyperopes likely be admitted to the Naval Academy). Moreover, Brown and Kronfeld's (1929) sample exhibited 10 percent prevalence of hyperopia +1.25 D. or above under atropine. Therefore the figure of 10 to 20 percent eligible based on prevalence figures for a general population appears valid. However, although no studies reveal the prevalence of hyperopia +~.25 D. Or above for young adults who could qualify for military cadet training, studies do show the prevalence of myopia among this group to be much greater than that found for young adult populations that include nonstudents and individuals who were not college bound. In a study of refractive errors among 836 entering West Point cadets, Brown (1986) found only 7 (less than 1 percent) who were +1.00 D. hyperopic or above, while 286 (34 percent) . . . ~~ . . , _ ~, . were myopic greater than-1.00 D. Gmelin (1976) found 0.05 percent cadets in the 1970 entering class were +1.00 D. or more hyperopic, while 44 percent were -1.00 D. or more myopic. West Point admission requirements include allowable refractive errors of +5.50 D. Biederman has estimated that the proportion of eligible males of +1.00 D. or greater may be as low as 1 percent of the population of 18-year-old males (personal communication, I. Beiderman, Department of Psychology, University of Minnesota). Of those eligible, based on a proposed spherical equivalent of hyperopic refractive error at entrance, what percentage could be expected to meet visual acuity standards upon graduation? Hynes (1956), in a study of the 1949 and 1950 graduating classes at the U.S. Naval Academy, found that 18 percent who had met unaided visual acuity requirements at entrance failed at graduation. He reported that a significant proportion of the acuity loss was due to development of myopia, older candidates reportedly being less likely to become myopic. Table 11 is a compilation from Hynes's data. Visual acuity data and refractions (retinoscopy under homatropine) were conducted under similar conditions at initial and final test periods. Hynes concluded that, if cycloplegic testing reveals hyperopia above +0.50 D., the attrition from onset of myopia would be less than 10 percent. What are the risk factors for the rate of progression of myopia? Shotwell (1981) conducted an investigation of myopia among Naval Academy students that suggested that those who spend more time reading, as opposed to outdoor activities, are more likely to show myopic onset and progression. However, he was unable to demonstrate any preventive effect in a study of the influence of convex reading lenses (Shotwell, 1984~. The Naval Academy studies show that those who initially exhibit 20/20 visual acuity, but who develop myopia of low degree when placed in the Academy environment, have two characteristics in common: they are more often at or near the lower end of the age spectrum (age 17 to 21 at entrance) and most are essentially emmetropic (range from +0.50 D. to -0.50 D. in at least one (often all) principal refractive meridians) rather than hyperopic at entrance. These studies also indicate that the myopia that develops is of low degree, and that many hyperopes above +0.50 D. may have a similar shift in refractive error, which results in a lowering of their hyperopia, at least for hyperopes up to +1.00 D. For hyperopes of higher refractive error, there is some evidence that they tend not to show decreases at this age (Baldwin, 1957; Bucklers, 1953; Sorsby, 1933; Sorsby et al., 1955~. By the 1950s, the military academies had begun to tolerate refractive errors and uncorrected visual acuity less than 20/20. Beginning in 1960 the U.S. Military Academy at West Point instituted liberal admission policies with respect to refractive errors. Since that

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31 TABLE 11 Refractive Error Change of 244 U.S. Naval Academy Graduates Who Retained 20/20 Acuity SphericalEquivalent Refraction At Entrance At Graduation N Percent N Percent + 1.00D and above 13856 46 19 +.62D to +1.00D 9439 112 46 +.SOD endless 125 86 35 Total 244100 244 100 SOURCE: Hynes (1956). time there have been a few studies of cadets, which compare refractive error at entrance to that at graduation. Sutton and Ditmars (1970) reported that, from 1964 to 1968, 45 to 51 percent of entering-cIasses were myopic based on the criterion that concave corrective lenses were required to improve visual acuity to 20/20. Based on the number of hyperopic corrections worn, their magnitude, and uncorrected visual acuity expected of hyperopes, this criterion would not lead to overrepresentation of myopia by more than 2 percent. At graduation the number of nonmyopes (by this criterion) who became myopic increased by an average of 15 percent for the four classes. It is probable that almost all visual acuity loss was due to a shift from emmetropia or low hyperopia into myopia. As a point of reference, 33 percent of the total Army active duty personnel at about that time wore corrective lenses to achieve visual acuity of 20/20 (Rengstorff, 1972~. Sutton and Ditmars also note that more than half of the admittees ranked academically in the top 10 percent of their high school classes and 80 percent won varsity letters. Gmelin (19763 compared refractive findings in the U.S. Military Academy class of 1974 at entrance and after four years: 67 percent wore spectacles or contact lenses at graduation; 28 percent of those who were from +0.50 D. to +1.50 D. hyperopic became sufficiently less hyperopic to be placed in a Tower refractive category at graduation. If we assume that all changes in category were toward myopia and were 1 D. or less, then 20 percent of emmetropes became myopic (-0.50 D. to -1.50 D.~. This is confirmed by the author's report that the number of nonmyopes who become myopic was 81 (22 percent). This slight increase would be expected because a few hyperopes above +0.50 D. become myopic (Hynes in 1956 found 7 percent). This confirmation helps us interpret refractive changes in studies that merely report difference in numbers at various refractive ranges with time. Brown (1986) compared the (noncycloplegic) refractive status of 418 West Point cadets determined prior to admission in 1975 and at the beginning of their senior year. A total of 418 subjects were randomly selected from the fourth-year class, and spherical equivalents were determined. The mean myopic change for the group was -0.66 D. Brown reported that the mean change of five subgroups, all of whom were myopic at entrance (entering spherical equivalent refractive errors of -0.50 D. to -6.50 D.~; the amount of mean myopic change was similar for each group (see Table 12~.

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32 TABLE 12 Mean Refractive Error Change of U.S. Military Academy Seniors Who Were Myopic at Entry Mean Spherical Equivalence at Entrance Myopic Change -.50 D. -1.50 D. -2.50 D. -3.50 D. -4.50 D. -5.50 D. -6.50 D. -.71 D. -.67 D. -.67 D. -.78 D. -.67 D. -.57 D. -.63 D. SOURCE: Brown (1986). An unpublished study of the U.S. Air Force Academy graduating class of 1980 (Goocison, 1983) and a recent published technical report on the U.S. Air Force Academy class of 1985 (O'Neal et al., 1986, 1987) provide refractive error data that permit some comparisons. Both classes showed similar percentages of entering myopes, both with a myopic change after two or three years of -0.50 D. or more (57 percent) and with a change of -1.00 D. or more (25 percent). The percentage of entering hyperopes, emmetropes, and myopes with no shift or a hyperopic or myopic shift of 0.25 D. or more is shown in Figure 1 for the 1985 group. For the entering emmetropes and hyperopes as a group, approximately 30 percent in both classes showed a negative change that resulted in a spherical equivalent myopia of no greater than -0.25 D. However, data forthis refractive group derived from the 1980 class show a more marked trend towards myopia of -0.50 D. or more at the second examination (43 percent) than for the 1985 class (15 percent). A myopic change of -1.00 D. or more was seen in 30 percent of the 1980 group of emmetropes and hyperopes, while only 6 percent of the 1985 group showed change of this degree. For the +0.75 to +1.50 D. hyperopes, a myopic change of -1.00 D. or more occurred in 43 percent (21) of the 1980 group, while only 2.5 percent of the 1985 group changed by this amount. The possible factors accounting for these differences in refractive error changes are the size and selectivity of the 1980 sample, and the added year in the academic environment in the Goodson study. In the 1980 class there were only 65 subjects in the group of emmetropes and hyperopes; in the 1985 group there were 245 emmetropes and hyperopes. O'Neal gives more detailed changes for specific ranges of entering refractive error. The mean refractive error change over the 2.5 year period in the O'Neal study is given in Table 10 for various levels of entering refractive error. This table also shows the percentage of eyes at each refractive error level that showed a myopic change of -0.25 D. or more, -0.50

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33 100 90- 80 - 70 ~n LL ~60 llJ 50- 11 o Z 40- C: 30 20 - Emil HYPEROPES O EMMETROPES MYO PES 18.5 02: 1 . USAF ACADEMY CLASS OF 1985 40 47.7 HYPEROPIC NO SHIFT MYOPIC TYPE SPEO REFRACTIVE ERROR SHIFT (20.25) FIGURE 1 Percentage eyes in each type of entering spherical equivalent (SPEQ) refractive error showing either a >0.25 D hyperopic or myopic shift or no shift in SPEQ between the entrance and third academic year exams (2.5 year period) for 994 eyes from the U.S. Air Force Academy class of 1985. Source: O'Neal et al., 1986. D. or more, and of -0.75 D. or more. Figure 2 shows that a larger proportion of myopic eyes shift toward greater myopia at all initial levels of myopia than do low hyperopic eyes. This figure also shows that the greater the initial myopia, the greater the myopic change is likely to be. A more meaningful description of the refractive error change is obtained by analysis of the myopic change data separately. Goodson's three-year data show that 16 percent of the myopic shifts were -~.50 D. or more, but the numbers were small: 5 of 40 hyperopes, 5 of 24 emmetropes, and 21 of 132 myopes. In O'Neal's study, only 7.5 percent of the myopic shifts were -1.50 D. or greater: 4 of 209 hyperopes, 3 of 99 emmetropes, and 43 of 356 myopes. In O'Neal's study, the highest myopic shift was -1.75 D. for the entering hyperopes or emmetropes and -3.00 D. for the entering myopes. The mean myopic shift of only those eyes showing a myopic change was -0.83 D. after three years for the 1980 group and -0.60 D. after two years for the 1985 group. The mean myopic shift by level of entering refractive error is shown in Figure 3. The mean myopic shift was similar (-0.50 D.) for the hyperopes, emmetropes, and very low myopes and was about double (-0.80 to -1.00 D.) for the -1.00 D. myopes and above. The higher amount of myopic change noted for the 1980 data than the 1985 study may be accounted for by the decision in the 1980 study to use only the data from the eye with the poorer uncorrected acuity. In addition, the 1980 study excluded a large number of subjects because the entering refractive data were not available for most of those who had 20/20 or better uncorrected visual acuity and did not wear spectacles. By 1985 refractive

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34 70.0 60 0 so.o - I1J ~40.0 L~ o ~30.0 I1J L;J of_ USAF ACADEMY CLASS OF 1985 CHANGE 2 - 0.50 - 1.00 D -1.50 D - 2.00 D f T T +100 +050 +02s ooo -02s -050 -1.oo -3.00 ENTERING SPEO REFRACTIVE ERROR ( D) FIGURE 2 Percentage of eyes in selected ranges of entering spherical equivalent (SPEQ) refractive error with a myopic shift in SPEQ greater than or equal to amounts shown for 994 eyes from the U.S. Air Force Academy class of 1985. Source: O'Neal et al., 1986. C] - 1.25 - ~: o ~, ~- 1.00 llJ > -0.75 LO LLJ Cal - 0.50 LL - 0.25- I En USAF ACADEMY CLASS OF 1985 I EMMETROPES E} HYPEROPES MYOPES 0.00 - 1 -0.58 \ -0.38 \ . ~ \ ~ ! ~ \ . V - 0.62 - 0.52 -0.43 - 1.00 -0,58 -'19 1.00 +0.50 0.00 - 0.25 - 0.50 - 1.00 -3.00 ENTERING SPEQ REFRACTIVE ERROR ( D) FIGURE 3 Mean myopic shift in spherical equivalent (SPEQ) refractive error between the entrance and third academic year exams (2.5 year period) for 664 eyes from the U.S. Air Force Academy class of 1985. Source: O'Neal et al., 1986.

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35 error findings were required on all records; consequently, a similar bias did not exist in the O'Neal study. It should be noted that the 1985 study excluded those wearing contact lenses or near-vision glasses in order to eliminate the effect of these devices on refractive changes from the findings. This bias could be expected to diminish against the inclusion of myopes, since most contact lens wearers are myopic. In addition, all refractive error measures made after entering the academy were cycloplegic refractions. Unfortunately, the military academy studies are often subject to additional biases that make interpretation of progression data difficult. For example, prior to entry into military academies, candidates receive refractive examinations from fanny doctors (optometrists and ophthalmologists), some of whom might be expected to empathize with applicants and underestimate the presence of myopia. Candidates with hyperopia are also more likely to be identified as emmetropic, since cycloplegic refractions are not routine prior to entry into the academies. To the extent that this is true, myopic shifts would be underestimated in hyperopia and overestimated for myopia. CHANGES IN THE OPTICAL COMPONENTS OF THE EYE The mechanism that produces myopic onset or increase in young adults is unclear. Goss and Erikson (1987) and Kent (1963) found steeper corneas after myopic progression in young adults. Adams (1987) found that asocial length change explained all of his own adult onset-myopia. Schell et al. (1986) reported that most adult myopic shifts found in a group of optometry students were accounted for by axial elongation. McBrien (1986) found that axial elongation accounted for myopic changes in his young adult sample. Change in accommodative tonus (also called dark focus or resting level of accommodation) between the light-adapted eye fixating a distance target and in total darkness has been implicated by some investigators. Owens and Harris (1986) reported that noncycloplegic refractions taken at the beginning and the end of each of two freshman semesters exhibited significant myopic shifts, as did accommodative focus measured in darkness (dark focus). Ebenholtz (1986b) found differences between hyperopes and myopes with respect to dark focus: hyperopes showed less mean difference when compared to myopes, while, after sustained near work, dark focus decreased in hyperopes and increased in myopes. McBrien and Millodot (1987) found that after sustained near work, dark focus increased in late-onset myopes; emmetropes and early-onset myopes showed no significant change in dark focus; and dark focus decreased in hyperopes. Ebenholtz (1983) raises the interesting thought that those individuals with consistent hysteresis effects may have a higher risk for myopia and proposes that this hypothesis be studied.