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2 Analysis of the Prevalence Literature This analysis primarily addresses the issue of whether there have been significant changes over time in the prevalence of myopia among young adults eligible for military academy training. Prevalence studies can shed light on this question if as we assume the preponderance of myopia develops at an early age, and if the groups compared are similar with respect to gender, socioeconomic status, and ethnicity. Our review of the literature suggests that the prevalence of myopia is not markedly different for populations under age 20 or 25 compared with those who are older. We know that numerous new cases of myopia appear between the ages of 7 and 13, at least among populations of schoolchildren in all ethnic groups. Nonschoo! populations have not been studied sufficiently to determine whether onset of myopia among children is a general phenomenon or primarily affects only schoolchildren. However, onset of myopia in young adulthood appears to affect few except those who are college students or who enter intensive near-work environments. Certain ethnic groups appear to have shown changes in prevalence over time. Myopia appears to have increased among Eskimo populations, to have varied over time in Japan, and by some reports to have decreased in Scandinavian countries. It appears that Caucasian populations display almost equal prevalence when clinic populations and more randomly collected samples are compared over time. INF[UENCIN(; FACTORS In the myopia literature, it is seldom possible to estimate the degree of influence that variables of age, gender, socioeconomic status and ethnicity contribute to comparisons of prevalence. Criteria of sample selection and characteristics of the sample are seldom defined sufficiently to assess their influence in any precise way; however, by reordering or extending the analysis of an author's data, at least the direction of some of these influences sometimes can be taken into account. Gender Gender differences probably have little effect on the comparability of data in large samples distributed over a wide range. However, slight but significant gender differences in prevalence of myopia have been found between ages 10 and 15 among a wide range of Caucasian and non-Caucasian ethnic groups (Alsbirk, 1979; Angle and Wissmann, 1980a; 8

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9 TABLE 1 Myopia Prevalence Among Children by Age and Gender, 1952 (percentage) Myopia Myopia Myopia Myopia -.12 D. or greater -1.00 D. or less -1.12 D. to -3.00 D (~,reatcr than -a () D. Age Females Males Females Males Females Males Fcmalcs Males 5-6 6.15 7.43 5.70 6.76 0.45 0.52 0.00 0.15 7-S 9.71 11.02 8.73 10.12 0.75 0.74 0.23 0.16 9-10 17.18 15.68 15.17 13.86 1.76 1.51 0.25 0.31 11-12 21.60 20.74 15.83 17.66 4.42 2.82 1.35 0.~6 13-14 25.36 22.53 19.58 17.45 4.17 5.22 1.61 1.37 Aclapted from Hirsch (1952). Baldwin, 1957; Bjerrum and Philipsen, 1884; Hirsch, 1952; Pendse, Bhave, and Dandekar, 1954; Sperduto et al., 1983~. Although Kempf et al. (1928) found no significant differences in Washington, D.C., schoolchildren, most other studies point to a slightly higher prevalence of myopia in females in this age group. Hirsch (1952), in a study of nearly 10,000 randomly selected California schoolchildren, determined prevalence of myopia at one-year intervals between ages 5 and 14 (Table 1~. Sperduto et al. (1983) examined gender differences in myopia prevalence among a group in the United States not selected for visual characteristics and found significant differences for age groups between age 12 and about 30. Above age 30 no gender differences were apparent. For all ages within this range, estimated prevalence of myopia was 27.1 percent for females and 22.8 percent for males. Angle and Wissmann (1980a) also found higher prevalence for teenage girls that was not apparent at older ages. The gender differences in the early teens may be associated with sample selection or, more likely, with different ages of onset of puberty. Other studies of the late teens and early 20s tend to show no significant differences with gender, suggesting that gender differences in myopia prevalence do not exist in young adult populations. For example, Pendse and Bhave (1951) found none at age 18, and Parnell (1951) found no differences among students of college age. For adult samples, ranging from ages in the 20s to age 70 or older, small but consistent differences are often reported, with males exhibiting slightly higher myopia prevalence. Witte (1923), for example, found that, in a sample of 34,000 adult subjects in Germany, 13.8 percent of the females and 16 percent of the males were myopic. Alsbirk (1979) also reports a higher prevalence of myopia at or above -0.12 D. for males above age 40 (48 percent males versus 26 percent females). Curiously, he reports this gender prevalence reversed for the age group 15-30 (42 percent females versus 29 percent males). The adult prevalence figures already cited by Sperduto et al. (1983) and Angle and Wissmann (1980a) suggest no significant gender differences. While the number of cases is sufficiently small to have little effect on-prevalence of myopia generally, all studies of severe myopia show much greater prevalence among females. In Hirsch's (1953) sample, female myopia above -7.00 D. was 2.5 times more prevalent. Blegvad (1927) and others (SchmerI, 1949; Schwartz and Andberg, 1954) found prevalence for myopia above -6.00 D. greater in females (2.65 versus 1.14 percent).

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10 Reports from various countries and studies of various ethnic groups suggest that severe myopia has been much more common among females worldwide for at least the last century, but that the incidence of severe myopia has decreased significantly in Europe and the United States. Socioeconomic and Educational Factore In the myopia literature, it is usually difficult to isolate socioeconomic factors unless the sample is controlled for ethnicity. Such studies show that certain social, economic, or cultural factors are related positively to myopic prevalence. In all countries for which data were reviewed (see Appendix B), the most consistently reported differences were associated with differences in educational level or attainment; some of these are confusing because of failure to separate the influence of educational level from that of age. Hirsch (1959) found that myopes scored above the mean on the Stanford-Binet test, and hyperopes below; Young (1963), Grosvenor (1970a), and others (Baldwin, 1981; Jahoda, 1962; Nadell and Hirsch, 1958) have reported that myopes are more successful in scholastic work. The latter result has been found more consistently than the former. Inconsistencies in studies equating intelligence with myopia have added to the controversy concerning whether myopes read more, or whether reading produces more myopes. One example of the relationship between family income and myopia is offered by the Angle and W~ssmann (1980a) study. When family income was at the upper end of the income range studied, they found the prevalence of myopia to be above 40 percent. When fondly income was at the lower end, myopia prevalence was about 20 percent. Sperduto et al. (1983) found a higher prevalence of myopia among children in higher-income families. Young et al. (1954a) found the prevalence of myopia significantly higher among children of a university faculty than among children of farmers living in the same region. Family income was not related to prevalence of myopia. They found a low positive correlation between time spent reading and myopia at ages 6 to 12, and a high correlation at ages 12 to 17. Et~nicity Attempts have been made to assess prevalence over time among single ethnic groups. The most comprehensive investigation was undertaken by Goldschmidt (1968), who com- pared prevalence data from Denmark and Norway collected in 1882 by Tscherning with the data he himself collected in 1968. Goldschmidt concluded that there had been no significant change in prevalence of myopia of low and moderate degree, but there was a sharp decline in severe myopia. FIedelius (1983) reached the same conclusions concerning mild, moder- ate, and severe myopia when he studied myopic prevalence of a patient population after excluding those who were diabetic. Other earlier reports suggest a decrease in prevalence of mild and moderate myopia among Scandinavian populations (Ask, 1904; Blegvad, 1918; Heinonen, 1924; Holm, 1925~. Significant decre~e in prevalence of severe myopia during this century has also been reported by several Scandinavian authors as well as those in other European countries (Barfoed, 1953; Heinonen, 1934; I`aatikainen and Erkkila, 1980~. Tamura (1932) found 12 percent of a clinic population in Japan had myopic greater ~ . ,, ,< ~ ~ than-1.00 D. and less than-10.00 D. He compared this figure with that calculated from patient files recorded in 1911, when 5 percent were found to be myopic within that range. He reported that severe myopia was less prevalent in 1932 than in 1911. Sato (1957) reported that prevalence had increased from 15 to 45 percent, when records of middle- schoo} children in Japan were examined for the years 1914 and 1955. However, both Sato

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11 (1965) and Otsuka (1956) concluded from their own studies and those of others that there was a marked decrease in prevalence of myopia in Japan between 1940 and 1950, followed by a reversion to earlier higher prevalence rates. The most consistent reports of significant increase in prevalence come from studies of Eskimo populations. Bind (1950) found almost no myopia among Eskimo children. Skeller (1954) reported that myopia was exceptionally rare among all Eskimos. Young et al. (1969) reported that, in a majority of the population living in one Eskimo community, virtually no myopia existed among Eskimo parents and grandparents, while more than half the children of school age were myopic. Morgan and Munro (1973) found a prevalence of 30 percent among an Eskimo population between ages 15 and 20 and less than 10 percent among those above 30 years of age. Alsbirk tl979) found relatively high prevalence among Greenland Eskimos at all ages. Otsuka and Sato disagree concerning the causes of myopia, but both attributed the temporary decrease in prevalence to changes in education in Japan during World War Il and its subsequent increase to resumption of the heavy reading demands placed on students. Increased prevalence of myopia among Eskimos has been attributed to the advent of compulsory education, although changes in diet and the introduction of artificial lighting may also be implicated. Goldschmidt's analysis (1968) and reports of other Scandinavian authors provide the most conclusive evidence that there has been a long-term stability of myopia prevalence in a population that might be compared to our own. The studies cited provide the only comparisons within ethnic groups that were found to offer clues concerning prevalence trends. ADULT SAMPL1:S In addition to variation in the selection factors listed above, comparison of data from dif- ferent studies is made even more hazardous by differences in measurement and classification criteria (see, for example, Grosvenor, 1987) and by failure to identify crucial design charac- teristics. Despite these problems, however, certain interpretations can be made. Prevalence results found by Scheerer (1928) and Betsch (1929) in Germany are similar to those of three other investigations conducted in the United States at about the same time (1925 to 1940~. Comparison suggests that adult populations, which are primarily Caucasian, show similar degrees of prevalence in different countries. Table 2 shows prevalence figures for adults in the four samples. Jackson (1932) and Tassman (1932) divided their samples into two age groups and found no significant difference in prevalence. Brown and Kronfeld (1929) reported that no difference in prevalence existed between a large sample of patients divided into two groups older than and younger than age 25. Four of the five broad population studies cited in Table 2 (Scheerer and Betsch [combined data], Jackson, and Tassman) are taken from records of ophthalmic clinics and private practices. The fifth (Walton, 1950) is made up of the adult residents of a home for the indigent. Scheerer and Betsch examined the records of more than 25,000 adults over age 25. They excluded records of patients showing more than 1.00 D. of astigmatism and patients who reported asthenopia as their chief complaint. The right eye subjective spherical components of the least plus or the highest minus were recorded. When compared with studies using spherical equivalents, this procedure would create a myopia bias; however, the degree of bias would be slight, because only astigmats of 1.00 D. or less were included in the study. Walton (1950) reported that only 1.8 percent of those in his study (n = 1,000) showed simple myopic astigmatism; 4.8 percent showed mixed astigmatism. All those with 1.00

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12 TABLE 2 Studies of Myopia Prevalence Among Adults in Various Populations, 1928-1950 (percentage) Prevalence Age at Lower Higher Which Age Total Age Age Group Invcstigator Year N Range Group Group Group Divided Scheerer (1928~; Betsch (1929) 1928 25,000 25-70+ yrs. 13.7 13.7 25 Jackson (1932) 1932 8,013 5-60+ yrs. 17.1 16.6 17.4 20 Tassman (1932) 1932 8,764 5-70+ yrs. 18.9 17.6 17.9 20 Walton (1950) 1950 1,000 30-90 yrs. 17.7 - 17.7 30 D. of astigmatism or less would be included in Scheerer's study; the portion of this small percentage who had spherical components of no greater than +0.25 D. sphere with -0.50 or -0.75 D. minus cylinder, or no more than +0.50 D. sphere with -0.75 or -1.00 D. Uranus cylinder, would be included in Scheerer's prevalence data. Since none of the studies to which the Scheerer data are compared included spherical equivalents of -0.25 D. or less in myopia prevalence data, the Scheerer and Betsch studies are not biased toward higher prevalence than actually exists; there is in fact a small bias in the opposite direction. The grouping technique of the other studies eliminates those myopes whose subjective refraction (usually without cycloplegia) was no greater than -0.37 D. in the meridian of the least plus (or the highest minus) power. Scheerer and Betsch's data can be classified according to prevalence of myopia of various degrees so that some comparisons can be made with Walton's data (Table 3~. Brown and Kronfeld took precautions to eliminate tonic accommodation in their study. Atropine was administered an average of three times each day for three days before refractive tests were taken. Despite the fact that only about one-third of Scheerer's patients were examined under cycloplegia using homatropine, the difference in mean refraction of the two groups is slight (+1.00 D. versus +0.50 D.~. The Brown and Kronfeld result presumably was the average for ah four meridians, with the same effect in the calculation of means as in determining spherical equivalent. Scheerer's mean is based on the least hyperopic or most myopic meridian of the right eye. The mean difference between the two principal meridians approximates 0.50 D. Some of the difference in means between the two studies is as likely to be the result of this factor as the result of differences produced by the use of a cycloplegic agent. Thorough atropinization prior to refraction can be expected to create some difference in subjects who are prepresbyopic when compared with cycloplegic refraction using homa- tropine or noncycloplegic refractions. The effect has usually been found to be greater for hyperopic subjects than for those who are myopic. Bothman (1932a) found that this effect is greater in both hyperopia and mixed astigmatism than in myopia (+~.00 D. mean difference versus +0.50 D. mean difference). Bannon (1947) found no significant difference between the refraction of myopes of any age without cycloplegia and with cycloplegia using homa- tropine. He found a mean difference of 0.50 D. among children and 0.25 D. among adults

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13 TABLE 3 Myopia Prevalence of Various Degrees Among Adults, 1929-1950 Spherical Equivalent Ref Faction Scheerer (1928) & Betsch (1929) Walton (1950) -.62 to -1.50 5.57.8 -1.62 to-2.50 2.53.4 -2.62 to-3.50 1.52.4 -3.62 to-4.50 1.00.7 -4.62 to -5.50 .50.5 (-5.50 to -8.00) -5.62 to -6.50 .51.0 (-8.25 to-11.00) -6.62 to - 1 0.50 2.0>0. 1 (, 1 1.25 to - 1 4.00) -10.62 and greater 1.0>0.1 (-14.25 and over) who were hyperopic. Both Bothman and Bannon used the same procedure for adm~nister- ing homatropine. Young et al. (1971) compared refractions performed with and without cycloplegia in Eskimos. They reported greater mean reduction of myopia after cycloplegia `1 ~. ~ _ fir ~ ~ '' . ~. ~ ~ . ~ .. ~ _ ~ ~ than did Cannon. Young and his colleagues utilized a ~ percent solution of C:yclogyl. Sato (1964), employing atropine in a study in Japan, reported greater differences for myopes than the mean change reported by Bothman. In separate studies involving a large number of children, Sato (1957) and Otsuka (1967) reported that hyperopic shifts can be observed following atropine cycloplegia in eyes exhibiting low myopia or ernmetropia during drugless refraction. (The English translations cited above did not indicate the magnitude of the hyperopic shift.) Jackson (1932) excluder] patients with eye injuries, ocular dmease, and those who were monocular. Cycloplegic examinations were conducted on all patients who were not advanced presbyopes. He employed spherical equivalents and identified myopia as -0.50 D. Or greater. Tassman (1932) determined spherical equivalents for more than 9,000 patients without ocular disease. He classified myopia in the same way as Jackson. Walton (1950) determined manifest refraction of 569 male and 431 female adults residing at a home for the indigent in Philadelphia. When astigmats are removed from the sample as they were in the Scheerer and Betsch study, 16.9 percent of Walton's group exhibited myopia greater than -0.25 D. Some proportion of the 1.8 percent who displayed simple astigmatism should be added to this figure for comparison with the other three studies, as should some indeterm~nant but small percentage of the 4.8 percent of mixed astigmats. As indicated earlier, Walton's data (excluding astigmats) is compared In Table 3 with those of Scheerer as to the percentage of the sample that exhibits myopia at various dioptric intervab. Brown and Kronfeld and Betsch and Scheerer reported that, while their samples were comprised of clinic patients, the distribution of refractive errors was similar to that of the general population, after the records of diseased eyes were removed. They based this on the

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14 high frequency of emmetropia in their samples and comparison of mean refractions within certain age groups in their samples with means of small random samples at the same age. These studies form a benchmark to which nineteenth-century and late twentieth-century investigations of prevalence can be compared. Several additional studies reinforce the conclusion that Caucasian adult samples are similar and have been stable throughout this century. Clarke (1925) reported 19 percent prevalence in an adult population in England, although his population characteristics and 1 ~ , . . ~ . .. . .. . . .. .. . . . . _ _ _ evaluation criteria were ~nsu~c~ently described to authenticate comparison. A 1962 survey by the British Ministry of Health (Bennett and Rabbetts, 1984) of 9,163 prescriptions for distance vision indicated that 23 percent were worn for myopia of -0.37 D. or greater (spherical equivalent, right eye). Rasmussen (1948) collected several hundred serial case records from ophthalmic practices in each of 68 cities in England and Scotland. He reported that prevalence of myopia among all ages was similar in the two countries (26 percent spherical equivalent greater than -0.25 D.~. Von Reuss (1877) and Witte (1923) found similar prevalence of myopia among two clinic populations in Germany (14.5 percent and 13.8 percent). They appear to have employed similar classification criteria (greater than -0.75 D.~. Hirsch (1958) found prevalence of myopia greater than -1.00 D. to be 8.8 percent in a U.S. clinical sample older than age 44. Prevalence of myopia of -0.50 D. and above in Hirsch's population is calculated to be 18.1 percent. CHILD POPULATIONS Prevalence studies have been conducted among groups of primarily Caucasian children as young as age 5 and as old as age 17. Compared over time these studies can provide some evidence concerning major changes, if any, in prevalence of myopia among young adults. Older studies clearly involve different populations compared with more recent studies, due to the effects of such factors as mandatory school attendance. Comparison of these studies should therefore be interpreted with caution. Children ages 5 to 17 and college-age students represent the largest number of in- vestigations reported. The earliest report discovered concerning schoolchildren is that of Schurmayer (1856), who between 1839 and 1850 examined 2,172 German schoolchildren between ages 8 and 16: 18 percent were reported overall to be myopic; 35 percent of those ages 14-16 were myopic. These and other prevalence figures among schoolchildren cited later are presented in Table 4. Van Jaeger (1861) reported that 55 percent of a group of boys in a school for orphans were myopic; his study was based on ophthalmoscopic estimates. Using the same technique, Cohn (1867) found that 1.4 percent of 5-6-year-olds in a village school were myopic greater than -1.00 D. but that this increased to 55.8 percent of 16- and 17-year-olds. After instilling atropine, Cohn (1886) found an average of 1.00 D. less myopia (or more hyperopia), which reduced the prevalence to about 20 percent. In the United States, Conrad (1874) compared the results of ophthalmoscopic estimates of refractive status with subjective refraction. He obtained higher prevalence rates of myopia from subjective testing (32 versus 22 percent). Cohn (1892) later made this same comparison and reported no significant difference in the prevalence rates obtained. Florschutz (1880) calculated myopia to be 21 percent among a group of 2,000 German schoolchildren ages 11 to 16. Hess and Diedricks (1894) found 26.8 percent prevalence among a group of German children ages 10 to 14. Studies in Sweden (Ask, 1904, 1925; Blegvad, 1927; Heinonen, 1924), Finland (I~aatikai- nen and Erkkila, 1980; Mantyjarvi, 1983), the Soviet Union (Mogilevchik and Bondareva, 1970; Popov, 1931), and Denmark (Bjerrum and Philipsen, 1884; Blegvad, 1918; FIedelius,

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15 TABLE 4 Studies of Myopia Prevalence Among Children in Europe and the United States, 1831-1980 Year Prevalence Age Range Investigator Country Reported (96) (years) N Relevant Criteria Schurmeyer Germany 1856 19.0 8-16 2,172 Refraction 35.0 14-16 Refraction Van Jaeger Germany 1861 55.0 9-16 Small Ophthalmoscope; orphan school 80.0 9-16 Small Ophthalmoscope; priorate school Cohn Germany 1864 1.4 5-7 10,060 Ophthalmoscope myopia >1 D. Cohn Germany 1867 9.9 6-16 Ophthalmoscope l?lorschutz Germany 1880 21.0 11-16 2,041 Ophthalmoscope Hess and Germany 1894 26.8 10-14 Diedricke Ware England 1831 <1.0 Lower school 1~300 Not known Thompson Scotland 1919 18.8 High school 3,249 Clinic sample McIllroy England 1932 21.6 6-14 1,702 Noncycloplegic and Hamilton Sorsby England 1932 4.0 4-14 Cycloplegic McNeill England 1955 21.2 5-15 1,066 Clinic sample Popover USSR 1931 2.4 8-10 Not known Mogilevchik USSR 1970 10.0 Not known and Bondareva 3.1 Lower grades Not known 23.7 Upper grades Not known Scats trance 1879 15.0 7-12 1~717 Not known Dor France 1878 22.0 12-16 Not known Bjerrum Denmark 1884 10.6 13-14 198 >-1.50 D. Philipeen Denmark 1884 12.4 13-14 210 >-1.50 D. Lastikainen Finland 1980 1.9 7-9 Not known and Erkkila 21.8 14-16 Not known (Conrad U.S. 1874 32.0 t-~u subjective refraction 22.0 6- 16 Ophthalmoscope Agnew U.~. Ib77 10.0 Grade 1-4 Ophthalmoscope 14.0 Grade 5-8 Opthalmoscope 16.0 Grade 9-12 Ophthalmoscope poring U.S. 1876 4.0 6 Not given 26.0 21 Not given Tempt ups. JAZZ 7.1 6-18 1,828 >-.25 D. (cycloplegia) 2.1 6-8 333 >-.25 D. (cycloplegia) 5.5 9-11 495 >-.25 D. (cycloplegia) 9.5 12-18 1,001 >-.25 D. (cycloplegia) Jackson up 1932 8.1 5-10 227 -.50 D. (cycloplegia) 16.1 10-15 414 -.50 D. (cycloplegia) 25.7 15-20 611 -.50 D. (cycloplegia) classman up soya;` 8.6 5-10 148 -.50 D. (cycloplegia) 19.5 10-15 403 -.50 D. (cycloplegia) 24.7 15-20 421 -.50 D. (cycloplegia) ~-~` .. CYST 24.0 13-14 >-.12 D. (noncyclopleg~c) 5.4 13-14 >-1.00 D. (noncycloplegic)

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16 TABLE 5 Studies of Myopia Prevalence Among Schoolchildren at Different Ages in the United States, 1877-1932 Kempf et al. Hirsch (1952) Agnew (1877) (1928) Jackson (1932) Tasemann (1932) (> -.12 D. and > -1.0 D.) (unknown) (> -.25 D.) (> -.50 D.) (> -.50 D.) % ,Myopica -.12D. -1.OD. ~Age % Age % Age 96 Age and and Grade Myopic (years) Myopic (Yeare) Myopic (years) Myopic (years) above above 1-4 10 6-7 1.4 5-9 8.1 5-9 8.6 5-6 6.8 0.6 5-8 14 8-9 4.2 10-15 16.1 10-15 19.5 7-8 10.4 1.0 9-12 16 10-11 8.5 5-15 13.3 5-15 14.6 9-10 16.7 2.0 12+ 9.1 11-12 21.2 4.5 13-14 24.0 5.4 aPrevalence at various ages for varying degrees of myopia constructed from Hirech's data. 1981b; Goldschmidt, 1968; Heinonen, 1924), as noted earlier, report similar prevalence rates. McIlroy and Hamilton (1932) found a prevalence rate of 21.6 percent among 1,700 English schoolchildren ages 5 to 14. McNeil (1955) found 21.2 percent among a group ages 16 to 18. Thompson's (1919) data for schoolchildren in Scotland are comparable. Table 4 lists prevalence findings of these and other European investigators as well as those from studies in the United States. The first U.S. study involving a large sample at a specific time was reported by Agnew in 1877. He examined 1,479 children by Cohn's ophthalmoscopic technique and found that this method did not yield a higher prevalence than did subjective refraction. Cohn (1892) and Derby (1880) also reported that ophthalmoscopic estimates of refraction gave results similar to those using other methods. Table 5 shows changes Agnew found with age along with those reported by later investigators. He concluded that prevalence in U.S. schools was sirn~lar to what Cohn (1886) found in Breslau and reported from studies in other European countries. Differences in prevalence reported in early studies are as likely to be due to variations in classification and testing methods as to actual differences in prevalence. Kempf et al. (1928) examined retinoscopically (under cycloplegia) 1,860 white school- children in Washington, D.C. Table 5 shows prevalence for each age group classified. The study included as myopic all those showing spherical equivalents of -0.25 D. or greater and all myopic astigmats. Mixed astigmats (less than 4 percent of Tassman's and Jackson's 1932 samples) were excluded. In Jackson's sample 13.3 percent of children in the age range 5 to 15 were -0.50 D. myopic or greater. Tassman's sample can be treated in the same way and yields a 14.6 percent prevalence rate. These figures, as well as prevalence rates from their studies for children ages 5 to 10 and 10 to 15, are presented in Table 5. Hirsch (1952) studied refractive characteristics in a large population of California schoolchildren. He did not report prevalence figures for comparable subgroups, but they can be calculated from his data (see Table 5~. Hirsch identified myopia as the spherical equivalent of -0.25 D. or greater. About 24 percent of the children between ages 13 and 14 (nearest birthday) were myopic by this criterion (18.5 percent were less than -1.00 D. myopic). The age group of 5-6 years had a prevalence rate of 6.8 percent (6.2 percent were less than -1.00 D. myopic). Angle and Wissmann (1980a) reported prevalence rates of

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17 29.9 percent at 12 years and 33.2 percent at 17 years. Sperduto et al. (1978) determined prevalence rates to be 24 percent at age 12, and 27.7 percent among the ages 18-24. They analyzed data from general health surveys completed several years earlier but did not adequately describe their classification criteria. Their samples were relatively unselected, as were those of Hirsch and Kempf. Some studies suggest that prematurity may be a risk factor for juvenile myopia. Castren (1955) examined 697 schoolchildren, 480 of whom were premature at birth. Those showing -0.50 D. spherical equivalent or greater were classified as myopic. Homatropine subjective refractions were performed. Approximately 5 percent of 109 children who had a birthweight between 1,000 and 2,000 grams were myopic, while 2.3 percent of the 217 children in the full-term control group (2,500 to 4,000 grams) were found to be myopic. Fletcher and Brandon (1955) and others (Birge, 1956; FIedelius, 1981a; Gerhard, 1983; Jain and Garg, 1970; Lledo, 1976) have reported that premature infants are at risk for myopia. In a retrospective study of high myopes (probably -6.00 or more), Fletcher and Brandon (1955) found that 90 percent of those whose birthweights could be determined weighed less than 1,500 grams at birth. While this is expected to have little influence on prevalence rates of large populations, it may be added to the list of risk factors for juvenile myopia (and conceivably young adult myopia as well). Variations among these U.S. studies can be interpreted to some degree. There is little if any evidence of major differences in prevalence of myopia among Caucasian schoolchildren over the past 100 years, after correction for age and selection and classification criteria are considered. This conclusion probably also applies to comparisons of U.S. and European studies. The conclusions of early investigators (Cohn, 1867; Dor, 1878; Florschutz, 1880; van Jaeger, 1861; Ware, 1813) that groups of children in more intensive educational environ- ments exhibit higher prevalence of myopia were not corroborated until recently. A recent study of New Zealand native children (Grosvenor, 1986) disclosed a significantly higher prevalence among those who were involved in intensive study than those who were not. YOUNG ADULT POPULATIONS Another large group of studies of myopia prevalence includes young adult populations. However, none can be identified as unselected samples. A few of the general clinic popula- tions can be classified into one or more age groups between ages 17 and 25 but most deal with highly selected populations usually military recruits or students. Clinic and General Populations Young adult clinic populations can sometimes be extrapolated from studies of broader age ranges. Differences are generally in directions that could be predicted when the sample characteristics and the study design criteria are given. For U.S. studies in the 1930s, the clinical populations in Jackson (1932) and Tassman (1932) were divided into two age groups: 15 to 19 and 20 to 29. Both applied the criterion of -0.50 D. or greater (spherical equivalent) to identify myopia. The 1~ to 19-year-olds exhibited 25.7 percent myopia in Jackson's study and 24.7 percent in Tassman's study. The 20- to 29-year-olds exhibited 19.6 percent myopia in Jackson's study and 22 percent in Tassman's study. All these figures are lower than those found in most studies involving undergraduates and all studies of older or upper division students. Table 6 lists studies that provide comparisons between two groups, each of which involves a significant number of subjects.

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18 TABLE 6 Studies of Myopia Prevalence Among Young Adults By Age and Other Variables, 1848-1953 Investigator Szokalskya Loring Loring Agnew D erby Collardb Collardb Tscherning Seggel J. ackson Tassman Boynton Nakamura Sutton and Ditmars ~ Myopia Year Reported Group A L848 1877 1877 1877 1880 1880 1881 1882 1884 1932 1932 1936 1954 1970 G melin 1976 Roberts and Rowlandl978 Sperduto 1983 Fledelius 1983 bCited by Cohn (1967) Both studies cited in Collard (1881~. 13.0 (1st yr. college) 26.0 (U.S.) 26.0 (U.S.) 29.0 (precollege) 35.4 (at entrance) 23.0 (scientific and professional) 30.0 (ages 17-22) 2.4 (farmere & fishermen) 2.4 (farm workers) 25.7 (ages 15-19) 24.7 (ages 15-19) 18.2 (at entrance) 20.0 (Caucasian) 45.0 (at entrance) 51.0 (at entrance) 29.9 (age 12) group B 17.0 (upper division) 63.0 (German) 44.0 (Soviet) 37.0 (seniore) 47.2 (at graduation) 42.0 (humanities) 27.0 (ages 24-27) 32.4 (advanced students) 56.7 (compositors & writers) 19.6 (ages 20-29) 22.0 (ages 20-29) 23.9 (at graduation) 30.0 (Nisei) 60.0 (at graduation) 67.0 (at graduation) S3.2 (age 17) 23.9 (ages 12-17) 27.7 (ages 18-26) 32.6 (-.25D. or above) 10.3 (-1.75D. or above) Source E`rance College students College students College students College students College students Military recruits Military conscriptees (Germany) Clinic sample Clinic sample College students with visual acuity 20/50 or worse -1.50 D. or above Military recruits West Point cadets West Point cadets General General Clinic sample Sperduto et al. (1983) analyzed a subset of the data from the National Health and Nutrition Examination Survey (NHANES) collected in the United States between 1971 and 1982 (Roberts and Rowland, 1978~. They report a slight decline in myopia prevalence, from 27.7 percent among 18- to 24-year-olds to 2~25 percent in the older groups. In school year 1919-1920, fewer than 17 percent of students enrolled in high school graduated (U.S. Department. of Education, 1981~. By the early 1960s this figure reached about 70 percent and has stayed at this level into the 1980s (U.S. Department of Education, 1981~. By the mid-1970s, a little over 50 percent of high school graduates enrolled in college (U.S. Department of Education, 1981~. Even if we make the unlikely conservative assumption that this was also true in 1920, we conclude that in the 1920s less than 8

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19 percent of the adult population had attended or were attending college; by comparison, in the mid-1970s the comparable percentage would be 35 percent. Despite this large difference in the adult populations of the 192~1930 and 1970 periods, studies of prevalence of myopia fall to reflect significant differences for the two periods (Tassman, 1932; Brown and Kronfeld, 1929; Jackson, 1932; Sperduto et al., 1983~. How can this be accounted for? First, it may be that, although the percentage of the population who attended college has increased since the 1920s, the proportion of individuals engaged in intensive near work may not have changed as dramatically. Unfortunately, good noncollege data for myopia prevalence are not available for those two periods. Second, it may be that the data used for these comparisons are simply not precise enough to reflect real changes in myopia prevalence between the two periods. Third, the myopic changes observed during the college years may be transient for a significant number of individuals and therefore do not appear in population studies that include a large proportion of individuals beyond college age. Finally, some combinations of these factors, and others, may be needed to account for the apparently paradoxical observations that, although myopia prevalence is significantly higher in college students than noncollege students and a much larger proportion of individuals attend college now compared with the 1920s, the limited adult population studies fail to reveal significant changes in the prevalence of myopia for the two periods. College Students College students represent the most accessible group for study within the young adult population. Studies of prevalence among college students in the United States and in Europe probably provide best evidence that prevalence is similar in both areas and over time. Among those prevalence rates listed in Table 6 are several for students ages 17-22. Ware (1813) was the first to report prevalence among college students: he found that 25.2 percent were myopic. His and other prevalence figures of college students are given in Table 7. Schuster (1911) reported that 18 percent of Oxford undergraduates "have distinctively bad eyesight" but gave no explanation concerning the derivation of this figure. This corresponds closely with Parnell's (1951) prevalence figure of Oxford undergraduate males with visual acuity of 6/60 or worse. Parnell also found 49.9 percent prevalence among a group of male students representing all undergraduate levels at the same university when he employed visual acuity worse than 6/9 as his criterion. It can be expected that almost all who exhibit this visual acuity at that age would be myopes of greater than -0.25 D. spherical equivalent (Peters, 1961~. Several studies in Germany provide prevalence data for college students (Cohn, 1881; Durr, 1883; Erismann, 1871; FIeischer, 1907; Pfluger, 1875; Van Anrooy, 1884~. The highest prevalence (58 percent) reported was by Seggel (1878~; the lowest (30 percent) was reported by Collard (1881~. Seggel's sample was comprised of older students. It may be presumed that they had spent more years in collegiate study. Several studies have reported a similar slight decrease in prevalence after age 20. This is most likely due to the addition of nonstudents to samples at older ages and, in clinic samples, to the failure of adults whose myopia is stable to seek care. Longitudinal studies seldom report significant decreases in myopia before age 40. Randall (1885b) examined reports from a large number of investigations in various European countries and in the United States. The data permitted comparison of prevalence of myopia among secondary school and college students; the total number of subjects was over 40,000. About 28 percent of the total group was calculated to meet his criterion for

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20 TABLE 7 Studies of Myopia Prevalence Among College Students In the United States and Other Countries, 1871-1985 Prevalence Investigator Year % Location College Students in the United States Derby 1877b 35.0 Harvard Derby 1880 35.0 Amherst (Iower division) Derby 1882 47.2 Amherst (tipper division) Randall 1885b 52.5 Composite of6 European and 2 U.S. studies (2,436 subjects) Hall 1935 35.0 Washington Dunphy 1968 44.0 Harvard (Business end Law Schools) Septon 1984 75.0 Optometry students Schell et al. 1985 81.0 Optometry students Agnew 1887 28.5 Brooklyn Polytechnic Agnew 1887 40.0 New York College Burnett 1911 15.0 University of California College Students in Other Countries Erismann 1871 43.0 Germany Pfluger 1875 40.0 Germany Kotelmanna 1877 49.0 England Seggel 1879 81.0 Germany Cohn 1881 53.0 Germany Collard 1881 30.0 Germany Durr 1883 35.0 Germany Van Anrooy 1884 31 Germany Fleischer 1907 50.0 Germany Ware 1813 25.2 England Smith 1880 20.0 England Clarke 1924 20.0 England Parnell 1951 32.0 England Emmerta 1876 76.0 Switzerland Francesschetti 1935 24.0 Switzerland Vogt 24.0 Switzerland Tscherning 1882 32.4 Denmark Tamura 1932 12.0 Japan Banerjee 1933 35.0 India Liand Rush 1920 55.0 China aCited by Randall (1885b).

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21 myopia (-0.25 D. or greater). Of college-age groups that permitted calculation, 52.5 percent met this criterion. According to Randall there were no clear differences in prevalence among entering college students. Table 7 presents prevalence figures from studies of college students in the United States and other countries for which some variations in study criteria could be assessed. Military Personnel Applicants to the U.S. Naval Academy and the U.S. Air Force Academy must meet special visual requirements that preclude their inclusion in prevalence evaluations. However, the U.S. Military Academy at West Point reduced vision requirements for admission in 1960. Socioeconomic and scholastic characteristics of those entering West Point in 1971 were compared with those enrolling in American colleges that year (Houston, 1972~. They appear to be similar in those characteristics surveyed that are related to myopia prevalence. Sutton and Ditmars (1970) determined the mean spherical equivalent refractions of those entering from 1964 to 1968. They reported 45 to S1 percent of members of classes entering during that period were myopic on the order of -0.25 D. or more. On the basis of differences in frequency of refractive errors between this amount and that found by other investigators (whose criteria for myopia are known), this range appears to be similar for college students and military cadets. Gmelin (1970) found the prevalence of mvonia among the ~nt.~rin~ class at West Point to be 51 percent. - 4 .1 1 ~ e ~ sir ~O O Several studies comparing young male military recruits of different backgrounds provide evidence concerning differences in prevalence among socioeconomic classes, similarities between Caucasians in Europe and the United States, and stability of prevalence with time. Ware (1813) reported that "among officers of the Queen's Guard many were myopic, while of 10,000 footguards less than half a dozen were myopic." Parnell (1951) compared unaided visual acuity of 279 male undergraduates at Oxford University to prevalence figures for over 90,000 l~year-old males from England and Wales who took National Service Board examinations in 1939. The Oxford students had a higher prevalence of low visual acuity at all levels (6/9 or less, 49.9 versus 11.4 percent; 6/60 or less, 16.8 versus 2.6 percent). Tscherning (1882) examined records of 38,670 Danish recruits and found a prevalence of myopia of -1.50 D. or greater in 8.3 percent of the group. Prevalence ranged from 2.4 percent of farmers and fishermen to 32.38 percent of advanced students. Segge! (1884) compared prevalence of myopia of the same magnitude among German conscripts and reported 2.4 percent for farm workers and 56.7 percent for compositors and writers. Goldschmidt (1968) designed a study to compare results with Tscherning's data. He reported a prevalence of 9.2 percent among a large group of Danish draftees. The differ- ences in prevalence between various occupational groups were similar to those reported by Tscherning 84 years earlier. After correcting his data for differences in representation of oc- cupational groups, he reported that the difference in prevalence (~.3 percent for Tscherning versus 9.2 percent for Goldschmidt) disappeared. Many of the factors affecting prevalence fortuitously or by design were similar in all three studies, including age, the mean of which was approximately 20 in each. Stromberg (1936) found myopia of -1.00 D. or greater in 4.8 percent of 2~year-old males who were entering the Swedish army. Steiger (1913) in Germany and Sorsby et al. (1960) in England found 13.1 percent and 11.6 percent, respectively, employing the same classification criterion for myopia. Steiger's sample was made up of males ages 20 to 30 from a clinic population. Sorsby's sample consisted of young men called up for national service. Nakamura (1954) compared prevalence of myopia among Caucasian and Nisei recruits

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in lO54 and Fund :0 and 30 percent, respectlvel~ Ibe Clarence between these two groups ~ probably read reRectlng an ethnic Clarence in prevalence. Ibe percentage exhibiting myopia in the C~c~lan group ~ Caller to those reported shove. Naka~rs considered aD myopic hobo bad ~ spheric equivalent of -0.50 D. or greater (see ale 6j.