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OCR for page 36
4
Conclusions and Recommendations
An exhaustive review of the relevant literature uncovered no experiment specifically
designed to answer the questions within our charge. Consequently, most of our conclusions
are necessarily tentative and inferred from considering multiple studies, each with different
purposes. Very few of the reports cited resulted from investigations undertaken to test a
hypothesis. Many authors drew from existing data collections or compared their own data
with those that had been assimilated by others for a different purpose and that employed
different criteria. In short, it is difficult to interpret the relevant studies. Consequently, the
most useful application of our efforts was considered to be the formulation of appropriate
hypotheses and suggested guidelines for future research. What is needed is considerably
more research targeted to answer specific questions, designed so that results of different
investigators can be compared and so that relatively small numbers of cohorts can be studied
over an extended time. In this section we offer our conclusions and recommendations,
although they are necessarily inferential and tentative for the reasons stated above.
CONCLUSIONS
1. Mom 1812 to the present, studies of prevalence of myopia among college students
and young adult males who entered military officer training indicate that myopia is much
more prevalent among these groups at entrance than among other populations at similar
ages.
2. Studies of the prevalence of myopia during the past 100 years lead us to conclude
that, among Caucasian populations, there has been no major thange in the prevalence of
myopia within selected populations of schoolchildren and college-eligible young adults. An
exception to this finding is that severe myopia appears to be less prevalent now at all ages.
This conclusion presumes that, even though it is impossible to directly compare data
from the studies available to us because of the variations that have been enumerated, a
significant trend probably would have been recognizable. Changes such as the proportion
of the general population falling within specified age ranges and the possible short-term
variations in risk factors could easily account for the magnitudes of difference in prevalence
extrapolated from the literature. By comparison, it is possible that significant changes in
prevalence of Tow and moderate myopia have occurred within certain other non-Caucasian
ethnic groups (notably Eskimos and Japanese) or in some very local regions.
36
OCR for page 37
37 ;
However, the proportion of the adult population attending college has changed markedly
in the last 5~60 years: less than 8 percent of the adult population had attended college in
the 1920s, whereas a comparable figure for the 1970s was 35 percent. Surprisingly, while
myopia prevalence remains considerably higher in college students than noncollege students
and about five times as many individuals attend college as did in the 1920s, the limited
adult population studies fad! to reveal significant changes in the prevalence of myopia. We
find no clear and single explanation, but it may be that, while college attendance has
changed, there has been a less dramatic change in the amount of intensive near work for
the entire population. (Unfortunately, good noncollege myopia prevalence data are not
available.) Alternatively, myopic changes in the college years may be small and transient for
a significant number of individuals and therefore bald to affect adult population prevalence
studies, which include a large proportion of individuals beyond college age. Finally, the
imprecision of the data used for comparison in the two periods, perhaps combined with the
above factors, does- not allow for real changes to be identified.
3. Myopia most often develops and progresses between the ages of 7 and 16 years,
then stabilizes In the late teens. Cross-sectional reh~acti~re data for individuals beyond age
16 suggest that onset and increase of myopia after that age occurs, although it is smaller
in-degree and appears limited to a subgroup. The degree of myopic shift among young
adults is small enough to mask myopic onset or nyopic change in the general population
deternimed by crose-sectional studies. The possibility that the shift is not permanent and
the fact that it does not occur m a large percentage of the general population would also
tend to mask the change m cross-sectional studies.
The traditional view is that myopia onset or progression rarely occurs among young
adults. Because available studies are limited to highly selectee} samples, we cannot assess
general trends concerning onset or increase during young adulthood of myopia acquired
during earlier years, nor can we ascertain whether this group of young adults experienced a
period during which their myopia was stable, then was reinitiated by the influence of some
new endogenous or exogenous factor. However, it seems clear that progression among young
adult myopes who enter an academic environment, while less in degree than among juvenile
myopes, occurs with significant frequency. While some young adult myopes may progress
because stabilization of juvenile myopia occurs at a later age, the frequency of progression
among college students and military cadets is much greater than would be predicted by
studies of the age at whith juvenile myopia stabilizes (Goss and Winkler, 1983; Saunders,
1984, 1986b).
4. Studies of college students and longitudinal studies of young adult males who
entered military officer training, which have been carried out aver the last 100 years, Show
that incidence and progression of myopia increase with tane spent in training environments.
There are too few data regarding randomly selected male populations to determine the
incidence or prevalence of myopia onset or progression in young adults (that which has
onset or increases between the ages of 17 and 35); however, it seems clear that myopia has
much lower prevalence among young males who do not pursue academic study or engage in
extensive near work. They also appear less likely to develop myopia.
5. Based on all studies reviewed, we estimate that, from populations in which selected
college and special near-work groups have been excluded, lese than 10 percent of emmetropes
and low hyperopes will develop myopia prior to becoming presbyopic. By comparison, as
many as 20 to 40 percent of low hyperopes and emmetropes who enter colleges and
OCR for page 38
38
military academies or pursue occupations with extensive near-work requirements are likely
to become myopic before the age of 25.
6. Prevalence of nyopia has been positively- related statistically to family income,
ferret of education of parents, refractive status of parents, reading ability, and scholastic
success.
It has been shown that applicants to West Point are similar in all these characteristics
, · . . · . . .. _ . .
to applicants to universities who, like West Point matriculants, are in the upper 10 percent
(scholastically) of their high school graduating classes (Houston, 1972~. While we know of
no study that specifically examines prevalence of myopia among high school graduates at
this scholastic level, indications are that it is significantly greater than among the general
population of similar age. The positive correlation between myopia and each of these other
factors associated with successful application to colleges further reduces the applicant pool
for military academies. While the direction of this influence is known, the magnitudes are
not.
7. Some portion of young adult Scopes progress after a period in which them myopia
has become stable. Others without myopia develop it as young adults. Although available
onset data are difficult to analyze, they generally indicate that shifts toward myopia among
low hyperopes and emmetropes are lese frequent and Snarler in degree than among low and
medium myopes.
Small measurement errors are more common in refraction of nonmyopes, especially
when cycloplegic and drugless refractions are compared. These factors combine to make
onset data unreliable. However, studies of uncorrected visual acuity loss indicate that at
least 20 percent of nonmyopes become myopic during the four-year period following entrance
to college or military academy. It appears that there is a distinct form of myopia (or myopic
shift) for which young adults who enter academic environments are at risk.
8. It is not known whether any of those several predictors of juvenile myopia are also
related to onset or progression of young adult myopia.
9. Virtually all students bound for military academies with noncycloplegic refractive
errors at entrance of +1.00 D. or more in any principal meridian retain ~co~Tected vimal
acuity of 20/20 or better In each eye at the end of four years of study. However, if this
criterion were to be used as a pass-fail basis for selecting military academy applicants, it
would provide an unrealistic constraint on the eligible pool.
This conclusion can be understood in terms of the following analysis, which does not
include an estimate for the increasing but still limited number of female applicants. There
are fewer than 2 million 18-year-old males in the U.S. population. If we take an ~Q cutoff
for selection into the academies of one standard deviation above the mean (i.e., As of 115
or greater) as generally reflecting the intellectual capacities required to meet a demanding
course of college instruction, then 16 percent of the general population, 320,000 mates, will
be eligible for consideration. Individuals in the Hayden (1941), Goodson (1983), and O'Neal
(1986) studies with refractive status between -0.25 D. and +0.50 D. had a 0.75 probability
of becoming myopic at -0.50 D. or more. The proportion of individuals with refractive
status of +1.00 D. or more in the relevant population (mainly bright 18-year-old males) will
then determine the~number of individuals who comprise the selection population. If this
proportion is as low as 1 percent, then only 3,200 (1 percent of 320,000) individuals make
OCR for page 39
39
up the pool of potential fighter pilot candidates. Clearly this is an unrealistic constraint on
the eligibility pool for fighter pilot candidates.
l~:COMM15NDATIONS
cations.
Based on the findings of this review the working group makes the following recommen
1. Investigation should be undertaken to determine what proportion of young adults
eligible for military academies and pHot training are hyperopic and which low hyperopes
and emmetropes exhibit myopic shifts after entering this environment.
Special attention should be given to those risk factors that have been associated with
juvenile myopia. The fact that young adult low hyperopes and emmetropes often develop
myopia of low magnitude, and the fact that young adult myopes more often than not show
increases in myopia when subjected to intense near-work environments suggest that there
may well be other initiating influences that are similar to those related to juvenile myopia.
The fact that the magnitude of myopic change in young adults is less than that of most
juveniles who become myopic may be related to maturation differences rather than to a
different mechanism. Determining risk factors could help identify the approximately 20
percent or more of military cadets whose uncorrected visual acuity becomes worse than
20/20.
2. The military academies showy consider the following four approaches to increasing
the pool of potential pilots:
A. Reducing the uncorrected visual acuity requirements for entrance. This should
be done in conjunction with developing a program that might make feasible the wearing
of corrective lenses (or contact lenses) while piloting. We recognize that, under current
practices, this may be unworkable and that this approach may require further advances in
medical technology.
B. Reducing the academic requirements for entrance. A slight downward shift in the
academic requirements of one-tenth of a standard deviation (equivalent to a drop of 1.5 A
points) Tom 115 to 113.5 womb rent in a 16 percent increase in the eligible population.
C. Searching for prophylactic measures that would reduce the shift toward myopia in
academy students whose refractive extort are less than +1.00 D.
D. Increasing the proportion of the target population that does apply to the academies,
through a `'talent identification program. Healthy, young (e.g., 13-year-olds) bright males
and females whose refractive status is +1.00 D. might be identified, counselled, and recruited
toward careers as Unitary pilots.
3. The professional tasks of military academy graduates should be studied to deter-
m~ne if differences in perfonnance of critical tasks can be related to refractive state. If so,
the extent to which wearing ophthahnic correction affects job performance should also be
studied.
Are the gains in requiring unaided visual acuity of 20/20 greater than if those who are
likely to become low myopes (or indeed those who are already myopic) are admitted and
wear ophthalmic corrections? One trend that bears on this question is the declining pool of
high school graduates, projected to decrease 25 percent by 1993 to 2,378,000 from its peak
of 3,154,000 in 1976.
OCR for page 40
40
4. The refractive status and changes In refraction of those who are high achievers
in various academic activities should be compared with those who do not chow significant
myopic shift. Comparison of refractive error and refractive changes should also be made
between high achievers and low achiever for various academic activities.
A specific issue raised but not resolved by this study is the nature of the relation
between myopia and special aptitude for reading and other near tasks. It may be that one
factor in the significant increase in number of myopes in collegiate and military graduating
classes is an association of certain college aptitude characteristics and myopia.
5. It is important to know what anatomical or functional changes are responsible for
young adult myopia. We therefore recommend that research be conducted to deternune how
each of the optical components of the eye contributes to refractive state and to refractive
change.
If adult onset or progression of myopia is principally a result of increase in the length of
the eye, as it is in juvenile myopia, does this involve a renewal of accelerated ocular growth
past the age at which such growth has normally declined? Questions such as this could be
answered by a longitudinal study of college students in which each of the relevant ocular
optical components among those who showed myopic progression could be compared with
those who did not. Hypotheses concerning appropriate preventive measures can be selected
more rationally after we have learned more about the optical components of myopic change
and its probable causers).
6. Military academies should collect information on standard refractive components
of the eye affecting refractive error under controlled conditions. The necessary "ethnology
and human resources are at hand to resolve the issues concerning causes, progression,
optical components, and consequences of adult-omet myopia. For example, excessive axial
elongation of the eye can usually be detected opEthaImoscopically by the presence of c onus
(crescent) or supertraction formation at the optic nerve head. It is possible that as a result
of carefid longitudinal studies of fundus characteristics, early changes might identify those
eyes at increased risk of undergoing fixture elongation. These measures need to be evaluated
against suspected behavioral risk factors, Inch as reading.
Rather than compile a lengthy list of such investigations, we provide an example to
illustrate. It has been shown (Betsch, 1929) that frequency of conus, an effect of axial
elongation, increases consistently from none in hyperopes above + 1.00 D. to 100 percent in
severe myopia (see Table 13~. We now need to know which hyperopes will become myopic
and which myopes will progress. The use of an automated refractor, ultrasound for axial
length measures, and other automated instrumentation would add efficiency and reliability.
Data should be collected at entrance and annually thereafter. Of particular interest is a test
of the hypothesis that those individuals who had experienced a period of axial elongation
of the eye during their early teens (whether or not it produced myopia) are those that are
most at risk for subsequent axial elongation during their college years.
7. Study designs should be standardized so that the results of various investigators
can be compared. The following initial suggestions for standardization are offered, more to
focus discussion that wiB lead to consensus than to provide final solutions.
A. Information of greatest value will be yielded by longitudinal studies. The number
of subjects required for most longitudinal studies is often not large; however, they do
necessitate selecting from groups in which the subjects will remain for the required period
of time and in which there is minimum attrition. Educational institutions within which the
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41
TABLE 13 Percentage of Eyes Exhibiting Conus at
Various Refractive Intervals
Refractive Status
Conus
Above +6.0D.
0 to + 6.0D.
-1.00
-2.00
-3.00
-4.00
-5.00
-6.00
-7.00
-8.00
-9.00
-10.00
-11.00 and above
Less than 1.0~o
Less than 2.0
3.3
16
29
50
47
70
75
80
90
85
100
SOURCE: Betsch (1929).
distribution of students approaches national norms for ethnicity and socioeconomic status
should be sought. Patients at health maintenance organizations and academically related
clinics, students, and military personnel are generally most accessible. Such groups can be
compared to determine if they demonstrate differences in prevalence or progression.
B. Classifications of age and age intervals should be annual with age determined by
nearest birthday until age 22, after which two-year intervals are recommended. Test periods
suggested are semiannual or annual for studies up to three years' duration, annually or
biannually for studies from 3 to 10 years' duration, and every two or four years for studies
of longer duration at older ages.
C. No study of refractive states should include less than a subjective or objective
measure of refraction, corneal curvatures (using a small mire keratometer), and axial length.
Regardless of whether an automated refractor, retinoscopic, or subjective measure is used,
the examiner should be formally trained and the examiner or interexaminer reliability
determined. Both cycloplegic and noncycloplegic refraction should be determined.
OCR for page 42
42
D. It is important to collect as much of the following data as circumstances permit:
anterior chamber depth, crystalline lens power, tonometry, and fundus photography. While
spherical equivalents most often have been used in tabulating data, it Is recommended that
principal meridians nearest vertical and nearest horizontal be tabulated separately and both
eyes separately. If full analysis is not to be considered, it is recommended that right eye
meridians nearest horizontal be chosen.
E. Emmetropia should be classified in two ways: +0.25 to -0.25 D. inclusive and
+0.75 to -0.75 D. inclusive. Proposed refractive categories are 0.50 D. intervals to +3.00
D. and -6.00 D.; 1.00 D. intervals to +6.00 and -10.00 D. Prevalence studies should include
all those with refractive errors greater than +6.00 D. of one category and those with greater
than -10.00 D. of another category. Unless designed to discover changes among very high
ametropes, progression studies need not include high refractive errors but should specify
whether they are included. Studies comparing refractive errors obtained by various methods
would permit better analysis of myopia research generally; most unportant among these
are automated instrumentation measured against retinoscopic and subjective methods, and
selected cycloplegic techniques against noncycloplegic subjective methods.
F. A retinoscopic method described by Hirsch (1950) and a cycloplegic refraction
method (e.g., Bannon, 1947) represent two highly reliable ways to determine refractive
states. The range of difference on retests and by a different examiner was ~ 0.25 D. by
both methods. Unless such methods are followed and described, measurement errors can
be expected to invalidate attempts to evaluate and classify refractive changes or differences
of +0.50 D. or less. It is recommended that the designation of emmetropia as +0.25 D. be
reserved for those studies that determine reliability to be at +0.25 D. and that +0.75 D.
define emmetropia in all other instances.
Representative terms from entire chapter:
military academies
