Context.— Exposure to UV-B radiation in sunlight has been shown to increase the
risk of cataract formation in high-risk occupational groups, but risk to the
population has not been quantified.
Objectives.— To determine the ocular exposure to UV-B radiation in sunlight for a
population of older persons and to determine the association between UV-B
and lens opacities.
Design.— The Salisbury Eye Evaluation project, a population-based cohort of older
adults.
Setting.— Salisbury, Md.
Participants.— A total of 2520 community-dwelling 65-year-old to 84-year- old adults
in Salisbury, Md, from 1993 to 1995, of whom 26.4% were African Americans.
Main Outcome Measure.— Association of photographically documented cortical opacity 3/16 or
greater in at least 1 eye with ocular UV-B exposure, reported in Maryland
sun-years of exposure.
Results.— The odds of cortical opacity increased with increasing ocular exposure
to UV-B (odds ratio [OR], 1.10; 95% confidence interval [CI], 1.02- 1.20).
The relationship was similar for women (OR, 1.14; 95% CI, 1.00- 1.30) and
for African Americans (OR, 1.18; 95% CI, 1.04-1.33). Analyses of the ocular
dose by each age group after the age of 30 years showed no vulnerable age
group, suggesting damage is based on cumulative exposure.
Conclusions.— Although this population of older Americans has relatively low ocular
exposure to UV-B in sunlight, there is still an association between ocular
exposure and increasing odds of cortical opacity. Our study found an association
among African Americans, which, to our knowledge, has not been reported previously.
All sex and racial groups would benefit from simple methods to avoid ocular
sun exposure.
DESPITE ADVANCES in surgical procedures and targeted programs, cataract
remains the leading cause of visual loss
worldwide.1
In the United States, cataract surgery is responsible for 12% of the Medicare
budget,2 or $3.4 billion annually in 1991.
Although cataract surgery rates have increased, there are still underserved
minority populations where cataract remains an important cause of visual impairment
(data available from authors).3 As the population
ages, both in the United States and globally, cataract surgery and loss of
vision from cataract will become an even more pressing issue for the eye care
community.
Considerable effort to elucidate risk factors for cataract has been
undertaken in hopes that simple, preventive strategies may be implemented
to avoid or delay the progression of lens opacification.4
In particular, several epidemiological studies have been carried out on the
relationship between risk of cataract and of sun exposure, especially UV-B
exposure.5-14 In general,
the ecological studies are suggestive of an association between cataract and
UV-B exposure, using crude approximations of exposure and various cataract
assessment techniques.6-8,10 Three case-control studies found
no association of cataract and proxy measures of sun exposures.12-14 Two of the studies used nonstandardized assessment of cataract
status, and one used insensitive measures of ocular exposure to UV-B. A detailed
assessment of ocular exposure to UV-B was carried out in the Chesapeake Bay
Waterman study,5 where increases in average
annual ocular exposure were associated with increasing risk of cortical opacity.
In this highly exposed group of predominantly white males, the evidence linking
cortical opacities to sunlight exposure was the strongest to date.
However, subsequent data from the Beaver Dam Eye study suggested the
risk may be confined to men.9 In that population-based
study, the exposures among women were lower than exposures among men, and
no association was seen. Moreover, there were no data linking sunlight exposure
to risk of cataract in African Americans, although other eye diseases have
different prevalences among the different racial groups, and cortical opacity
appears to be higher in African Americans compared with whites.15,16
The purposes of this component of the Salisbury Eye Evaluation (SEE)
project were to quantify, for the first time, the levels of ocular exposure
to UV-B and visible light for a population, as opposed to high- risk occupational
groups, and to determine the association of these levels of exposure with
the risk of cortical opacity separately for women and African Americans. In
previous publications, we have described the methods and results of our ocular
exposure models.17-19 In this article, we describe the association of UV-B with
lens opacities.
The SEE project is a population-based, longitudinal study of the impact
of visual impairment and age-related eye diseases on functional status in
older, community-dwelling adults.20 To achieve
the aims of this project, a random sample of residents of Salisbury, Md, aged
65 to 84 years was recruited for a home interview and an examination at the
SEE clinic, which included lens photography and administration of a questionnaire
about sun exposure during leisure and work times over the lifetime of the
participant since the age of 30 years. The sample was selected from the Health
Care Financing Administration Medicare database, which is reported to include
98% of persons 65 years or older.21 The sample
included a 100% sample of the African American population, a 56% sample of
whites aged 65 to 74 years, and a 62% sample of whites aged 75 to 84 years.
The older age group was oversampled in anticipation of a higher refusal rate
among older ages. We included all African Americans aged 65 to 74 years to
have sufficient numbers for race-specific analyses. Exclusion criteria included
those who were institutionalized, or completely house bound, and those who
scored less than 18 on the Mini-Mental State Examination.22
Written, informed consent was obtained at the home interview in accordance
with the tenets of the Declaration of Helsinki. Details on the population
and recruitment are described elsewhere.21 In summary, of the original sample, 73% participated in the
home interview and 65% participated in both the interview and the clinical
examination.
Permission was also sought to administer a 12-question screener questionnaire
of both the refusals and the participants in order to investigate the comparability
between those for whom data were available and those who refused. Of the 1301
refusals to the clinic examination or the home questionnaire, 65% agreed to
answer the questions on the screener. There were no differences by age, race,
or sex between the refusals who answered the screener and those who did not
answer the screener.22 The screener included
the following question related to sun exposure: "For the job you held longest
in your life, did you spend more than 2 hours outside during daylight in the
summer months?" Another question asked respondents to self-rate their vision
status on a scale of 1 to 10, with 10 being excellent. There was no difference
in participation rates by race; participation rates declined with age from
68% in the age group 65 to 69 years to 55% in the age group 80 to 84 years.
There was no difference between the refusals and the participants in the proportion
who reported being outside in the summer months in the job held longest in
their lifetime. Among participants, 41% reported being outside more than 2
hours per day, compared with 40% among refusals. There was also no difference
in the sex- and age-adjusted proportion rating their vision as 6 or better;
82.9% were 6 or better among participants compared with 84.0% among refusals.
These data provide some assurances that the sample of 2520 participants did
not seem to be biased on either sun exposure or self-reported vision status.
At the examination center, each participant's eyes were dilated and
lens photographs were taken using standardized protocols previously described.15 Photographs were graded for the type and severity
of opacity using the Wilmer grading scheme.23,24 Nuclear opacification was graded against a standard
photograph for the integer grade, with decimalized interpolation between the
standards.25 Cortical opacification was graded
by estimating the amount of pupillary area obscured, in 1/16th sectors.21 Posterior subcapsular (PSC) opacities were graded
as present or absent.
All grading was done for each eye independently by 2 trained graders
masked to each other's grade and the grade of the fellow eye. Differences
greater than 0.3 in nuclear, 1/16th in cortical, and present or absent in
PSC were openly adjudicated with a third grader. Interobserver variation was
monitored over the course of the study by the periodic circulation of 53 photographs.
The interobserver agreement at baseline was κ=0.92 (95% confidence interval
[CI], 0.85-0.99) for nuclear and κ=0.95 (95% CI, 0.91-1.0) for cortical.
The average κ value for agreement over time was 0.83 for nuclear and
0.81 for cortical.15
Nuclear opacity was defined as present if at least 1 eye had a grade
of 2.0 or higher. Cortical opacity was defined as present if at least 1 eye
had a grade of 3/16 or higher. Bilateral surgery occurred in 245 participants
and the type of opacity was obtained from the surgeon's records in 185 participants
(76%). The data were analyzed with and without the inclusion of the bilateral
surgical cases and the results were unchanged; therefore, the analyses are
presented based on the photograph data alone.
Measurement of Sun Exposure
An empirical model to estimate ocular exposure in the UV-B wavelength
band has been extensively described in previous publications.17-19 The model for cumulative exposure for a single day is as
follows:
Graphic Jump Location![Image description not available.]()
where Roa indicates the ocular-ambient exposure ratio (fixed for the
day but variable with season); Ft (ti), the fraction of time spent outdoors in
the ith period of the day (can be variable by month); Ha(ti),
the global ambient exposure during this day (variable by month and hour of
day); Thats and Teye, fixed factors (between 0 and 1) that reflect the dimunitions
conferred by the use of hats and eyewear; G, a geographic correction factor
that relates the total yearly ambient exposures seen in the Maryland area
with locations elsewhere in the world; and i, generic
time index. The following formula then is the implementation of this model
during a day and over the course of a year:
Graphic Jump Location![Image description not available.]()
where m indicates index that runs
over the months, and t indicates index that runs
over the hours of the day from 5:00 AM (5 hours) to 6:00 PM (18 hours). The
exposure units are in Maryland sun- years (MSYs) or the equivalent of 75.9-J/cm2 effective integrated energy density (erythemal spectral weighting).
The exposure for each person was then summed for each year of life since the
age of 30 years, and a cumulative lifetime ocular exposure derived. Those
whose cumulative exposure was equal to 0 were those reporting less than 1
hour per day outside in job or leisure activities. Although this group probably
has a finite, very low exposure, our model categorizes them as zero.
The ambient exposure levels were obtained from 2 years of measurements
made on the Eastern Shore using a UV-B pyranometer (Solar Light Company, Philadelphia,
Pa).17 The geographic correction factor for
jobs and leisure time spent outside Maryland was developed as the result of
a semiempirical model developed at the National Aeronautics and Space Administration
and described in detail in an earlier publication.17
The model does correct for cloud cover in various locations around the globe.
The ocular ambient exposure ratios were determined through a series of measurements
made on residents of the population of Salisbury as they carried out usual
daily activities. The ocular ambient exposure ratios were allowed to vary
by season.18 Diminution factors for hat use
were also based on our measurements in the Salisbury population.18
Diminution factors for glasses were based on previous experiments on UV-B
attenuation for plastic and glass sunglasses and eyeglasses.26
The fraction of time spent outdoors and the use of glasses, sunglasses, and
hats were derived from the job and leisure history questionnaire administered
to participants. This questionnaire asked participants about their job history
since the age of 30 years, time spent outside during the job and leisure time,
geographic location of job and leisure activity, and glasses and hat use while
outside. In our pilot studies, we were unable to obtain reliable data from
this age group on time spent outside and job history, prior to the age of
30 years, so our job history questionnaire began at the age of 30 years.19 Based on data from optometrists in Salisbury, plastic
lenses were introduced in 1970, and, currently, approximately 85% of the sample
who use any eyewear use plastic lenses; the remaining 15% use glass lenses.
Therefore, we presume that prior to 1970, all spectacles had glass lenses,
and between 1970 and the present, a yearly linear increment used plastic lenses.
Data on age, race, and sex of participants were available from the home
questionnaire. Data on diabetes were based on a self-report, validated by
use of insulin or oral hypoglycemics or by hospital or physician records.
For those who denied presence of diabetes, we included as diabetic patients
those patients with a hemoglobin A1c value greater than 7%.27 In addition, other variables, such as educational
level, smoking, and alcohol use, were collected via an interviewer- administered
home questionnaire. These variables were evaluated for the relationship to
the different types of lens opacities.
Review of our models suggested that original, untransformed data on
UV-B exposure were acceptable. Therefore, all analyses are based on untransformed
average annual exposure. Analyses were also carried out using cumulative exposure,
which is highly correlated with age. By using average annual exposure, the
age component of exposure is removed, and the addition of age alone permits
other age-related effects, which are also significant, to be in the model
of risk of opacities. Differences in exposure by sex and race were assessed
using a median test for 2 samples, where a simple linear rank statistic was
calculated on the median score.
Data were analyzed initially using simple bivariate analyses. χ2 Tests and trend tests for significance were performed. Logistic regression
models with opacity as the dependent variable were created to evaluate the
effect of increasing exposure to UV-B, adjusting for other risk factors. In
order to present the most parsimonious models, factors that were not associated
with the opacities of interest were not entered into the models.
A serially additive expected-dose model was used to further determine
possible associations between age of exposure and risk of lens opacities.28 In this model, the expected yearly exposure to UV-B
is calculated based on all the controls without lens opacities of the same
age as the cases. The actual observed exposure for each subject is then compared
with the expected exposure calculated from the controls. A paired t test is used to test the cumulative difference in observed vs expected
exposures. In addition, differences at each age or age category can be explored.
The data from the short questionnaire showed similar distributions in
refusals and participants of responses to questions on sun exposure and self-report
of vision. Therefore, these variables were not used to determine adjustments
for differential response rates. Participants differed from nonparticipants
in having more disability, less education, and reporting poorer health status.22 The association between UV-B and opacity was evaluated
within strata of each of these variables to determine if differential response
rate might explain our findings. However, there were no differences in the
associations between UV-B exposure and opacities within these factors. Thus,
we did not make any further adjustments for refusal rates.
Of the 2520 participants, 26.4% were African American and 58% were female.
Of those without cataract surgery, cortical photographs on at least 1 eye
were obtained on 94% and nuclear photographs obtained on 93%.15
Photographs could not be obtained on the rest primarily due to equipment failure
or medical contraindication to dilation. There was no statistically significant
difference in the clinical grade of opacities in those with and without photographs.15 There was a pronounced difference in prevalence of
opacity types by racial group (Table 1),
with more cortical opacities and fewer nuclear opacities in African Americans
(P<.05).
There were no pronounced differences in the distribution of average
annual ocular UV-B exposure by racial group. However, women had significantly
less ocular exposure compared with men (P<.05)
(Table 2). The average annual
exposure for this population-based sample of older men and women was significantly
lower than the average annual exposure reported for the occupational group
of the Chesapeake Bay watermen, 0.011 MSY vs 0.022 MSY (P<.05).
The prevalence of cortical opacity by quartiles of average annual ocular
UV-B exposure increased from the lowest to the highest quartile, with a significant
difference in the prevalence comparing the highest with the lowest (Table 3). With adjustment for age, race,
sex, diabetes status, and education, the test for linear trend of increasing
odds with increasing quartile of exposure was significant (P=.03). Smoking, education, and alcohol use were not significantly
related to cortical opacity and were not included in the model.
With ocular exposure modeled as a continuous variable, the odds of cortical
opacity increased 10% with each 0.01 MSY increase in average annual ocular
UV-B exposure (Table 4). The relationship
was similar when analyses were confined to women and to African Americans,
(ORs, 1.14 and 1.18, respectively)
(Table 5 and
Table 6). There
was no evidence of any relationship between nuclear opacity or PSC opacity
and exposure to UV-B, either age adjusted or in multivariate models, which
also included race, sex, smoking, diabetes, education, and steroid use (data
not shown).
We used the serially additive expected-dose model, where differences
in the annual dose by age between cases of cortical opacity and controls without
cortical opacity can be compared. There was no age (>30 years) where exposure
for cases was much greater than the exposures at other ages (Figure). There were relatively few participants aged 80 years and
older, especially among controls, so the differences are less stable in the
very oldest age groups. These results are consistent with a cumulative exposure
model in which the potential for lens damage is not confined to a particular
stage of life.
In a population-based study of older Americans in Salisbury, a detailed
model was developed for the assessment of ocular exposure to UV- B.17-19 Using this model
for exposure assessment, a significant association between cortical opacities
and average annual UV-B exposure was found. The excess risk was observed at
each age from 30 years and older, suggesting no particular age of life (>30
years) is more important than others in determining risk but rather the risk
is a cumulative dose phenomenon.
Previous studies, notably the study of Chesapeake Bay waterman, have
demonstrated an association between cortical opacity and increasing ocular
exposure to UV-B.5 However, it was not clear
that the association would be observed with lower exposures more characteristic
of the general population. Our data in the older population suggest there
is a consistent risk, even in the lower exposures. The contribution of childhood
exposure was not evaluated in this study and remains to be determined.
The association was observed among women; although, in general, women
had less exposure than men in this population. The previous work showing no
association among women was also population based but did not use a detailed
assessment of ocular UV exposure.9 In that
study, differences in exposure were generated as a result of differences in
the latitude of residence, and the women were less likely to live outside
the home state and thus had little variation in the exposures. In the Salisbury
study, women also had lower average annual exposures, but the association
of cortical cataract with exposure was still evident and marginally significant.
Our study is the first to document the relationship between ocular exposure
to UV-B and risk of cortical opacity in African Americans. There are racial
differences in the prevalences of ocular diseases, notably glaucoma and age-related
macular degeneration.29,30 We
and others have previously shown a pronounced racial variation in types of
lens opacities,15,16 which is
unlikely due to differential rates of cataract surgery or other methodological
issues. It is conceivable that risk factors may operate differently in different
racial groups. However, there is still an excess of cortical opacity in the
African American population that is not attributable to UV-B exposure or the
other risk factors evaluated, such as age, sex, and diabetes status. It is
clear that UV-B exposure is a risk factor for cortical opacity in this racial
group as well. Further work on risk factors within racial groups is warranted.
Our study did not find any association between UV-B exposure and nuclear
opacity, a finding consistent with the Chesapeake Bay Waterman study and other
studies as well.5,7,9 The lens absorbs UV-B radiation, with a gradient of absorption
from the anterior to the posterior plane of the lens. The anterior cortical
surface receives the most radiant energy and thus would be the most likely
target for damage. In animal experiments, anterior opacities were also the
most common opacities observed.31
Our study also did not find an association with PSC opacities, although
a previous case-control study of PSC opacities in a sample from the same geographic
locale found an increased risk associated with UV-B exposure.32
The previous study also used a detailed assessment of ocular UV-B exposure,
although using assumptions more appropriate for the Chesapeake Bay Waterman
study. We analyzed our data to mimic the analyses done in the prior study
in an effort to reproduce the findings, but there was still no association.
The control selection for the earlier study was persons without PSC who had
visited an eye care professional at the same time as a case of PSC. Half those
controls had no opacities, with cases of PSC having more concomitant cortical
opacities than controls. It is possible that the presence of cortical opacities
in the cases explained the association.
Age was also an independent predictor of cortical opacity, after adjustment
for other factors. This finding suggests that UV-B exposure over the lifetime
does not explain the "age" effects in cataractogenesis. Myriad changes occur
in the lens with age, including pronounced physical and metabolic changes.33 Some of these changes are accelerated in cataractogenesis,
which may explain the "age, " beyond chronologic age, effects.33
This study has found a significant association between cortical opacity
with even the lower levels of ocular UV-B exposure likely to be found in the
general population of older persons. The estimate of increased risk from the
lowest to the highest quartile was 1.6 in this population-based study. If
25% of our study population is in the highest quartile of exposure (>0.024
average annual exposure in MSYs), and the odds of cortical opacity, relative
to the lowest group, are 1.6, an estimate of the attributable risk can be
derived as34:
Graphic Jump Location![Image description not available.]()
where Pe indicates
proportion of population exposed to highest quartile; OR, estimate of risk; and 1, attributable risk. For this population of
older persons in Salisbury, the attributable risk for cortical opacity due
to higher levels of UV-B exposure is 13%. These data add to the growing body
of knowledge that suggests even low levels of UV-B can harm the lens. Measures
to avoid ocular exposure to UV-B in sunlight are simple. The wearing of plastic
glasses or sunglasses confers excellent protection, and the simple wearing
of a hat with a brim decreases ocular exposure by 30% to 50%. These measures
should be part of any public health program to increase awareness of sun damage
and avoid unhealthy consequences.
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