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Causes of Incident Visual Impairment
The Blue Mountains Eye Study
Arch Ophthalmol. 2002;120:613-619.
ABSTRACT
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Objective To describe the causes of 5-year incident visual impairment and doubling
of the visual angle in a population-based cohort.
Methods Of the 3654 participants aged older than 50 years who participated in
the Blue Mountains Eye Study (BMES I, 1992-1994), 543 died and 2335 were reexamined
between 1997 and 1999 (BMES II). Visual acuity was measured using a logMAR
chart before and after refraction. Pupils were dilated, and a detailed eye
examination was performed. For participants with incident visual impairment
or doubling of the visual angle, an ophthalmologist attributed and proportioned
causes. Primary causes were defined as those responsible for 50% or more of
the impairment.
Results After refractive correction, the proportion of incident bilateral impairment
worse than 20/40, worse than 20/70, and worse than 20/200 that were caused
primarily by cataract decreased from 51.4% (n = 19) to 40.0% (n = 6) to 0%;
while the proportion of cases caused primarily by age-related maculopathy
increased from 24.3% (n = 9) to 33.3% (n = 5) to 100.0% (n = 2). Similarly,
the corresponding proportions of incident unilateral impairment caused primarily
by cataract decreased from 53.7% (n = 72) to 36.9% (n = 31) to 13.6% (n =
6); meanwhile, the proportion of cases caused primarily by age-related maculopathy
increased from 19.4% (n = 26) to 32.1% (n = 27) to 54.5% (n = 24). The proportions
of persons with incident bilateral impairment worse than 20/40, worse than
20/70, and worse than 20/200 that could be improved with refraction were 79.4%
(n = 143), 73.6% (n = 42), and 0%, respectively. The corresponding proportions
of incident unilateral impairment improved by refraction were 66.7% (n = 269),
59.0% (n = 121), and 21.4% (n = 12).
Conclusion This study has documented the 5-year incidence and causes of visual
impairment in an older Australian population.
INTRODUCTION
DATA FROM large cross-sectional population-based studies in the United
States, Europe, and Australia show a similar profile of the leading causes
of prevalent visual impairment in older, developed communities.1-9
The Wisconsin Beaver Dam Eye Study reported data on the 5-year incidence of
visual impairment and doubling of the visual angle.10
To our knowledge, however, no large population-based studies of older persons
have reported details of the causes of incident visual impairment. Cause-specific
incidence of visual impairment may identify changing trends in the mix of
diseases leading to visual impairment and could also provide useful data for
health care planners in predicting the need for eye care services and in allocating
limited resources. This report from the Blue Mountains Eye Study (BMES) aims
to describe the causes of incident visual impairment and of doubling of the
visual angle in a representative older cohort of the Australian population.
PARTICIPANTS AND METHODS
THE POPULATION STUDIED
The Blue Mountains Eye Study is a population-based survey of vision
and eye diseases in an urban population aged 49 years and older, residing
in 2 postal codes of the Blue Mountains region west of Sydney, Australia.
A previous report from the study explained the reasons for selecting and the
methods used to identify the target population, and described the population.9 All noninstitutionalized, permanent residents born
before January 1, 1943, at the time of the census were eligible.
Of the 4433 eligible residents, 3654 (82.4%) participated in the examinations
from 1992 to 1994. Of the 779 persons who did not participate, 353 (45.3%)
permitted only an interview, 148 (19.0%) refused, 210 (27.0%) had moved out
of the area, and 68 (8.7%) had died. A previous report compared participants
and nonparticipants at baseline.9 Surviving
members of the cohort were invited to return for a 5-year follow-up examination
conducted between 1997 and 1999. Of the 3654 persons seen at baseline, 383
(10.5%) had moved, 394 (10.8%) refused to participate, and 543 (14.9%) had
died. Thus, 2335 (75.1%) persons returned for a follow-up examination. Table 1 compares participants and nonparticipants.
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Table 1. Baseline Characteristics of Participants and Nonparticipants
in the Blue Mountains Eye Study 5-Year Examinations*
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PROCEDURES
Previous BMES reports9, 11-12
describe the survey methods and procedures, which were similar for the 2 examinations.
Participants provided written informed consent on both occasions. The Western
Sydney Human Ethics Committee provided ethical approval.
Visual acuity was measured by a logarithm of the minimum angle of resolution
(logMAR) chart, retroilluminated with automatic calibration to 85 cd/m2 (Vectorvision CSV-100TM; Vectorvision Inc, Dayton, Ohio), and read
at 8 ft (244 cm).9 Distance visual acuity in
each eye was initially measured with current distance glasses if worn. A Humphrey
autorefractor (Model 530; Humphrey-Zeiss, Dublin, Calif) provided an objective
refraction. Subjective refraction was performed according to the Beaver Dam
Eye Study modification of the Early Treatment Diabetic Retinopathy Study protocol.13 For each eye, visual acuity was recorded as the number
of letters read correctly from 0 to 70. If no letters on the chart could be
identified, visual acuity was assessed as counting fingers at 2 ft (61 cm),
hand motions, light perception, or no light perception.
Applanation intraocular pressures were measured. Automated perimetry
of both eyes used the Humphrey 76-point test. Persons with a hemifield defect
of 5 or more points, or other suspicious features were asked to return for
Humphrey 30-2 tests. Pupils were dilated, and the detailed eye examination
included slitlamp (Topcon America Corp, Paramus, NJ) and retroillumination
(Neitz Instrument Co, Tokyo, Japan) photographs of the lens, and stereoscopic
photographs (Humphrey-Zeiss) of the optic disc and retina.
For participants with incident visual impairment or doubling of the
visual angle, an ophthalmologist (P.M.) reviewed their baseline and follow-up
notes and photographs and determined causes and their proportional contributions.
No intraobserver study was performed to assess the reproducibility of these
estimates or drift over time. We also did not attempt to validate the causes
of visual impairment with other ophthalmologists. The primary cause was defined
as that cause estimated to be responsible for 50% or more of the impairment
or deterioration. Where amblyopia coexisted with another cause, development
of visual impairment was arbitrarily assigned to the other cause. Cases with
baseline amblyopia and incident impairment for which no other cause was evident
were arbitrarily assigned as undetermined.
VISUAL IMPAIRMENT DEFINITIONS
Table 2 provides a summary
of the definitions for persons at risk and incident cases used in this report.
We defined visual impairment as worse than 20/40, based on the minimum legal
visual acuity requirement for obtaining a driver's license in Australia. This
was taken in this report as fewer than 39 letters read on the logMAR chart
(allowing one incorrect letter on the 20/40 line). The World Health Organization
(WHO) International Classification of Diseases, 10th Revision
(ICD-10) category 1 or higher defines visual impairment as worse than
20/70 in the better eye, taken as fewer than 29 letters read. The WHO ICD-10 category 2 or higher defines visual impairment as
worse than 20/200, taken as fewer than 4 letters read.14
Blindness in the United States is defined by visual impairment worse than
or equal to 20/200 in both eyes, taken in this report as fewer than 5 letters
read. Unless otherwise specified, visual acuity refers to best-corrected acuity.
Age was defined as age at the baseline examination.
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Table 2. Definitions for Persons at Risk for Visual Impairment and
for Incident Visual Impairment Used in This Report*
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Persons developed bilateral impairment if both eyes were at risk at
baseline and if both eyes had incident impairment at follow-up. Persons developed
unilateral impairment if both eyes were at risk at baseline and only 1 eye
had incident impairment at follow-up.
In keeping with the Beaver Dam Eye Study, deterioration or doubling
of the visual angle was defined as a loss of 15 letters or more read correctly
at follow-up compared with baseline. An eye was considered to be at risk for
deterioration if at baseline it had an acuity of light perception or better.
As with unilateral and bilateral visual impairment, persons were considered
to have unilateral or bilateral deterioration.
DATA HANDLING AND STATISTICAL ANALYSIS
Data were entered into dBase (Borland International Inc, Scotts Valley,
Calif) for BMES I and Microsoft Access (Microsoft Corporation, Redmond, Wash)
for BMES II. Statistical Analysis System (SAS Institute Inc, Cary, NC) was
used for analyses, including  2 and logistic regression.
RESULTS
The mean age of the 2335 BMES II participants at baseline was 64.5 years;
their mean follow-up period was 5.1 years. Table 1 presents the baseline characteristics of participants.
PRESENTING VS BEST-CORRECTED INCIDENT VISUAL IMPAIRMENT
Among persons at risk for incident bilateral visual impairment worse
than 20/40, worse than 20/70, and worse than 20/200, bilateral impairment
occurred in 180 (8.4%), 57 (2.5%), and 2 persons (0.1%), respectively, based
on their presenting vision in the better eye at the follow-up examination.
After refraction, 143 persons (79.4%) and 42 persons (73.6%) with incident
bilateral impairment worse than 20/40 and worse than 20/70, respectively,
improved. Neither of the 2 persons with incident bilateral impairment better
than 20/200 improved following refraction. Table 3 presents the levels of visual acuity following refraction.
Of the 143 persons correctable to at least 20/40, 23 (16.1%) had also had
bilateral impairment, and 75 (52.4%) had also had unilateral visual impairment
worse than 20/40 based on presenting vision at the baseline examination. Of
the 42 persons with vision correctable to at least 20/70, 1 (2.4%) had also
had bilateral impairment, and 12 (28.6%) had also had unilateral visual impairment
worse than 20/70 based on presenting vision at the baseline examination.
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Table 3. Primary Causes of Incident Bilateral Visual Impairment Worse
Than 20/40, Worse Than 20/70, and Worse Than 20/200 Following Subjective Refraction*
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Among persons at risk for incident unilateral visual impairment worse
than 20/40, worse than 20/70, and worse than 20/200, unilateral impairment
occurred in 403 (18.8%), 205 (9.2%), and 56 persons (2.5%), respectively,
based on their presenting vision in the better eye at the follow-up examination.
After refraction, at each of these levels of impairment, 269 (66.7%),
121 (59.0%), and 12 persons (21.4%) improved. Table 4 shows the level of visual acuity following refraction. Of
the 269, 121, and 12 persons who improved, refractive correction to at least
20/40, 20/70, and 20/200 at the baseline examination was also possible in
60 (22.3%), 21 (17.4%), and 2 persons (16.7%), respectively. Table 5 and Table 6
show the breakdown by age and level of visual impairment of persons with unilateral
impairment worse than 20/40.
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Table 4. Primary Causes of Incident Unilateral Visual Impairment Worse
Than 20/40, Worse Than 20/70, and Worse Than 20/200 Following Subjective Refraction*
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Table 5. Age-Specific Primary Attributable Cause of Unilateral Incident
Visual Impairment Worse Than 20/40*
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Table 6. Primary Causes of Incident Unilateral Visual Impairment Worse
Than 20/40 and Subsets Worse Than 20/40 to 20/70, Worse Than 20/70 to 20/200,
and Worse Than 20/200*
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AGE-SPECIFIC CAUSES OF INCIDENT VISUAL IMPAIRMENT WORSE THAN 20/40
Table 5 shows the age-specific
causes of incident unilateral impairment worse than 20/40. Overall, the primary
causes were cataract (53.7%) and age-related macular degeneration (ARM) (19.4%).
In the age groups 49 to 59 years, 60 to 69 years, 70 to 79 years, and older
than 80 years, cataract was the primary cause in 41.7%, 58.7%, 49.2%, and
69.2% of cases, respectively; meanwhile, ARM was responsible for 8.3%, 15.2%,
23.8%, and 23.1% of cases, respectively. Other retinal disorders and retinal
vascular occlusions were jointly the next most frequent cause of incident
visual impairment.
CAUSES OF INCIDENT UNILATERAL VISUAL IMPAIRMENT
Unilateral Visual Impairment (<20/40-20/70)
Table 6 presents the causes
of unilateral visual impairment worse than 20/40, and the 3 subsets worse
than 20/40 to 20/70, worse than 20/70 to 20/200, and worse than 20/200. After
refraction, 81 persons had incident visual impairment of worse than 20/40
to 20/70; 53 were women (65.4%), and 28 were men (34.6%). Of these persons,
5 (6.2%) were aged between 49 and 59 years, 34 (42.0%) were aged between 60
and 69 years, 35 (43.2%) were aged between 70 and 79 years, and 7 (8.6%) were
80 years and older. Cataract was the primary cause in 52 persons (64.2%),
2 of whom had postcataract surgical complications (posterior capsular opacity).
Age-related macular degeneration was the primary cause in 11 persons (13.6%).
Additional primary causes included other retinal disease, glaucoma, retinal
vascular occlusion, corneal disease, and diabetic retinopathy.
Unilateral Visual Impairment (<20/70-20/200, WHO ICD-10 Category 1)
Of the 32 persons with incident unilateral visual impairment worse than
20/70 to 20/200, 18 were women (56.3%), 14 were men (43.7%). Five persons
(15.6%) were aged between 49 and 59 years, 9 (28.1%) were aged between 60
and 69 years, 15 (46.7%) were aged between 70 and 79 years, and 3 (9.4%) were
older than 80 years. Cataract was the primary cause in 16 persons (50.0%),
1 of whom had posterior capsular opacity. Age-related macular degeneration
was the primary cause in 4 persons (12.5%). Additional primary causes included
other retinal disease, retinal vascular occlusion, glaucoma, and corneal disease.
Unilateral Visual Impairment (<20/200, WHO ICD-10 Category 2 or Worse)
Of the 21 persons with incident unilateral visual impairment worse than
20/200, 16 (76.2%) were women and 5 (23.8%) were men. Two persons (9.5%) were
aged between 49 and 59 years, 3 (14.3%) were aged between 60 and 69 years,
13 (61.9%) were aged between 70 and 79 years, and 3 (14.3%) were older than
80 years. Age-related macular degeneration was the primary cause in 11 persons
(52.4%). Cataract was the primary cause in 4 persons (19.0%), 1 of whom had
posterior capsular opacity. Additional primary causes were retinal vascular
occlusion, corneal disease, optic neuropathy, and enucleation after trauma.
Nonexclusive Visual Impairment Categories
Table 3 and Table 4 present the primary causes for the 3 overlapping incident
bilateral and unilateral visual impairment categories (<20/40, <20/70,
and <20/200, respectively). They show that the proportions of participants
with cataract as the primary cause of incident impairment decrease with increasing
severity of visual impairment, while the proportions of cases with ARM as
the primary cause of incident impairment increase with increasing severity
of visual impairment.
VISUAL IMPAIRMENT ( 20/200, US DEFINITION OF BLINDNESS)
Applying the US definition for blindness (20/200) identified no
more persons than the 2 with incident bilateral visual impairment worse than
20/200 as shown in Table 3. An
identical number of cases of incident unilateral impairment were found at
these 2 levels (44 persons), and there were the same proportions due to ARM
and cataract. Only minor differences were seen for additional causes.
DOUBLING OF THE VISUAL ANGLE (DETERIORATION)
Doubling of the visual angle (a decrease of 15 or more letters read
correctly) occurred bilaterally in 31 persons (1.3%) and unilaterally in 149
persons (6.4%). Of the 31 persons with bilateral visual angle doubling, 23
(74.2%) also developed incident bilateral visual impairment. The primary cause
in both eyes was ARM in 12 participants (38.7%) and cataract in 10 (32.2%).
Combined cataract and/or ARM developed in 29 persons (93.5%).
Of the 149 persons with incident unilateral deterioration, 129 persons
(86.6%) had no visual impairment in that eye at baseline and 9 (6.0 %) had
preexisting severe visual impairment. Concurrent incident visual impairment
developed in 116 participants (77.9%). The primary cause was cataract in 61
persons (40.9%) and posterior capsular opacity in 4 persons (2.7%). Age-related
macular degeneration was the primary cause in 38 persons (25.5%). Additional
principal causes included other retinal pathology in 18 persons (12.1%), retinal
vascular occlusion in 7 (4.7%), glaucoma in 5 (3.4%), corneal disease in 3
(2.0%), eye injury in 3 (2.0%), and enucleation in 2 (1.3%). Optic neuropathy,
diabetic retinopathy, and neurological pathology were causal in 1 person (0.7%)
each. Causes were undetermined in 5 persons (3.4%).
COMMENT
A better understanding of changes in the underlying causes of visual
impairment over time among older persons will be important in the development
of strategies that could reduce both the incidence and the various impacts
of visual impairment.15-19
It could also provide a basis for prioritizing research activity, which may
be a relevant consideration with current population aging trends. Although
data on incident blindness may be derived indirectly from blind registries,
these are known substantially to underestimate the number of elderly persons
affected and can provide information only about severe levels of visual impairment.
To our knowledge, no data providing a detailed analysis of the causes
of incident visual impairment from large, older population-based samples have
been reported previously. Comparison of our data with reported prevalence
findings on the causes of visual impairment from large, population-based cross-sectional
studies, including our own,20 would be useful
in confirming the frequency of specific diseases leading to incident visual
impairment.
In keeping with previously reported data from the BMES20
and several other studies,2-4,6, 8
our findings confirmed cataract and ARM as the 2 principal causes of incident
bilateral visual impairment after refraction. Cataract was the principal cause
of incident visual impairment worse than 20/40, while ARM was increasingly
important as the principal cause of the 2 more severe levels of incident visual
impairment worse than 20/70 and worse than 20/200. In the BMES baseline examination,
ARM was the predominant cause of bilateral moderate (20/80-20/200) and severe
visual impairment (<20/200) in persons aged 70 years or older.9, 20 Cataract was the most frequent cause
of bilateral mild impairment (<20/40-20/70).
Other cross-sectional population-based studies have also assessed the
causes of visual impairment and blindness following refraction. The Baltimore
Eye Survey reported in a multiracial US community aged 40 years or older that
cataract was the leading cause of visual impairment (<20/20 to >20/200),
followed by ARM, then diabetic retinopathy and glaucoma.1
The leading causes of blindness (20/200) were cataract, glaucoma, and
ARM.21 The more recent Salisbury Study2 examined persons aged 65 to 84 years and defined visual
impairment and blindness using similar criteria to Baltimore.1
The leading causes of visual impairment were cataract, then ARM, diabetic
retinopathy, and glaucoma. Blindness was caused by ARM (43%), followed by
optic atrophy, diabetic retinopathy, glaucoma, and trauma. In the Rotterdam
Study, the leading causes of visual impairment (<20/60 to  20/400) among
persons aged 55 years or older were cataract, followed by ARM, then myopic
retinal degeneration.3 Blindness (<20/400)
was predominantly due to ARM (in 58% of blind eyes), followed by glaucoma,
cataract, and myopic retinal degeneration.
It is likely that some of the differences found in these cross-sectional
studies of visual impairment may be due to the inherent difficulty in determining
the cause(s) of visual impairment and their proportional contribution. Attributing
the causes of visual impairment is a difficult and potentially imprecise exercise,
particularly when lens opacity obscures a clear view of the macula or optic
disc. Estimating the proportional contribution to visual impairment when more
than one cause is present, or when only minimal visual impairment develops,
can be especially difficult. Attribution of cause is inherently subjective,
so our estimates need to be viewed with caution. In our study, we could not
attribute the cause of visual impairment or deterioration in approximately
3% of cases. The variability in the causes of blindness also reflects the
relatively small number of blind participants in each of these studies. The
development of cognitive impairment could also have influenced the assessment
of visual impairment. Although we measured cognitive impairment with the Mini-Mental
State Examination, we have not yet explored this possible association.
Two recent reports from the Visual Impairment Project (VIP) from Melbourne,
Australia, highlighted the high frequency of undercorrected or uncorrected
refractive error as a cause of visual impairment in older persons.8, 22 This study examined persons 40 years
or older and assessed the causes of visual impairment (<20/40) and blindness
(either <20/200 or <20/400) for both presenting best-corrected visual
acuity and visual acuity at initial visit. Many visual field criteria were
incorporated into each level of impairment. Undercorrected or uncorrected
refraction was the leading cause of visual impairment (58%). Nonrefractive
causes of visual impairment, however, were similar to those reported by most
other studies. These included ARM, followed by cataract, glaucoma, neurological
causes, and diabetic retinopathy. Age-related macular degeneration and glaucoma
were equal principal causes of both levels of blindness in this study.22
In our study, undercorrected refraction was frequently associated (in
approximately 75% of cases) with incident bilateral visual impairment (based
on presenting visual acuity at initial visit) at lower levels of visual impairment,
worse than 20/40 and worse than 20/70, but it was not associated with incident
blindness (Table 3). After refractive
correction, the leading cause of incident bilateral visual impairment worse
than 20/40 was cataract, followed by ARM. Following refraction, ARM and cataract
each caused or contributed to a similar proportion of cases (46.7%) of incident
bilateral visual impairment worse than 20/70. Age-related macular degeneration
caused each of the 2 cases with incident bilateral visual impairment worse
than 20/200. The proportion of incident cases with unilateral impairment worse
than 20/40, worse than 20/70, and worse than 20/200 that could be improved
with refraction declined with increasing severity of impairment (66.7%, 59.0%,
and 21.4%, respectively). As best-corrected visual impairment increased across
these 3 levels, the role of cataract as the predominant cause decreased (53.7%,
36.9%, and 13.6%, respectively) and ARM increased in importance as the leading
cause of incident impairment (19.4%, 32.1%, and 54.5%). These opposite trends
were not unexpected based on previous prevalence reports from older white
communities indicating cataract as the leading cause of mild visual impairment
and ARM as the cause of severe visual impairment.3-4,8, 10, 20, 22
This study has confirmed that undercorrected refraction is often associated
with incident visual impairment, as recent prevalence findings from the Visual
Impairment Project (Victoria, Australia)8, 22
and our previous report suggest.9 Our study
and others have described many different impacts on daily living activities
and ability to live independently resulting from visual impairment in older
persons. These have included effects on the use of community support services,16 nursing home placement,23
falls,15, 24-25 mortality,18, 26 and factors determining self-rated
health.27 Most studies have assessed the impacts
from noncorrectable visual impairment; however, in several recent reports,
visual impairment associated with uncorrected refraction was also associated
with substantial impacts, though generally at a lower magnitude.16, 18-19
At the baseline examination, persons whose vision improved by 1 or more
lines following refraction were advised by the examining physician, and later
in the examination report, that their vision could be improved by glasses
or a change in their distance glasses if worn. Around 20% of participants
whom we classified as developing incident unilateral visual impairment correctable
with refraction (based on their best-corrected visual acuity at baseline and
presenting visual acuity at follow-up) were not true incident cases because
they had also had correctable visual impairment at the baseline examination.
Many people with undercorrected refraction do not feel they need, or dislike
wearing, distance glasses. Many stated that they had distance glasses but
did not wear them. Thus, while simple optical measures may correct a large
proportion of visual impairments, people may not feel the need for these (particularly
if one eye is unimpaired) until other abnormalities intervene. Nevertheless,
the accumulating evidence regarding impacts of undercorrected refraction on
independent living indicate the potential public health importance of refractive
correction.
The BMES examined 75.1% of survivors from the initial study after 5
years. A potential limitation of our study is that we may have underestimated
the decrease in visual acuity or the incidence of visual impairment because
persons returning had significantly less visual impairment and late ARM at
baseline than nonparticipants, as indicated in Table 1.
This report confirmed ARM as the preeminent cause of moderate to severe
incident visual impairment in an older white population. Cataract was the
principal remediable disease causing visual impairment. Undercorrected refraction
seems to play a potentially important role as a frequent associated factor
that could be readily addressed. Appropriately targeted refractive services
could substantially reduce the morbidity from visual impairment in this age
group.
AUTHOR INFORMATION
Submitted for publication October 12, 2001; final revision received
January 14, 2002; accepted January 24, 2002.
This study was supported by research grants 974159 and 991407 from the
Australian National Health & Medical Research Council, Canberra, Australia.
Corresponding author and reprints: Paul Mitchell, MD, PhD, Department
of Ophthalmology, University of Sydney, Hawkesbury Road, Westmead, New South
Wales, Australia 2145 (e-mail: paulmi{at}westgate.wh.usyd.edu.au).
Suriya Foran, MBBS, MPH;
Jie Jin Wang, MMed, PhD;
Paul Mitchell, MD, PhD
From the Department of Ophthalmology, Centre for Vision Research, Westmead
Hospital, Sydney, Australia, and the Save Sight and Westmead Millennium Institutes,
the University of Sydney, Sydney.
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