- Email: [email protected]
Cause-specific prevalence of bilateral visual impairment in Victoria, Australia
Cause-specific Prevalence of Bilateral Visual Impairment in Victoria, Australia The Visual Impairment Project Mylan R. VanNewkirk, MD, FRACO, LeAnn Weih, PhD, MS, Catherine A. McCarty, PhD, MPH, Hugh R. Taylor, MD, FRACO Purpose: To study the cause-specific prevalence of eye diseases causing bilateral visual impairment in Australian adults. Design: Two-site, population-based cross-sectional study. Participants: Participants were aged 40 years and older and resident in their homes at the time of recruitment for the study. The study was conducted during 1992 through 1996. Methods: The study uses a cluster stratified random sample of 4744 participants from two cohorts, urban, and rural Victoria. Participants completed a standardized interview and eye examination, including presenting and best-corrected visual acuity, visual fields, and dilated ocular examination. The major cause of vision loss was identified for all participants found to be visually impaired. Population-based prevalence estimates are weighted to reflect the age and gender distribution of the two cohorts in Victoria. Main Outcome Measures: Visual impairment was defined by four levels of severity on the basis of best-corrected visual acuity or visual field: ⬍6/18 ⱖ6/60 and/or ⬍20° ⱖ10° radius field, moderate vision impairment; severe vision impairment, ⬍6/60 ⱖ3/60 and/or ⬍10° ⱖ5° radius field; and profound vision impairment ⬍3/60 and/or ⬍5° radius field. In addition, less-than-legal driving vision, ⬍6/12 ⱖ6/18, and/or homonymous hemianopia were defined as mild vision impairment. In Australia, legal blindness includes severe and profound vision impairment. Results: The population-weighted prevalence of diseases causing less-than-legal driving or worse impairment in the better eye was 42.48/1000 (95% confidence interval [CI], 30.11, 54.86). Uncorrected refractive error was the most frequent cause of bilateral vision impairment, 24.68/1000 (95% CI, 16.12, 33.25), followed by age-related macular degeneration (AMD), 3.86/1000 (95% CI, 2.17, 5.55); other retinal diseases, 2.91/1000 (95% CI, 0.74, 5.08); other disorders, 2.80/1000 (95% CI, 1.17, 4.43); cataract, 2.57/1000 (95% CI, 1.38, 3.76); glaucoma, 2.32/1000 (95% CI, 0.72, 3.92); neuro-ophthalmic disorders, 1.80/1000 (95% CI, 0, 4.11); and diabetic retinopathy, 1.53/1000 (95% CI, 0.71, 2.36). The prevalence of legal blindness was 5.30/1000 (95% CI, 3.24, 7.36). Although not significantly different, the causes of legal blindness were uncorrected refractive errors, AMD, glaucoma, other retinal conditions, and other diseases. Conclusions: Significant reduction of visual impairment may be attained with the application of current knowledge in refractive errors, diabetes mellitus, cataract, and glaucoma. Although easily preventable, uncorrected refractive error remains a major cause of vision impairment. Ophthalmology 2001;108:960 –967 © 2001 by the American Academy of Ophthalmology. Appropriate eye health care planning, allocation of resources, and prioritization of research require accurate estimates of the prevalence and causes of blindness. Well-
Originally received: October 5, 1998. Accepted: January 3, 2001. Manuscript no. 98663. Centre For Eye Research Australia, University of Melbourne, Melbourne, Australia. Supported by National Health and Medical Research Council, Canberra, ACT, Australia; Victorian Health Promotion Foundation, Melbourne, Victoria, Australia; Estate of the late Dorothy Edols, Melbourne, Victoria, Australia; Ansell Ophthalmology Foundation, Melbourne, Victoria, Australia; Jack Brockhoff Foundation, Melbourne, Victoria, Australia. Reprint requests to LeAnn Weih, CERA, Locked Bag 8, East Melbourne, 3002 VIC, Australia.
960
© 2001 by the American Academy of Ophthalmology Published by Elsevier Science Inc.
designed, population-based studies of eye diseases have come from single urban or rural communities in the United States, Australia, and The Netherlands.1– 6 The Visual Impairment Project (VIP) is a population-based study of eye disease that aims to fully represent the Victorian population by examining two groups, the urban and rural residential populations. The most densely populated Australian state, Victoria is located in the southeastern corner of Australia and comprises almost 25% of its total population.7 The clusterstratified sample of 4744 adults 40 years of age and older consists of 3271 urban and 1473 rural participants. Confusion exists because of the many definitions of visual impairment that have been previously described.8 The United States definition of legal blindness, ⱕ20/200 or ISSN 0161-6420/01/$–see front matter PII S0161-6420(01)00554-1
VanNewkirk et al 䡠 Causes of Bilateral Visual Impairment in Victoria 6/60 and/or ⱕ20° visual field, differs from both the Australian and the World Health Organization (WHO) definitions of blindness. This study defined moderate bilateral vision impairment by use of the WHO definition for low vision (⬍6/18 ⱖ6/60, ⬍20° ⱖ10° field,9 severe vision impairment (⬍6/60 ⱖ3/60, ⬍10° ⱖ5° field), and the WHO definition of blindness9 or profound vision impairment (⬍3/ 60, ⬍5° field). The Australian definition of legal blindness encompasses severe and profound categories. In addition, we defined visual impairment at less-than-driving vision, ⬍6/12 ⱖ6/18, and/or homonymous hemianopia in the two noninstitutional cohorts of our study, the cohorts most likely to drive. The aim of this study is to describe the causes of bilateral mild, moderate, severe, and profound visual impairment. The population-weighted cause-specific prevalences of visual impairment are reported individually and cumulatively.
Methods A detailed method has been reported elsewhere.10,11 The clusterstratified sample was drawn from nine randomly selected pairs of adjacent census collector districts in urban Melbourne and four pairs of randomly selected adjacent census collector districts in four rural communities in Victoria. Target sample size for the study was based on the ability to measure the prevalence of visual impairment (⬍6/18 ⱖ3/60) in Victoria with relative precision of 80% (alpha ⫽ 0.05), assuming that the prevalence was similar to that of white participants of the Baltimore Eye survey, 0.0127.12 The sample size required was 2772. A door-to-door household census was conducted to identify all eligible persons, those aged 40 years and older in the calendar year of the examination who had lived at that address for at least 6 months. The first interview of eligible residents, conducted at the time of the household census, used a standardized questionnaire that included basic demographic data, current medical history, most recent visit to an ophthalmologist or an optometrist, and visual symptoms. The participants were encouraged to attend the local test site for a second interview and an eye examination. The urban study was carried out from 1992 to 1995, the rural study in 1996. The study was conducted with ethics approval from the Royal Victorian Eye and Ear Hospital. All participants gave signed consent for examination after being informed of the nature of the examination and the use of the data collected.
Visual Assessment A participant’s presenting distance visual acuity was measured with a logarithm of the minimum angle of resolution chart at 4 m and, if necessary, at 3, 2 and 1 m on each eye separately while the participant wore his or her current spectacles (if worn). Visual acuity was recorded as the total number of characters read correctly. The logarithm of the minimum angle of resolution chart measures a range of Snellen equivalent visual acuity from at best 6/3 at 4 m to at worst 1/60 at 1 m. An E chart was used for participants who could not read English and for non-Englishspeaking participants where translation was not possible. If visual acuity could not be measured with the logarithm of the minimum angle of resolution chart, a sequential approach was used with the following tests: counting fingers, hand movement, clinical fixation on a target image, and light perception. Test/retest of visual acuity measurement was not performed. Current spectacles were analyzed, and autorefraction was performed with Humphrey instru-
ments. A subjective refraction was performed if presenting visual acuity was less than 6/6, and the participant’s mental status was adequate to respond to the test. Visual field assessment was performed with a Humphrey Field Analyzer by use of the 24-2 Fastpac statistical package (Humphrey Instruments Inc, San Leandro, CA). If the participant was unable to complete threshold visual field testing, a Bjerrum tangent screen visual field was done; as a final resort, confrontation visual fields were performed. Threshold visual field tests were inspected for reliability at the time of the examination, and unreliable tests were repeated or alternative visual field tests were performed. Test results on the Humphrey Field Analyzer were considered inadequate for use in data analysis when there were 20% or greater fixation losses or 33% or greater false-positive or false-negative errors. However, a few reliable threshold visual field tests had no obvious clinical disease correlation, and these fields had not been repeated. This may be due to perimetric inexperience13 and may result in an overestimation of visual impairment caused by visual field constriction. Standardized ophthalmologic examinations were performed on all participants. The biomicroscopic examination included clinical lens nucleus grading by visual comparison with a standard photograph. Cortical and posterior subcapsular cataracts were measured and graded visually with biomicroscopic retroillumination by the ophthalmologist.14,15 Lens nucleus and retroillumination photographs were taken and graded separately by two trained observers, and any differences were adjudicated. Vertical cup/disc ratios were measured and recorded, as were abnormalities of the choroid and retina. Age-related maculopathy, age-related macular degeneration (AMD), and diabetic retinopathy (DR) were graded clinically with indirect biomicroscopy and ophthalmoscopy and by photo graders according to standardized classification schemes.16,17 Glaucoma was defined by a consensus group of six ophthalmologists using photos of the optic disc and visual fields.18 Other diseases causing visual impairment were classified using International Classification of Diseases-9.19 The category of visual impairment was classified by vision in the better eye for all causes other than uncorrected refractive error. Level of visual impairment was defined by presenting acuity for uncorrected refractive error. The major predisposing condition for each person, by better eye, was assigned as the cause of visual impairment (e.g., if a motor vehicle accident fractured the optic foramen resulting in optic atrophy, the cause of visual impairment was listed as trauma). If two or more diseases were present, the disease with the most clinically significant effect on vision was assigned as the principal cause of visual loss. For example, when AMD with geographic atrophy coexisted with nuclear cataract, the cause of vision loss was attributed to AMD. When both eyes had the same level of visual impairment and each eye had a different cause of visual impairment, consistent clinical principles were used by one of us (MRVN) to assign the cause reflecting the principal disease. For example, if AMD and corneal opacity occurred in different eyes of one participant, AMD was assigned as the cause of visual impairment for that individual. Uncorrected refractive error was measured by the difference between presenting and best-corrected visual acuity. A case of uncorrected refractive error required an improvement of one or more categories of visual acuity (Table 1). For example, the visual acuity of a participant must improve with best correction from ⬍6/18 ⱖ6/60 presenting vision to at least ⬍6/12 ⱖ6/18 to be counted in the uncorrected refractive error category of moderate visual impairment.
Data Analysis Interview data were collected by direct computer entry using a questionnaire programmed in Paradox (Borland International,
961
Ophthalmology Volume 108, Number 5, May 2001 Table 1. Level of Visual Acuity and/or Visual Field in the Better Eye Vision Impairment
Visual Acuity
Visual Field
Equivalent
Mild
⬍6/12 ⱖ6/18
Moderate
⬍6/18 ⱖ6/60
Homonymous hemianopia ⬍20° ⱖ10° radius
Severe
⬍6/60 ⱖ3/60
⬍10° ⱖ5° radius
Profound
⬍3/60
⬍5° radius
⬍AU and US driving acuity WHO low vision ⬍6/18 ⱖ 3/60, ⬍20° ⱖ5° radius AU legal blindness ⬍6/60, ⬍10° radius WHO blindness
AU ⫽ Australia; US ⫽ United States; WHO ⫽ World Health Organization.
Scotts Valley, CA) or on self-coding forms. Open-ended responses were coded at a later time. Visual assessment and ophthalmic examination data were doubly entered. Classification of visual impairment used Snellen equivalent visual acuity based on reading a full line, five letters. Population-weighted prevalences were calculated by weighting the data in each cohort according to the 1996 Australian Bureau of Census data by the proportion of each cohort, by decade of age and gender, residing in the Victorian population.20 Ninety-five percent confidence intervals around cause-specific prevalences were calculated according to Cochran21 to account for the effect of the cluster sampling. The design effect was 1.56. Participants who did not have visual acuity measured were not included in the analyses. Odds ratios were calculated with multivariate logistic regression controlling for age, gender, and accounting for cluster design effects using SAS software version 6.10 (SAS Institute, Cary, NC). Confidence intervals around age-standardized rates for each cohort were calculated using the standard error of the directly standardized rate. When a participant was observed to have both visual acuity impairment and visual field defect, the category with the most severe disability was used to classify the participant. That is, if visual acuity was ⬍6/18 but ⱖ6/60 and visual field was ⬍10°, then the participant was included only in the severe visual impairment category. Similarly, if the visual acuity was ⬍3/60 but the visual fields were full, the participant was defined as having profound visual impairment. The cumulative prevalence includes all participants with more than the stated level of visual impairment. For example, the analysis of legal blindness in Australia includes all participants with visual impairment in the severe or profound levels.
Definitions of Visual Impairment The VIP has defined four levels of decreasing visual impairment expanding the WHO criteria (Table 1).22 Each level of visual
impairment has a visual acuity and a visual field criterion of classification. The visual field criterion is well established for profound visual impairment and to a lesser extent for severe and moderate visual impairment. The addition of homonymous hemianopia as a criterion for mild visual impairment is an attempt to be consistent and to classify the visual impairment associated with homonymous hemianopia.
Results The numbers of eligible persons identified were 5520, 3912 in the urban residential cohort and 1608 in the rural residential cohort. A total of 4744 (86%) eligible persons, 3271 (84%) urban residents, and 1473 (92%) rural residents participated in interviews and examinations. Participation rates for those aged 65 years and older (1513 of 1786, 85%) compared with those less than 65 years of age (3231 of 3734, 86%) were not statistically different (chi square, 1 df, 3.2, P ⫽ 0.07). The age and gender distribution of each cohort is detailed in Table 2. A total of 3079 (65%) participants were born in Australia, 413 (9%) in the United Kingdom, 316 (7%) in Europe, and 634 (13%) in either Greece or Italy. A smaller number of participants were born in Southeast Asia, India, the United States, African countries, the Middle East, and South America. Participants and nonparticipants were similar with respect to age and gender. Visual acuity was determined in 4734 (99.7%) participants, 3266 (99.8%) urban residential and 1468 (99.6%) rural residents. Logarithm of the minimum angle of resolution visual acuity was measured in 4711 (97%) participants, 3244 (99%) urban residential and 1467 (99%) rural residential participants. Refraction was performed in 1246 (26%) participants, 843 (26%) urban participants and 403 (27%) rural participants when presenting vision was ⬍6/6. Visual field testing was completed for 4669 (98.4%) participants, 3211 (98%) urban and 1458 (99%) rural participants. Visual field assessment was completed using the Humphrey Field
Table 2. Visual Impairment Project Population by Age Group, Gender, and Cohort
Age Group
Men
Women
Men
Women
Visual Impairment Project Total
40–49 50–59 60–69 70–79 80–89 90⫹ Total
357 447 426 222 56 3 1511
466 531 437 223 89 14 1760
200 180 150 132 32 7 701
237 177 174 140 41 3 772
1260 1335 1187 717 218 27 4744
Urban
962
Rural
VanNewkirk et al 䡠 Causes of Bilateral Visual Impairment in Victoria Table 3. Cause-specific Prevalence by Age and Gender of Visual Acuity ⬍6/12 ⱖ 6/18 and/or Hemianopia Visual Impairment Project Cohorts Urban
Rural
ⱖ40–⬍65 yrs ⬎65 yrs ⱖ40–⬍65 yrs Cause
N
AMD 6 Glaucoma 1 Cataract 11 DR 2 Other retinal 4 Neuro-oph 7 Other causes 2 Ref error 87 All causes 120
Population-weighted Prevalence/1000 ⬎65 yrs
M
F
M
F
M
F
M
F
0 0 0 0 1 0 1 13 15
0 0 0 0 0 0 0 13 13
0 1 2 0 1 1 (1) 0 18 23
5 0 6 0 2 0 1 27 41
0 0 0 1 0 0 0 3 4
0 0 0 0 0 2 (1) 0 2 4
0 0 2 1 0 2 (2) 0 6 11
1 0 1 0 0 2 (2) 0 6 9
ⱖ40–⬍65 yrs
⬎65 yrs
ⱖ40 yrs
All All All (95% Confidence Interval) (95% Confidence Interval) (95% Confidence Interval) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.28 (0, 0.86) 0.31 (0, 0.91) 1.15 (0, 1.58) 0.43 (0, 0.91) 9.55 (6.20, 12.89) 11.23 (7.60, 14.86)
4.07 (1.87, 6.26) 0.59 (0, 1.41) 6.68 (3.87, 9.48) 0.43 (0, 1.14) 1.76 (0.31, 3.19) 3.72 (1.62, 5.81) 0.59 (0, 1.41) 18.60 (28.33, 40.92) 46.56 (39.30, 53.80)
1.28 (0.34, 2.23) 0.21 (0, 0.64) 2.10 (1.13, 3.07) 0.46 (0, 1.12) 0.86 (0, 1.85) 1.15 (0, 2.64) 0.43 (0, 0.96) 18.61 (12.07, 25.14) 25.10 (17.69, 32.51)
indicates number of participants included because of visual field defects. N ⫽ number; M ⫽ male; F ⫽ female; AMD ⫽ age-related macular degeneration; DR ⫽ diabetic retinopathy; Neuro-oph ⫽ neuro-ophthalmic disease; Ref error ⫽ uncorrected refractive error.
Analyzer for 4637 (98%) participants, 3211 (98%) urban residents and 1426 (97%) rural residents. Eighty-two (1.7%) urban and rural participants were observed with moderate, severe, or profound visual impairment in the better eye. An additional 120 participants were observed to have less than legal driving vision, 92 in the urban cohort and 28 in the rural cohort. In two (2.4%) of the participants the cause of moderate, severe, or profound visual impairment was different in the two eyes. In one participant, moderate visual impairment was due to myopic choroidal degeneration in one eye and retinal detachment in the other eye. The cause was assigned to myopic degeneration, because myopia may have contributed to the retinal detachment. In the other participant, bilateral severe visual impairment was due to optic atrophy in one eye and posterior capsule opacification in the other eye; visual loss was assigned to optic atrophy. Different causes of mild bilateral visual impairment occurred in four (3.3%) participants, two urban and two rural. AMD and corneal opacity occurred in one participant, and AMD was assigned as cause for that individual. Optic atrophy and cataract were present in one participant, and optic atrophy was assigned as cause. Posterior capsule opacification and cataract were present in one participant, and cataract was assigned as cause. Cataract and amblyopia were present in one participant, and cataract was assigned as cause.
Mild Vision Impairment Uncorrected refractive error was the most common diagnosis, present in 87 participants, an overall prevalence of 18.6/1000 (95% confidence interval [CI] 12.07, 25.14) (Table 3). The number of participants already wearing glasses was 30 (34.5%). Among those wearing glasses, one (3.3%) improved to 6/6 vision after refraction. Among those not wearing glasses, 17 (29.8%) improved to 6/6 vision after refraction. Uncorrected hyperopia, greater than ⫹0.5 diopters (D) but less than ⫹3.0 D was present in 56% (49 of the 87). The uncorrected refractive error in three participants was greater than ⫹3 D, 3.5%. Uncorrected myopia (greater than ⫺0.5 D but less than ⫺10 D) was observed in 29%, 25 of the 87. Cataract 2.1/1000 (95% CI, 1.13, 3.07) followed by AMD 1.28/ 1000 (95% CI, 0.34, 2.23) were the next most prevalent disorders. In total, 6 (5%) of 120 participants had mild visual impairment because of visual field loss. All had had cerebrovascular accidents,
and five of the six had a history of hypertension. Homonymous hemianopia was observed in one rural participant ⬍65 years of age and in four rural and one urban participant ⱖ65 years of age.
Moderate Vision Impairment Moderate visual impairment was observed in 58 participants in the two cohorts, an overall prevalence of 12.08/1000 (95% CI, 7.56, 16.59) (Table 4). Uncorrected refractive error accounted for the highest overall weighted prevalence, 4.73/1000 (95% CI, 2.49, 6.97). Uncorrected myopia greater than ⫺0.5 D was observed in 50%; 12 of the 24 participants were in this category. Uncorrected hyperopia greater than ⫹0.5 D but less than ⫹6 D was present in 46%, 11 of 24. The prevalence of all diseases was greater in the participants aged 65 years and older. Moderate visual impairment caused by AMD, glaucoma, and cataract was not observed in participants ⬍65 years of age. Moderate visual impairment from DR was observed to have a population-weighted prevalence of 1.07/1000 (95% CI, 0.35, 1.78) in participants aged 40 years and older. Three of five urban participants with DR had retained reasonably good visual acuity, but their visual fields were ⬍20° radius, probably because of the effects of extensive panretinal photocoagulation and the effect of DR on the peripheral retina. Visual field constriction to ⬍20° causing moderate visual impairment was observed in 13 (22%) of the 58 participants with moderate visual impairment. In addition to DR, glaucoma (n ⫽ 3), neuro-ophthalmic disorder (n ⫽ 1), and other disorders (n ⫽ 6) were the cause of visual field reduction.
Severe Vision Impairment Severe visual impairment was observed in 16 participants, a population-weighted prevalence of 3.52/1000 (95% CI, 1.75, 5.28) (Table 5). Only four (25%) participants with severe visual impairment were ⬍65 years of age. Uncorrected refractive error, 1.14/ 1000 (95% CI, 0.33, 1.95), was the most prevalent cause of visual impairment. Uncorrected hyperopia greater than ⫹0.5 D was observed in four of the five participants initially seen with uncorrected refractive error. Four (9%) participants were classified as having severe vision impairment because of bilateral constriction of visual field to ⬍10°
963
Ophthalmology Volume 108, Number 5, May 2001 Table 4. Cause-specific Prevalence by Age and Gender of Visual Acuity ⬍6/18 ⱖ 6/60 and/or ⬍20° Visual Impairment Project Cohorts Urban ⱖ40–⬍65 yrs Cause AMD Glaucoma Cataract DR Other retinal Neuro-oph Other causes Ref error All causes
N M 7 5 3 5 5 4 7 22 58
F
0 0 0 1 (1) 0 0 0 1 2
Rural ⬎65 yrs M
ⱖ40–⬍65 yrs
F
0 4 0 2 (1) 0 0 1 (1) 2 (1) 0 0 0 1 (1) 2 (2) 1 (1) 1 6 4 16
3 0 1 1 4 0 4 (3) 10 23
Population-weighted Prevalence/1000 ⬎65 yrs
M
F
M
F
0 0 0 0 0 1 0 0 1
0 0 0 0 0 0 0 0 0
0 1 (1) 1 0 0 1 0 3 6
0 2 (1) 1 0 1 1 0 1 6
ⱖ40–⬍65 yrs
⬎65 yrs
ⱖ40 yrs
All All All (95% Confidence Interval) (95% Confidence Interval) (95% Confidence Interval) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.62 (0, 1.47) 0 (0, 0) 0.32 (0.86) 0.62 (0, 1.47) 0.62 (0, 1.47) 2.16 (0.54, 3.72)
5.03 (1.90, 6.30) 4.07 (1.73, 6.01) 2.73 (0.55, 3.74) 1.76 (0.31, 3.19) 3.56 (1.45, 5.51) 2.57 (0.55, 3.75) 3.11 (1.06, 4.78) 11.80 (8.08, 15.51) 30.58 (22.43, 33.81)
1.50 (0.45, 2.55) 0.98 (0, 2.15) 0.47 (0, 1.04) 1.07 (0.35, 1.79) 1.02 (0.02, 2.01) 0.65 (0, 1.58) 1.66 (0.58, 2.74) 4.73 (2.49, 6.97) 12.08 (7.56, 16.59)
indicates number of participants included because of visual field defects. N ⫽ number; M ⫽ male; F ⫽ female; AMD ⫽ age-related macular degeneration; DR ⫽ diabetic retinopathy; Neuro-oph ⫽ neuro-ophthalmic disease; Ref error ⫽ uncorrected refractive error.
radius but ⱖ5° radius. Three of these four had severe glaucoma, one from the urban cohort and two from the rural cohort.
Profound Vision Impairment
field to ⬍5°. Cataract was not observed as a cause of profound visual impairment in either the urban or the rural cohort.
Cumulative Prevalence of Vision Impairment
Profound visual impairment was observed in eight participants. The population-weighted prevalence of profound visual impairment was 1.56/1000 (95% CI, 0.58, 2.55). Seven (87%) of the participants with profound visual impairment were 65 years of age or older, a prevalence of 8.0/1000, and four (50%) were women (Table 6). AMD and glaucoma accounted for 75% of the WHO blindness or profound visual impairment observed in the VIP. Although not statistically different from other causes, AMD is the most prevalent cause of profound visual impairment. One participant with severe glaucoma from the rural cohort was classified with profound visual impairment because of bilateral constriction of visual
The prevalence of vision less than that required for driving (mild to profound) is 42.48/1000 (95% CI, 30.11, 54.86), and the leading cause of vision loss is uncorrected refractive error, 24.68/1000 (95% CI, 16.12, 33.25) (Table 7). The prevalence of other causes are 6 to 16 times less than that of uncorrected refractive error. The prevalence of low vision or worse is 17.68/1000 (95% CI, 11.75, 23.01) and uncorrected refractive error continues to be the dominant cause of vision loss, 6.08/1000 (95% CI, 3.36, 8.79). The prevalence of legal blindness is 5.30/1000 (95% CI, 3.24, 7.36). Although uncorrected refractive error continues to be the most prevalent cause in this range of vision impairment, it is not
Table 5. Cause-specific Prevalence by Age and Gender of Visual Acuity ⬍6/60 ⱖ 3/60 and/or ⬍10° Visual Impairment Project Cohorts Urban
Rural
ⱖ40–⬍65 yrs ⬎65 yrs ⱖ40–⬍65 yrs Cause
N
M
F
M
AMD Glaucoma Cataract DR Other retinal Neuro-oph Other causes Ref error All causes
2 3 0 0 4 0 2 5 16
0 0 0 0 0 0 1 0 1
0 0 0 0 1 0 0 0 1
1 0 0 0 0 0 0 1 2
F 0 1 (1) 0 0 2 0 0 2 5
Population-weighted Prevalence/1000 ⬎65 yrs
M
F
M
0 0 0 0 0 0 1 (1) 1 2
0 0 0 0 0 0 0 0 0
1 1 (1) 0 0 1 0 0 0 3
F 0 1 (1) 0 0 0 0 0 1 2
ⱖ40–⬍65 yrs
⬎65 yrs
ⱖ40 yrs
All All All (95% Confidence Interval) (95% Confidence Interval) (95% Confidence Interval) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.31 (0, 0.91) 0 (0, 0) 0.59 (0, 1.42) 0.28 (0, 0.86) 1.18 (0, 2.36)
1.40 (0, 2.10) 2.27 (0.55, 3.74) 0 (0, 0) 0 (0, 0) 1.64 (0.22, 2.97) 0 (0, 0) 0 (0, 0) 2.90 (1.04, 4.74) 7.72 (3.68, 9.18)
0.39 (0, 0.89) 0.74 (0, 1.84) 0 (0, 0) 0 (0, 0) 0.82 (0, 1.84) 0 (0, 0) 0.43 (0, 1.03) 1.14 (0.33, 1.95) 3.52 (1.75, 5.28)
indicates number of participants included because of visual field defects. N ⫽ number; M ⫽ male; F ⫽ female; AMD ⫽ age-related macular degeneration; DR ⫽ diabetic retinopathy; Neuro-oph ⫽ neuro-ophthalmic disease; Ref error ⫽ uncorrected refractive error.
964
VanNewkirk et al 䡠 Causes of Bilateral Visual Impairment in Victoria Table 6. Cause-specific Prevalence by Age and Gender of Visual Acuity ⬍3/60 and/or ⬍5° Visual Impairment Project Cohorts Urban ⱖ40–⬍65 yrs
Rural ⬎65 yrs
ⱖ40–⬍65 yrs
Population-weighted Prevalence/1000 ⬎65 yrs
Cause
N
M
F
M
F
M
F
M
AMD Glaucoma Cataract DR Other retinal Neuro-oph Other causes Ref error All causes
4 2 0 0 1 0 1 0 8
0 0 0 0 1 0 0 0 1
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
2 1 0 0 0 0 0 0 3
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
1 1 (1) 0 0 0 0 1 0 3
ⱖ40–⬍65 yrs
⬎65 yrs
ⱖ40 yrs
All All All F (95% Confidence Interval) (95% Confidence Interval) (95% Confidence Interval) 1 0 0 0 0 0 0 0 1
0 (0, 0) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.31 (0, 0.91) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.31 (0, 0.91)
2.74 (0.94, 4.53) 1.01 (0, 2.10) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.43 (0, 1.14) 0 (0, 0) 3.78 (1.66, 5.89)
0.68 (0, 1.39) 0.39 (0, 0.85) 0 (0, 0) 0 (0, 0) 0.21 (0, 0.64) 0 (0, 0) 0.28 (0, 0.83) 0 (0, 0) 1.56 (0.58, 2.55)
indicates number of participants included because of visual field defects. N ⫽ number; M ⫽ male; F ⫽ female; AMD ⫽ age-related macular degeneration; DR ⫽ diabetic retinopathy; Neuro-oph ⫽ neuro-ophthalmic disease; Ref error ⫽ uncorrected refractive error.
discernibly greater than AMD, glaucoma, retinal disorders, and other causes.
Discussion This study describes the cause-specific population-weighted prevalence of vision impairment in the state of Victoria. Across all levels of vision impairment, the prevalence of visual impairment is greatest in those aged 65 years and older. Uncorrected refractive error is the leading cause of mild through moderate vision impairment. Severe to profound vision impairment is caused by a number of eye conditions, including uncorrected refractive error. The population-weighted prevalence of uncorrected refractive error as a cause of mild, moderate, and severe visual impairment was surprisingly high. Uncorrected hyperopia, greater than ⫹0.5 D, accounted for 59%, 67 of the 114 participants with mild, moderate, and severe visual impairment. VIP participants with nuclear cataract were more likely than those without nuclear cataract to have uncorrected refractive error, odds ratio 1.51 (95% CI, 1.14, 2.00). Myopia induced by nuclear sclerosis was the most frequent
uncorrected refractive error in the older age group. For many older individuals, uncorrected myopia is not considered a problem; because most of their activities are nearassociated, the decrease in distant visual acuity appears not to bother them. Uncorrected refractive error has been identified as a major source of visual impairment in previous studies.4,5,12 Many possible explanations have been proposed, but the reality is that some people do not wish or feel the need to wear spectacles.23 The effects of hyperopia may be reduced by the availability and use of nonprescription reading glasses. Certainly this group of visually impaired lends itself to the most simple correction. AMD is a major cause of disabling vision impairment in elderly white populations.2,3,5,6,24,25 In the VIP, early agerelated maculopathy was not associated with vision loss. The prevalence of legal blindness caused by AMD is similar to that found in the Baltimore Eye Survey, 2.7/1000 (95% CI, 1.2, 5.4).2 The difference in the definition of legal blindness in the United States should be noted; the United States definition includes 6/60 vision. The prevalence of legal blindness caused by glaucoma is also similar to other predominantly white populations, such as the Baltimore Eye Survey, 0.7/1000 (95% CI, 0.1, 2.5).2
Table 7. Cumulative Prevalence/1000 (95% Confidence Interval) for Specific Causes of Visual Impairment Cause
Less-Than-Driving Vision (95% Confidence Interval)
Low Vision (95% Confidence Interval)
Legal Blindness (95% Confidence Interval)
Blindness (95% Confidence Interval)
AMD Glaucoma Cataract DR Other retinal Neuro-oph Other causes Ref error All causes
3.86 (2.17, 5.55) 2.32 (0.72, 3.92) 2.57 (1.38, 3.76) 1.53 (0.71, 2.36) 2.91 (0.74, 5.08) 1.80 (0, 4.11) 2.80 (1.17, 4.43) 24.68 (16.12, 33.25) 42.48 (30.11, 54.86)
2.58 (1.21, 3.94) 2.10 (0.49, 3.72) 0.47 (0, 1.03) 1.07 (0.35, 1.79) 2.05 (0.59, 3.52) 0.65 (0, 1.58) 2.37 (1.04, 3.70) 6.08 (3.36, 8.79) 17.38 (11.75, 23.01)
1.07 (0.14, 2.01) 1.13 (0.05, 2.21) 0 (0, 0) 0 (0, 0) 1.04 (0, 2.12) 0 (0, 0) 0.71 (0, 1.48) 1.35 (0.57, 2.13) 5.30 (3.24, 7.36)
0.68 (0, 1.39) 0.68 (0, 1.39) 0.39 (0, 0.85) 0 (0, 0) 0 (0, 0) 0.21 (0, 0.64) 0 (0, 0) 0 (0, 0) 1.56 (0.58, 2.55)
AMD ⫽ age-related macular degeneration; DR ⫽ diabetic retinopathy; Neuro-oph ⫽ neuro-ophthalmic disease; Ref error ⫽ uncorrected refractive error.
965
Ophthalmology Volume 108, Number 5, May 2001 The VIP prevalence of visual impairment caused by glaucoma, which includes one rural participant with angleclosure glaucoma, is markedly increased in the older age group. Typically, visual acuity remains good until very late stages of glaucomatous optic neuropathy, and early visual impairment would be missed without visual field assessment. Among those with legal blindness caused by glaucoma, only one of five participants from the urban and rural cohorts was classified according to visual acuity criteria. Four of the five VIP participants with visual field constriction to ⬍10° had glaucoma; three participants were from the rural cohort. This may indicate that diagnosis of glaucoma in the rural cohort is delayed compared with the urban cohort. Possibly early detection, different treatment, and better compliance with treatment could reduce the prevalence of visual impairment caused by glaucoma. The age-standardized prevalence of previous cataract surgery was not statistically different between the rural cohort, 44.2/1000 (95% CI, 26.5, 62.0), and the urban cohort, 33.7/1000 (95% CI, 27.4, 40.0). Seventy percent of those who had previously had cataract surgery had 6/12 or better visual acuity. Complications of cataract surgery causing visual impairment were infrequent, occurring in one eye of two participants. One participant had pseudophakic bullous keratopathy and one participant had posterior capsule opacification. As in the Rotterdam Study,6 visual impairment from DR was limited to the mild and moderate levels of visual impairment. Of the seven VIP cases with bilateral proliferative DR, six from the urban cohort and one from the rural cohort had been treated with panretinal photocoagulation and appeared to be in a stable posttreatment state. The failure to detect legal blindness from DR may be due to sampling error, because Harper et al (1998 Abstract, Scientific Program Melbourne Ophthalmic Alumni Scientific Meeting) reported that 39% (68 of 175) of consecutive first-visit patients with DR seen at the Royal Victorian Eye and Ear Hospital retina clinic have significant loss of vision (⬍6 of 18). The overall rate of diabetes in the VIP was 4.9%, with a rate of 5.2% in the urban and 4.9% in the rural cohort. Among diabetics, 19.6% had DR, 21.8% urban residential and 16.9% rural residential diabetic participants. Previous reports from the VIP of the age-specific causes of vision loss, which included a cohort of 400 nursing home and hostel residents, demonstrated the age dependence of causes of vision loss.26 Consistent with this report, uncorrected refractive error was a major cause of vision loss at all ages, with a prevalence rising from 0.5% of 40- to 49-yearolds to 13% of those aged 80 years and older. However, the predominant causes of vision loss other than refractive error were shown to change with age. Vision impairment caused by DR and glaucoma was observed in participants 50 to 59 years of age, whereas vision loss caused by AMD was not present in participants less than 70 years of age. AMD was the cause of vision impairment in 11% of participants older than 90 years of age. Cataract was a common cause of severe and profound vision impairment among nursing home residents.
966
Conclusions Uncorrected refractive error remains a major cause of vision impairment even though it is easily prevented. Program development to reduce the vision impairment caused by uncorrected refractive error, cataract, glaucoma, and diabetes mellitus should be attainable with current resources and technology.
References 1. Leibowitz HM, Krueger DE, Maunder LR, et al. The Framingham Eye Study monograph: An ophthalmological and epidemiological study of cataract, glaucoma, diabetic retinopathy, macular degeneration, and visual acuity in a general population of 2631 adults, 1973–1975. Surv Ophthalmol 1980; 24(Suppl):335– 610. 2. Sommer A, Tielsch JM, Katz J, et al. Racial differences in the cause-specific prevalence of blindness in east Baltimore. N Engl J Med 1991;325:1412–7. 3. Dana MR, Tielsch JM, Enger C, et al. Visual impairment in a rural Appalachian community. Prevalence and causes. JAMA 1990;264:2400 –5. 4. Klein R, Klein BEK, Linton KLP, De Mets DL. The Beaver Dam Eye Study: visual acuity. Ophthalmology 1991;98: 1310 –5. 5. Attebo K, Mitchell P, Smith W. Visual acuity and the causes of visual loss in Australia. The Blue Mountains Eye Study. Ophthalmology 1996;103:357– 64. 6. Klaver CCW, Wolfs RCW, Vingerling JR, et al. Age-specific prevalence and causes of blindness and visual impairment in an older population: the Rotterdam Study. Arch Ophthalmol 1998;116:653– 8. 7. The Victorian Yearbook, Number 110. Australian Bureau of Statistics, 1998. 8. Sorsby A. The Incidence and Causes of Blindness in England and Wales, 1948 –1962. London: HMSO, 1966. Great Britain Ministry of Health. Reports on Public Health and Medical Subjects; No. 114. 9. Strategies for the Prevention of Blindness in National Programmes: A Primary Health Care Approach. Geneva: World Health Organization, 1984. 10. Livingston PM, Carson CA, Stanislavsky YL, et al. Methods for a population-based study: the Melbourne Visual Impairment Project. Ophthalmic Epidemiol 1994;1:139 – 48. 11. Taylor HR, Livingston PM, Stanislavsky YL, McCarty CA. Visual impairment in Australia: distance visual acuity, near vision, and visual field findings of the Melbourne Visual Impairment Project. Am J Ophthalmol 1997;123:328 –37. 12. Tielsch JM, Sommer A, Witt K, et al. Blindness and visual impairment in an American urban population. The Baltimore Eye Survey. Arch Ophthalmol 1990;108:286 –90. 13. Heijl A, Lindgren G, Olsson J. The effect of perimetric experience in normal subjects. Arch Ophthalmol 1989;107:81– 6. 14. Taylor HR, West SK, Rosenthal FS, et al. Effect of ultraviolet radiation on cataract formation. N Engl J Med 1988;319: 1429 –33. 15. Bailey IL, Bullimore MA, Raasch TW, Taylor HR. Clinical grading and the effects of scaling. Invest Ophthalmol Vis Sci 1991;32:422–32. 16. Bird AC, Bressler NM, Bressler SB, et al. An international classification and grading system for age-related maculopathy and age-related macular degeneration. The International ARM Epidemiological Study Group. Surv Ophthalmol 1995;39: 367–74.
VanNewkirk et al 䡠 Causes of Bilateral Visual Impairment in Victoria 17. Report 7. A modification of the Airlie House classification of diabetic retinopathy. Diabetic Retinopathy Study Research Group. Invest Ophthalmol Vis Sci 1981;21:210 –26. 18. Wensor MD, McCarty CA, Stanislavsky L, et al. The prevalence of glaucoma in the Melbourne Visual Impairment Project. Ophthalmology 1989;105:733–9. 19. World Health Organisation. International Classification of Diseases, Ninth Revision (ICD-9). Geneva, Switzerland: World Health Organisation; 1977. 20. Australian Bureau of Statistics. 1996 Census of Population and Housing. Canberra, Australia: 1996. 21. Cochran WG. Sampling Techniques, 3rd ed. New York: John Wiley & Sons, 1977;249 –73.
22. Thylfors B, Negrel AS, Pararajasegarm R, Dadzie KY. Global Data on Blindness. WHO/PBL/94.40 Geneva, 1994. 23. Lichter PR. Resiliency as it affects treatment for refractive errors [editorial]. Ophthalmology 1994;101:221–2. 24. Klein R, Wang Q, Klein BEK, et al. The relationship of age-related maculopathy, cataract, and glaucoma to visual acuity. Invest Ophthalmol Vis Sci 1995;36:182–91. 25. Newland HS, Hiller JE, Casson RJ, Obermeder S. Prevalence and causes of blindness in the South Australian population aged 50 and over. Ophthalmic Epidemiol 1996;3:97–107. 26. Weih LM, VanNewkirk MR, McCarty CA, Taylor HR. Agespecific causes of bilateral vision impairment. Arch Ophthalmol 2000;118:264 –9.
967
Originally received: October 5, 1998. Accepted: January 3, 2001. Manuscript no. 98663. Centre For Eye Research Australia, University of Melbourne, Melbourne, Australia. Supported by National Health and Medical Research Council, Canberra, ACT, Australia; Victorian Health Promotion Foundation, Melbourne, Victoria, Australia; Estate of the late Dorothy Edols, Melbourne, Victoria, Australia; Ansell Ophthalmology Foundation, Melbourne, Victoria, Australia; Jack Brockhoff Foundation, Melbourne, Victoria, Australia. Reprint requests to LeAnn Weih, CERA, Locked Bag 8, East Melbourne, 3002 VIC, Australia.
960
© 2001 by the American Academy of Ophthalmology Published by Elsevier Science Inc.
designed, population-based studies of eye diseases have come from single urban or rural communities in the United States, Australia, and The Netherlands.1– 6 The Visual Impairment Project (VIP) is a population-based study of eye disease that aims to fully represent the Victorian population by examining two groups, the urban and rural residential populations. The most densely populated Australian state, Victoria is located in the southeastern corner of Australia and comprises almost 25% of its total population.7 The clusterstratified sample of 4744 adults 40 years of age and older consists of 3271 urban and 1473 rural participants. Confusion exists because of the many definitions of visual impairment that have been previously described.8 The United States definition of legal blindness, ⱕ20/200 or ISSN 0161-6420/01/$–see front matter PII S0161-6420(01)00554-1
VanNewkirk et al 䡠 Causes of Bilateral Visual Impairment in Victoria 6/60 and/or ⱕ20° visual field, differs from both the Australian and the World Health Organization (WHO) definitions of blindness. This study defined moderate bilateral vision impairment by use of the WHO definition for low vision (⬍6/18 ⱖ6/60, ⬍20° ⱖ10° field,9 severe vision impairment (⬍6/60 ⱖ3/60, ⬍10° ⱖ5° field), and the WHO definition of blindness9 or profound vision impairment (⬍3/ 60, ⬍5° field). The Australian definition of legal blindness encompasses severe and profound categories. In addition, we defined visual impairment at less-than-driving vision, ⬍6/12 ⱖ6/18, and/or homonymous hemianopia in the two noninstitutional cohorts of our study, the cohorts most likely to drive. The aim of this study is to describe the causes of bilateral mild, moderate, severe, and profound visual impairment. The population-weighted cause-specific prevalences of visual impairment are reported individually and cumulatively.
Methods A detailed method has been reported elsewhere.10,11 The clusterstratified sample was drawn from nine randomly selected pairs of adjacent census collector districts in urban Melbourne and four pairs of randomly selected adjacent census collector districts in four rural communities in Victoria. Target sample size for the study was based on the ability to measure the prevalence of visual impairment (⬍6/18 ⱖ3/60) in Victoria with relative precision of 80% (alpha ⫽ 0.05), assuming that the prevalence was similar to that of white participants of the Baltimore Eye survey, 0.0127.12 The sample size required was 2772. A door-to-door household census was conducted to identify all eligible persons, those aged 40 years and older in the calendar year of the examination who had lived at that address for at least 6 months. The first interview of eligible residents, conducted at the time of the household census, used a standardized questionnaire that included basic demographic data, current medical history, most recent visit to an ophthalmologist or an optometrist, and visual symptoms. The participants were encouraged to attend the local test site for a second interview and an eye examination. The urban study was carried out from 1992 to 1995, the rural study in 1996. The study was conducted with ethics approval from the Royal Victorian Eye and Ear Hospital. All participants gave signed consent for examination after being informed of the nature of the examination and the use of the data collected.
Visual Assessment A participant’s presenting distance visual acuity was measured with a logarithm of the minimum angle of resolution chart at 4 m and, if necessary, at 3, 2 and 1 m on each eye separately while the participant wore his or her current spectacles (if worn). Visual acuity was recorded as the total number of characters read correctly. The logarithm of the minimum angle of resolution chart measures a range of Snellen equivalent visual acuity from at best 6/3 at 4 m to at worst 1/60 at 1 m. An E chart was used for participants who could not read English and for non-Englishspeaking participants where translation was not possible. If visual acuity could not be measured with the logarithm of the minimum angle of resolution chart, a sequential approach was used with the following tests: counting fingers, hand movement, clinical fixation on a target image, and light perception. Test/retest of visual acuity measurement was not performed. Current spectacles were analyzed, and autorefraction was performed with Humphrey instru-
ments. A subjective refraction was performed if presenting visual acuity was less than 6/6, and the participant’s mental status was adequate to respond to the test. Visual field assessment was performed with a Humphrey Field Analyzer by use of the 24-2 Fastpac statistical package (Humphrey Instruments Inc, San Leandro, CA). If the participant was unable to complete threshold visual field testing, a Bjerrum tangent screen visual field was done; as a final resort, confrontation visual fields were performed. Threshold visual field tests were inspected for reliability at the time of the examination, and unreliable tests were repeated or alternative visual field tests were performed. Test results on the Humphrey Field Analyzer were considered inadequate for use in data analysis when there were 20% or greater fixation losses or 33% or greater false-positive or false-negative errors. However, a few reliable threshold visual field tests had no obvious clinical disease correlation, and these fields had not been repeated. This may be due to perimetric inexperience13 and may result in an overestimation of visual impairment caused by visual field constriction. Standardized ophthalmologic examinations were performed on all participants. The biomicroscopic examination included clinical lens nucleus grading by visual comparison with a standard photograph. Cortical and posterior subcapsular cataracts were measured and graded visually with biomicroscopic retroillumination by the ophthalmologist.14,15 Lens nucleus and retroillumination photographs were taken and graded separately by two trained observers, and any differences were adjudicated. Vertical cup/disc ratios were measured and recorded, as were abnormalities of the choroid and retina. Age-related maculopathy, age-related macular degeneration (AMD), and diabetic retinopathy (DR) were graded clinically with indirect biomicroscopy and ophthalmoscopy and by photo graders according to standardized classification schemes.16,17 Glaucoma was defined by a consensus group of six ophthalmologists using photos of the optic disc and visual fields.18 Other diseases causing visual impairment were classified using International Classification of Diseases-9.19 The category of visual impairment was classified by vision in the better eye for all causes other than uncorrected refractive error. Level of visual impairment was defined by presenting acuity for uncorrected refractive error. The major predisposing condition for each person, by better eye, was assigned as the cause of visual impairment (e.g., if a motor vehicle accident fractured the optic foramen resulting in optic atrophy, the cause of visual impairment was listed as trauma). If two or more diseases were present, the disease with the most clinically significant effect on vision was assigned as the principal cause of visual loss. For example, when AMD with geographic atrophy coexisted with nuclear cataract, the cause of vision loss was attributed to AMD. When both eyes had the same level of visual impairment and each eye had a different cause of visual impairment, consistent clinical principles were used by one of us (MRVN) to assign the cause reflecting the principal disease. For example, if AMD and corneal opacity occurred in different eyes of one participant, AMD was assigned as the cause of visual impairment for that individual. Uncorrected refractive error was measured by the difference between presenting and best-corrected visual acuity. A case of uncorrected refractive error required an improvement of one or more categories of visual acuity (Table 1). For example, the visual acuity of a participant must improve with best correction from ⬍6/18 ⱖ6/60 presenting vision to at least ⬍6/12 ⱖ6/18 to be counted in the uncorrected refractive error category of moderate visual impairment.
Data Analysis Interview data were collected by direct computer entry using a questionnaire programmed in Paradox (Borland International,
961
Ophthalmology Volume 108, Number 5, May 2001 Table 1. Level of Visual Acuity and/or Visual Field in the Better Eye Vision Impairment
Visual Acuity
Visual Field
Equivalent
Mild
⬍6/12 ⱖ6/18
Moderate
⬍6/18 ⱖ6/60
Homonymous hemianopia ⬍20° ⱖ10° radius
Severe
⬍6/60 ⱖ3/60
⬍10° ⱖ5° radius
Profound
⬍3/60
⬍5° radius
⬍AU and US driving acuity WHO low vision ⬍6/18 ⱖ 3/60, ⬍20° ⱖ5° radius AU legal blindness ⬍6/60, ⬍10° radius WHO blindness
AU ⫽ Australia; US ⫽ United States; WHO ⫽ World Health Organization.
Scotts Valley, CA) or on self-coding forms. Open-ended responses were coded at a later time. Visual assessment and ophthalmic examination data were doubly entered. Classification of visual impairment used Snellen equivalent visual acuity based on reading a full line, five letters. Population-weighted prevalences were calculated by weighting the data in each cohort according to the 1996 Australian Bureau of Census data by the proportion of each cohort, by decade of age and gender, residing in the Victorian population.20 Ninety-five percent confidence intervals around cause-specific prevalences were calculated according to Cochran21 to account for the effect of the cluster sampling. The design effect was 1.56. Participants who did not have visual acuity measured were not included in the analyses. Odds ratios were calculated with multivariate logistic regression controlling for age, gender, and accounting for cluster design effects using SAS software version 6.10 (SAS Institute, Cary, NC). Confidence intervals around age-standardized rates for each cohort were calculated using the standard error of the directly standardized rate. When a participant was observed to have both visual acuity impairment and visual field defect, the category with the most severe disability was used to classify the participant. That is, if visual acuity was ⬍6/18 but ⱖ6/60 and visual field was ⬍10°, then the participant was included only in the severe visual impairment category. Similarly, if the visual acuity was ⬍3/60 but the visual fields were full, the participant was defined as having profound visual impairment. The cumulative prevalence includes all participants with more than the stated level of visual impairment. For example, the analysis of legal blindness in Australia includes all participants with visual impairment in the severe or profound levels.
Definitions of Visual Impairment The VIP has defined four levels of decreasing visual impairment expanding the WHO criteria (Table 1).22 Each level of visual
impairment has a visual acuity and a visual field criterion of classification. The visual field criterion is well established for profound visual impairment and to a lesser extent for severe and moderate visual impairment. The addition of homonymous hemianopia as a criterion for mild visual impairment is an attempt to be consistent and to classify the visual impairment associated with homonymous hemianopia.
Results The numbers of eligible persons identified were 5520, 3912 in the urban residential cohort and 1608 in the rural residential cohort. A total of 4744 (86%) eligible persons, 3271 (84%) urban residents, and 1473 (92%) rural residents participated in interviews and examinations. Participation rates for those aged 65 years and older (1513 of 1786, 85%) compared with those less than 65 years of age (3231 of 3734, 86%) were not statistically different (chi square, 1 df, 3.2, P ⫽ 0.07). The age and gender distribution of each cohort is detailed in Table 2. A total of 3079 (65%) participants were born in Australia, 413 (9%) in the United Kingdom, 316 (7%) in Europe, and 634 (13%) in either Greece or Italy. A smaller number of participants were born in Southeast Asia, India, the United States, African countries, the Middle East, and South America. Participants and nonparticipants were similar with respect to age and gender. Visual acuity was determined in 4734 (99.7%) participants, 3266 (99.8%) urban residential and 1468 (99.6%) rural residents. Logarithm of the minimum angle of resolution visual acuity was measured in 4711 (97%) participants, 3244 (99%) urban residential and 1467 (99%) rural residential participants. Refraction was performed in 1246 (26%) participants, 843 (26%) urban participants and 403 (27%) rural participants when presenting vision was ⬍6/6. Visual field testing was completed for 4669 (98.4%) participants, 3211 (98%) urban and 1458 (99%) rural participants. Visual field assessment was completed using the Humphrey Field
Table 2. Visual Impairment Project Population by Age Group, Gender, and Cohort
Age Group
Men
Women
Men
Women
Visual Impairment Project Total
40–49 50–59 60–69 70–79 80–89 90⫹ Total
357 447 426 222 56 3 1511
466 531 437 223 89 14 1760
200 180 150 132 32 7 701
237 177 174 140 41 3 772
1260 1335 1187 717 218 27 4744
Urban
962
Rural
VanNewkirk et al 䡠 Causes of Bilateral Visual Impairment in Victoria Table 3. Cause-specific Prevalence by Age and Gender of Visual Acuity ⬍6/12 ⱖ 6/18 and/or Hemianopia Visual Impairment Project Cohorts Urban
Rural
ⱖ40–⬍65 yrs ⬎65 yrs ⱖ40–⬍65 yrs Cause
N
AMD 6 Glaucoma 1 Cataract 11 DR 2 Other retinal 4 Neuro-oph 7 Other causes 2 Ref error 87 All causes 120
Population-weighted Prevalence/1000 ⬎65 yrs
M
F
M
F
M
F
M
F
0 0 0 0 1 0 1 13 15
0 0 0 0 0 0 0 13 13
0 1 2 0 1 1 (1) 0 18 23
5 0 6 0 2 0 1 27 41
0 0 0 1 0 0 0 3 4
0 0 0 0 0 2 (1) 0 2 4
0 0 2 1 0 2 (2) 0 6 11
1 0 1 0 0 2 (2) 0 6 9
ⱖ40–⬍65 yrs
⬎65 yrs
ⱖ40 yrs
All All All (95% Confidence Interval) (95% Confidence Interval) (95% Confidence Interval) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.28 (0, 0.86) 0.31 (0, 0.91) 1.15 (0, 1.58) 0.43 (0, 0.91) 9.55 (6.20, 12.89) 11.23 (7.60, 14.86)
4.07 (1.87, 6.26) 0.59 (0, 1.41) 6.68 (3.87, 9.48) 0.43 (0, 1.14) 1.76 (0.31, 3.19) 3.72 (1.62, 5.81) 0.59 (0, 1.41) 18.60 (28.33, 40.92) 46.56 (39.30, 53.80)
1.28 (0.34, 2.23) 0.21 (0, 0.64) 2.10 (1.13, 3.07) 0.46 (0, 1.12) 0.86 (0, 1.85) 1.15 (0, 2.64) 0.43 (0, 0.96) 18.61 (12.07, 25.14) 25.10 (17.69, 32.51)
indicates number of participants included because of visual field defects. N ⫽ number; M ⫽ male; F ⫽ female; AMD ⫽ age-related macular degeneration; DR ⫽ diabetic retinopathy; Neuro-oph ⫽ neuro-ophthalmic disease; Ref error ⫽ uncorrected refractive error.
Analyzer for 4637 (98%) participants, 3211 (98%) urban residents and 1426 (97%) rural residents. Eighty-two (1.7%) urban and rural participants were observed with moderate, severe, or profound visual impairment in the better eye. An additional 120 participants were observed to have less than legal driving vision, 92 in the urban cohort and 28 in the rural cohort. In two (2.4%) of the participants the cause of moderate, severe, or profound visual impairment was different in the two eyes. In one participant, moderate visual impairment was due to myopic choroidal degeneration in one eye and retinal detachment in the other eye. The cause was assigned to myopic degeneration, because myopia may have contributed to the retinal detachment. In the other participant, bilateral severe visual impairment was due to optic atrophy in one eye and posterior capsule opacification in the other eye; visual loss was assigned to optic atrophy. Different causes of mild bilateral visual impairment occurred in four (3.3%) participants, two urban and two rural. AMD and corneal opacity occurred in one participant, and AMD was assigned as cause for that individual. Optic atrophy and cataract were present in one participant, and optic atrophy was assigned as cause. Posterior capsule opacification and cataract were present in one participant, and cataract was assigned as cause. Cataract and amblyopia were present in one participant, and cataract was assigned as cause.
Mild Vision Impairment Uncorrected refractive error was the most common diagnosis, present in 87 participants, an overall prevalence of 18.6/1000 (95% confidence interval [CI] 12.07, 25.14) (Table 3). The number of participants already wearing glasses was 30 (34.5%). Among those wearing glasses, one (3.3%) improved to 6/6 vision after refraction. Among those not wearing glasses, 17 (29.8%) improved to 6/6 vision after refraction. Uncorrected hyperopia, greater than ⫹0.5 diopters (D) but less than ⫹3.0 D was present in 56% (49 of the 87). The uncorrected refractive error in three participants was greater than ⫹3 D, 3.5%. Uncorrected myopia (greater than ⫺0.5 D but less than ⫺10 D) was observed in 29%, 25 of the 87. Cataract 2.1/1000 (95% CI, 1.13, 3.07) followed by AMD 1.28/ 1000 (95% CI, 0.34, 2.23) were the next most prevalent disorders. In total, 6 (5%) of 120 participants had mild visual impairment because of visual field loss. All had had cerebrovascular accidents,
and five of the six had a history of hypertension. Homonymous hemianopia was observed in one rural participant ⬍65 years of age and in four rural and one urban participant ⱖ65 years of age.
Moderate Vision Impairment Moderate visual impairment was observed in 58 participants in the two cohorts, an overall prevalence of 12.08/1000 (95% CI, 7.56, 16.59) (Table 4). Uncorrected refractive error accounted for the highest overall weighted prevalence, 4.73/1000 (95% CI, 2.49, 6.97). Uncorrected myopia greater than ⫺0.5 D was observed in 50%; 12 of the 24 participants were in this category. Uncorrected hyperopia greater than ⫹0.5 D but less than ⫹6 D was present in 46%, 11 of 24. The prevalence of all diseases was greater in the participants aged 65 years and older. Moderate visual impairment caused by AMD, glaucoma, and cataract was not observed in participants ⬍65 years of age. Moderate visual impairment from DR was observed to have a population-weighted prevalence of 1.07/1000 (95% CI, 0.35, 1.78) in participants aged 40 years and older. Three of five urban participants with DR had retained reasonably good visual acuity, but their visual fields were ⬍20° radius, probably because of the effects of extensive panretinal photocoagulation and the effect of DR on the peripheral retina. Visual field constriction to ⬍20° causing moderate visual impairment was observed in 13 (22%) of the 58 participants with moderate visual impairment. In addition to DR, glaucoma (n ⫽ 3), neuro-ophthalmic disorder (n ⫽ 1), and other disorders (n ⫽ 6) were the cause of visual field reduction.
Severe Vision Impairment Severe visual impairment was observed in 16 participants, a population-weighted prevalence of 3.52/1000 (95% CI, 1.75, 5.28) (Table 5). Only four (25%) participants with severe visual impairment were ⬍65 years of age. Uncorrected refractive error, 1.14/ 1000 (95% CI, 0.33, 1.95), was the most prevalent cause of visual impairment. Uncorrected hyperopia greater than ⫹0.5 D was observed in four of the five participants initially seen with uncorrected refractive error. Four (9%) participants were classified as having severe vision impairment because of bilateral constriction of visual field to ⬍10°
963
Ophthalmology Volume 108, Number 5, May 2001 Table 4. Cause-specific Prevalence by Age and Gender of Visual Acuity ⬍6/18 ⱖ 6/60 and/or ⬍20° Visual Impairment Project Cohorts Urban ⱖ40–⬍65 yrs Cause AMD Glaucoma Cataract DR Other retinal Neuro-oph Other causes Ref error All causes
N M 7 5 3 5 5 4 7 22 58
F
0 0 0 1 (1) 0 0 0 1 2
Rural ⬎65 yrs M
ⱖ40–⬍65 yrs
F
0 4 0 2 (1) 0 0 1 (1) 2 (1) 0 0 0 1 (1) 2 (2) 1 (1) 1 6 4 16
3 0 1 1 4 0 4 (3) 10 23
Population-weighted Prevalence/1000 ⬎65 yrs
M
F
M
F
0 0 0 0 0 1 0 0 1
0 0 0 0 0 0 0 0 0
0 1 (1) 1 0 0 1 0 3 6
0 2 (1) 1 0 1 1 0 1 6
ⱖ40–⬍65 yrs
⬎65 yrs
ⱖ40 yrs
All All All (95% Confidence Interval) (95% Confidence Interval) (95% Confidence Interval) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.62 (0, 1.47) 0 (0, 0) 0.32 (0.86) 0.62 (0, 1.47) 0.62 (0, 1.47) 2.16 (0.54, 3.72)
5.03 (1.90, 6.30) 4.07 (1.73, 6.01) 2.73 (0.55, 3.74) 1.76 (0.31, 3.19) 3.56 (1.45, 5.51) 2.57 (0.55, 3.75) 3.11 (1.06, 4.78) 11.80 (8.08, 15.51) 30.58 (22.43, 33.81)
1.50 (0.45, 2.55) 0.98 (0, 2.15) 0.47 (0, 1.04) 1.07 (0.35, 1.79) 1.02 (0.02, 2.01) 0.65 (0, 1.58) 1.66 (0.58, 2.74) 4.73 (2.49, 6.97) 12.08 (7.56, 16.59)
indicates number of participants included because of visual field defects. N ⫽ number; M ⫽ male; F ⫽ female; AMD ⫽ age-related macular degeneration; DR ⫽ diabetic retinopathy; Neuro-oph ⫽ neuro-ophthalmic disease; Ref error ⫽ uncorrected refractive error.
radius but ⱖ5° radius. Three of these four had severe glaucoma, one from the urban cohort and two from the rural cohort.
Profound Vision Impairment
field to ⬍5°. Cataract was not observed as a cause of profound visual impairment in either the urban or the rural cohort.
Cumulative Prevalence of Vision Impairment
Profound visual impairment was observed in eight participants. The population-weighted prevalence of profound visual impairment was 1.56/1000 (95% CI, 0.58, 2.55). Seven (87%) of the participants with profound visual impairment were 65 years of age or older, a prevalence of 8.0/1000, and four (50%) were women (Table 6). AMD and glaucoma accounted for 75% of the WHO blindness or profound visual impairment observed in the VIP. Although not statistically different from other causes, AMD is the most prevalent cause of profound visual impairment. One participant with severe glaucoma from the rural cohort was classified with profound visual impairment because of bilateral constriction of visual
The prevalence of vision less than that required for driving (mild to profound) is 42.48/1000 (95% CI, 30.11, 54.86), and the leading cause of vision loss is uncorrected refractive error, 24.68/1000 (95% CI, 16.12, 33.25) (Table 7). The prevalence of other causes are 6 to 16 times less than that of uncorrected refractive error. The prevalence of low vision or worse is 17.68/1000 (95% CI, 11.75, 23.01) and uncorrected refractive error continues to be the dominant cause of vision loss, 6.08/1000 (95% CI, 3.36, 8.79). The prevalence of legal blindness is 5.30/1000 (95% CI, 3.24, 7.36). Although uncorrected refractive error continues to be the most prevalent cause in this range of vision impairment, it is not
Table 5. Cause-specific Prevalence by Age and Gender of Visual Acuity ⬍6/60 ⱖ 3/60 and/or ⬍10° Visual Impairment Project Cohorts Urban
Rural
ⱖ40–⬍65 yrs ⬎65 yrs ⱖ40–⬍65 yrs Cause
N
M
F
M
AMD Glaucoma Cataract DR Other retinal Neuro-oph Other causes Ref error All causes
2 3 0 0 4 0 2 5 16
0 0 0 0 0 0 1 0 1
0 0 0 0 1 0 0 0 1
1 0 0 0 0 0 0 1 2
F 0 1 (1) 0 0 2 0 0 2 5
Population-weighted Prevalence/1000 ⬎65 yrs
M
F
M
0 0 0 0 0 0 1 (1) 1 2
0 0 0 0 0 0 0 0 0
1 1 (1) 0 0 1 0 0 0 3
F 0 1 (1) 0 0 0 0 0 1 2
ⱖ40–⬍65 yrs
⬎65 yrs
ⱖ40 yrs
All All All (95% Confidence Interval) (95% Confidence Interval) (95% Confidence Interval) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.31 (0, 0.91) 0 (0, 0) 0.59 (0, 1.42) 0.28 (0, 0.86) 1.18 (0, 2.36)
1.40 (0, 2.10) 2.27 (0.55, 3.74) 0 (0, 0) 0 (0, 0) 1.64 (0.22, 2.97) 0 (0, 0) 0 (0, 0) 2.90 (1.04, 4.74) 7.72 (3.68, 9.18)
0.39 (0, 0.89) 0.74 (0, 1.84) 0 (0, 0) 0 (0, 0) 0.82 (0, 1.84) 0 (0, 0) 0.43 (0, 1.03) 1.14 (0.33, 1.95) 3.52 (1.75, 5.28)
indicates number of participants included because of visual field defects. N ⫽ number; M ⫽ male; F ⫽ female; AMD ⫽ age-related macular degeneration; DR ⫽ diabetic retinopathy; Neuro-oph ⫽ neuro-ophthalmic disease; Ref error ⫽ uncorrected refractive error.
964
VanNewkirk et al 䡠 Causes of Bilateral Visual Impairment in Victoria Table 6. Cause-specific Prevalence by Age and Gender of Visual Acuity ⬍3/60 and/or ⬍5° Visual Impairment Project Cohorts Urban ⱖ40–⬍65 yrs
Rural ⬎65 yrs
ⱖ40–⬍65 yrs
Population-weighted Prevalence/1000 ⬎65 yrs
Cause
N
M
F
M
F
M
F
M
AMD Glaucoma Cataract DR Other retinal Neuro-oph Other causes Ref error All causes
4 2 0 0 1 0 1 0 8
0 0 0 0 1 0 0 0 1
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
2 1 0 0 0 0 0 0 3
0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
1 1 (1) 0 0 0 0 1 0 3
ⱖ40–⬍65 yrs
⬎65 yrs
ⱖ40 yrs
All All All F (95% Confidence Interval) (95% Confidence Interval) (95% Confidence Interval) 1 0 0 0 0 0 0 0 1
0 (0, 0) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.31 (0, 0.91) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.31 (0, 0.91)
2.74 (0.94, 4.53) 1.01 (0, 2.10) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0 (0, 0) 0.43 (0, 1.14) 0 (0, 0) 3.78 (1.66, 5.89)
0.68 (0, 1.39) 0.39 (0, 0.85) 0 (0, 0) 0 (0, 0) 0.21 (0, 0.64) 0 (0, 0) 0.28 (0, 0.83) 0 (0, 0) 1.56 (0.58, 2.55)
indicates number of participants included because of visual field defects. N ⫽ number; M ⫽ male; F ⫽ female; AMD ⫽ age-related macular degeneration; DR ⫽ diabetic retinopathy; Neuro-oph ⫽ neuro-ophthalmic disease; Ref error ⫽ uncorrected refractive error.
discernibly greater than AMD, glaucoma, retinal disorders, and other causes.
Discussion This study describes the cause-specific population-weighted prevalence of vision impairment in the state of Victoria. Across all levels of vision impairment, the prevalence of visual impairment is greatest in those aged 65 years and older. Uncorrected refractive error is the leading cause of mild through moderate vision impairment. Severe to profound vision impairment is caused by a number of eye conditions, including uncorrected refractive error. The population-weighted prevalence of uncorrected refractive error as a cause of mild, moderate, and severe visual impairment was surprisingly high. Uncorrected hyperopia, greater than ⫹0.5 D, accounted for 59%, 67 of the 114 participants with mild, moderate, and severe visual impairment. VIP participants with nuclear cataract were more likely than those without nuclear cataract to have uncorrected refractive error, odds ratio 1.51 (95% CI, 1.14, 2.00). Myopia induced by nuclear sclerosis was the most frequent
uncorrected refractive error in the older age group. For many older individuals, uncorrected myopia is not considered a problem; because most of their activities are nearassociated, the decrease in distant visual acuity appears not to bother them. Uncorrected refractive error has been identified as a major source of visual impairment in previous studies.4,5,12 Many possible explanations have been proposed, but the reality is that some people do not wish or feel the need to wear spectacles.23 The effects of hyperopia may be reduced by the availability and use of nonprescription reading glasses. Certainly this group of visually impaired lends itself to the most simple correction. AMD is a major cause of disabling vision impairment in elderly white populations.2,3,5,6,24,25 In the VIP, early agerelated maculopathy was not associated with vision loss. The prevalence of legal blindness caused by AMD is similar to that found in the Baltimore Eye Survey, 2.7/1000 (95% CI, 1.2, 5.4).2 The difference in the definition of legal blindness in the United States should be noted; the United States definition includes 6/60 vision. The prevalence of legal blindness caused by glaucoma is also similar to other predominantly white populations, such as the Baltimore Eye Survey, 0.7/1000 (95% CI, 0.1, 2.5).2
Table 7. Cumulative Prevalence/1000 (95% Confidence Interval) for Specific Causes of Visual Impairment Cause
Less-Than-Driving Vision (95% Confidence Interval)
Low Vision (95% Confidence Interval)
Legal Blindness (95% Confidence Interval)
Blindness (95% Confidence Interval)
AMD Glaucoma Cataract DR Other retinal Neuro-oph Other causes Ref error All causes
3.86 (2.17, 5.55) 2.32 (0.72, 3.92) 2.57 (1.38, 3.76) 1.53 (0.71, 2.36) 2.91 (0.74, 5.08) 1.80 (0, 4.11) 2.80 (1.17, 4.43) 24.68 (16.12, 33.25) 42.48 (30.11, 54.86)
2.58 (1.21, 3.94) 2.10 (0.49, 3.72) 0.47 (0, 1.03) 1.07 (0.35, 1.79) 2.05 (0.59, 3.52) 0.65 (0, 1.58) 2.37 (1.04, 3.70) 6.08 (3.36, 8.79) 17.38 (11.75, 23.01)
1.07 (0.14, 2.01) 1.13 (0.05, 2.21) 0 (0, 0) 0 (0, 0) 1.04 (0, 2.12) 0 (0, 0) 0.71 (0, 1.48) 1.35 (0.57, 2.13) 5.30 (3.24, 7.36)
0.68 (0, 1.39) 0.68 (0, 1.39) 0.39 (0, 0.85) 0 (0, 0) 0 (0, 0) 0.21 (0, 0.64) 0 (0, 0) 0 (0, 0) 1.56 (0.58, 2.55)
AMD ⫽ age-related macular degeneration; DR ⫽ diabetic retinopathy; Neuro-oph ⫽ neuro-ophthalmic disease; Ref error ⫽ uncorrected refractive error.
965
Ophthalmology Volume 108, Number 5, May 2001 The VIP prevalence of visual impairment caused by glaucoma, which includes one rural participant with angleclosure glaucoma, is markedly increased in the older age group. Typically, visual acuity remains good until very late stages of glaucomatous optic neuropathy, and early visual impairment would be missed without visual field assessment. Among those with legal blindness caused by glaucoma, only one of five participants from the urban and rural cohorts was classified according to visual acuity criteria. Four of the five VIP participants with visual field constriction to ⬍10° had glaucoma; three participants were from the rural cohort. This may indicate that diagnosis of glaucoma in the rural cohort is delayed compared with the urban cohort. Possibly early detection, different treatment, and better compliance with treatment could reduce the prevalence of visual impairment caused by glaucoma. The age-standardized prevalence of previous cataract surgery was not statistically different between the rural cohort, 44.2/1000 (95% CI, 26.5, 62.0), and the urban cohort, 33.7/1000 (95% CI, 27.4, 40.0). Seventy percent of those who had previously had cataract surgery had 6/12 or better visual acuity. Complications of cataract surgery causing visual impairment were infrequent, occurring in one eye of two participants. One participant had pseudophakic bullous keratopathy and one participant had posterior capsule opacification. As in the Rotterdam Study,6 visual impairment from DR was limited to the mild and moderate levels of visual impairment. Of the seven VIP cases with bilateral proliferative DR, six from the urban cohort and one from the rural cohort had been treated with panretinal photocoagulation and appeared to be in a stable posttreatment state. The failure to detect legal blindness from DR may be due to sampling error, because Harper et al (1998 Abstract, Scientific Program Melbourne Ophthalmic Alumni Scientific Meeting) reported that 39% (68 of 175) of consecutive first-visit patients with DR seen at the Royal Victorian Eye and Ear Hospital retina clinic have significant loss of vision (⬍6 of 18). The overall rate of diabetes in the VIP was 4.9%, with a rate of 5.2% in the urban and 4.9% in the rural cohort. Among diabetics, 19.6% had DR, 21.8% urban residential and 16.9% rural residential diabetic participants. Previous reports from the VIP of the age-specific causes of vision loss, which included a cohort of 400 nursing home and hostel residents, demonstrated the age dependence of causes of vision loss.26 Consistent with this report, uncorrected refractive error was a major cause of vision loss at all ages, with a prevalence rising from 0.5% of 40- to 49-yearolds to 13% of those aged 80 years and older. However, the predominant causes of vision loss other than refractive error were shown to change with age. Vision impairment caused by DR and glaucoma was observed in participants 50 to 59 years of age, whereas vision loss caused by AMD was not present in participants less than 70 years of age. AMD was the cause of vision impairment in 11% of participants older than 90 years of age. Cataract was a common cause of severe and profound vision impairment among nursing home residents.
966
Conclusions Uncorrected refractive error remains a major cause of vision impairment even though it is easily prevented. Program development to reduce the vision impairment caused by uncorrected refractive error, cataract, glaucoma, and diabetes mellitus should be attainable with current resources and technology.
References 1. Leibowitz HM, Krueger DE, Maunder LR, et al. The Framingham Eye Study monograph: An ophthalmological and epidemiological study of cataract, glaucoma, diabetic retinopathy, macular degeneration, and visual acuity in a general population of 2631 adults, 1973–1975. Surv Ophthalmol 1980; 24(Suppl):335– 610. 2. Sommer A, Tielsch JM, Katz J, et al. Racial differences in the cause-specific prevalence of blindness in east Baltimore. N Engl J Med 1991;325:1412–7. 3. Dana MR, Tielsch JM, Enger C, et al. Visual impairment in a rural Appalachian community. Prevalence and causes. JAMA 1990;264:2400 –5. 4. Klein R, Klein BEK, Linton KLP, De Mets DL. The Beaver Dam Eye Study: visual acuity. Ophthalmology 1991;98: 1310 –5. 5. Attebo K, Mitchell P, Smith W. Visual acuity and the causes of visual loss in Australia. The Blue Mountains Eye Study. Ophthalmology 1996;103:357– 64. 6. Klaver CCW, Wolfs RCW, Vingerling JR, et al. Age-specific prevalence and causes of blindness and visual impairment in an older population: the Rotterdam Study. Arch Ophthalmol 1998;116:653– 8. 7. The Victorian Yearbook, Number 110. Australian Bureau of Statistics, 1998. 8. Sorsby A. The Incidence and Causes of Blindness in England and Wales, 1948 –1962. London: HMSO, 1966. Great Britain Ministry of Health. Reports on Public Health and Medical Subjects; No. 114. 9. Strategies for the Prevention of Blindness in National Programmes: A Primary Health Care Approach. Geneva: World Health Organization, 1984. 10. Livingston PM, Carson CA, Stanislavsky YL, et al. Methods for a population-based study: the Melbourne Visual Impairment Project. Ophthalmic Epidemiol 1994;1:139 – 48. 11. Taylor HR, Livingston PM, Stanislavsky YL, McCarty CA. Visual impairment in Australia: distance visual acuity, near vision, and visual field findings of the Melbourne Visual Impairment Project. Am J Ophthalmol 1997;123:328 –37. 12. Tielsch JM, Sommer A, Witt K, et al. Blindness and visual impairment in an American urban population. The Baltimore Eye Survey. Arch Ophthalmol 1990;108:286 –90. 13. Heijl A, Lindgren G, Olsson J. The effect of perimetric experience in normal subjects. Arch Ophthalmol 1989;107:81– 6. 14. Taylor HR, West SK, Rosenthal FS, et al. Effect of ultraviolet radiation on cataract formation. N Engl J Med 1988;319: 1429 –33. 15. Bailey IL, Bullimore MA, Raasch TW, Taylor HR. Clinical grading and the effects of scaling. Invest Ophthalmol Vis Sci 1991;32:422–32. 16. Bird AC, Bressler NM, Bressler SB, et al. An international classification and grading system for age-related maculopathy and age-related macular degeneration. The International ARM Epidemiological Study Group. Surv Ophthalmol 1995;39: 367–74.
VanNewkirk et al 䡠 Causes of Bilateral Visual Impairment in Victoria 17. Report 7. A modification of the Airlie House classification of diabetic retinopathy. Diabetic Retinopathy Study Research Group. Invest Ophthalmol Vis Sci 1981;21:210 –26. 18. Wensor MD, McCarty CA, Stanislavsky L, et al. The prevalence of glaucoma in the Melbourne Visual Impairment Project. Ophthalmology 1989;105:733–9. 19. World Health Organisation. International Classification of Diseases, Ninth Revision (ICD-9). Geneva, Switzerland: World Health Organisation; 1977. 20. Australian Bureau of Statistics. 1996 Census of Population and Housing. Canberra, Australia: 1996. 21. Cochran WG. Sampling Techniques, 3rd ed. New York: John Wiley & Sons, 1977;249 –73.
22. Thylfors B, Negrel AS, Pararajasegarm R, Dadzie KY. Global Data on Blindness. WHO/PBL/94.40 Geneva, 1994. 23. Lichter PR. Resiliency as it affects treatment for refractive errors [editorial]. Ophthalmology 1994;101:221–2. 24. Klein R, Wang Q, Klein BEK, et al. The relationship of age-related maculopathy, cataract, and glaucoma to visual acuity. Invest Ophthalmol Vis Sci 1995;36:182–91. 25. Newland HS, Hiller JE, Casson RJ, Obermeder S. Prevalence and causes of blindness in the South Australian population aged 50 and over. Ophthalmic Epidemiol 1996;3:97–107. 26. Weih LM, VanNewkirk MR, McCarty CA, Taylor HR. Agespecific causes of bilateral vision impairment. Arch Ophthalmol 2000;118:264 –9.
967