- Email: [email protected]
Visual Impairment in Australia: Distance Visual Acuity, Near Vision, and Visual Field Findings of the Melbourne Visual Impairment Project
Visual Impairment in Australia: Distance Visual Acuity, Near Vision, and Visual Field Findings of the Melbourne Visual Impairment Project HUGH R. TAYLOR, MD, FRACO, PATRICIA M. LIVINGSTON, PHD, BA(HONS), YURY L. STANISLAVSKY, MD, AND CATHERINE A. McCARTY, PHD, MPH
• PURPOSE: To describe the age-specific and gen der-specific rates of blindness and visual impair ment in urban adults aged 4 0 years and older. • METHODS: A population-based sample of resi dents was recruited. Presenting and best-corrected distance visual acuities were assessed. Functional near vision was measured at each participant's pre ferred distance. Visual field examination was per formed with a Humphrey Field Analyzer (HFA); those unable to perform the field analyzer test attempted a Bjerrum screen or confrontation field. • RESULTS: The study population comprised 3,271 residents (83% of eligible) from ages 4 0 to 98 years; 54% were women. Overall, 56% of the study population wore distance correction; this was significantly lower in men but higher in the older age groups. Age-adjusted rates of blindness were 0.066% in men and 0.170% in women. Vision with current correction improved after refraction by gender and age. Direct agestandardized rates of functional near vision did not
Accepted for publication Oct 10, 1996. From the Department of Ophthalmology, University of Melbourne, Royal Victorian Eye and Ear Hospital, East Melbourne, Victoria, Austral ia. The Melbourne Visual Impairment Project is supported in part by the Victorian Health Promotion Foundation, Melbourne, Australia; the Ansell Ophthalmology Foundation, Melbourne, Australia; and the National Health and Medical Research Council, Canberra, Australia, including the Sir John Eccles Award (to Prof Taylor). Reprint requests to Hugh R. Taylor, MD, FRACO, University of Melbourne, Department of Ophthalmology, Royal Victorian Eye and Ear Hospital, 32 Gisborne St, East Melbourne, 3002 Victoria, Australia; fax: (613) 9662 3859; e-mail: [email protected]
328
vary significantly by gender. Forty-six people had significant visual field loss in their better eye. The proportion of participants with constriction of the visual field to within 2 0 degrees of fixation was similar for men and women when controlled for age (odds ratio, 0.81; 95% confidence interval, 0.44 to 1.49) but increased significantly with age con trolled for gender. Visual field abnormalities were detected in 5 4 8 right eyes (17%) and 533 left eyes (16%). • CONCLUSIONS: Although overall rates of blind ness because of visual acuity loss were relatively low, nearly three times more people had visual impairment because of visual field loss than visual acuity loss. These results highlight the need to target blindness prevention programs to the aging population, with a special emphasis on women.
T
O PLAN APPROPRIATE PROGRAMS OR INTERVEN-
tions aimed at the prevention or cure of avoidable blindness, it is necessary to know the magnitude, distribution, and causes of visual impair ment and blindness. In addition, knowledge of the functional impact of visual loss and the availability and utilization of existing health services is important to prioritize potential interventions. To provide this information was the broad aim of the Melbourne Visual Impairment Project (Melbourne VIP), a popu lation-based survey of visual impairment and eye disease.1 Data on visual impairment or blindness are avail able from many different sources, including registries
© AMERICAN JOURNAL OF OPHTHALMOLOGY 1997;123:328-337
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of the blind,2'4 that have differing degrees of rigor.5'8 Much useful information can be extracted from data obtained by these differing methods, such as the worldwide figures on blindness prepared by the World Health Organization. 9 However, for planning purpos es, for the evaluation of health service utilization, or for risk-factor analysis, much more specific and popu lation-based data are desired. In almost every report on visual impairment or blindness, vision is assessed only in terms of visual acuity, even though constriction or loss of the visual field is also recognized as causing severe visual impair ment.10 In this report, we present the findings on the age-specific and gender-specific prevalence of visual impairment and blindness as assessed by both visual acuity and visual field testing in a population-based sample of adults living in Melbourne, Australia. Near vision of all participants was also tested.
METHODS THE MELBOURNE VIP WAS A POPULATION-BASED SURVEY
of noninstitutionalized permanent residents of the Melbourne metropolitan area. T h e detailed method ology has been reported elsewhere.1 A manual of operations was prepared to standardize procedures. In brief, nine pairs of adjacent census collector districts were randomly selected from the Melbourne Statisti cal Division.11 A door-to-door household census was taken to identify all eligible persons aged 40 years or older in the calendar year of examination who had lived at their address for 6 months or more. Information about household characteristics and basic demographic data were collected during a brief interview conducted at the household. Eligible indi viduals were then invited to a local examination site for a more detailed interview and an ophthalmic examination.1,12 Participants were asked to bring to the examination site all corrective spectacles or lenses currently being used. The power of these lenses was measured. Distance visual acuity was measured with a logMAR letter chart set at 4 m under standardized illumina tion.1 A n E chart was used for illiterate and n o n English-speaking participants. Presenting visual acuity was measured with the participant's current distance correction; when distance correction was not VOL.123, No. 3
used, the unaided distant visual acuity was measured. When the presenting acuity was less than 53 letters (that is, <6/6), further refraction was undertaken. Best-corrected visual acuity was determined after objective autorefraction and subjective refinement. Functional near vision was measured with a log MAR word reading card designed for use at 25 cm, although the test was performed at the participant's preferred reading distance. Near correction was worn for testing if the participant normally used it. Illiterate or non-English-speaking participants were asked to spell the letters on the near chart or were tested with a near E chart. Visual field examination was performed with a Humphrey Field Analyzer (HFA), (Humphrey Instru ments Inc, San Leandro, California), on all capable participants. In the first 6 months of field work, the visual field was first examined with a screening 80-point suprathreshold test. If the results of this test were considered abnormal in either eye, a 24-2 (FastPac) threshold test was then performed in both eyes. The 80-point test was considered abnormal when two adjacent points or any three points were missed in the central 20 degrees. Although this combined strategy of visual field testing provided some time saving in young individuals, it was logistically difficult; subsequently, 24-2 FastPac tests were routinely performed on both eyes. Where possible, this test was repeated when the results of the first test were unreliable or diagnostically equivocal. Participants not capable of performing the field analyzer test and those examined at their homes were tested with a Bjerrum screen at a 1-m distance with 2/1,000 and 10/1,000 objects. Visual fields were assessed by confrontation for those who were unable to perform a Bjerrum screen because of either poor vision or mental or physical difficulties. The shape of a field analyzer defect was analyzed using the total deviation and pattern deviation plots. Points with a probability below 1% were considered abnormal. Abnormal fields were classified as constric tion within 5 degrees, 10 degrees, or 20 degrees from the fixation point, hemianopia, quadrantic defects, central scotomata of 3-degree, 6-degree, 9-degree, or more than 9-degree radius, paracentral defects in four grades of severity, and nonspecific defects. Results of the Bjerrum or confrontation tests were classified similarly.
VISUAL IMPAIRMENT IN AUSTRALIA
329
Visual impairment was defined as a best-corrected visual acuity score of less than 6/18 or visual field constriction to within 20 degrees of fixation, or both. Legal blindness (Australian definition) was defined as a best-corrected visual acuity score of less than 6/60 or visual field constriction to within 10 degrees of fixation, or both, and blindness by the World Health Organization was defined as a best-corrected visual acuity score of less than 3/60 or visual field constric tion to within 10 degrees of fixation, or both. Data were collected either by direct computer entry using a questionnaire programmed in Paradox (Borland International, Scotts Valley, California) or on self-coding forms. Open-ended responses were coded at a later time. Data were entered into a computer using double data entry with reconciliation of any inconsistencies. Data range and consistency checks were performed on the whole data set. Chi-square analyses were employed to assess signifi cant relationships among categoric variables. Analysis of covariance and multiple linear regression were used for continuous variables. The following variables were transformed before inclusion in parametric analyses: number of letters read (cubic transformation), num ber of letters improved (square root transformation), and number of words read (square transformation). Poisson regression analysis was used to calculate the 95% confidence intervals (Cls) around the agestandardized rates of blindness. Prevalence rates were directly age-standardized using the entire study popu lation.13 Confidence intervals around the agestandardized rates of blindness (United States defini tion, <6/60) were calculated using the standard error of the directly standardized rate. Age-standardized comparisons were extrapolated to derive the blindness rate ( ^ 6/60) for the Appalachian community. 7 Data analyses were performed using SAS software (SAS Institute, Cary, North Carolina).
RESULTS A TOTAL OF 4,273 HOUSES WERE IDENTIFIED IN THE NINE
study clusters.14 Of these, 4,033 (94%) provided information and 2,391 (59%) had eligible residents. In the eligible houses were 3,946 eligible people, of whom 3,271 (83%) agreed to participate and 3,266 (83%) had complete vision assessment. N o n -
Age
Males
Females
90+ 80-89 70-79 60-69 50-59 40-49
::^;::,;fj|-;"f • -;:.; -]..:.'i; ■;, •;.'. :;;.-.^L;:<;C.
600 500 400 300 200 100 0
100 200 300 400 500 600
Number of participants Figure 1. The sample population by age and gender, with those examined shown in the shaded bars.
English-speaking persons were significantly less likely to participate (odds ratio [OR], 0.61; 95% CI, 0.48 to 0.77), but there were no other significant differences between participants and nonparticipants. 14 The par ticipants ranged in age from 40 to 98 years, and 54% were women (Figure 1). Three (0.12%) of the 3,271 participants did not have the distance, functional near vision, and visual field examinations (two refused and one person was affected by Alzheimer's disease). A n additional seven persons (0.21%) did not complete the near visual acuity assessment (six refused and one was blind). A further 19 participants (0.58%) were not tested for visual fields (nine refused; seven were not tested because of mental constraints; and three were not tested because of language disabilities). Fifteen of the 19 had normal or near-normal vision; two had visual acuities of less than 6/12; one was visually impaired (<6/18); and one was legally blind (Australian definition, <6/60; U.S. definition, <20/200). The proportion of participants wearing distance correction for the examination was significantly lower for men when controlling for age (OR = 0.78; 95% CI, 0.67 to 0.91) and increased significantly with age when controlling for gender (chi-square = 2.47; P = .0001) (Figure 2). Overall, 56% of the participants had distance correction. The mean (±SD) number of letters read (right eye) ranged from 57.4 ± 7.7 among men and 55.8 ± 9.2 among women in the 40- to 49-year-old age group to 17.7 ± 15.4 among men and 34.4 ± 17.9 among
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Table 1. Distribution of initial Visual Acuity in Best Eye by Age and Gender No. (%) Age (yrs)
Gender
n
<3/60
40-49
Male Female Male Female Male Female Male Female Male Female Male Female
357 466 446 532 426 436 221 223 56 89 3. 13 3,268
0(0) 0(0) 0 (0.2) 0(0) 1 (0.2) 0(0) 0(0) 0(0)
50-59 60-69 70-79 80-89 &90
—
Total
100
1 (1-8) 1(1.1) 0(0) 2(15.4) 5(0.15)
<6/60 to £3/60
<6/18
<6/12
to 2:6/60
tO 2:6/18
<6/6 to 2:6/12
0(0) 0(0) 1 (0.2) 0(0) 0(0) 1 (0.2) 0 (0.0) 3(1.3) 0(0) 1(1.1) 1 (33.3) 0(0) 7(0.21)
0(0) 1 (0.2) 0(0) 0(0) 4 (0.9) 4 (0.9) 3(1.4) 5 (2.2) 4 (7.1) 8 (9.0) 1 (33.3) 0(0) 30 (0.92)
2 (0.6) 2 (0.4) 9 (2.0) 5 (0.9) 10(2.3) 10(2.3) 11 (5.0) 13 (5.8) 3 (5.4) 16(18.0) 1 (33.3) 3 (25.0) 85 (2.60)
52(15) 87(18.7) 90 (20.2) 141 (26.5) 145(34.0) 182(41.7) 109(49.3) 132(59.2) 36 (64.3) 50 (56.2) 0(0) 7 (58.3) 1,031 (31.56)
%
J""
"
^
80
60
/
40
20
. '^Z' '^:;;'""" ""' "" * ""' * -jr
*»* /
40.49
Near:
Male Female Distance: Male Female 50-69
60-69
70-79
80-89
« : ■■ *mm ^^m
90+
Age Group
Figure 2. Age-specific and gender-specific frequency of the use of distance correction and near correction.
women in the group aged 90 years or more. The mean number of letters read after refraction, when neces sary (right eye), ranged from 58.3 ± 6.5 among men and 57.0 ± 7.1 among women in the 40- to 49-yearold age group to 24.0 ± 16.1 among men and 36.4 ± 18.5 among women in the group aged 90 years or more. Best correction was higher in men than in women after controlling for age (presenting, F = 52.8, P = .0001; best-corrected, F = 85.7, P = .0001) and decreased with age after controlling for gender (pre senting, F = 1,005, P = .0001; best-corrected, F = 1,000, P = .0001). There was a small but statistically significant difference in the mean number of letters Vot. 123,
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2:6/6
303 (85) 376 (80.7) 346 (77.6) 386 (72.7) 266 (62.4) 239 (54.8) 98 (44.3) 70(31.4) 12(21.4) 13(14.6) 0(0) 1 (8.3) 2,110(64.60)
read for both presenting and best-corrected acuity between right and left eyes after controlling for age and gender (F = 4.12, P = .04 and F = 8.68, P = .003, respectively). On presentation, five (0.15%) of the participants were blind by the World Health Organization defini tion for visual acuity (<3/60) and 42 (1.28%) had visual impairment (<6/18) (Table 1). After correc tion, when that was possible, four (0.12%) remained blind and 23 (0.70%) were visually impaired (Table 2). The direct age-standardized rates of both present ing and best-corrected visual impairment were signifi cantly higher for women than men (Table 3). The age-standardized blindness rate (<3/60) with best correction was 0.066% (95% CI, 0.032 to 0.100) in men and 0.170% (95% CI, 0.116 to 0.220) in women. Improvement in the number of letters read with best correction also varied significantly by gender after controlling for age (F = 19.3, P = .0001) and by age after controlling for gender (F = 44.3, P = .0001). The mean number of letters improved in the best eye of the men was 8.4 (7.29), whereas the mean number in women was 7.0 (6.39). The majority of both the right eyes (71%) and the left eyes (70%) that had presenting visual acuity of less than 6/18 improved to at least 6/12 (driving vision) with refraction. Overall, 60% of people improved by at least 1 line in their better eye (Table 4).
VISUAL IMPAIRMENT IN AUSTRALIA
331
Table 2. Distribution of Best-corrected Visual Acuity in Best Eye by Age and Gender No. (%) Age (yrs)
40-49
Gender
Male Female
50-59
Male Female
60-69
Male Female
70-79
Male Female
80-89
Male Female
==90
Male Female
Total
—
n
357 466 446 532 426 436 221 223 56 89 3 13 3,268
<3/60
<6/60 to ==3/60
<6/12
to ==6/60
toa6/18
<6/6 to ==6/12
26/6
321 (90.2)
0(0)
0(0)
0(0)
1 (0.3)
35 (9.8)
0(0)
0(0)
0(0)
65(13.9)
401 (86.1)
0(0)
0(0) 1 (0.2)
0(0)
1 (0.2)
59 (13.2)
385 (86.3)
0(0)
0(0)
0(0)
0(0)
100(18.8)
432(81.4)
1 (0.2)
0(0)
1 (0.2)
1 (0.2)
106 (24.9)
317(74.4)
0(0)
1 (0.2)
2 (0.5)
0(0)
146(33.5)
287 (65.8)
0(0)
0(0)
3(1.4)
1 (0.5)
91 (41.2)
126 (57.0)
0(0)
2 (0.9)
2 (0.9)
3(1.3)
126(56.5)
90 (40.4)
0(0)
1 (1.8)
1 (1.8)
1(1.8)
36 (64.3)
17(30.4)
1(1.1)
0(0)
4 (4.5)
9(10.1)
60 (67.4)
15(16.9)
0(0)
0(0)
1 (33.3)
1 (33.3)
1 (33.3)
2(15.4)
0(0)
0(0)
2(16.7)
8 (66.7)
4 (0.12)
5(0.15)
Twenty-one persons required home visits because of mobility restrictions; the vision of these persons was significantly worse than the vision of those attending the test site. O n presentation, the mean number of letters read by the right eyes of the 21 persons with a home visit was 36.5, compared with 52.9 in those who attended the test site (unpaired t test, - 3 . 6 8 , P = .002). Overall, 87% of women and 85% of men wore eyeglasses for near-vision assessment (OR, 0.71; 95% CI, 0.57 to 0.90; P = .006, controlling for age), and the proportion of people wearing eyeglasses increased with age (chi-square = 3.61, P = .0001, controlling for gender) (Figure 2). The mean number of words ranged from 68.0 ± 7.7 among men and 67.9 ± 7.8 among women in the 40- to 49-year-old age group to 42.7 ± 14-6 among men and 50.3 ± 20.6 among women in the group aged 90 years or more. The mean number of words read was higher for men after controlling for age (F = 7.4, P = .007) and decreased significantly with age after controlling for gender (F = 40.9, P = .0001). Direct age-standardized rates of near vision did not vary significantly by gender (chi-square = 3.3, P = .19), with more than 98% of the population able to read N8 (Table 5). Near and distance vision were significantly related. Controlling for age, the correlation of the number of letters read on the distance chart on presentation was
332
<6/18
14 (0.43)
20(0.61)
833 (25.49)
0(0) 1 (8.3) 2,392 (73.23)
significantly related to the number of words read on the near test (Pearson's r = .44, P — .0001) (Table 6). The number of people who maintained very good functional near vision despite distance visual impair ment was striking. Nearly all right eyes (n = 3,249 [99.33%]) and left eyes (n = 3,250 [99.36%]) had an assessment of visual fields; 2,919 (89%) were field analyzer 24-2 tests, 255 (8%) were field analyzer 80-point tests, 61 (2%) were Bjerrum tests, and 14 (0.4%) were by confrontation. Visual field abnormalities were detect ed in 548 right eyes (17%) and 533 left eyes (16%); the most common identifiable defect was a paracentral arcuate or sectoral field (Table 7). Altogether, 45 persons (1.38%) had visual field constriction to within 20 degrees of fixation (total deviation or equivalent) in the better eye (Table 8). Similarly, 26 persons (0.80%) had visual field con striction to within 10 degrees of fixation (total deviation or equivalent) in the better eye. Taking the more stringent and possibly more logical criterion of pattern loss on field analyzer testing or field loss detected by Bjerrum or confrontation testing, 26 (0.80%) had visual field constriction to within 20 degrees of fixation (pattern deviation or equivalent) and seven (0.21%) had visual field constriction to within 10 degrees of fixation (pattern deviation or equivalent).
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Table 3. Direct Age-standardized Prevalence of Initial and Best-corrected Visual Acuity in the Better Eye by Gender Prevalence (%) (95% Confidence Interval) Initial
Best-corrected
Visual Acuity
Men
Women
Men
Women
<3/60 <6/60 to >3/60 <6/18toa6/60 <6/12to >6/18 <6/6 to z=6/12 ^6/6
0.132 (0.081-0.178) 0.132(0.081-0.178) 0.795 (0.706-0.884) 2.385(2.258-2.511) 28.628 (28.397-28.858) 67.925(67.642-68.197)
0.170(0.116-0.223) 0.284(0.218-0.350) 1.023(0.922-1.120) 2.785 (2.645-2.922) 34.050 (33.799-34.300) 61.682(61.400-61.963)
0.066(0.032-0.100) 0.132(0.081-0.178) 0.397(0.315-0.464) 0.400(0.313-0.480) 21.763(21.512-21.947) 77.269 (76.970-77.548)
0.170(0.116-0.223) 0.170(0.116-0.223) 0.454 (0.374-0.880) 0.796(0.711-0.880) 28.709 (28.453-28.946) 69.698 (69.405-69.988)
Table 4. Improvement of at Least 1 Snellen Line (Five Letters) of Visual Acuity With Refraction in the Better Eye Initial Visual Acuity
N
No. (%) Improved
No light perception Light perception Hand movements <3/60 <6/60 <6/18 <6/12 <6/6 Total
0 0 0 4 7 30 85 434 560
0(0) 0(0) 0(0) 1 (25) 3(43) 18(60) 71 (84) 244 (56) 337 (60)
The proportion of participants with visual field loss constriction to within 20 degrees of fixation (pattern deviation or equivalent) was similar for men and women when controlling for age (OR, 0.76; 95% CI, 0.33 to 1.69) but increased significantly with age when controlling for gender (chi-square = 43.55, P = .001) (Table 9). The overlap of visual impairment caused by visual acuity loss and visual field loss highlighted that four persons (0.12%) were affected by a combination of visual acuity of less than 6/60 and visual field constriction to within 20 degrees of fixation, and that three persons (0.09%) were affected by a combination of visual acuity of less than 3/60 and visual field constriction to within 10 degrees of fixation (Table 10). However, the number of people with visual field constriction to within 10 degrees of fixation was 1.75 times higher than for those with severe visual acuity loss (<3/60) and 2.9 times higher
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Table 5. Direct Age-standardized Estimates of Functional Near Vision by Gender
Near Vision
Male
Female
0.08 0.12 1.70 98.2
0.09 0.18 1.56 98.1
for those with visual field constriction to within 20 degrees of fixation compared with people with visual acuity loss measuring less than 6/60. If visual field constriction (total deviation or equivalent) were tak en rather than pattern field deviation or equivalent, these proportions would be 6.5 and 5.0, respectively. To estimate the potential magnitude of visual impairment in Australia, age-standardized rates of visual impairment based on best-corrected visual acuity and significant visual field loss results were applied to the estimated 1996 population of persons aged 40 years and older.15 These estimates indicate that approximately 110,000 adults 40 years of age or older in Australia are visually impaired because of a best-corrected visual acuity of less than 6/18 or visual field constriction to within 20 degrees of fixation (pattern deviation or equivalent), or both. Of these, approximately 30,000 adults would be eligible for the blind pension because they would have a bestcorrected visual acuity less than 6/60 or visual field constriction to within 10 degrees of fixation, or both. Approximately 12,000 people would meet the more
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333
Table 8. Frequency of Visual Field Loss in the Better Eye
Table 6. Relationship Between Near and Distance Vision at Initial Examination Near Vision (no.) Visual Acuity
N48
N20
N8
Total No.
<3/60 <6/60 <6/18 <6/12 <6/6 >6/6 Total
2 2 0 0 0 0 4
1 0 4 1 0 0 6
4 1 9 13 21 3 51
2 0 28 72 1,216 1,882 3,200
9 3 41 86 1,237 1,885 3,261
Table 7. Visual Field Defects Present in Right and Left Eyes No Type of Field Defect
Right Eye (n = 3,249)
Visual Field Loss in Both Eyes
Humphrey Field Analyzer
Constriction to within 20 degrees of fixation (total deviation) Constriction to within 20 degrees of fixation (pattern deviation) Constriction to within 10 degrees of fixation (total deviation) Constriction to within 10 degrees of fixation (pattern deviation)
32
Bjerrum Confrontation
13
20
1
(%) Left Eye (n = 3,250)
2,661 (81.90) 2,673(82.25) None Constriction <20 degrees 27 (0.83) 28 (0.86) Constriction <10 degrees 12 (0.43) 13 (0.46) Constriction <5 degrees 20(0.61) 21 (0.65) 3 (0.09) 3 (0.09) No light perception (or enucleated) 103(3.11) Paracentral arcuate or sectoral 110(3.38) Hemianopic defect 7 (0.22) 4(0.12) Quadrant defect 4 (0.12) 2 (0.06) 11 (0.34) 14(0.43) Central scotoma Nonspecific 349 (10.83) 344(10.58) Uncertain classification 40(1.23) 44(1.36) Total 3,249(100) 3,250(100)
rigid World Health Organization definition of blind ness, that is, best-corrected visual acuity of less than 3/60 or visual field constriction to within 10 degrees of fixation (pattern deviation or equivalent), or both. We estimate that by 2021, there will be a 1.90 times increase in the number of people aged 40 years or older who will be visually impaired (approximately 210,000), a twofold increase in the number of people who will be eligible for the blind pension (approxi mately 64,000), and a two-and-half-fold increase in the number of people who will meet the World Health Organization definition of blindness (approxi mately 30,000).
334
No.
Table 9. Number of People Affected and Age-specific Prevalence of Visual Field Constriction to Within 20 Degrees of Fixation (Pattern Deviation or Equivalent) in the Better Eye Age Range (yrs)
No. (%) Male
Female
>90
1 (0.28) 0(0) 6(1.40) 0(0) 2 (3.67) 0(0)
Total
9 (0.60)
0(0) 2 (0.56) 2 (0.46) 5 (2.69) 5 (5.62) 3(14.30) 17(1.00)
40-49 50-59 60-69
70-79 80-89
DISCUSSION WE SYSTEMATICALLY ASSESSED BOTH VISUAL ACUITY
and visual field loss to identify community rates of visual impairment in Australia. Other studies of vis ual impairment have presented data on visual acuity without reference to the status of peripheral vision. With regard to visual acuity, the Melbourne VIP found that the age-standardized rate of blindness (United States definition, <6/60) was 0.34%. This rate is not significantly different from other (agestandardized to the Melbourne VIP) rates reported in white cohorts from three studies conducted in urban
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Table 10. Correlation Between Visual Impairment Caused by Visual Acuity (VA) Loss and Visual Field (VF) Constriction to Within 20 Degrees of Fixation (Pattern Deviation or Equivalent) in the Better Eye No. (%) Assessment VA &6/60 VA <6/60 to a3/60 VA <3/60 VA not tested Total
VFa20 Degrees
VF <20 Degrees
VF<10 Degrees
3,219(98.41) 3 (0.09) 1 (0.03)
19(0.58)
3 (0.09) 1 (0.03) 3 (0.09)
18(0.55) 1 (0.03)
—
3 (0.09) 22 (0.67)
— 3,223 (98.53)
19(0.58)
Melb VIP BMES BDES BES (Whites) BES (Blacks) Appalachian * 0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 % Prevalence of blindness (<6/60)
* extrapolated to derive £6/60 rate
Figure 3. Prevalence and 95% confidence intervals of visual impairment from various population-based studies compared with the present study (Melbourne VIP). BMES = Blue Mountains Eye Study18 (rural Australia); BDES = Beaver Dam Eye Study16 (rural United States); BES = Baltimore Eye Study8 (urban United States); Appalachian Community7 (rural United States). and rural areas in the United States: the Beaver Dam Eye Study (0.42%), 16 the Baltimore Eye Survey (0.79% in whites),17 and the Appalachian communi ty (0.61%). 7 This rate is also similar to the rate from the Blue Mountains Eye Study in New South Wales (0.39%) 18 (Figure 3). As in the Beaver Dam Eye Study,16 the mean number of letters read was slightly higher in the left eye than in the right eye. There is not an obvious biologic explanation for this. One might expect right eyes to have slightly better visual acuity than left eyes because right eyes are more frequently the dominant eye. However, the opposite was found. It seems quite likely that this observation could be explained by
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7 (0.21)
VF Not Tested
—
3,259 (99.63) 5(0.15) 4 (0.12) 3 (0.09) 3,271 (100)
measurement bias caused by participants' learning chart letters or symbols because the right eye was always tested first. As has been shown in previous studies, the rates of visual impairment rise dramatically with age. The higher rate of visual impairment in women than men that was observed was also documented in the Beaver Dam Eye Study16 and the Blue Mountains Eye Study.18 Further investigation is needed to determine why women have higher rates of visual impairment and whether the causes of visual impairment differ by gender. This could have implications for health service delivery and effective targeting of public health messages. It is quite extraordinary that the number of people with visual impairment could be halved simply by the provision of new spectacle correction. This is particu larly unexpected in Australia, where refraction is covered by the national health insurance system, Medicare. Two relative limitations of the near vision examina tion are noted. First, near vision was measured at the participants' preferred reading distance rather than at a standardized distance, and second, the participants' presenting near vision was tested rather than their best-corrected near vision. However, this examina tion method gives more informative data about cur rent function at near. The retention of useful near vision by those with poor distance vision is notewor thy and of great functional importance. Discordance between near and distance vision was also demon strated by Salive and associates.19 The Humphrey Field Analyzer is widely used and may produce threshold visual fields that are more
VISUAL IMPAIRMENT IN AUSTRALIA
335
reliable than those produced by other perimeters.20,21 However, it is not without its limitations. Because of test duration and participant fatigue, not everyone can be examined. This is especially a problem in those who are blind or elderly or who have mental or physical constraints. Of interest are the four persons (18%) who had visual acuities less than 6/12 and who could not complete the visual field assessment. The inability to test all able subjects is a significant factor in a study such as this, in which the numerator may be very small despite the large number of people in the denominator. Visual field constriction to within 20 degrees of fixation is considered by some to cause severe visual impairment.22 In the present study, there were nearly three times as many people with visual impairment caused by visual field constriction to within 20 degrees of fixation (pattern deviation or equivalent) compared with visual acuity loss (<6/60). These results highlight the potential magnitude of signifi cant visual field loss in the community, which we believe has been unrecognized in the past. Significant visual field loss is classified by the World Health Organization as constriction of the visual field to within 10 degrees of fixation. This is incorporated into the level 3 or "blindness" category and is comparable to a visual acuity of less than 3/60.10 This classification is based on the results of a Bjerrum screen, a simple apparatus for standardized perimetry that was used primarily to test the central 30-degree field. A reassessment of the World Health Organiza tion classification system to incorporate constriction of the visual field to within 20 degrees of fixation in level 2, the "low vision" category of visual impairment comparable to visual acuity of less than 6/60, is warranted. This would better recognize the frequency and importance of visual impairment caused by visual field loss. Visual impairment is likely to double by 2021 because of the aging population. Although these figures are a conservative estimate because they do not account for people resident in nursing homes, where there is a far greater prevalence of visual impairment in the age group older than 60 years,16,17 they do highlight the potential magnitude of the problem. Public health programs such as screening, intervention, and education programs need to be established to prevent the likely increase in future
336
cases of vision loss and to provide for adequate health service allocation. In conclusion, although the overall rate of visual impairment was low, it could be avoided by simple refraction in a surprisingly large proportion of those with visual impairment. There were nearly three times more people with visual impairment caused by visual field loss compared with visual acuity loss. These results highlight the need to target blindness prevention programs to the aging population, with a special emphasis on women. ACKNOWLEDGMENTS
The authors wish to acknowledge the support of Carl Zeiss, Melbourne, Australia, for providing the Hum phrey Field Analyzer equipment used in this study.
REFERENCES 1. Livingston PM, Carson CA, Stanislavsky YL, Lee SE, Guest CS, Taylor HR. Methods for a population-based study: the Melbourne Visual Impairment Project. Ophthalmic Epidemi ol 1994;1:139-148. 2. Bruce IW, McKennell AC, Walker EC. Blind and partially sighted adults in Britain: the Royal National Institute for the Blind Survey. Volume 1. London: HMSO, 1991. 3. Walker EC, Tobin MJ, McKennell AC. Blind and partially sighted children in Britain: the Royal National Institute for the Blind Survey. Volume 2. London: HMSO, 1992. 4. Robinson R, Deutsch J, Jones HS, et al. Unrecognised and unregistered visual impairment. Br J Ophthalmol 1994; 78:736-740. 5. Report of the National Trachoma and Eye Health Program. Sydney: Royal Australian College of Ophthalmologists, 1980. 6. Taylor HR. Prevalence and causes of blindness in Australian aborigines. Med J Aust 1980;1:71-76. 7. Dana MR, Tielsch JM, Enger C, Joyce E, Santoli JM, Taylor HR. Visual impairment in a rural Appalachian community. JAMA 1990;264:2400-2405. 8. Sommer A, Tielsch JM, Katz J, et al. Racial differences in the cause-specific prevalence of blindness in east Baltimore. N EnglJ Med 1991;325:1412-1417. 9. Thylefors B, Negrel AD, Pararajesegaram R, Dadzie KY. Global data in blindness. Bull WHO 1995;73:115-121. 10. World Health Organization. Guidelines for programs for the prevention of blindness. Geneva: World Health Organiza tion, 1979. 11. Castles I. How Australia takes a census. Canberra: Austra lian Bureau of Statistics, 1991. 12. Livingston PM, Guest CS, Bateman A, Woodcock N, Taylor HR. Cost effectiveness of recruitment methods in a popula tion-based epidemiological study. Aust J Public Health 1994;18:314-318. 13. Fleiss JL. Statistical methods for rates and proportions. New York: John Wiley and Sons, 1981.
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14. Livingston PM, Lee SE, McCarty CA, Taylor HR. A comparison of participants with non-participants in a popu lation-based epidemiologic study: the Melbourne Visual Impairment Project. Ophthalmic Epidemiol. Forthcoming. 15. Australian Bureau of Statistics. Projections of the populations of Australia, states and territories, 1993 to 2041- Canberra: Australian Bureau of Statistics, 1994. 16. Klein R, Klein BEK, Linton KLP, De Mets DL. The Beaver Dam Eye Study: visual acuity. Ophthalmology 1991;98: 1310-1315. 17. Tielsch JM, Sommer A, Witt K, Katz J, Royall RM, Baltimore Eye Survey Research Group. Blindness and visual impair ment in an American urban population: the Baltimore Eye Survey. Arch Ophthalmol 1990;108:286-290. 18. Attebo K, Mitchell P, Smith W. Visual acuity and the causes
19.
20. 21. 22.
of visual loss in Australia: the Blue Mountains Eye Study. Ophthalmology 1996;103:357-364. Salive ME, Guralnik J, Christen W, Glynn RJ, Colsher P, Ostfield AM. Functional blindness and visual impair ment from three communities. Ophthalmology 1992;99: 1840-1847. Lewis RA, Johnson CA, Keltner JL, Labermeier PK. Variabil ity of quantitative automated perimetry in normal observers. Ophthalmology 1986;93:878-881. Brenton RS, Argus WA. Fluctuations on the Humphrey and Octopus perimeters. Invest Ophthalmol Vis Sci 1987;28: 767-771. Hyvarinen L. Classification of visual impairment. Chibret International Journal of Ophthalmology 1989;6:1-8.
Authors Interactive™ Comments and questions of 100 words or less regarding this article may be addressed to authors via THE JOURNAL'S Web Site on the Internet. Selected questions and comments along with the authors' statements and responses will be posted on the Authors Interactive™ Bulletin Board, a feature of THE JOURNAL'S home page. Readers may access THE JOURNAL'S home page and the Authors Interactive™ Bulletin Board at http://www.ajo.com/
V0L.123, N o . 3
VISUAL IMPAIRMENT IN AUSTRALIA
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• PURPOSE: To describe the age-specific and gen der-specific rates of blindness and visual impair ment in urban adults aged 4 0 years and older. • METHODS: A population-based sample of resi dents was recruited. Presenting and best-corrected distance visual acuities were assessed. Functional near vision was measured at each participant's pre ferred distance. Visual field examination was per formed with a Humphrey Field Analyzer (HFA); those unable to perform the field analyzer test attempted a Bjerrum screen or confrontation field. • RESULTS: The study population comprised 3,271 residents (83% of eligible) from ages 4 0 to 98 years; 54% were women. Overall, 56% of the study population wore distance correction; this was significantly lower in men but higher in the older age groups. Age-adjusted rates of blindness were 0.066% in men and 0.170% in women. Vision with current correction improved after refraction by gender and age. Direct agestandardized rates of functional near vision did not
Accepted for publication Oct 10, 1996. From the Department of Ophthalmology, University of Melbourne, Royal Victorian Eye and Ear Hospital, East Melbourne, Victoria, Austral ia. The Melbourne Visual Impairment Project is supported in part by the Victorian Health Promotion Foundation, Melbourne, Australia; the Ansell Ophthalmology Foundation, Melbourne, Australia; and the National Health and Medical Research Council, Canberra, Australia, including the Sir John Eccles Award (to Prof Taylor). Reprint requests to Hugh R. Taylor, MD, FRACO, University of Melbourne, Department of Ophthalmology, Royal Victorian Eye and Ear Hospital, 32 Gisborne St, East Melbourne, 3002 Victoria, Australia; fax: (613) 9662 3859; e-mail: [email protected]
328
vary significantly by gender. Forty-six people had significant visual field loss in their better eye. The proportion of participants with constriction of the visual field to within 2 0 degrees of fixation was similar for men and women when controlled for age (odds ratio, 0.81; 95% confidence interval, 0.44 to 1.49) but increased significantly with age con trolled for gender. Visual field abnormalities were detected in 5 4 8 right eyes (17%) and 533 left eyes (16%). • CONCLUSIONS: Although overall rates of blind ness because of visual acuity loss were relatively low, nearly three times more people had visual impairment because of visual field loss than visual acuity loss. These results highlight the need to target blindness prevention programs to the aging population, with a special emphasis on women.
T
O PLAN APPROPRIATE PROGRAMS OR INTERVEN-
tions aimed at the prevention or cure of avoidable blindness, it is necessary to know the magnitude, distribution, and causes of visual impair ment and blindness. In addition, knowledge of the functional impact of visual loss and the availability and utilization of existing health services is important to prioritize potential interventions. To provide this information was the broad aim of the Melbourne Visual Impairment Project (Melbourne VIP), a popu lation-based survey of visual impairment and eye disease.1 Data on visual impairment or blindness are avail able from many different sources, including registries
© AMERICAN JOURNAL OF OPHTHALMOLOGY 1997;123:328-337
MARCH 1997
of the blind,2'4 that have differing degrees of rigor.5'8 Much useful information can be extracted from data obtained by these differing methods, such as the worldwide figures on blindness prepared by the World Health Organization. 9 However, for planning purpos es, for the evaluation of health service utilization, or for risk-factor analysis, much more specific and popu lation-based data are desired. In almost every report on visual impairment or blindness, vision is assessed only in terms of visual acuity, even though constriction or loss of the visual field is also recognized as causing severe visual impair ment.10 In this report, we present the findings on the age-specific and gender-specific prevalence of visual impairment and blindness as assessed by both visual acuity and visual field testing in a population-based sample of adults living in Melbourne, Australia. Near vision of all participants was also tested.
METHODS THE MELBOURNE VIP WAS A POPULATION-BASED SURVEY
of noninstitutionalized permanent residents of the Melbourne metropolitan area. T h e detailed method ology has been reported elsewhere.1 A manual of operations was prepared to standardize procedures. In brief, nine pairs of adjacent census collector districts were randomly selected from the Melbourne Statisti cal Division.11 A door-to-door household census was taken to identify all eligible persons aged 40 years or older in the calendar year of examination who had lived at their address for 6 months or more. Information about household characteristics and basic demographic data were collected during a brief interview conducted at the household. Eligible indi viduals were then invited to a local examination site for a more detailed interview and an ophthalmic examination.1,12 Participants were asked to bring to the examination site all corrective spectacles or lenses currently being used. The power of these lenses was measured. Distance visual acuity was measured with a logMAR letter chart set at 4 m under standardized illumina tion.1 A n E chart was used for illiterate and n o n English-speaking participants. Presenting visual acuity was measured with the participant's current distance correction; when distance correction was not VOL.123, No. 3
used, the unaided distant visual acuity was measured. When the presenting acuity was less than 53 letters (that is, <6/6), further refraction was undertaken. Best-corrected visual acuity was determined after objective autorefraction and subjective refinement. Functional near vision was measured with a log MAR word reading card designed for use at 25 cm, although the test was performed at the participant's preferred reading distance. Near correction was worn for testing if the participant normally used it. Illiterate or non-English-speaking participants were asked to spell the letters on the near chart or were tested with a near E chart. Visual field examination was performed with a Humphrey Field Analyzer (HFA), (Humphrey Instru ments Inc, San Leandro, California), on all capable participants. In the first 6 months of field work, the visual field was first examined with a screening 80-point suprathreshold test. If the results of this test were considered abnormal in either eye, a 24-2 (FastPac) threshold test was then performed in both eyes. The 80-point test was considered abnormal when two adjacent points or any three points were missed in the central 20 degrees. Although this combined strategy of visual field testing provided some time saving in young individuals, it was logistically difficult; subsequently, 24-2 FastPac tests were routinely performed on both eyes. Where possible, this test was repeated when the results of the first test were unreliable or diagnostically equivocal. Participants not capable of performing the field analyzer test and those examined at their homes were tested with a Bjerrum screen at a 1-m distance with 2/1,000 and 10/1,000 objects. Visual fields were assessed by confrontation for those who were unable to perform a Bjerrum screen because of either poor vision or mental or physical difficulties. The shape of a field analyzer defect was analyzed using the total deviation and pattern deviation plots. Points with a probability below 1% were considered abnormal. Abnormal fields were classified as constric tion within 5 degrees, 10 degrees, or 20 degrees from the fixation point, hemianopia, quadrantic defects, central scotomata of 3-degree, 6-degree, 9-degree, or more than 9-degree radius, paracentral defects in four grades of severity, and nonspecific defects. Results of the Bjerrum or confrontation tests were classified similarly.
VISUAL IMPAIRMENT IN AUSTRALIA
329
Visual impairment was defined as a best-corrected visual acuity score of less than 6/18 or visual field constriction to within 20 degrees of fixation, or both. Legal blindness (Australian definition) was defined as a best-corrected visual acuity score of less than 6/60 or visual field constriction to within 10 degrees of fixation, or both, and blindness by the World Health Organization was defined as a best-corrected visual acuity score of less than 3/60 or visual field constric tion to within 10 degrees of fixation, or both. Data were collected either by direct computer entry using a questionnaire programmed in Paradox (Borland International, Scotts Valley, California) or on self-coding forms. Open-ended responses were coded at a later time. Data were entered into a computer using double data entry with reconciliation of any inconsistencies. Data range and consistency checks were performed on the whole data set. Chi-square analyses were employed to assess signifi cant relationships among categoric variables. Analysis of covariance and multiple linear regression were used for continuous variables. The following variables were transformed before inclusion in parametric analyses: number of letters read (cubic transformation), num ber of letters improved (square root transformation), and number of words read (square transformation). Poisson regression analysis was used to calculate the 95% confidence intervals (Cls) around the agestandardized rates of blindness. Prevalence rates were directly age-standardized using the entire study popu lation.13 Confidence intervals around the agestandardized rates of blindness (United States defini tion, <6/60) were calculated using the standard error of the directly standardized rate. Age-standardized comparisons were extrapolated to derive the blindness rate ( ^ 6/60) for the Appalachian community. 7 Data analyses were performed using SAS software (SAS Institute, Cary, North Carolina).
RESULTS A TOTAL OF 4,273 HOUSES WERE IDENTIFIED IN THE NINE
study clusters.14 Of these, 4,033 (94%) provided information and 2,391 (59%) had eligible residents. In the eligible houses were 3,946 eligible people, of whom 3,271 (83%) agreed to participate and 3,266 (83%) had complete vision assessment. N o n -
Age
Males
Females
90+ 80-89 70-79 60-69 50-59 40-49
::^;::,;fj|-;"f • -;:.; -]..:.'i; ■;, •;.'. :;;.-.^L;:<;C.
600 500 400 300 200 100 0
100 200 300 400 500 600
Number of participants Figure 1. The sample population by age and gender, with those examined shown in the shaded bars.
English-speaking persons were significantly less likely to participate (odds ratio [OR], 0.61; 95% CI, 0.48 to 0.77), but there were no other significant differences between participants and nonparticipants. 14 The par ticipants ranged in age from 40 to 98 years, and 54% were women (Figure 1). Three (0.12%) of the 3,271 participants did not have the distance, functional near vision, and visual field examinations (two refused and one person was affected by Alzheimer's disease). A n additional seven persons (0.21%) did not complete the near visual acuity assessment (six refused and one was blind). A further 19 participants (0.58%) were not tested for visual fields (nine refused; seven were not tested because of mental constraints; and three were not tested because of language disabilities). Fifteen of the 19 had normal or near-normal vision; two had visual acuities of less than 6/12; one was visually impaired (<6/18); and one was legally blind (Australian definition, <6/60; U.S. definition, <20/200). The proportion of participants wearing distance correction for the examination was significantly lower for men when controlling for age (OR = 0.78; 95% CI, 0.67 to 0.91) and increased significantly with age when controlling for gender (chi-square = 2.47; P = .0001) (Figure 2). Overall, 56% of the participants had distance correction. The mean (±SD) number of letters read (right eye) ranged from 57.4 ± 7.7 among men and 55.8 ± 9.2 among women in the 40- to 49-year-old age group to 17.7 ± 15.4 among men and 34.4 ± 17.9 among
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Table 1. Distribution of initial Visual Acuity in Best Eye by Age and Gender No. (%) Age (yrs)
Gender
n
<3/60
40-49
Male Female Male Female Male Female Male Female Male Female Male Female
357 466 446 532 426 436 221 223 56 89 3. 13 3,268
0(0) 0(0) 0 (0.2) 0(0) 1 (0.2) 0(0) 0(0) 0(0)
50-59 60-69 70-79 80-89 &90
—
Total
100
1 (1-8) 1(1.1) 0(0) 2(15.4) 5(0.15)
<6/60 to £3/60
<6/18
<6/12
to 2:6/60
tO 2:6/18
<6/6 to 2:6/12
0(0) 0(0) 1 (0.2) 0(0) 0(0) 1 (0.2) 0 (0.0) 3(1.3) 0(0) 1(1.1) 1 (33.3) 0(0) 7(0.21)
0(0) 1 (0.2) 0(0) 0(0) 4 (0.9) 4 (0.9) 3(1.4) 5 (2.2) 4 (7.1) 8 (9.0) 1 (33.3) 0(0) 30 (0.92)
2 (0.6) 2 (0.4) 9 (2.0) 5 (0.9) 10(2.3) 10(2.3) 11 (5.0) 13 (5.8) 3 (5.4) 16(18.0) 1 (33.3) 3 (25.0) 85 (2.60)
52(15) 87(18.7) 90 (20.2) 141 (26.5) 145(34.0) 182(41.7) 109(49.3) 132(59.2) 36 (64.3) 50 (56.2) 0(0) 7 (58.3) 1,031 (31.56)
%
J""
"
^
80
60
/
40
20
. '^Z' '^:;;'""" ""' "" * ""' * -jr
*»* /
40.49
Near:
Male Female Distance: Male Female 50-69
60-69
70-79
80-89
« : ■■ *mm ^^m
90+
Age Group
Figure 2. Age-specific and gender-specific frequency of the use of distance correction and near correction.
women in the group aged 90 years or more. The mean number of letters read after refraction, when neces sary (right eye), ranged from 58.3 ± 6.5 among men and 57.0 ± 7.1 among women in the 40- to 49-yearold age group to 24.0 ± 16.1 among men and 36.4 ± 18.5 among women in the group aged 90 years or more. Best correction was higher in men than in women after controlling for age (presenting, F = 52.8, P = .0001; best-corrected, F = 85.7, P = .0001) and decreased with age after controlling for gender (pre senting, F = 1,005, P = .0001; best-corrected, F = 1,000, P = .0001). There was a small but statistically significant difference in the mean number of letters Vot. 123,
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2:6/6
303 (85) 376 (80.7) 346 (77.6) 386 (72.7) 266 (62.4) 239 (54.8) 98 (44.3) 70(31.4) 12(21.4) 13(14.6) 0(0) 1 (8.3) 2,110(64.60)
read for both presenting and best-corrected acuity between right and left eyes after controlling for age and gender (F = 4.12, P = .04 and F = 8.68, P = .003, respectively). On presentation, five (0.15%) of the participants were blind by the World Health Organization defini tion for visual acuity (<3/60) and 42 (1.28%) had visual impairment (<6/18) (Table 1). After correc tion, when that was possible, four (0.12%) remained blind and 23 (0.70%) were visually impaired (Table 2). The direct age-standardized rates of both present ing and best-corrected visual impairment were signifi cantly higher for women than men (Table 3). The age-standardized blindness rate (<3/60) with best correction was 0.066% (95% CI, 0.032 to 0.100) in men and 0.170% (95% CI, 0.116 to 0.220) in women. Improvement in the number of letters read with best correction also varied significantly by gender after controlling for age (F = 19.3, P = .0001) and by age after controlling for gender (F = 44.3, P = .0001). The mean number of letters improved in the best eye of the men was 8.4 (7.29), whereas the mean number in women was 7.0 (6.39). The majority of both the right eyes (71%) and the left eyes (70%) that had presenting visual acuity of less than 6/18 improved to at least 6/12 (driving vision) with refraction. Overall, 60% of people improved by at least 1 line in their better eye (Table 4).
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Table 2. Distribution of Best-corrected Visual Acuity in Best Eye by Age and Gender No. (%) Age (yrs)
40-49
Gender
Male Female
50-59
Male Female
60-69
Male Female
70-79
Male Female
80-89
Male Female
==90
Male Female
Total
—
n
357 466 446 532 426 436 221 223 56 89 3 13 3,268
<3/60
<6/60 to ==3/60
<6/12
to ==6/60
toa6/18
<6/6 to ==6/12
26/6
321 (90.2)
0(0)
0(0)
0(0)
1 (0.3)
35 (9.8)
0(0)
0(0)
0(0)
65(13.9)
401 (86.1)
0(0)
0(0) 1 (0.2)
0(0)
1 (0.2)
59 (13.2)
385 (86.3)
0(0)
0(0)
0(0)
0(0)
100(18.8)
432(81.4)
1 (0.2)
0(0)
1 (0.2)
1 (0.2)
106 (24.9)
317(74.4)
0(0)
1 (0.2)
2 (0.5)
0(0)
146(33.5)
287 (65.8)
0(0)
0(0)
3(1.4)
1 (0.5)
91 (41.2)
126 (57.0)
0(0)
2 (0.9)
2 (0.9)
3(1.3)
126(56.5)
90 (40.4)
0(0)
1 (1.8)
1 (1.8)
1(1.8)
36 (64.3)
17(30.4)
1(1.1)
0(0)
4 (4.5)
9(10.1)
60 (67.4)
15(16.9)
0(0)
0(0)
1 (33.3)
1 (33.3)
1 (33.3)
2(15.4)
0(0)
0(0)
2(16.7)
8 (66.7)
4 (0.12)
5(0.15)
Twenty-one persons required home visits because of mobility restrictions; the vision of these persons was significantly worse than the vision of those attending the test site. O n presentation, the mean number of letters read by the right eyes of the 21 persons with a home visit was 36.5, compared with 52.9 in those who attended the test site (unpaired t test, - 3 . 6 8 , P = .002). Overall, 87% of women and 85% of men wore eyeglasses for near-vision assessment (OR, 0.71; 95% CI, 0.57 to 0.90; P = .006, controlling for age), and the proportion of people wearing eyeglasses increased with age (chi-square = 3.61, P = .0001, controlling for gender) (Figure 2). The mean number of words ranged from 68.0 ± 7.7 among men and 67.9 ± 7.8 among women in the 40- to 49-year-old age group to 42.7 ± 14-6 among men and 50.3 ± 20.6 among women in the group aged 90 years or more. The mean number of words read was higher for men after controlling for age (F = 7.4, P = .007) and decreased significantly with age after controlling for gender (F = 40.9, P = .0001). Direct age-standardized rates of near vision did not vary significantly by gender (chi-square = 3.3, P = .19), with more than 98% of the population able to read N8 (Table 5). Near and distance vision were significantly related. Controlling for age, the correlation of the number of letters read on the distance chart on presentation was
332
<6/18
14 (0.43)
20(0.61)
833 (25.49)
0(0) 1 (8.3) 2,392 (73.23)
significantly related to the number of words read on the near test (Pearson's r = .44, P — .0001) (Table 6). The number of people who maintained very good functional near vision despite distance visual impair ment was striking. Nearly all right eyes (n = 3,249 [99.33%]) and left eyes (n = 3,250 [99.36%]) had an assessment of visual fields; 2,919 (89%) were field analyzer 24-2 tests, 255 (8%) were field analyzer 80-point tests, 61 (2%) were Bjerrum tests, and 14 (0.4%) were by confrontation. Visual field abnormalities were detect ed in 548 right eyes (17%) and 533 left eyes (16%); the most common identifiable defect was a paracentral arcuate or sectoral field (Table 7). Altogether, 45 persons (1.38%) had visual field constriction to within 20 degrees of fixation (total deviation or equivalent) in the better eye (Table 8). Similarly, 26 persons (0.80%) had visual field con striction to within 10 degrees of fixation (total deviation or equivalent) in the better eye. Taking the more stringent and possibly more logical criterion of pattern loss on field analyzer testing or field loss detected by Bjerrum or confrontation testing, 26 (0.80%) had visual field constriction to within 20 degrees of fixation (pattern deviation or equivalent) and seven (0.21%) had visual field constriction to within 10 degrees of fixation (pattern deviation or equivalent).
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Table 3. Direct Age-standardized Prevalence of Initial and Best-corrected Visual Acuity in the Better Eye by Gender Prevalence (%) (95% Confidence Interval) Initial
Best-corrected
Visual Acuity
Men
Women
Men
Women
<3/60 <6/60 to >3/60 <6/18toa6/60 <6/12to >6/18 <6/6 to z=6/12 ^6/6
0.132 (0.081-0.178) 0.132(0.081-0.178) 0.795 (0.706-0.884) 2.385(2.258-2.511) 28.628 (28.397-28.858) 67.925(67.642-68.197)
0.170(0.116-0.223) 0.284(0.218-0.350) 1.023(0.922-1.120) 2.785 (2.645-2.922) 34.050 (33.799-34.300) 61.682(61.400-61.963)
0.066(0.032-0.100) 0.132(0.081-0.178) 0.397(0.315-0.464) 0.400(0.313-0.480) 21.763(21.512-21.947) 77.269 (76.970-77.548)
0.170(0.116-0.223) 0.170(0.116-0.223) 0.454 (0.374-0.880) 0.796(0.711-0.880) 28.709 (28.453-28.946) 69.698 (69.405-69.988)
Table 4. Improvement of at Least 1 Snellen Line (Five Letters) of Visual Acuity With Refraction in the Better Eye Initial Visual Acuity
N
No. (%) Improved
No light perception Light perception Hand movements <3/60 <6/60 <6/18 <6/12 <6/6 Total
0 0 0 4 7 30 85 434 560
0(0) 0(0) 0(0) 1 (25) 3(43) 18(60) 71 (84) 244 (56) 337 (60)
The proportion of participants with visual field loss constriction to within 20 degrees of fixation (pattern deviation or equivalent) was similar for men and women when controlling for age (OR, 0.76; 95% CI, 0.33 to 1.69) but increased significantly with age when controlling for gender (chi-square = 43.55, P = .001) (Table 9). The overlap of visual impairment caused by visual acuity loss and visual field loss highlighted that four persons (0.12%) were affected by a combination of visual acuity of less than 6/60 and visual field constriction to within 20 degrees of fixation, and that three persons (0.09%) were affected by a combination of visual acuity of less than 3/60 and visual field constriction to within 10 degrees of fixation (Table 10). However, the number of people with visual field constriction to within 10 degrees of fixation was 1.75 times higher than for those with severe visual acuity loss (<3/60) and 2.9 times higher
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Table 5. Direct Age-standardized Estimates of Functional Near Vision by Gender
Near Vision
Male
Female
0.08 0.12 1.70 98.2
0.09 0.18 1.56 98.1
for those with visual field constriction to within 20 degrees of fixation compared with people with visual acuity loss measuring less than 6/60. If visual field constriction (total deviation or equivalent) were tak en rather than pattern field deviation or equivalent, these proportions would be 6.5 and 5.0, respectively. To estimate the potential magnitude of visual impairment in Australia, age-standardized rates of visual impairment based on best-corrected visual acuity and significant visual field loss results were applied to the estimated 1996 population of persons aged 40 years and older.15 These estimates indicate that approximately 110,000 adults 40 years of age or older in Australia are visually impaired because of a best-corrected visual acuity of less than 6/18 or visual field constriction to within 20 degrees of fixation (pattern deviation or equivalent), or both. Of these, approximately 30,000 adults would be eligible for the blind pension because they would have a bestcorrected visual acuity less than 6/60 or visual field constriction to within 10 degrees of fixation, or both. Approximately 12,000 people would meet the more
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Table 8. Frequency of Visual Field Loss in the Better Eye
Table 6. Relationship Between Near and Distance Vision at Initial Examination Near Vision (no.) Visual Acuity
N48
N20
N8
Total No.
<3/60 <6/60 <6/18 <6/12 <6/6 >6/6 Total
2 2 0 0 0 0 4
1 0 4 1 0 0 6
4 1 9 13 21 3 51
2 0 28 72 1,216 1,882 3,200
9 3 41 86 1,237 1,885 3,261
Table 7. Visual Field Defects Present in Right and Left Eyes No Type of Field Defect
Right Eye (n = 3,249)
Visual Field Loss in Both Eyes
Humphrey Field Analyzer
Constriction to within 20 degrees of fixation (total deviation) Constriction to within 20 degrees of fixation (pattern deviation) Constriction to within 10 degrees of fixation (total deviation) Constriction to within 10 degrees of fixation (pattern deviation)
32
Bjerrum Confrontation
13
20
1
(%) Left Eye (n = 3,250)
2,661 (81.90) 2,673(82.25) None Constriction <20 degrees 27 (0.83) 28 (0.86) Constriction <10 degrees 12 (0.43) 13 (0.46) Constriction <5 degrees 20(0.61) 21 (0.65) 3 (0.09) 3 (0.09) No light perception (or enucleated) 103(3.11) Paracentral arcuate or sectoral 110(3.38) Hemianopic defect 7 (0.22) 4(0.12) Quadrant defect 4 (0.12) 2 (0.06) 11 (0.34) 14(0.43) Central scotoma Nonspecific 349 (10.83) 344(10.58) Uncertain classification 40(1.23) 44(1.36) Total 3,249(100) 3,250(100)
rigid World Health Organization definition of blind ness, that is, best-corrected visual acuity of less than 3/60 or visual field constriction to within 10 degrees of fixation (pattern deviation or equivalent), or both. We estimate that by 2021, there will be a 1.90 times increase in the number of people aged 40 years or older who will be visually impaired (approximately 210,000), a twofold increase in the number of people who will be eligible for the blind pension (approxi mately 64,000), and a two-and-half-fold increase in the number of people who will meet the World Health Organization definition of blindness (approxi mately 30,000).
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No.
Table 9. Number of People Affected and Age-specific Prevalence of Visual Field Constriction to Within 20 Degrees of Fixation (Pattern Deviation or Equivalent) in the Better Eye Age Range (yrs)
No. (%) Male
Female
>90
1 (0.28) 0(0) 6(1.40) 0(0) 2 (3.67) 0(0)
Total
9 (0.60)
0(0) 2 (0.56) 2 (0.46) 5 (2.69) 5 (5.62) 3(14.30) 17(1.00)
40-49 50-59 60-69
70-79 80-89
DISCUSSION WE SYSTEMATICALLY ASSESSED BOTH VISUAL ACUITY
and visual field loss to identify community rates of visual impairment in Australia. Other studies of vis ual impairment have presented data on visual acuity without reference to the status of peripheral vision. With regard to visual acuity, the Melbourne VIP found that the age-standardized rate of blindness (United States definition, <6/60) was 0.34%. This rate is not significantly different from other (agestandardized to the Melbourne VIP) rates reported in white cohorts from three studies conducted in urban
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Table 10. Correlation Between Visual Impairment Caused by Visual Acuity (VA) Loss and Visual Field (VF) Constriction to Within 20 Degrees of Fixation (Pattern Deviation or Equivalent) in the Better Eye No. (%) Assessment VA &6/60 VA <6/60 to a3/60 VA <3/60 VA not tested Total
VFa20 Degrees
VF <20 Degrees
VF<10 Degrees
3,219(98.41) 3 (0.09) 1 (0.03)
19(0.58)
3 (0.09) 1 (0.03) 3 (0.09)
18(0.55) 1 (0.03)
—
3 (0.09) 22 (0.67)
— 3,223 (98.53)
19(0.58)
Melb VIP BMES BDES BES (Whites) BES (Blacks) Appalachian * 0
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 % Prevalence of blindness (<6/60)
* extrapolated to derive £6/60 rate
Figure 3. Prevalence and 95% confidence intervals of visual impairment from various population-based studies compared with the present study (Melbourne VIP). BMES = Blue Mountains Eye Study18 (rural Australia); BDES = Beaver Dam Eye Study16 (rural United States); BES = Baltimore Eye Study8 (urban United States); Appalachian Community7 (rural United States). and rural areas in the United States: the Beaver Dam Eye Study (0.42%), 16 the Baltimore Eye Survey (0.79% in whites),17 and the Appalachian communi ty (0.61%). 7 This rate is also similar to the rate from the Blue Mountains Eye Study in New South Wales (0.39%) 18 (Figure 3). As in the Beaver Dam Eye Study,16 the mean number of letters read was slightly higher in the left eye than in the right eye. There is not an obvious biologic explanation for this. One might expect right eyes to have slightly better visual acuity than left eyes because right eyes are more frequently the dominant eye. However, the opposite was found. It seems quite likely that this observation could be explained by
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7 (0.21)
VF Not Tested
—
3,259 (99.63) 5(0.15) 4 (0.12) 3 (0.09) 3,271 (100)
measurement bias caused by participants' learning chart letters or symbols because the right eye was always tested first. As has been shown in previous studies, the rates of visual impairment rise dramatically with age. The higher rate of visual impairment in women than men that was observed was also documented in the Beaver Dam Eye Study16 and the Blue Mountains Eye Study.18 Further investigation is needed to determine why women have higher rates of visual impairment and whether the causes of visual impairment differ by gender. This could have implications for health service delivery and effective targeting of public health messages. It is quite extraordinary that the number of people with visual impairment could be halved simply by the provision of new spectacle correction. This is particu larly unexpected in Australia, where refraction is covered by the national health insurance system, Medicare. Two relative limitations of the near vision examina tion are noted. First, near vision was measured at the participants' preferred reading distance rather than at a standardized distance, and second, the participants' presenting near vision was tested rather than their best-corrected near vision. However, this examina tion method gives more informative data about cur rent function at near. The retention of useful near vision by those with poor distance vision is notewor thy and of great functional importance. Discordance between near and distance vision was also demon strated by Salive and associates.19 The Humphrey Field Analyzer is widely used and may produce threshold visual fields that are more
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reliable than those produced by other perimeters.20,21 However, it is not without its limitations. Because of test duration and participant fatigue, not everyone can be examined. This is especially a problem in those who are blind or elderly or who have mental or physical constraints. Of interest are the four persons (18%) who had visual acuities less than 6/12 and who could not complete the visual field assessment. The inability to test all able subjects is a significant factor in a study such as this, in which the numerator may be very small despite the large number of people in the denominator. Visual field constriction to within 20 degrees of fixation is considered by some to cause severe visual impairment.22 In the present study, there were nearly three times as many people with visual impairment caused by visual field constriction to within 20 degrees of fixation (pattern deviation or equivalent) compared with visual acuity loss (<6/60). These results highlight the potential magnitude of signifi cant visual field loss in the community, which we believe has been unrecognized in the past. Significant visual field loss is classified by the World Health Organization as constriction of the visual field to within 10 degrees of fixation. This is incorporated into the level 3 or "blindness" category and is comparable to a visual acuity of less than 3/60.10 This classification is based on the results of a Bjerrum screen, a simple apparatus for standardized perimetry that was used primarily to test the central 30-degree field. A reassessment of the World Health Organiza tion classification system to incorporate constriction of the visual field to within 20 degrees of fixation in level 2, the "low vision" category of visual impairment comparable to visual acuity of less than 6/60, is warranted. This would better recognize the frequency and importance of visual impairment caused by visual field loss. Visual impairment is likely to double by 2021 because of the aging population. Although these figures are a conservative estimate because they do not account for people resident in nursing homes, where there is a far greater prevalence of visual impairment in the age group older than 60 years,16,17 they do highlight the potential magnitude of the problem. Public health programs such as screening, intervention, and education programs need to be established to prevent the likely increase in future
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cases of vision loss and to provide for adequate health service allocation. In conclusion, although the overall rate of visual impairment was low, it could be avoided by simple refraction in a surprisingly large proportion of those with visual impairment. There were nearly three times more people with visual impairment caused by visual field loss compared with visual acuity loss. These results highlight the need to target blindness prevention programs to the aging population, with a special emphasis on women. ACKNOWLEDGMENTS
The authors wish to acknowledge the support of Carl Zeiss, Melbourne, Australia, for providing the Hum phrey Field Analyzer equipment used in this study.
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