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Visual Acuity Thresholds for Cataract Surgery and the Changing Australian Population
Hugh R. Taylor, AC, MD, FRANZCO;
Hien T. V. Vu, PhD;
Jill E. Keeffe, PhD
Arch Ophthalmol. 2006;124:1750-1753.
ABSTRACT
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Objective To examine the relation between visual acuity thresholds for cataract surgery for the changing Australian population.
Methods Population-based 5-year incidence data for 2594 people were used to calculate age-specific prevalence of incident cataract and existing cataract backlog. The cataract surgery rates (CSRs) per million people were estimated for different acuity thresholds for the Australian population at different times.
Results In 2005, after adjusting by age, the estimated CSRs for Australia were 1470, 3110, 5080, and 6440 for visual acuities of less than 6/60, less than 6/18, less than 6/12, and less than 6/9, respectively, whereas the actual CSR was 9000. Although the acuity threshold had the greatest impact, the CSR for less than 6/12 increased 3-fold from 2300 in 1950 to 7210 in 2020 because of demographic changes.
Conclusions Demographic changes, with an increasing proportion of older people, will continue to increase the need for cataract surgery, as will a reduction in the visual impairment threshold. Plans for the future for eye care services should take these factors into account.
INTRODUCTION
Restoring sight to the blind has biblical roots and cataract surgery of one type or another has been practiced for millennia. However, the introduction of modern small-incision cataract surgery with intraocular lens implantation has revolutionized cataract surgery and dramatically changed its indications and the frequency with which it is performed.
The amount of cataract surgery performed is usually termed the cataract surgery rate (CSR), which is the number of cataract operations performed per million people per year. The World Health Organization has set a CSR of 3000 as being the minimum necessary to eliminate cataract blindness.1 However, this rate is greatly exceeded in many developed countries, including Australia.2 The higher CSR in developed countries is explained in large part by the reduced threshold of visual impairment used as an indication for surgery. As the results of modern cataract surgery have improved, the threshold level of visual acuity has dropped progressively from legal blindness (<6/60) some 30 or 40 years ago to driving vision (<6/12) or even "on change" (<6/6).
Although the change in visual threshold has a major impact on the CSR, we were interested in exploring the impact that the changing demographic structure may also have. The demographic structure, and especially the proportion of older people, is changing quite markedly with increased life expectancy and a decreased birth rate. If we assume that there is no significant change in the age-specific rates of cataract, then the change in the proportion of older people in a population will increase the need for cataract surgery and so alter the CSR.
METHODS
The Melbourne Visual Impairment Project is a population-based study of residents of Victoria, Australia, aged 40 years or older. The detailed methods have previously been published.3 Briefly, in 1992-1994, a door-to-door household census was conducted in 9 randomly selected pairs of adjacent census collector districts in urban Melbourne, Australia, to identify eligible residents who had lived in their homes for at least 6 months and were 40 years or older in the year of examination. A 5-year follow-up examination was conducted in 1997-1999.4
In both surveys, participants underwent a standard ophthalmic examination at a local examination center and completed a standardized questionnaire covering sociodemographic characteristics, symptoms of eye disease, medical history, and medication use. Home visits were conducted for participants who were unable to attend the local examination center. The Human Research and Ethics Committee of the Royal Victorian Eye and Ear Hospital approved the protocol, and all participants gave written informed consent.
The ophthalmic examination included initial visual acuity and photography of the lens; best-corrected visual acuity was measured with an objective refraction refined by subjective refraction for those with a visual acuity less than 6/6. The cortical and posterior subcapsular cataract opacities were photographed with a retroillumination camera, and a slitlamp was used to photograph nuclear opacities. Lens opacities were graded clinically and from photographs by the Wilmer cataract photograph grading system, and clinical grades were used only if corresponding photographic grades were missing.5-6 Two observers graded photographs separately, and an independent reviewer adjudicated discrepancies. Cortical cataract was defined as a cortical opacity of 4/16 or greater, posterior subcapsular cataract was defined as opacity of 1 mm2 or greater, and nuclear cataract was defined as opacity of Wilmer standard 2.03 or greater.
Participants were divided into 5-year age groups except for those 85 years or older who were all included in the 85 years and older age group. The age-specific prevalence of incident cataract and backlog cataract was derived using the presence of cataract as defined earlier and the visual acuity thresholds of less than 6/6, less than 6/9, less than 6/12, less than 6/18, less than 6/36, less than 6/60, and less than 3/60. The baseline visual acuity was used for eyes that underwent cataract surgery between the 2 examinations. For each age group, the denominator was the number of participants in that age group who had cataract gradings in the follow-up survey. For a given threshold of visual acuity, the numerators were the number of eyes with either true incident cataract or backlog cataract for that threshold from participants in the same age group.
For incident cataract, an eye diagnosed with cataract at the follow-up survey contributed to the numerators for all thresholds worse than or equal to its visual acuity at the follow-up survey if it had no cataract at the baseline survey. Such an eye contributed to the numerators for thresholds not only worse than or equal to its visual acuity at the follow-up survey but also worse than its visual acuity at the baseline survey. For backlog cataract, an eye diagnosed with cataract at the baseline survey contributed to numerators for all thresholds worse than or equal to its visual acuity at the follow-up survey.
Cataract surgery rates were calculated for both incident cataract and backlog cataract associated with different levels of visual impairment. The Australian population or its projection was provided by the Australian Bureau of Statistics.7 SAS (version 8.2 for Windows; SAS Institute, Cary, NC) was used to carry out all calculations.
The Australian Bureau of Statistics provided the exact populations for 1950, 1960, 1980, and 2000 in 5-year age groups, with all people 85 years or older included in the same age group. We also used the medium projections (series B) of Australian populations in 2005, 2010, 2015, and 2020 provided by the Australian Bureau of Statistics (Figure 1). The Australian population in 1950 was 8.2 million; in 2000, 19.2 million; and in 2005, it is projected to be 23.2 million.
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Figure 1. Population pyramids for Australia for 1950, 2000, and 2020.
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RESULTS
Of 3271 urban residents recruited in 1992-1994, 2594 participants (79%) were reexamined in 1997-1999. In all, 2302 participants (89%) had cataract grading in at least 1 eye at the follow-up survey. Of 4566 eyes with cataract grading at the follow-up surveys, 1372 eyes (30%) had cataract at the follow-up surveys. This included 599 eyes (13%) that had cataract at the baseline survey.
For different levels of visual acuity, we calculated the age-specific prevalence of incident cataract and backlog cataract and used the estimated prevalence to obtain the estimated rates per million people for Australian populations in 1950, 1960, 2000, 2005, 2010, 2015, and 2020 (Table 1 and Table 2) (Figure 2). Both incident and backlog CSRs increased with poorer visual acuity and also with increasingly aging Australian populations. Most interestingly, the incident cataract rates were consistently smaller than the backlog rates.
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Table 1. Cataract Surgery Rate for Incident Cataract (Operations per Million People)
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Table 2. Cataract Surgery Rate for Backlog Cataract (Operations per Million People)
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Figure 2. The relation between the prevalent cataract surgery rate (CSR) (incident and backlog) and visual acuity thresholds for the Australian population in 1950, 2000, and 2020.
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In 2005, the actual CSR in Australia was reported as 9000 (Warwick Kitt, oral communication, January 2006), but the CSR for all prevalent cases (backlog and incident) was 1470 for less than 6/60, 3110 for less than 6/18, 5080 for less than 6/12, and 6440 for less than 6/9 (Table 1 and Table 2).
COMMENT
This analysis shows 3 things. First, the CSR increases almost 5-fold as the visual acuity impairment threshold for surgery goes from less than 6/60 to less than 6/9. Second, it shows that the demographic change in Australia from 1950 to 2020 would lead to an almost 3-fold increase in the CSR. Third, it suggests that backlog cataract numbers exceed those of incident cataract in Australia. This may be surprising given the estimates of the cataract backlog in Australia being about 45 000,8 or less than 3 months of the current surgical volume. It seems likely to reflect the fact that early lens opacity may be present for some time before it causes sufficient disability for the individual to seek eye care and decide to proceed to cataract surgery. This is consistent with the finding that almost half (51%) of people with visually significant cataract (visual acuity <6/12) had had an eye examination in the previous year but had not been scheduled for cataract surgery.9
This study has as strengths its population basis, the standardized assessment of cataract status and visual acuity, and its ability to project the derived rates using reliable national census data. Its weaknesses include the fact that not all cataracts with an associated level of visual acuity actually require cataract surgery and that a cataract will not be the only cause of visual impairment. However, today most surgeons would usually recommend cataract surgery for visually significant cataract occurring in an eye that also has significant vision loss for another cause, such as age-related macular degeneration or glaucoma, because of the expected improvement in function. Therefore, even though cataract may not be the only cause of vision impairment, its presence is still likely to lead to surgery in many patients.
What do these observations tell us? In Australia, the number of cataract operations has increased from 130 000 in 2000 to 180 000 in 2005 (Warwick Kitt, oral communication, July 2005). This gives an increase in CSR from 6500 to 9000. This CSR is higher than the predicted rate for incident and backlog cataract combined for a visual acuity threshold of less than 6/9 (predicted CSR, 6440). This would suggest that a significant number of cataracts with a visual acuity of 6/9 or better are being operated on, in which case some consideration should be given to the allocation of resources and the prioritization of cataract surgery. Second, and more importantly, these observations indicate the need to continue to expand our cataract surgery capacity to enable us to handle the increasing need with the future demographic change.
Data on the CSR in the United States are hard to find. The number of cataract surgeries in Medicare beneficiaries in 2001 would give a rate of about 4400,10 but other estimates suggest possibly 1.8 million cataract operations were performed in the United States in 2005, which would give a CSR of 6060.
Although our data are specific for Australia, the findings and proportional changes will be highly relevant to other developed areas, including the United States. Our data highlight the need to ensure sufficient capacity is built to meet the continuing growth in both need and demand for cataract surgery. Cataract surgery has been shown to be extraordinarily cost-effective at about AU$3000 per quality-adjusted life-year.11 We as a community cannot afford to have people with visual impairment from cataract waiting long periods for cataract surgery.12
AUTHOR INFORMATION
Correspondence: Hugh R. Taylor, AC, MD, FRANZCO, Centre for Eye Research Australia, Royal Victorian Eye and Ear Hospital, 32 Gisborne St, East Melbourne, Victoria 3002, Australia (h.taylor{at}unimelb.edu.au).
Submitted for Publication: November 27, 2005; final revision received June 9, 2006; accepted July 4, 2006.
Financial Disclosure: None reported.
Funding/Support: This research was supported by the Australian Government Cooperative Research Centre Program.
Author Affiliations: Centre for Eye Research Australia, University of Melbourne, World Health Organization Collaborating Centre for the Prevention of Blindness, East Melbourne, Victoria; and Vision CRC, University of New South Wales, Sydney, New South Wales, Australia.
REFERENCES
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1. WHO. Global Initiative for the Elimination of Avoidable Blindness. Geneva, Switzerland: World Health Organization; 2000. WHO/PBL/97.61 Rev 2.
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10. West ES, Behrens A, McDonnell PJ, et al. The incidence of endophthalmitis after cataract surgery among the US Medicare population increased between 1994 and 2001. Ophthalmology. 2005;112:1388-1394.
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11. Busbee BG, Brown MM, Brown GC, Sharma S. Incremental cost-effectiveness of initial cataract surgery. Ophthalmology. 2002;109:606-613.
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12. Centre for Eye Research Australia. Investing in sight: strategic interventions to prevent vision loss in Australia 2004: a report prepared by Access Economics Pty Ltd. www.cera.org.au. Accessed October 5, 2006.
SECTION EDITOR: LESLIE HYMAN, PhD
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