5. Epidemiology
- 5.1 Introduction
- 5.2 Final Estimates of New Zealand Blind and Vision impaired Populations (June 2004)
- 5.3 Projections of New Zealand Blind and Vision impaired Populations (2011 and 2021)
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5.1 Introduction
The following section estimates the number of blind and vision impaired individuals living in New Zealand and provides future incidence estimates.
The estimates are structured by the World Health Organisation's (WHO) Classification of Visual Performance (Colenbrander, 1977). This framework categorises vision impairment according to visual acuity and field of vision. Three broad categories of visual performance are defined:
1. Normal: (6/4-6/18 visual acuity, 180-80 degrees field of vision).
2. Low Vision (6/24-3/60 visual acuity, 60-15 degrees field of vision).
3. Blindness (6/150-No Light Perception visual acuity, 10-0 degrees field of vision).
The Classification of Visual Performance provides an internationally accepted definition of blindness and vision impairment relating to scientifically measured variables rather than qualitative assessments of vision.
A variety of estimates for the occurrence of blindness and vision impairment in New Zealand exist in the public arena, but no organisation collects information regarding visual acuity and field of vision for every New Zealander. The current study has relied on the following sources for estimates of blind and vision impaired New Zealanders:
- Royal New Zealand Foundation of the Blind Membership Database;
- Statistics New Zealand Disability Survey 2001;
- Vision Education Agency National Database for Learners who are Blind and Vision impaired;
- Land Transport Safety Authority Driver Licensing;
- Anecdotal data from New Zealand ophthalmologists; and
- Price Waterhouse Coopers and Diabetes New Zealand Incorporated (2001).
Other sources investigated include client records from the National Health Index, Health Insurers, Low Vision Clinics and individual optometrists and ophthalmologists. These organisations indicate that individual records are not easily accessible and time, financial and confidentiality issues prevented the release of data.
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5.2 Final Estimates of New Zealand Blind and Vision impaired Populations (June 2004)
Each of the data sources discussed throughout this section measures blindness and vision impairment using different definitions. At the low end of the spectrum the RNZFB membership data indicates that there are 11,293 blind and vision impaired New Zealanders in 2004. At the high end of the spectrum, the Disability Survey estimates there were 94,600 blind and vision impaired New Zealanders in 2001.
RNZFB data is collected according to acuity and field of vision measurements recorded by ophthalmologists. The membership criterion is visual acuity worse than 6/24. Therefore it is known that there are 11,293 New Zealanders that fit the WHO Classification of Visual Performance criteria for Low Vision and Blindness. However, not all New Zealanders with significant Low Vision or Blindness will be members of RNZFB. There are a number of reasons why individuals may choose not to access services from RNZFB. For example, older people who have progressively lost their vision may see their vision loss as a part of the ageing process and decide to deal with impediments in their own manner. Other people may not want to accept that they are blind, because they perceive there is a social stigma attached to blindness or may feel that they have no need of the RNZFB services.
The Statistics NZ Disability Survey (2001) asks respondents subjective questions to rate their visual impairment. There is no scientific basis for respondents' answers, which means that optimistic blind people may say that they have little difficulty undertaking the screening questions, whilst pessimistic vision impaired people may say that they cannot do tasks in the screening questions at all. This means that the results from the Disability Survey cannot be easily matched with data from RNZFB, VEA or LTSA. For example, the Statistics New Zealand estimates of blind and vision impaired populations for children (0-15 year olds) of 13,200 differs significantly from the RNZFB membership (727) and the VEA estimates (1,110), highlighting the conflict of comparing scientifically measured variables with subjective responses to questionnaires. However, LTSA records show that there are significant proportions (60-85%) of failed drivers licence applications in young people (aged 15-35 years) due to poor vision. Nevertheless, government organisations rely on these estimates for strategic policy decisions.
Anecdotal evidence from leading New Zealand ophthalmologists suggests that the number of blind and vision impaired New Zealanders is more likely to be closer to 30,000-35,000. Keefe et al (2002) estimate that New Zealand has a low vision rate of 8,600 people per million. This indicates that New Zealand would have approximately 37,600 New Zealanders with Low Vision in 2004. Keefe et al (2002) present WHO estimates of prevalence of low vision from the Global Update 1994, which concluded that there are three people with low vision for every blind person. Whilst this ratio varies by country, the ratio is widely accepted for industrialised nations. This would indicate that there are approximately 12,500 blind New Zealanders in 2004.
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In order to deal with the diverse estimates of blindness and vision impairment uncovered throughout the research process, three estimates of the blind and vision impaired population have been considered.
Scenario One:
Low Estimate: 11,293 RNZFB members in June 2004. (3161 blind and 8132 vision impaired).
Scenario Two:
Medium Estimate: 12,000 blind and 34,700 vision impaired people in 2004, a total of 46,700.
Scenario Three:
High Estimate: A June 2004 equivalent for the 2001 estimated 98,400 blind and vision impaired New Zealanders identified by the Statistics New Zealand Disability Survey.
For the purposes of the costs presented in this report, the low estimate is adopted. Many of the costs in the report were derived from discussions with, and a survey of, blind and vision impaired people selected from the RNZFB member database. There is validity in extrapolating these costs across the RNZFB membership. There is however little knowledge about the composition of the larger groups represented by the medium and high estimate, and there can be no presumption that the characteristics found in the sample of RNZFB members can be extrapolated across these larger populations, even though they have been either professionally identified or self-identified as having a vision impairment.
5.3 Projections of New Zealand Blind and Vision impaired Populations (2011 and 2021)
5.3.1 Analysis Methods
Statistics New Zealand provides estimates of population growth according to age, sex and ethnic distribution to 2021. In order to estimate the number of blind and vision impaired people in New Zealand in 2011 and 2021, we have applied the age, sex, ethnicity, visual status ratios for 2004 to the projected populations.
5.3.2 Scenario One - Low Estimate
- By 2021 the blind population will grow by 36% from 3,200 in 2004 to 4,300.
- By 2021 the vision impaired population will reach 11,500, up 3,400 (42%) from 2004.
- The proportion of blind people older than 65 years will increase from 56% in 2004 to 67% in 2021.
- The proportion of vision impaired people older than 65 years will increase from 72% in 2004 to 80% in 2021 (refer Figure A).
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Figure A: Estimated growth in blind and vision impaired population, Low Estimate (2011, 2021)
5.3.3 Scenario Two - Medium Estimate
- By 2021 the blind population will grow by 33% from 12,000 in 2004 to 16,000.
- By 2021 the vision impaired population will reach 44,600, up 10,000 (29%) from 2004.
- The proportion of blind people older than 65 years will increase from 58% in 2004 to 69% in 2021.
- The proportion of vision impaired people older than 65 years will increase from 45% in 2004 to 56% in 2021 (refer Figure B).
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Figure B: Estimated growth in blind and vision impaired population, Medium Estimate (2011, 2021)
5.3.4 Scenario Three - High Estimate
- By 2021 the blind population will grow by 34% from 10,900 in 2004 to 14,600.
- By 2021 the vision impaired population will reach 113,000, up 25,000 (29%) from 2004.
- The proportion of blind people older than 65 years will increase from 59% in 2004 to 70% in 2021.
- The proportion of vision impaired people older than 65 years will increase from 46% in 2004 to 57% in 2021 (refer Figure C).
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Figure C: Estimated growth in blind and vision impaired population, High Estimate (2011, 2021)
Increasing proportions of blindness and vision impairment are caused by age related macular degeneration and diabetic retinopathy in western countries. Diabetic retinopathy is particularly important for New Zealand because of the high prevalence of Type 2 diabetes in Maori and Pacific Island populations.
Price Waterhouse Coopers and the Diabetes New Zealand Incorporated (2001) estimate the prevalence of Type 2 diabetes will grow in the Pacific Island population by 109%, from 8,760 diagnosed cases in 2001 to 18,260 diagnosed cases in 2021. There will be a 90% increase in diagnosed cases in the Maori population, up from 24,820 cases in 2001 to 47,070 cases in 2021. European and other ethnic groups are expected to increase by 39%, from 72,830 diagnosed cases in 2001 to 101,420 diagnosed cases in 2021. The report estimates that for every two diagnosed cases of Type 2 diabetes in 2001, there is one undiagnosed case. This ratio is projected to remain the same in 2021.
The New Zealand Health Strategy claims that Type 2 diabetes is the leading cause of preventable blindness in NZ. The strategy cites international studies that have estimated 70 people become legally blind every year as a result of diabetes in New Zealand, and this number will grow in line with increases in the rate of diabetes across the population.
The Price Waterhouse Coopers report estimated that 7% of Maori, 8% of Pacific Island and 2% of European and Other ethnic populations diagnosed with Type 2 diabetes are
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likely to become blind as a complication of the disease. This means in 2021 there will be approximately 600-1,000 cases of blindness caused by diabetic retinopathy.
Age related macular degeneration is significant in the New Zealand environment as well, because New Zealand's population is increasingly ageing. Whilst the blindness and vision impairment estimates have not estimated incidence according to disease, the estimates assume that the age related macular degeneration has the same influence on people aged over 65 years in the future. Therefore the estimates presented in this report take into account the influence that age related macular degeneration and diabetic retinopathy will have on the blind and vision impaired populations overall.