The Prevalence of Visual Impairment: a Capture-Recapture Study of Potential Recipients

The prevalence of visual impairment: a capture-recapture study of potential recipients of vision rehabilitation in threeNew Zealand urban regions

Simon J. Thornley, Keith Gordon, Chris Shelton, Roger Marshall.

Background/Purpose:

As New Zealand’s population ages, the number of people requiring vision rehabilitation services is increasing. Planning for the delivery of these services necessitates a knowledge of the prevalence of visual impairment in New Zealand. Traditional methods for obtaining this information involves the usage of population health studies which are generally time consuming and expensive. For these reasons, there have been no studies of the prevalence of visual impairment conducted in New Zealand. Past estimates of the burden of visual impairment have been obtained by extrapolating overseas surveys to the New Zealand population.

Capture-recapture is a relatively new method that enables the rapid estimation of prevalence of disease. It is being used increasingly for this purpose in many diseases and conditions. With this method, the total population with acondition can be determined from lists of patients who have received healthcare services from more than one source e.g. an ophthalmology clinic and the New Zealand Blind Foundation (NZBF).

The aim of this study was to use the capture-recapture method to estimate the number of potential recipients of vision rehabilitation services by the NZBF in three regions of New Zealand.

Methods:

Capture-recapture methods were used to estimate the prevalence of visual impairment (as defined by NZBF membership criteria, a best corrected visual acuity of ≤ 6/24) in Auckland-Waitemata; Canterbury and Counties Manukau, three different urban regions of New Zealand. Lists of people 15 years of age or over, who underwent eye health assessments in ophthalmology and/or optometry clinics, as well as those applying for Blind Foundation services, during the period September 2011 to September 2014 were obtained in these regions. Log-linear models were used to estimate the incidence of new cases throughout the study period. Corresponding estimated resident census figures were used from 2013 to derive a three-year cumulative incidence, and prevalence was extrapolated based on estimates of survival in people with visual impairment.

Results

In the Auckland District Health Board and Waitemata regions, where three lists were available, the prevalence of visual impairment was 0.49% (95% confidence interval [CI]: 0.41 to 0.60) for the population over the age of 15 and 0.86% (95% CI: 0.72 to 1.06) for the population over the age of 40. In Canterbury and Counties Manukau, where two lists were available, the prevalence of visual impairment was 0.52% (95% CI: 0.45 to 0.60) and 1.96% (95% CI: 1.20 to 3.61) respectively for the population ≥15,and 0.84% (95% CI: 0.73 to 0.98) and 3.35% (95% CI: 2.02 – 6.4) for the population ≥40. The prevalence of people meeting the NZBF criteria for membership (≤6/24) for all three regions combined was 0.82% for people ≥15 and 1.41% for people ≥40.

Conclusion

This study is the first to use capture-recapture methods for measuring the prevalence of visual impairment in any region of New Zealand. It is estimated that there are 30,000 people over the age of 15 that meet the NZBF criteria for membership. The results are useful for planning vision health and rehabilitation services in this country.