Incidence estimation using a single cross-sectional age-specific prevalence survey with differential mortality.

Here, we present a method for incidence estimation of a curable, non-recurring disease when data from a single cross-sectional survey are used together with population-level mortality rates and an assumption of differential mortality of diseased versus non-diseased individuals. The motivating example is cataract, and the VISION2020 goal to eliminate avoidable blindness globally by 2020. Reliable estimates of current and future cataract disease burden are required to predict how many surgeries would need to be performed to meet the VISION2020 goals. However, incidence estimates, needed to derive future burden, are not as easily available, due to the cost of conducting cohort studies. Disease is defined at the person-level in accordance with the WHO person-level definition of blindness. An extension of the standard time homogeneous illness-death model to a four-state model is described, which allows the disease to be cured, whereby surgery is performed on at least one diseased eye. Incidence is estimated, and the four-state model is used to predict disease burden assuming different surgical strategies whilst accounting for the competing risk of death. The method is applied to data from approximately 10,000 people from a survey of visual impairment in Nigeria.

Preoperative visual acuity among cataract surgery patients and countries' state of development: a global study.

OBJECTIVE: To describe the preoperative surgical case mix among patients undergoing cataract extraction and explore associations between case mix, country level of development (as measured by the Human Development Index, HDI) and cataract surgery rates (CSRs). METHODS: Ophthalmologists in 50 countries were invited to join the newly-established International Eye Research Network and asked to complete a web-based questionnaire about their eye hospitals. Those who complied received a data collection form for recording demographic and clinical data on 100 consecutive patients about to undergo cataract surgery. Countries were ranked into five HDI categories and multivariable regression was used to explore associations. FINDINGS: Ophthalmologists at 112 eye hospitals (54% of them nongovernmental) in 50 countries provided data on 11,048 cataract procedures over 9 months in 2008. Patients whose visual acuity (VA) before surgery was < 6/60 in the better eye comprised 47% of the total case mix in poorly developed countries and 1% in developed countries (P < 0.001). Overall, 72% of the eyes undergoing surgery had a VA < 6/60. Very low VA before cataract surgery was strongly associated with poor development at the country level and inversely associated with national CSR. CONCLUSION: The proportion of patients with very poor preoperative VA is a simple indicator that can be easily measured periodically to monitor progress in ophthalmological services. Additionally, the internet can be an effective tool for developing and supporting an ophthalmological research network capable of providing a global snapshot of service activity, particularly in developing countries.