The depletion of susceptibles effect in the assessment of burden-of-illness: the example of age-related macular degeneration in the community-dwelling elderly population of Quebec.

BACKGROUND: With the aging of the population, age-related macular degeneration (AMD) is becoming a public health concern. Few studies have assessed its consequences on morbidity and mortality, and the findings are conflicting.
OBJECTIVES: To assess the risk of depression, fracture, institutionalization, and death among elderly patients with suspected exudative AMD and the impact of the depletion of susceptibles effect in a burden-of-illness study.
METHODS: A population-based retrospective cohort study was conducted in the community-dwelling elderly population of Quebec. The cohort was assembled through the Quebec medical claims database (RAMQ). Among patients age 65 and older with a claim involving a diagnosis of AMD over the years 2000 to 2004, those with suspected exudative AMD (n=2,071) were retained, using fluorescein angiography as a marker. The reference cohort consisted of a sample of 16,932 elderly without a claim involving AMD or visual impairment.
RESULTS: Suspected exudative AMD was associated with an increased risk of depression (hazard ratio HR=1.3, 95%CI 1.18-1.43) and fracture (HR=1.19, 95%CI 1.03-1.37), but a decreased risk of institutionalization (HR=0.55, 95%CI 0.42-0.71) and death (HR=0.68, 95%CI 0.59-0.78). After adjustment for the incident/prevalent status of the AMD, the association between suspected exudative AMD and institutionalization was no longer statistically significant (HR=0.75, 95%CI 0.5-1.12).
CONCLUSIONS: These findings enhance the need to detect visual loss and to consider patients' ability to adapt to AMD, to maintain their quality of life. Failure to account for duration of illness and the depletion of susceptibles effect may bias results of burden-of-illness studies.