THE POTENTIALLY CONFOUNDING INFLUENCE OF "PERFECT VISION" IN ASSESSMENT OF HEALTH-RELATED UTILITY IN PEOPLE WITH VISUAL IMPAIRMENT
Steven M. Kymes, Ph.D., Bryan S. Lee, JD, Walton Sumner, M.D., and Mae O Gordon. Washington University School of Medicine, Saint Louis, MO
Purpose: Assessment of utility is common in many specialties. Utility studies for visual impairment (VI) have only been conducted recently. In people with VI, there may be difficulties on a policy scale due to the difficulty in characterizing impairment of visual function as a “health state.” We attempt to determine what impact this cognitive difficulty might have on utility assessment in people with VI. Methods: We interviewed 443 patients: diabetic retinopathy (DR) =59, glaucoma=99, macular degeneration (AMD)=44, cataract=132, correctable refractive error (RE)=109. A wide range of disease was represented in each subsample. Utilities were estimated using the standard gamble (SG) for two scenarios: 1) assuming current health status including current visual function, what risk of death would be accepted to gain perfect health (PH scenario); 2) assuming current health status and assuming perfect visual function what risk of death would be accepted to gain perfect health (CP scenario). People who reported a lower score on “CP” than “PH” were considered to have cognitive difficulty. Scores stratified by disease severity were examined to determine if cognitive difficulty threatened construct validity. Characteristics of people with cognitive difficulty were compared to those who did not using chi-square and Wilcoxon tests. Results: Stratified scale scores for people with vision-threatening diseases (DR, glaucoma, AMD) showed modest construct validity. Scores for people with cataract and RE showed poor construct validity. The proportion of people with cognitive difficulty ranged from 9% among those with DR and AMD to 20% among those with cataract. For all diseases, the mean scores of the CP scenario were similar to the PH. For people with AMD and cataract, the mean score for CP was lower than PH. Among participants with vision threatening disease, cognitive difficulty was associated with worse visual acuity. For those with non-threatening disease, people with worse comorbidity and quality of life were more likely to have cognitive difficulty. Other factors such as age, income, education and severity of ocular disease were not associated with cognitive difficulty for any disease group. Conclusions: It is likely that utility assessment among people with visual impairment is confounded by difficulty in understanding the construct of “perfect vision” as a health state. This has important implications for cost-utility analyses of the treatment and prevention of ocular disease.