Estimating quality-adjusted life year losses associated with visual field deficits using methodological approaches

Purpose: To develop a function to describe quality-adjusted life year ( QALY) losses associated with visual field losses for use in cost-effectiveness models. Methods: We estimated the patient visual field defect in decibels (dBs) that corresponded to visual acuity values of 20/40, 20/70, 20/200 ( US blindness), 20/400 ( WHO blindness), and 20/1,000 ( extreme impairment) using two methods. The first method categorized visual field defects by applying the logic of the Snellen minimum angle of resolution ( MAR) acuity scale to the ability to perceive luminance adjusting for the differences in the visual acuity and luminance scales (MAR-based). The second method categorized visual field defects based on assumptions. We then assigned each visual field category the same QALY value associated with the analogous visual acuity category. Finally, for each method, we calculated a function that described QALY losses as a function of the absolute value of patient mean deviation ( MD) values measured in dBs. Results: The MAR-based method estimated that MDs of -16, -18, -23, -24, and -30 or greater dBs corresponded to visual acuity values of 20/40, 20/70, 20/ 200, 20/400, and 20/1,000, respectively. For the same visual acuity values, the assumption-based method resulted in MD estimates of -13, -17, 22, -24, and -30. Our MAR-based method yielded the function, QALYs = 0.98991 + 0.0022. dBs - 0.00080518. dBs(2), setting dBs equal to the absolute value of the mean deviation in the better-seeing eye. Our assumption-based method yielded the piece-wise function, QALYs = 10 - 0.0155. dBs for dBs < 22, and 1.815-0.0525. dBs for dBs >= 22. Conclusions: Results from two estimation methods suggest a convex and downward sloping relationship between dBs of visual field loss and QALY decrements. QALY losses accumulate slowly at first and then increase at an accelerating rate following an MD of approximately - 10 dBs. This relationship is consistent with recent empirical findings reporting minimal QALY losses associated with small visual field defects.