The proximal-distal continuum of multiple health outcome measures: the case of cataract surgery.

This paper presents the concept of a proximal-distal continuum in health outcome measures. It indicates how this continuum can be used in the selection of outcome measures in health technology evaluation studies. Finally, it demonstrates several ways in which the placement of a specific health outcome measure in the proximal-distal continuum determines the overall statistical model of treatment and nontreatment variables. We identify five principles that relate to the above issues. The first three principles state that a larger effect of treatment on health outcomes will be seen when the following occur: 1) more proximal (e.g., signs and symptoms, disease-specific outcomes) measures are examined, 2) the initial illness is more severe; and 3) pretreatment distal (e.g., role functioning, life satisfaction) outcome measures show relatively high impairment. Principle four indicates that distal outcomes are influenced more heavily by external (i.e., nontreatment) factors. Principle five states that a causal chain links each outcome measure in the continuum to the next more distal outcome measure. This last principle enables the determination of indirect relationships between treatment and outcomes. These principles are illustrated with data from a study on the effects of cataract surgery with intraocular lens implantation on patient outcome variables across the proximal-distal continuum.