In previous work
13 we showed that the NEI-VFQ-25 was not only influenced by visual function, but by other demographic factors as well: age, gender, level of acculturation, income level, education level, location (Nogales or Tucson), and self-report of having medical insurance. These confounders were chosen because of their demonstrated effects on self-reported quality of life in this population and their association with eye disease. We adjusted for all these variables in the ensuing multivariate regression models to obtain more precise estimates of the relationship of vision with quality-of-life domains. Although diabetes was an important comorbid condition in this population, the models do not include diabetes as a covariate, because of its high association with DR; however, we adjusted for the presence of hypertension as a representative of comorbid conditions. All covariates, except age, were modeled as categorical variables.
We used presenting acuity in the better-seeing eye to characterize vision used on a daily basis. This approach was reasonable, because acuity in the better eye has been shown to be as good a predictor of self-reported functional status as other approaches of summarizing better- and worse-eye acuity.
20 We modeled presenting acuity as a continuous variable (logMAR units) and analyzed the strength of the association between lines lost and subscale score.
We also explored whether monocular impairment (presenting acuity in the worse eye worse than 20/40 and acuity in the fellow eye of 20/40 or better) influenced quality-of-life domains. Participants with presenting acuity in the better eye of 20/40 or better were used in the analysis, and comparisons were made between those with no impairment (both eyes had 20/40 or better acuity) and those who had monocular impairment (one eye had worse than 20/40 acuity). In addition, we adjusted for acuity in the better eye, because those with monocular impairment were more likely to have lower presenting acuity in the better eye than those without impairment.
Also assessed were the individual contributions of glaucoma, DR, cataract, and refractive error on quality-of-life-domains. In a first model, we assessed these additive contributions without accounting for acuity impairment, to determine the contribution a particular disease has on quality of life. Those with any of these eye diseases or refractive error were compared with those with no evidence of eye disease nor refractive error. However, eye disease per se would not be expected to impact on quality of life unless there were an associated impact on some aspect of vision. In a second model, we tested this hypothesis by evaluating the additive contributions of eye disease (glaucoma, DR, and/or cataract) on quality of life, with adjustment for presenting acuity. In a third analysis, we added to the model second-order interaction terms between eye disease variables, to observe whether decrements in quality of life due to eye disease acted additively, or whether extra decrements occurred when two diseases were present. Participants with vision loss not due to glaucoma, DR, or cataract (e.g., macular degeneration, amblyopia, trauma) were excluded from these analyses because there were small numbers of people in these categories.
Because the data are from a cross-sectional sample, all inferences regarding the relationship of acuity and quality-of-life scores refer to the comparison of quality-of-life measures between individuals with different levels of acuity. Use of the words “decrement,” “decline,” or “loss” are meant only to describe the relationship between the measures of interest, rather than to imply a longitudinal shift in quality of life with a change in vision (or other measure).
Preliminary analysis showed that some NEI-VFQ-25 subscales had a potentially nonlinear relationship with presenting acuity: Distance Vision, Driving, and Mental Health. For these subscales, we allowed the slope of presenting acuity to change for acuity worse than 20/40 (spline regression).
Subscale score distributions were skewed toward the higher scores (e.g., ratings of excellent or no difficulty), resulting in linear model residuals that were independent but non-normally distributed. Confidence intervals will suggest false significance, because the standard error calculation from such models is based on residuals that are normally distributed. To better characterize the variation of the parameter estimates, we performed a nonparametric bootstrap on all models. A participant’s information was drawn at random, with replacement, from the observed values. Estimates from the linear models were calculated (a bootstrap estimate), and this process was repeated to create 1000 bootstrap estimates. A 95% bias-corrected confidence interval, was calculated, using the 2.5th and 97.5th percentiles of the bootstrap distributions.