Table 1 summarizes the baseline characteristics of two sets of subjects: those with at least 12 months of follow-up used in longitudinal MID estimates (
n = 590), and the subset with follow-up at 66 months used in cross-sectional and most distribution-based MID estimates (
n = 420). Subjects in the two samples were comparable in age (mean 58 years, SD = 11), sex (54%–55% male), better eye VA (88 letters, or approximately 20/20 Snellen VA), and global health impact rating (4.3 of 5.0, SD = 1). The few dropouts before 12 months (
n = 17) did not differ from those who were active on any of these characteristics. Those with 12-month follow-up who were inactive at 66 months (
n = 170) were more likely to be nonwhite (
P = 0.0011, Fisher exact test), reported worse global health impact (
P = 0.0036,
t-test), and had worse baseline MD in the better eye (
P = 0.0010,
t-test) than those who were active (
n = 420).
Anchor-based cross-sectional MIDs estimated from the 66-month sample are summarized in
Table 2 for both anchors: better eye MD and better eye VA. Subscales most relevant for each anchor are noted by a footnote in the Table. The VAQ total score and NEI-VFQ total score, respectively, showed MIDs of 2.6 and 2.3 for the MD anchor, and 5.2 and 3.8 for the VA anchor. For all but one scale, MIDs for the VA anchor were larger than those for the MD anchor. Among the various subscales of the two HR-QOL questionnaires, the peripheral vision and acuity/spatial vision subscales of the VAQ showed the largest MIDs for the MD anchor (3.4 and 3.3, respectively), whereas for the NEI-VFQ, the driving (4.4), role difficulties (4.1), and general vision (3.5) subscales showed the largest MIDs for the MD anchor. With regard to the VA anchor, the VAQ subscale that had the highest MID was the acuity/spatial vision subscale (7.3); for the NEI-VFQ, the same three subscales with the highest MID for the MD anchor also were highest for the VA anchor. Models adjusted for treatment effect (which was significant only in 5 of the models) yielded almost identical MIDs.
Table 3 summarizes the average change from baseline to last follow-up of three summary HR-QOL measures from the VAQ (total, acuity/spatial vision, and peripheral vision) within categories of anchor change during the same time period. The mean changes indicated that, on average, those subjects who had decreasing (worsening) MD over time also had decreasing HR-QOL over time for all three summary measures; likewise, subjects who had increasing MD also had increasing HR-QOL measures (
Fig. 1). With regard to VA, loss of 10 or more letters was reflected by a mean decrease of 2.9 units for the VAQ total score. The other loss category (5.1–9.9 letters), the no change category, and the gain category (>5 letters) all showed an increase in the VAQ total score of approximately 3 units. Similar trends in QOL change within categories of VA change also were seen for the VAQ's acuity/spatial and peripheral vision subscales. However, the VA change patterns did not show the ordinal relationships seen with the MD anchor. As shown in the box plots for MD (
Fig. 1), there is much variation in these data.
Anchor-based longitudinal MIDs, estimated from change-in-QOL versus change-in-anchor regression models, are summarized in
Table 4. These change scores were calculated as the difference between baseline and the last time point where clinical and HR-QOL data were collected on a subject. These subjects had a median of 6.8 years of follow-up (range, 1–9 years). The MIDs were calculated for a 3 dB change in MD and for a 10-letter change in VA. The MIDs for the VAQ total score were 3.4 (MD anchor) and 3.2 (VA anchor). For the acuity/spatial score they were 3.9 and 2.5, and for the peripheral vision score they were 3.1 and 3.7. The MIDs for the MD and VA anchors were similar for the VAQ total and all subscales. Models adjusted for treatment effect (which was not significant in any of the models) yielded almost identical MIDs.
Distribution-based MIDs for the VAQ subscales were similar regardless of whether they were calculated at the 66-month cross-section or based on the change from baseline to last follow-up (
Table 5). The 66-month MIDs were 6.5 for the VAQ total score, 8.5 for acuity/spatial vision, and 7.0 for peripheral vision. The MIDs for the NEI-VFQ total score and subscales were smaller than those from the VAQ at 66 months. Medicine and surgery specific MIDs were similar to each other. In addition, Supplementary Table S1 gives MID estimates using ½ SD, as well as the standardized response means, effect sizes, and responsiveness statistics for the VAQ total and all VAQ subscales.
Figure 2 displays the MIDs by anchor and method for the VAQ and NEI-VFQ total scores, and the 2 subscales that are similar in content between the questionnaires (acuity/spatial vision for VAQ and near activities for NEI-VFQ; peripheral vision for VAQ and NEI-VFQ). The range of estimated MIDs for the VAQ total score was 2.6 to 6.5 units and for the NEI-VFQ total score it was 2.3 to 3.8 units. For the acuity/spatial subscale of the VAQ, MIDs ranged from 2.5 to 8.5, and for the near activities subscale of the NEI-VFQ MIDs ranged from 2.1 to 3.7. For the peripheral vision subscale, MIDs ranged from 3.1 to 7.0 for the VAQ and 2.1 to 4.3 for the NEI-VFQ. Distribution-based MIDs were somewhat larger than those from anchor-based methods.