Association of Vision-related Quality of Life with Visual Function in Age-Related Macular Degeneration

The purpose of this study was to assess which visual function measures are most strongly associated with vision-related quality of life (VRQoL) in age-related macular degeneration (AMD). A cross-sectional study of subjects with early AMD (n = 10), intermediate AMD (n = 42) and late AMD (n = 38) was conducted. Subjects were interviewed with the Impact of Vision Impairment (IVI) questionnaire. Functional tests performed included best-corrected visual acuity (BCVA), low luminance visual acuity (LLVA), visual acuity measured with the Moorfields Acuity Charts (MAC), contrast sensitivity, reading speed, mesopic and dark-adapted microperimetry. The relationship between VRQoL and visual function was assessed with multiple regressions controlling for confounders. Rasch analysis demonstrated the validity of the IVI to assess VRQoL through three subscales: reading and accessing information, mobility and independence, and emotional well-being. Subjects with late AMD had significant lower IVI scores on all subscales compared with intermediate and early AMD (p < 0.011). In the overall cohort, IVI subscales were associated with BCVA, LLVA, MAC-VA and contrast sensitivity (all p < 0.001). Among the subgroup of early and intermediate AMD subjects, reading and mobility subscales were significantly associated with MAC-VA (p < 0.013). These results suggest that MAC-VA is a useful, patient-relevant measure of visual impairment in AMD.

functional testing. All participants underwent the following visual function tests: Best-corrected visual acuity (BCVA) in Early Treatment Diabetic Retinopathy Study (ETDRS) letters, low luminance VA (LLVA), BCVA in Moorfields Acuity Chart (MAC) letters and contrast sensitivity measurement using Pelli-Robson Charts.
Visual acuity and functional tests were performed before fundus imaging. BCVA (in letters) was assessed according to the ETDRS method 14 at a testing distance of 4 m. If the patient was unable to read the first four rows of the chart, the distance was reduced to one meter 14 . LLVA was assessed in the same manner, but with a 2.0-log unit neutral density filter that reduces luminance by 100 fold 15 placed in the trial frame. VA measurement with a MAC chart followed the same procedure as BCVA. The MAC charts are based on the ETDRS charts and employ a high-contrast, high-pass letter design with a gray background of the same mean luminance as the letters to simulate lower contrast situations 16 . The letters are also called "vanishing optotypes", because, for normal vision, the detection and recognition thresholds are very similar and the letters seem to disappear soon after the recognition limit has been reached 16 . Contrast sensitivity was measured using a Pelli-Robson Chart presented at 1 m distance [17][18][19] . To avoid fatigue patients were allowed to take small breaks (maximum five minutes) between the tests if required.
In patients with early and intermediate AMD, we additionally assessed reading speed using the International Reading Speed Texts (IReST) 20 and macular sensitivity via mesopic and dark-adapted microperimetry. For the IReST, patients wore their best near correction and were asked to read one paragraph aloud while they were timed with a stopwatch. Mesopic and dark-adapted microperimetry were performed after pupillary dilation with 1.0% tropicamide. Macular sensitivity was measured using the modified S-MAIA device (S-MAIA, CenterVue, Padova, Italy), which performs fundus tracking using a line-scanning laser ophthalmoscope (SLO) with a super-luminescent diode illumination with a central wave light of 850 nm for mesopic testing and with an additional LED projecting red (627 nm) stimuli for dark-adapted testing. As previously described, a customized stimulus grid was used that consisted of 33 points located at 0°, 1°, 3°, 5° and 7° from fixation 21 . First, mesopic testing was performed, followed by dark-adapted testing after 30 minutes of dark adaptation while waiting in the examination room (light was switched off, light level <0.1 lx). The microperimetric outcome measure was the mean sensitivity (MS) in dB. Due to feasibility issues (fixation stability, grid centration and age/patient fatigue) patients with late AMD did not undergo microperimetry examination. We did also not test reading speed in late AMD patients as most of them did not reach the minimum required visual acuity of 55 letters 22 . All tests were performed in one eye with the non-study eye covered with an eye-patch. psychometric evaluation of the iVi. Rasch analysis was used to evaluate the instrument in our cohort. It is a psychometric method that transforms ordinal scales into interval-level scales (expressed in logits) 23,24 . Item difficulty (item measure) in relation to person ability (person measure) is calculated by placing both in the same linear continuum. The ability of the scale to discriminate different strata of person ability was assessed using person separation index (PSI) and person reliability coefficient (PR) 25 . Values of >2.0 and >0.8, respectively, were considered adequate and represented the capacity of the scale to distinguish three levels of person ability 26,27 .
Unidimensionality -the ability that a scale measures a single underlying latent trait and that the items "fit" the underlying trait -was assessed in two ways. First, we determined item fit through an "infit" mean square standardized residuals statistic 28 . Values between 0.7 and 1.3 are considered acceptable, while lower or higher values may indicate redundancy or an unacceptable level of "noise" in the responses 28 . Second, the principle component analysis (PCA) of the residuals was examined to test for local independence. The PCA of residuals for the first factor should explain at least 50% of the variance and the first contrast of residuals should be <2.0 eigenvalue 29 .
The targeting of the instrument, i.e. how well item difficulty targets person ability, was assessed by visual inspection of the person-item map and the difference between person and item mean logits. A difference of >1.0 logits indicates notable mistargeting 30 .
Furthermore, each item was assessed for differential item functioning (DIF), which is a statistical method for detecting whether sample subgroups (sex and age groups) systematically respond differently to certain items, despite having similar underlying ability. A DIF contrast of >1.0 logits is notable and suggests that the item may be biased for some participants subgroups.
Person measures (in logits) were recalibrated to a 0 to 100 scale. Higher values indicate lower visual ability and indicate poorer VRQoL. Rasch analysis was performed using commercial software (Winsteps software, ver. 3.92.1.2, Chicago, IL) 31 . The Andrich rating scale model was used for analysis 29 . Statistical analysis. Descriptive statistics were performed to assess baseline demographic variables for the AMD groups. The nonparametric Wilcoxon rank sum test was used to compare person measures on the three subscales of the IVI in early, intermediate AMD and late AMD subjects. Univariate linear regression was carried out to assess the relationship between the person measures and demographic variables. In the overall cohort, separate univariate linear regression models against the IVI subscale person measures were performed with each of the visual function test. If the relationship between a function test and average person measures reached a p-value < 0.05 in univariate analysis, multiple regression was used to ensure that the findings were not confounded by AMD stage and age. We additionally performed a subgroup analysis, which included only patients with early and intermediate AMD. Statistical analyses were performed using the statistical software STATA 32 .

Results
Sociodemographic and clinical characteristics of the participants. A total of 90 participants were recruited comprising 10 patients with early AMD (11%), 42 (47%) with intermediate AMD and 38 (42%) patients with late AMD. Participants' mean age was 73.9 ± 8.4 years and there were more female (69%) than male participants. Patients in the late AMD group were significantly older compared to early and intermediate AMD patients (p < 0.05), while patients with early and intermediate AMD were in the same age range (p = 0.96) ( Table 1). All functional tests were significantly decreased in intermediate and late AMD compared to the early AMD group (all p-values < 0.001). There was no significant difference in BCVA between early and intermediate AMD (p = 0.553), as well as in reading speed (p = 0.617) and mesopic and dark-adapted microperimetry (p = 0.274 and p = 0.141). However, LLVA, MAC-VA and contrast sensitivity were significantly decreased in intermediate AMD compared to the early AMD group (p = 0.023, p = 0.041 and p < 0.001 respectively). Detailed results can be found in Table  6 in Supplement 1.
psychometric validation of the iVi questionnaire. The data for the IVI were fitted to the Rasch model and key indicators of fit were explored. Overall, the psychometric testing supported the use of the three IVI subscales in this sample and demonstrated satisfactory PSI and PR for all subscales. No items had to be removed due to misfit or DIF. Detailed results of the Rasch analysis can be found in Supplement 2.  (Table 2). Age and AMD stage were also significantly associated with all three scales of the IVI in univariate linear regression analysis (Table 3).

Relationship between visual function tests and
In univariate linear regression, person measures of all three scales were negatively associated with BCVA, LLVA, MAC-VA and contrast sensitivity in the overall cohort. After controlling for age and AMD stage, multiple regression analysis showed that BCVA and MAC-VA remained significantly associated with all three IVI scores. LLVA was still significantly associated with the reading and mobility scales and contrast sensitivity only with the mobility scale (Table 4). Contrast sensitivity and MAC-VA had the strongest associations with all scales.
Analyzing only subjects with early and intermediate AMD, BCVA, LLVA and MAC-VA were associated with the reading scale and BCVA and MAC-VA with the mobility scale. In the adjusted analysis BCVA, LLVA and MAC-VA were still significantly associated with the reading scale and MAC-VA with the mobility scale ( Table 5).
The IReST and macular sensitivity on mesopic and dark-adapted microperimetry showed no association with any of the scales of the IVI.

Discussion
In our study we found that especially functional tests of central retinal function under low luminance and challenging contrast conditions were associated with VRQoL. LLVA, MAC-VA and contrast sensitivity were significantly reduced in patients with intermediate and late AMD compared to early AMD, while no significant difference was found for BCVA between intermediate and early AMD. www.nature.com/scientificreports www.nature.com/scientificreports/ This is in accordance with previous studies, which investigated these tests in early and intermediate AMD, with any visual impairment in early stages of AMD being present on LLVA but not BCVA 33 . Similarly, Feigl and associates reported decreased contrast sensitivity in early AMD compared to healthy controls 34 . In line with previous other studies we found late AMD and older age to be associated with decreased VRQoL on all three subscales, namely the reading, mobility and emotional scales 35,36 . MAC-VA and contrast sensitivity both showed a stronger relationship with the subscales of the IVI in the overall cohort than BCVA. This is of special relevance because poor contrast sensitivity has been shown to be sensitive in discriminating early stages of AMD 34,37 . Furthermore, in previous studies contrast sensitivity was a factor impacting VRQoL of AMD patients: Roh and colleagues 38 demonstrated that contrast sensitivity was an important factor affecting VRQoL in patients with   www.nature.com/scientificreports www.nature.com/scientificreports/ vision impairment due to bilateral advanced AMD. Bansback et al. 39 found a relationship between contrast sensitivity and health-related quality of life, suggesting that benefits of ocular treatments may be underestimated if contrast sensitivity is not taken into account.
A subgroup analysis, only including early and intermediate AMD subjects, revealed a noticeable association between MAC-VA and VRQoL for both the IVI reading and mobility subscales. The MAC-VA is a relatively recent functional test which simulates lower contrast situations. Shah et al. 16 demonstrated the MAC-VA to be more sensitive in detecting early AMD compared to conventional BCVA and hypothesized that recognition of the high-pass letters is more vulnerable to photoreceptor dysfunction than conventional high luminance and high contrast letters. This is in accordance with our findings as MAC-VA was not only significantly associated with VRQoL in the overall cohort, but also in the subgroup of early and intermediate AMD. As the MAC-VA is a relatively new test, no previous studies have investigated whether it is associated with VRQoL. We could show that MAC-VA as well as LLVA and contrast sensitivity had a stronger effect on VRQoL compared to BCVA. A review by Mones and colleagues revealed that the use of contrast sensitivity as an outcome measure in clinical trials may be a better predictor of activities of daily living, mobility and orientation than BCVA 40 . Also in the Blue Mountains Eye Study, contrast sensitivity was strongly associated with self-reported measures of visual disability 41 .
We did not find significant associations between any of the IVI scales with mesopic or dark-adapted microperimetry mean sensitivity. This is in accordance with the findings of Wu and co-authors 42 . They evaluated subjects with bilateral intermediate AMD using a shorter 10-item Night Vision Questionnaire (NVQ-10) 42 and assessed the relationship of the NVQ scores with LLVA (also using a standard 2.0.-log neutral density filter), low luminance deficit (LLD), mesopic microperimetric mean sensitivity and central sensitivity. NVQ-10 person measures were significantly associated with LLD, but not with BCVA, LLVA, microperimetric mean sensitivity or central sensitivity. Thompson et al., who determined in their study whether Low Luminance Questionnaire scores were associated with objective measures of visual function in early and intermediate AMD, did also not find any association with mesopic microperimetry measures 36 . Interestingly, we did not find reading speed to be correlated with VRQoL.
Strengths of our study include the wide range of functional tests including the relatively new MAC charts for which little data are available to date as well as reading performance and dark-adapted microperimetry. VRQoL was assessed using a validated instrument available in German -the German IVI. We re-evaluated its psychometric performance and transformed responses into an interval-based scale for further statistical testing using modern psychometric methods. Participants were phenotyped based on current gold-standard retinal imaging in combination with a clinical examination. One of the major limitations of our study is the relatively small  www.nature.com/scientificreports www.nature.com/scientificreports/ sample size, especially for the early AMD group. For microperimetry and reading speed measures the sample size was even smaller, as we only performed these test in subjects with early and intermediate AMD. As common with exploratory studies, no adjustment for multiple testing was done which might lead to an over-estimation of statistical power. The IVI assesses overall VRQoL with no particular focus on activities under low luminance and low contrast which might have decreased our ability to detect any associations with functional tests under those conditions. However, general VRQoL is most closely related to daily visual functioning and thus very suitable to assess patient-relevance of visual function tests.
In conclusion, our study showed that performance on BCVA, LLVA, MAC-VA and contrast sensitivity are associated with all aspects of VRQoL in overall AMD. In patients with earlier stages of AMD, BCVA, LLVA and the MAC-VA are associated with VRQoL on the reading scale. In addition, MAC-VA is also correlated with VRQoL on the mobility scale in these patients, which suggests that the MAC-VA might be a useful and patient-relevant measure of visual impairment in AMD, in particular in earlier stages.

Data availability
The datasets generated during and analyzed during the current study are available from the corresponding author on reasonable request.