Purpose: To assess if the Impact of Vision Impairment (IVI) is a valid instrument to measure participation in daily activities and rehabilitation in patients with age-related macular degeneration (AMD) and varying levels of visual impairment.
Methods: Participants, recruited from a public eye hospital and low vision centers, completed the IVI questionnaire. The IVI and its three subscales were assessed for fit to the Rasch model. Unidimensionality, item fit, response category performance, and targeting of items to patients were assessed. Confirmatory factor analysis (CFA) was used to assess the three-factor model of the IVI in this sample of AMD patients.
Results: 219 patients (mean +/- SD age = 83.5 +/- 7.4 yr) were recruited. Of these, 22%, 55% and 23% had mild (< 6/12-6/18), moderate (< 6/18-6/60) and severe (< 6/60) vision loss, respectively. The IVI total and three subscales displayed discrete thresholds indicating that the respondents understood the response categories. The IVI items fitted the scale and unidimensionality was established. Person separation reliability for the IVI score was substantial (0.94) indicating that the scale can discriminate between several groups of AMD patients. The IVI items were significantly targeted to the AMD patients with the means of the two distributions shown to be very close (0.0 and 0.1, respectively). Substantial targeting was also evident for the subscales. Poorer visual acuity was significantly associated (ANOVA; F (2, 216) = 23.4; p < 0.001) with greater restriction of participation suggesting that the IVI has substantial construct validity. CFA supported the IVI three-factor model which includes items from the "emotional well-being, "reading and accessing information" and "mobility and independence" subscales.
Conclusions: Clinicians and researchers can reliably use the IVI to assess the impact on daily life and the effectiveness of clinical trials and rehabilitation interventions in patients with AMD across a range of vision loss.