OBJECTIVES: RPE65-mediated inherited retinal dystrophy (IRD) is a rare condition that causes retinal degeneration. There is an unmet need for a patient-reported outcome (PRO) to assess functional vision among patients with this condition. A modified version of the Visual Function Questionnaire (mVFQ-25) was developed for paediatric populations with visual impairments, and was administered to patients with RPE65-mediated IRD in a phase III trial. This retrospective study evaluated the psychometric properties of this mVFQ-25.

METHODS: A mVFQ-25 was completed by 31 patients and/or their caregivers at baseline, days 30, 90 and 180 and 1-year. Patients completed the Multi-Luminance Mobility Test (MLMT), an endpoint to assess functional vision in different lighting conditions, at the same timepoints. Psychometric analyses assessed item properties, dimensionality, reliability, construct validity, responsiveness and interpretation of scores.

RESULTS: Floor and ceiling effects were present across timepoints, however the pattern indicated improvements in functional vision due to treatment. Inter-item correlations were weak-to-moderate (0.00-0.66) at baseline and slightly stronger at 1-year (0.00-0.87). Internal consistency was excellent across timepoints (Cronbach’s alpha=0.854-0.950). Test-retest reliability was strong (Intraclass Correlation Coefficient=0.912) for stable patients between days 90-180. Convergent validity was supported by moderate correlations with concurrent measures post-baseline (r=-0.48 to 0.76). There were statistically significant differences post-baseline (p<0.001) in mean mVFQ-25 scores between known-groups defined by MLMT scores indicating good discriminant ability and supporting construct validity, but not between groups differing in visual field scores. Statistically significant differences in improvements in mVFQ-25 scores between groups defined as improved and stable/worsened using MLMT scores, provided evidence of responsiveness. Distribution and anchor-based analyses were triangulated to suggest that changes of 1.5 points or greater can be considered meaningful for the total score.

CONCLUSIONS: Results support the reliability, validity and responsiveness of this mVFQ-25 as a measure of functional vision impacts in IRD due to RPE65 mutations.