Properties of Patient-Reported Outcome Measures in Recessive Stargardt Disease

Introduction: The study aimed to explore the psychometric properties of the National Eye Institute Visual Function Questionnaire (NEI VFQ) and Impact of Vision Impairment (IVI) profile in recessive Stargardt disease (STGD1). Methods: The NEI VFQ-25 and IVI-28 were administered to individuals with STGD1. Responses were analyzed following psychometrically established dimension structures of the NEI VFQ (visual function [VF] subscale; socioemotional [SE] subscale) and of the IVI (functional [F] subscale; emotional [E] subscale). We analyzed internal consistency, dimensionality, item fit, and differential item functioning (DIF), using latent trait models. Criterion validity was assessed using Pearson correlation coefficients. Results: Seventy-one participants (42 females, 29 males; mean age, 44 ± 19 years) were included. Self-reported difficulty levels were lower than the mean difficulty of items in both instruments. Person reliability and person separation index of the instruments were 0.85 and 2.40 (NEI VFQ-VF), 0.69 and 1.49 (NEI-VFQ-SE), 0.88 and 2.77 (IVI-F), and 0.72 and 1.62 (IVI-E). No items showed misfit at a level distorting the measurement system. One IVI item showed DIF by gender but was retained as person measures were largely unaffected by its removal. NEI VFQ-VF and IVI-F as well as NEI VFQ-SE and IVI-E were positively correlated (r = 0.79 and 0.64, respectively). Conclusion: The NEI VFQ and IVI have acceptable psychometric properties in STGD1 with the IVI allowing more sensitive person stratification. Targeting of questionnaires to individuals with STGD1 might be improved by including additional content domains specific to the disease.