Psychometric validation techniques applied to the IND-VFQ-33 visual function questionnaire: the Hyderabad ocular morbidity in the elderly study (HOMES).

BACKGROUND: Over 2 billion people suffer from vision impairment or blindness globally, and access to validated visual measurement tools in imperative in accurately describing and managing the burden of eye disease. The present study applies contemporary psychometric validation techniques to the widely used 33-item Indian Visual Function Questionnaire (IND-VFQ-33). METHODS: We first estimated the polychoric correlation between each pair of items. Next, an unrotated and oblique Promax rotated factor analysis, item response theory (IRT, using a graded response model (GRM)), and differential item functioning (DIF) testing were applied to the IND-VFQ-33. We subsequently propose a validated IND-VFQ-33 questionnaire after psychometric testing, data reduction, and adjustment. RESULTS: Exploratory unrotated factor analysis identified two factors; one with a particularly high eigenvalue (18.1) and a second with a lower eigenvalue still above our threshold (1.1). A subsequent oblique Promax factor rotation was undertaken for a 2-factor solution, revealing two moderately correlated factors (+ 0.68) with clinically discrete item loadings onto either Factor 1 (21 items; collectively labelled "daily activities") or Factor 2 (5 items; collectively labelled "bright lights"). IRT confirmed high item discrimination for all remaining items with good separation between difficulty thresholds. We found significant DIF on depression for six items in Factor 1 (all uniform DIF, except item 21 (non-uniform DIF) with no substantive difference in beta thresholds for any item and no substantive difference in expected individual or sum score, by depression at baseline. For Factor 2, only one item demonstrated significant uniform DIF on gender, similarly without major differences in beta thresholds or expected total score between gender at baseline. Consequently, no further item recalibration or reduction was undertaken after IRT and DIF analysis. CONCLUSION: Applying IRT and DIF validation techniques to the IND-VFQ-33 identified 2 discrete factors with 26 uniquely-loading items, clinically representative of difficulty performing daily activities and experiencing difficulty due to bright lights/glare respectively. The proposed modified scale may be useful in evaluating symptomatic disease progression or response to treatment in an Indian population.

Prevalence of visual hallucinations.

BACKGROUND: To determine the prevalence of visual hallucinations and to report its associations with demographic and clinical factors in a population-based survey in India. METHODS: Prevalence of visual hallucinations was determined as a part of a large prospective, cross-sectional, population-based study that was carried out in two districts of Telangana, namely Khammam and Warangal. Multi-stage cluster random sampling was used in this study. The administration of the questionnaire preceded the clinical examination of the subjects. Associations with factors such as age, sex, education level and visual impairment categories were studied using univariate analysis and multiple logistic regression. RESULTS: Out of the 6,000 subjects who were enumerated from the 120 clusters in Khammam and Warangal districts, a total of 4,936 (82.3 per cent) responded. The mean age of the participants was 55.6 ± 11.8 years. Only the older age group (> 70 years of age) was associated with visual hallucinations on univariate analysis (p < 0.001). On multiple logistic regression, subjects with moderate visual impairment were found to have higher odds of experiencing visual hallucinations (odds ratio 1.48; 95% confidence interval 1.12-1.96, p = 0.006) and those who were older also had higher odds (odds ratio 1.35, p = 0.06). CONCLUSION: The prevalence of visual hallucinations among those with visual impairment in this population-based study was 13.8 per cent. The older age group and moderate visual impairment were noted to be the associated factors among individuals experiencing visual hallucinations.