Structural equation modelling and its application to quality of life of visually impaired people

The main objective of the thesis was to construct a model of relationships among perceived visual ability (VF), quality of life (QOL) and visual acuity (VA) using the structural equation modeling (SEM) technique in an older, visually impaired population of India.The World Health Organization QOL questionnaire, adopted as a QOL questionnaire, and a 16-item visual function questionnaire (VFQ) were administered to adults in the population-based Andhra Pradesh Eye Disease Study prior to their detailed eye examination. Participants aged 40 years and older (n=3,702), who underwent an interview by trained professionals, were included in this study. A total of 1,333 visually impaired subjects (presenting VA < 6/18) responded to both questionnaires. The VFQ and QOL questionnaires were evaluated using Rasch analysis. The general linear model analysis (GLM) was performed to identify the factors associated with the measures of perceived visual ability and QOL. The direct and indirect relationships between VA, perceived visual ability and QOL were established using the SEM after adjusting for socio-demographic factors identified to be significant in GLM.The mean age of the visually impaired participants was 60.5 years; 753 (56%) were female. There were 760 visually impaired people with cataract in at least one eye, 524 with uncorrected refractive error, 84 with retinal disease, 41 with glaucoma and 60 with corneal disease. The VFQ and QOL measures were found to be valid and unidimensional for those with retinal disease, glaucoma, corneal disease and those who are blind due to cataract or any other cause. In QOL questionnaire the item that required the most ability was "chance to acquire new skills" and the item that required the least ability was "dressing" for the sample with retinal disease, glaucoma, corneal disease and cataract. In the VFQ, the items that required the most visual ability were "recognizing small objects" and "recognizing people across the street". The items that required least visual ability were "reaching an object that is farther or closer than you thought" and "identifying colors".Perceived visual ability to perform the functional vision activities mediated the relationship between VA and QOL, a new diagram for the International Classification of Functioning (ICF), however, the percentage of change mediated through perceived functional ability varied for different eye diseases. The total change that was mediated through perceived functional ability was 100%, 100%, 52%, 52% and 63% for people with retinal disease, glaucoma, corneal disease and those who are blind due to cataract or any cause of disease, respectively. Decreased QOL and perceived visual ability was associated with the presence of glaucoma or corneal disease independent of VA and with cataract and retinal disease as a function of VA.Perceived visual ability to perfol111 the functional vision activities mediates the relationship between VA and QOL. Improving the perceived visual ability would improve the QOL of low vision persons. Further longitudinal studies on the impact of therapy and rehabilitation for glaucoma, corneal disease and retinal disease on QOL and VF are needed