Comparison of methodologies for predicting mobility performance in adults with low vision
 
PSW Fuhr, O.D., Ph.D., KH Chung, B.S., JL Elliott, M.A., TK Kuyk, Ph.D.
 
Birmingham VA Medical Center, Birmingham, AL
 
Objectives: Better methods for serving increasing numbers of visually impaired veterans may be devised with knowledge of factors that contribute to performance of essential activities of daily living. High in importance to functional independence is orientation to and mobility within the environment. We have previously reported on visual correlates of obstacle avoidance under photopic and mesopic conditions in veterans with low vision (Kuyk, Elliott & Fuhr, 1998). The purpose of this study was to determine if measuring and combining visual acuity and visual field measures in the manner proposed in the 1999 Guide for the Evaluation of Visual Impairment (Guidebook) would better predict mobility performance on an obstacle course than the measures of visual function we used in our previous research.
 
Methods: Visual acuity, visual field, contrast sensitivity, and scanning data from 156 legally blind veterans in our previous mobility study (described in detail in previous publications) were re-analyzed. Binocular visual fields were measured with Goldmann kinetic perimetry, III4e stimulus at standard background luminance along 12 meridians. Amount of visual field remaining (visual field extend, VFE) was determined by the sum of the 12 meridians, corrected for intervening scotomas. The same visual field tests were re-scored by the methodology proposed in the Guidebook to determine the Functional Field Score (FFS). Monocular and binocular visual acuities that contribute to the Functional Acuity Score (FAS) were obtained from the subject’s records, and these were combined (multiplied) to determine the Functional Vision Score (FVS). Mobility performance measures included time required to walk the course and the total number of errors made during the walk. Correlation and regression analyses were performed. Step-wise linear regression was used to compare the strength of multi-predictor models composed of the new functional scores (FFS, FAS and FVS) versus those composed of the measures of visual function used in the original study.
 
Results: Of the two field (VFE & FFS) and two acuity (high contrast and FAS) measures, only the field measures were significantly correlated with performance. Combining FFS and FAS to form FVS, did not improve correlations, but instead lowered them slightly below those for FFS alone. In general the visual field measure from our original study (VFE) was more highly correlated with the performance variables that FFS. The best predictive models of time to complete and errors made on the obstacle course under photopic and mesopic lighting conditions combined VFE and scanning ability and these could account for upward of approximately 40% of the variance in performance.
 
Conclusions: The method of combining field and acuity measures as outlined in the Guidebook did not provide any advantage in predicting mobility performance over either variable alone. If anything it lowered correlations. This may be due to the fact FVS is obtained by multiplying two variables, one of which (FAS) has almost no correlation with performance. Future plans are to evaluate FVS as a predictor of other outcome measures where both acuity and field may be important determiners of performance.
 
Funding Acknowledgment: This study was funded by the VA Rehabilitation Research and Development Service, project # C775-RA and the VAMC, Birmingham, AL