J E Lovie-Kitchin¹, R L Woods², S E Hassan3, G P Soong4
¹Queensland University of Technology, Centre for Eye Research, Brisbane, Australia; ²Schepens Eye Research Institute, Harvard Medical School, Boston, United States; 3Johns Hopkins University, Wilmer Eye Institute, Baltimore, United States; 4University of Auckland, Department of Optometry and Vision Science, Auckland, New Zealand
Background: Previous studies have shown that the remaining visual field (VF) size has a significant effect on mobility performance of people with low vision (LV). However, the degree of VF loss at which mobility is impaired is not known. This study attempted to determine the VF size for safe and unsafe mobility performances for adults with LV. Methods: Subjects were aged 25 to 90 years; 79 subjects had LV and 20 age-matched subjects had normal vision. Kinetic visual fields were measured with the Humphrey Field Analyser IV 4 barE target. Subjects' preferred walking speeds (PWS) were measured on an unobstructed corridor and mobility was assessed as Percent Preferred Walking Speed (PPWS) and number of errors on a 79-metre, obstacle-rich, indoor course. The weighted kappa coefficient of association (rescaled sensitivity and specificity) of mobility scores that discriminated between subjects with normal vision and LV was used to determine the VF size at which mobility became unsafe (many errors) and inefficient (low PPWS). Results: While a VF diameter of about 85° discriminated between LV and normal vision subjects, more specific criteria (fewer false positives) of PPWS <37% and errors >6 were preferred. Among the LV subjects, a VF diameter <52° identified those subjects who failed both mobility criteria (21% of LV subjects) with an accuracy of 83%. Conclusions: Safe and efficient mobility begins to be compromised at a VF diameter of about 85°, but orientation and mobility assessment is indicated when VF diameter is reduced to about 50° or less.
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