Aspects of vision loss – visual functions and functional vision

To promote consistency in terminology, this paper explores the distinction between Visual Functions, which describe how the eye functions, and Functional Vision, which describes how the person functions in vision-related activities. Adequate assessment of Functional Vision is essential to measure outcomes of vision rehabilitation. Consistent measurement will require the development of a unified set of criteria, better suited to use in rehabilitation than the current ‘participation’ categories in International Classification of Functioning (ICF). Whether vision tests are used to explore the underlying causes of eye disease or their functional consequences affects the ways in which they are administered and interpreted. This is explored in detail for letter-chart acuity (the most common visual function test) and reading (the most common functional vision test). At the group level a statistical relationship between visual functions and functional vision can be shown, but individual variations are too great to use this relationship for individual predictions. Definitions of Ranges of Vision Loss are discussed that fit the visual function aspect as well as the functional vision aspect. In this context, the proper use of the terms ‘blindness,’ and ‘low vision’ and the more inclusive term ‘vision loss’ is discussed. Labeling patients with residual vision as ‘blind’ may have adverse psychological consequences. For calculating a statistical estimate of functional vision, based on visual function measurements, the revised AMA Guides can be used. The AMA approach is useful for certain statistical and administrative applications, but should not be used to predict individual rehabilitative needs.