Vision is a complex sensation that provides us with a personal conscious representation of our surrounding environment. Loss of vision or blindness may limit a person’s ability to complete normal, daily activities and decrease overall quality of life. Visual impairment can impose various limitations on a person’s functional ability including reading, mobility (which includes driving), visual information processing (also called “seeing”), and visually guided motor behavior (also called “manipulation”).
Loss of vision due to disease or disorders of the visual system can result in problems associated with resolution of detail, field of view, appearance of contrast, appearance of colors, appearance of motion, resolution of depth, seeing in dim light, and disablement from glare. While each of these aspects of vision is important, visual acuity and visual fields are key aspects of vision that are commonly tested in clinical settings. Visual acuity tests are used to measure impairments in visual resolution or focus that can be caused by blurring of the retinal image, neural processing disorders, or damage to neurons in the retina or other parts of the visual pathway. Visual acuity is measured as the ratio of the viewing distance to the size of the smallest letters that can be read or smallest symbols that can be recognized on a standard vision chart. Visual field tests are measures of impairments in the person’s field of view that can be caused by disorders of the retina (e.g., retinal detachments), optic nerve (e.g., glaucoma), or visual parts of the brain (e.g., stroke). Visual fields are measured by mapping out blind or visually impaired areas in visual space. The person being tested is asked to report when spots of light or various types of patterns presented at different viewing angles in every direction can be seen while the person is looking straight ahead.
The impact of vision loss on functional abilities can be evaluated with visual performance measures or patient-reported measures. A questionnaire can be used to assess a patient’s own perception of his or her ability to perform specific types of activities in everyday life. Functioning and well-being are often assessed by using questionnaires such as the National Eye Institute’s Visual Function Questionnaire (NEI VFQ). The instrument typically uses a list of statements about specific activities that are answered in terms of frequency of occurrence, level of agreement, perceived difficulty or some other response to each item using a categorical response rating scale.
For the NIH Toolbox, we have selected three instruments for vision assessment: a measure of visual acuity, a measure of visual field, and a self-reported measure of functional ability and well-being. Visual acuity will be evaluated using the ETDRS visual acuity test, an existing standardized instrument designed to allow for comparison of data across research studies. The measure will be embedded within the Dynamic Visual Acuity Test being used to assess vestibular balance. For younger children, the letters H-O-T-V and symbols will be used.
Visual field detection will be measured with a modified motion perimetry instrument specifically designed for NIH Toolbox. Motion detection perimetry is a computer program developed at the University of Iowa that maps the visual field by measuring a subject’s ability to detect a coherent shift in position of dots in a defined circular area against a background of fixed dots. The main outcome measure, motion size threshold, is defined as the smallest detectable circular area in which dot motion is detected. The NIH Toolbox version is a binocular test of the central 40° of the visual field. The next steps in the development of the motion perimetry instrument include test validation and determination of normative limits.
A modified instrument to measure visual function based on existing vision-specific quality of life instruments was developed for NIH Toolbox using the PROMIS approach (De Walt, 2007).