Functional Vision Assessment Service
What is visual function?
Visual function refers to how well a person sees. In a clinical setting, visual function is determined through a number of tests that measure different aspects of vision. These include measurements of visual acuity (sharpness of central vision), visual fields (peripheral vision) and other measures such as the ability to distinguish contrast between objects (contrast sensitivity), binocular vision (for depth perception) and color vision.
What is a visual field and what are visual field defects?
The visual field is the total area in which objects can be seen in peripheral vision while the eye is focused on a central point. If you think of your field of vision as a wide half circle visual field restriction can block the right or left portions of the hemisphere, or one of the corners or the lower portion of the hemisphere. To see for yourself what it may be like to have a visual field defect, visit our visual field simulations developed for e-advisor, a resource for families and teachers of visual impaired children.
How do visual field defects affect how a child functions?
In children with certain neurological problems or eye problems, field of vision may be restricted and this can cause difficulties in the child's mobility, reading ability and spatial awareness. Children with severe field defects often also experience developmental delays. It may be difficult for parents to know that there is a problem with the child's peripheral vision because central vision (measured in visual acuity tests) may be normal.
These visual field restrictions can affect function in a number of ways. For instance, they impact scanning eye movements used in reading and thus make reading more inefficient. A child with a visual field defect may only see part of a large object and must compensate by turning or tilting his or her head. Children with a visual field defect may frequently bump into things and have difficulties localizing objects that fall into the non-seeing field. They may have difficulty seeing objects and people at a distance, even when their visual acuity is normal. A child with an inferior field miss objects in the lower field of vision, such as food on the table, or may have trouble anticipating stairs.
How are visual fields measured in a baby or young child?
Testing infants and young children for visual field defects has traditionally been very difficult to do. The Visual Function Service at Children's Hospital is one of the only places in the country where specialized tests created specifically for this young age group are available. These methods are used in accordance with a child's age, developmental status and the known level of visual impairment. They were developed in research studies here at Children's that involved infants and young children both with and without eye disorders and in patients with developmental disabilities.
All forms of testing for visual field loss measure a person's response to stimuli presented at various locations in side vision. In adults, visual fields are tested using a hemispheric perimeter. The adult sits before a concave dome and stares at a central target within the dome. A small light is presented a various peripheral locations on the surface of the dome and the adult indicates when the light is detected. Responses are then plotted on a visual field map and compared to age-matched controls to determine whether there is a field defect. Children and infants are tested with similar perimeter instruments using larger lights that flash. The child's natural response is an orienting look to the peripheral light. A video camera in the center of the perimeter records the child' looks. A visual field defect is indicated if the child does not look consistently to lights in one part of the perimeter.
In addition, examiners also make informal observations, using special games that involve puppets, dolls and other toys to evoke visual responses from young children. These creative games help us to observe how a child uses vision to function.
How precise are the results of these tests?
A baby or child's visual field is very difficult to measure. It is not possible to measure the visual field precisely in children until they can cooperate for standard perimetric tests used for adults. Our goal with tests developed here at Children's is to identify major field defects, that is, half or quarter field defects, in young children. Milder field defects can be detected based on our observations, but not with absolute precision.
Determining that a child has a visual field restriction then leads to specific recommendations that help a child function in day to day life. Finding that a child does not have a major field defect means that apparent difficulties in peripheral vision must be due to other factors.
What is visual acuity and contrast sensitivity and how will they be measured?
Visual acuity refers to clarity or sharpness of central vision, rather then the extent or clarity of peripheral vision. Most people who are familiar with going to the eye doctor, have acuity measured using the Snellen Eye Chart, which is made up of rows of letters, numbers or other symbols that become progressively smaller. At our Visual Function Service, a variety of specialized methods, such as visual acuity cards and preferential looking methods, are used to measure acuity. These methods are also adapted for age, developmental level and visual impairment and are complemented by electroretinogram and visually evoked potential as needed.
In addition, we measure contrast sensitivity, which is the ability to see objects that have low contrast features or contrast poorly with other objects or a background. When possible low contrast detection is assessed using simple schematic faces and symbols or letters. Contrast sensitivity testing is important because in some conditions, it is much more reduced than visual acuity and it has important implications for functional vision. For more information, see visual acuity and contrast sensitivity on the e-advisor.us website.
What happens after testing?
The results of our assessments are communicated directly to parents, educators and any therapists who are involved with the child's care. We make a concerted effort to interpret real-world behaviors of our patients and contribute to their rehabilitation and educational interventions. Communication with teachers of visually impaired children who provide services through the child's early intervention and school programs is essential and enables us to translate the results of our assessment in the practical realm.
How long is the examination?
The actual testing will take anywhere from 45 minutes to an hour and a half, depending on the age of the child, and the child's willingness to cooperate. Our testing methods are very child-friendly and our staff is trained and knowledgeable in communicating with children of all ages. However, we recognize coming to the hospital may be a daunting experience for children and some children have difficulty cooperating. In these cases, we do our best to accommodate the child. If necessary we reschedule for a time and day in which the child is more likely to cooperate in testing.