L. Hyvärinen

Lea-Test Limited,
Apollonkatu 6 A 4,
FIN-00100 Helsinki,
Finland

Present classification of low vision states that a person has low vision if his visual acuity is less than 0.3 or visual field less than 10 degrees from the point of fixation. It does not define the methods used in these measurements.

The best known group of visually impaired people with normal visual acuity values are persons with multipple sclerosis whose "hidden visual loss" was described by Regan and coworkers in 1977. Other common disorders that may cause loss of low contrast vision without loss of visual acuity are cataract, diabetes and glaucoma. Measurement of visual acuity at low ( 1-2% ) contrast levels requires less than a minute testing time and depicts function at low contrast levels that are important in communication and orientation.

If visual acuity is measured with single symbol tests at distance, normal or near normal values may be present, yet the child’s visual acuity measured with line tests may be half of the single symbol value and visual acuity measured with more crowded symbols at near may be less than 0.1. Such a child may need to use a CCTV for reading despite a visual acuity value 0.6 – 1.0. In the assessment of vision of children and adults with brain damage related vision loss, simple measurement of visual acuity using tests with varying degrees of crowding reveals the quality of central vision and should thus become a routine investigation in low vision.

Boys and men with mild forms of retinoschisis usually have visual acuity 0.2 - 0.25, yet function like normally sighted at school, in sports and in road traffic where many of them have driven cars more than a million kilometers without accidents or tickets.

Another typical group of "normally sighted persons with low vision" are persons with ’hemianopia’ caused by circulatory accidents in the posterior parts of the optic radiation. Although Goldman-perimetry shows loss of visual field there may be normal sensitivity to flickering stimuli. In retinitis pigmentosa it is possible to record flicker sensitivity within an ’absolute’ ring scotoma. Magnocellular information is used in detection of objects in the peripheral visual field more efficiently than sustained visual information and thus its quality should be evaluated in occupational assessments.

The above three groups need to be recognized as nondisabled because they perform like normally sighted people in visual tasks. In low vision, evaluation of visual field should consider both parvo- and magnocellular information and the functional quality of a person’s field of vision.

Visual acuity value and the size of the visual field depict only two of the many aspects of visual function. We see forms, colours and movement. Low vision evaluation should cover all three main functional areas and use test situations that reveal capabilities and deficiencies in different visual tasks. Uncritical use of visual acuity values and visual field findings now prevents numerous people from working in some occupations although they are able to perform well in those tasks. On the other hand, for the same reason visually impaired people are denied services they need.