Visual disability in this group of children is caused by changes in the anterior part of the visual pathways, i.e. eyes and optic nerves causing changes in the image quality but the use of visual information in the higher visual function is normal. Children with disorders of their higher visual functions are discussed in Chapter 7.

Down Syndrome is the most common cause of intellectual disability associated with anterior visual pathway disorders in children. If the developmental delay is mild to moderate, it does not affect the assessment in any notable way. The techniques are chosen to suit the communication and cognitive abilities of the child, i.e. tests are those for younger children.

Quite often the training for the test situations is needed in the day care or class room teaching for weeks or even months before formal testing is possible. Often we start with such simple exercises as moving a finger along a straight line. Since the concepts of 'similar' and 'different' are taught in colours and forms as a usual component of standard teaching programs, it is easy to include the colours and forms used in the test situation in classroom teaching. Measurement of visual field is based on observations in play situations and confrontation fields that may be performed as a game.

In each group of children with different developmental delays there are specific features that should be assessed. For example many children with Down Syndrome have strabismus, which should be treated early, and they may have impaired focusing of the eyes (poor accommodation). In many countries there is no tradition of treating insufficient accommodation in infancy and early childhood, and treatment of strabismus is commonly postponed in children with developmental delay. Careful measurement of visual acuity at distance and at near is therefore necessary, the near vision acuity being more important for learning. If the child cannot accommodate, visual acuity at near distance is worse than visual acuity at greater distances. In such cases reading lenses are needed in near work.

While testing of visual acuity, it may be found that a child with Down Syndrome performs well with line test measurement at near but cannot respond at the distance of three meters. Distance visual acuity at three meters needs then to be measured with single symbols instead. A response like this clearly demonstrates to the teacher that the child is unable to look at words, or other crowded information on the blackboard but is able to appreciate the same words at near.

Cataractous changes and changes in refraction may develop at any age in people with Down's Syndrome. Therefore the ophthalmological examination should be repeated at the beginning of each school year.

Spectacle corrections are often problematic. The reasons for not accepting the glasses vary. If a high minus correction is given as full correction, the dramatic change in the size of objects and distances and the change in the image quality may be too much to be tolerated. Myopic correction (= minus glasses) does not need to be full correction for distance vision (undercorrection of myopia improves the quality of near vision). For the same reason hyperopic correction (= plus glasses) may be 1-2 diopters overcorrected, sometimes more if the child's cognitive sphere is limited. It is then important to notice that when the child starts to pull the plus glasses away this can be due to an increase in the cognitive space and thus bifocal glasses may be needed.

Correction of an astigmatic refractive error may initially cause disturbing tilt of lines and therefore be unacceptable and initial undercorrection is required. The introduction of a spectacle correction can be easier if the child has used empty spectacle frames for some time. Like most other problems in medicine, problems with glasses are individual; some children accept full correction of astigmatic error and spherical correction. When the child reacts negatively, we should find out 'why'.

During courses for special educators on visual disability, the effect of spectacle correction has been demonstrated by fitting high minus or plus contact lenses and then giving corresponding opposite spectacle lenses to correct the induced refractive error. In each case the adult person could not tolerate the glasses because of the distortions of the image and preferred the blurred image to a clear image with distortion. If adults react in this way, we should expect similar reactions from children if high refractive errors are fully corrected at once. Correction can be made stepwise when the child has become accustomed to the use of simpler lenses.

Some children with intellectual disability and myopia learn to use the "pin hole" effect to see clearly. They turn their head and look through the small hole created by the edge of the pupil and the corner of the eyelid. A small hole or pin hole, effectively compensates for refractive errors and gives a clear enough image for orientation. The child sees where to go and once close enough to the object can see it quite well because of nearsightedness. In cases like this, glasses may be acceptable only outside, if at all. This phenomenon is quite common in children with Cornelia de Lange Syndrome.

In the correction of refractive errors the functional visual sphere should be kept in mind and the spectacle correction fitted to function within that sphere. Usual distance glasses become useful only when the cognitive sphere of the child increases to include objects at greater distances.

The spectacle frames provided for some children with intellectual disability are of poor quality, too big and clumsy. This is sad because large lenses cause distortions of the peripheral image and thick, heavy lenses make the frames to slip down the nose. Both features disturb visual function, especially vision for orientation and mobility.

If a child does not accept spectacles, all demanding tasks must be kept at the distance where the child sees best, and within the dimension of the uncorrected visual acuity for that distance.

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