Cone adaptation problems
Loss of rod function and changes in cone function may start at any time in pre-school years. Fear of darkness and clumsiness in twilight may be signs of beginning night blindness. If parents are aware of this they will recognise and understand the first symptoms when the child clings to them in twilight or turns on lights before other children.
Changes in visual adaptation can be estimated in a play situation using red, blue and white pieces small chips (Precision Vision's CONE Adaptation Test Game). The chips are mixed on a dark surface and the child is asked to pick them and to put each colour in a separate pile. A normal two-year-old child can do this with little training. When the child has demonstrated his/her ability to follow the instructions, he/she is told to play the same game in semidarkness.
The luminance value of the red and blue chips is close to equal at low mesopic (twilight) luminance, so that the luminance difference cannot be used to separate the chips but the colours must be seen. We are more sensitive to blue light in twilight than we are to red, therefore the blue pieces are darker in photopic (daylight) vision. The mesopic light level of the test situation is chosen so low that the tester (with his/her normal vision) starts to see the colour difference between the red and the blue chips after 4-5 seconds of adaptation.
Another practical test of cone adaptation in deaf children can be arranged after watching TV during a dark afternoon. When the program is over, the TV-set is switched off and the room lights are not turned on. The children are signed a short question or asked to do something immediately. The child who does not respond is examined for dark adaptation.
Photophobic children need absorptive lenses, darkened spectacle lenses, usually more than one pair, if non-photochromatic lenses are used. Because of variations in the loss of sensory cells, there is no single lens type that can be recommended for all retinitis pigmentosa children. Several absorptive lenses should be tested, preferably by letting the child borrow them for a week-end or longer and deciding for him/herself which are the best. If photochromatic lenses are available, they may be a good choice. Some children prefer filter lenses before school-age.
Cosmetic factors are important, especially in teenage years. It is possible to hide the socially unacceptable but otherwise good red filter lenses with a polarising surface. Filter lenses with the polarised surface are pleasant brown (Multilens, 435 25 Mölnlycke, Sweden) . They are plastic lenses and thus lighter and less breakable than the older glass lenses. Side shields and upper shields can be made rather inconspicuous.
Filter lenses should always be tested in case of loss of cone function at the macula, because this may lead to increase of rod function in daylight and therefore photophobia and decrease of image quality. Some of these filter lenses have transmission in blue, so blue cones adapt. If there is no transmission in blue, the sky and other blue surfaces are seen dirty gray which many persons experience unpleasant or even depressing. Transmission of light within the range of rod absorption is minimal, so rods function at the mesopic level. Thus the filter allows the child to use both cone and rod vision in daylight, which results in better quality of information than if regular absorption lenses were used.