3.1. ASSESSMENT OF VISION IN DISORDERS AFFECTING ROD AND CONE CELLS INCLUDING CHILDREN WITH HEARING IMPAIRMENT
Assessment of visual functions in children who have retinal disorders affecting rod and cone cells requires several specific test situations that were briefly discussed in Part I. Slowly progressive retinal dystrophies and degenerations occur also in hearing impaired children, sometimes as part of a syndrome. In Scandinavia and in some other parts of the world, Usher Syndrome is common and therefore is given a lengthy description in this chapter. The visual problems of hearing children are the same as those of hearing impaired children but communication during testing is a special feature in assessing deaf children.
Deaf children are amazing in their ability to understand poor sign language. They seem to find it amusing that a grown up doctor cannot express herself any better. Therefore it is possible to have direct communication with a deaf child even with rudimentary capabilities in sign language. However, a person with good communication in sign language needs to be available, either an interpreter accustomed to ophthalmologic examinations or a teacher with similar communication capability (if an interpreter is not available) to make sure that information is correctly understood by both parties.
In both normally hearing and hearing impaired children retinitis pigmentosa is usually the only retinal disorder they have. However, it is possible to have retinitis pigmentosa and simultaneously another retinal disorder. A child may have lost hearing and have retinopathy due to maternal rubella. Rubella retinopathy is a stable condition. If that child has inherited retinitis pigmentosa, the development of visual field defects and night blindness may occur at an earlier age than usually.
When retinitis pigmentosa and sensory hearing loss occur in a child, they may be caused by Usher Syndrome or may be unrelated. Hearing impairment can be caused by an infection or may even be traumatic. The appearance (phenotype) is the same in all these cases, dual sensory impairment but in terms of prognosis and genetic counseling they are separate entities.
In a sporadic case of dual sensory impairment in a 10-year old child with asymmetric loss of visual field, the likely cause is recessive retinitis pigmentosa combined with an unrelated disorder or disease causing hearing loss and possibly some damage to the retina. In Usher syndrome the changes are usually more symmetric.
In hearing impaired children, retinitis pigmentosa should always be remembered when the diagnosis of severe hearing loss is made during the first year of life. It should also be kept in mind in an older child with a less severe hearing loss. Retinitis pigmentosa and related retinal disorders start to affect retinal functions during the first year of life but functional changes are usually not detected before the age of 18 months. However, in families with an older child diagnosed having retinal degeneration, the parents may notice a decrease in the speed of dark adaptation during the first year of life.