Post by rickismom on Jul 23, 2005 16:32:14 GMT -5
I recently saw this and thought that maybe not everyone here would know about it. It is necessary, when getting your child's vision checked, that the also check for accomadation in close vision:
Accommodation (focusing at near)
Conventionally, we measure visual acuity and refractive errors for distance targets. However, children spend relatively little of their time looking into the distance. Children's interests are mostly close at hand, and this is where most of children's learning takes place. It is here that we find the greatest differences between vision in children with Down's Syndrome and children who do not have Down's Syndrome..
Usually, children focus very easily and very accurately on near targets and it is only as we approach middle age that we expect to experience difficulty. We find, however, that most children with Down's Syndrome focus very poorly- they tend to under-accommodate by quite a large amount, whatever the distance of the target. This is consistent for any individual child, and persists even when the children wear their glasses to correct long sight. This means that close work, especially in school, must be more difficult for these children because it is out-of-focus.
We do not yet know the reason for the poor focusing. One explanation might be that the children's visual system does not recognise blur as easily as the visual systems of people who do not have Down's Syndrome. An alternative explanation is that the co-ordination between the two eyes is weak and might mean that the focusing mechanism does not get the usual feedback from the alignment of the two eyes. We have studies underway to examine these possibilities.
Whatever the reason behind this poor focusing, there may be a link with the other problems that the children develop. Emmetropisation is not well understood, but seems usually to occur because the eyes recognise that the image is out of focus, and that can signal to the growth mechanisms that adjustment in size and shape of the eyes is needed. It may be that inadequate recognition of blur causes poor accommodation in people with Down's Syndrome and the same fault prevents the eyes from achieving proper growth towards emmetropia. Similarly, the poor focusing means that the eyes have an out-of-focus image whenever a child looks at near objects, which is most of the time in young children. This may mean that fine discrimination mechanisms do not have the opportunity to develop, and that visual acuity is always poorer than it might otherwise be. The above links are, for the present, speculative, but we do have some evidence from our study to back up these possibilities. Some of the children in our study focus accurately, and have done consistently since an early age. Some other children are improving their focusing as they get older. These children and young people are much less likely to have refractive errors and are much more likely to have good visual acuity, than the children who focus poorly.
Our latest study shows that it is possible to dramatically improve the children's focusing with bifocal spectacles. In a controlled trial, we supplied bifocals to a group of 17 primary school children with Down's syndrome, and conventional spectacles to a second group (the control group). The two groups were matched for all of the factors that might influence spectacle use or near work, such as age, cognitive ability, school placement, etc. Over a 20 week trial, the children in the bifocal group consistently focused more accurately on near work than did the children in the control group.
In the trial, and now that we prescribe bifocals clinically, we find that children with Down's syndrome wear bifocals very successfully. None, so far, have encountered any problems, and several of the children prefer to wear their bifocals all of the time rather than keep the for school use. Two children were very reluctant to wear glasses when they had conventional ones, but now wear bifocals very happily. In some cases, teachers and classroom assistants have reported improvement in concentration and quality of work when the children wear bifocals. We are now, therefore, recommending that all children with Down's syndrome who show poor focusing are prescribed bifocals.
The positioning of the bifocal is very important. The top of the bifocal should lie across the child's pupil (this is a much higher position than usual for bifocals) so that the child can look down through the bifocal without effort.
see:
www.cddg-downs.org.uk/doctors/parenthandout_vision.doc
for full info and oic of bifocal size
Accommodation (focusing at near)
Conventionally, we measure visual acuity and refractive errors for distance targets. However, children spend relatively little of their time looking into the distance. Children's interests are mostly close at hand, and this is where most of children's learning takes place. It is here that we find the greatest differences between vision in children with Down's Syndrome and children who do not have Down's Syndrome..
Usually, children focus very easily and very accurately on near targets and it is only as we approach middle age that we expect to experience difficulty. We find, however, that most children with Down's Syndrome focus very poorly- they tend to under-accommodate by quite a large amount, whatever the distance of the target. This is consistent for any individual child, and persists even when the children wear their glasses to correct long sight. This means that close work, especially in school, must be more difficult for these children because it is out-of-focus.
We do not yet know the reason for the poor focusing. One explanation might be that the children's visual system does not recognise blur as easily as the visual systems of people who do not have Down's Syndrome. An alternative explanation is that the co-ordination between the two eyes is weak and might mean that the focusing mechanism does not get the usual feedback from the alignment of the two eyes. We have studies underway to examine these possibilities.
Whatever the reason behind this poor focusing, there may be a link with the other problems that the children develop. Emmetropisation is not well understood, but seems usually to occur because the eyes recognise that the image is out of focus, and that can signal to the growth mechanisms that adjustment in size and shape of the eyes is needed. It may be that inadequate recognition of blur causes poor accommodation in people with Down's Syndrome and the same fault prevents the eyes from achieving proper growth towards emmetropia. Similarly, the poor focusing means that the eyes have an out-of-focus image whenever a child looks at near objects, which is most of the time in young children. This may mean that fine discrimination mechanisms do not have the opportunity to develop, and that visual acuity is always poorer than it might otherwise be. The above links are, for the present, speculative, but we do have some evidence from our study to back up these possibilities. Some of the children in our study focus accurately, and have done consistently since an early age. Some other children are improving their focusing as they get older. These children and young people are much less likely to have refractive errors and are much more likely to have good visual acuity, than the children who focus poorly.
Our latest study shows that it is possible to dramatically improve the children's focusing with bifocal spectacles. In a controlled trial, we supplied bifocals to a group of 17 primary school children with Down's syndrome, and conventional spectacles to a second group (the control group). The two groups were matched for all of the factors that might influence spectacle use or near work, such as age, cognitive ability, school placement, etc. Over a 20 week trial, the children in the bifocal group consistently focused more accurately on near work than did the children in the control group.
In the trial, and now that we prescribe bifocals clinically, we find that children with Down's syndrome wear bifocals very successfully. None, so far, have encountered any problems, and several of the children prefer to wear their bifocals all of the time rather than keep the for school use. Two children were very reluctant to wear glasses when they had conventional ones, but now wear bifocals very happily. In some cases, teachers and classroom assistants have reported improvement in concentration and quality of work when the children wear bifocals. We are now, therefore, recommending that all children with Down's syndrome who show poor focusing are prescribed bifocals.
The positioning of the bifocal is very important. The top of the bifocal should lie across the child's pupil (this is a much higher position than usual for bifocals) so that the child can look down through the bifocal without effort.
see:
www.cddg-downs.org.uk/doctors/parenthandout_vision.doc
for full info and oic of bifocal size
