In a new series about children who have difficulties with processing visual information, Janet Harwood describes the impact of CVI.
Cerebral visual impairment (CVI) is the commonest cause of visual impairment in children in the developed world, yet often goes unrecognised or is little understood.
In addition to our eyes, a large part of the brain is involved in “seeing”. So conditions which affect the structure or function of the brain often influence how a person “sees” and responds.
The image made by our eyes is converted to electrical signals and sent to the occipital lobes at the back of the brain to be processed.
This visual information is then carried to the posterior parietal lobes of the brain, which are responsible for guiding our movement, being able to see lots of things at once and allowing us to give attention to different components of a visual scene.
Information is then passed on to the frontal lobes of the brain where decisions about where to move to, and what to pick up are made, accompanied by the rapid eye movements needed to examine items closely.
This pathway is called the dorsal stream and when the visual processing system works properly, it operates in a split second and is unconscious.
The temporal lobes in our brains are the visual “filing cabinet”. This is a conscious system known as the ventral stream. Here, from the day we are born, visual “memories” are stored in appropriate files and then retrieved when that image is met again. We have areas for recognising faces and facial expression, shape and form and finding our way around.
The dorsal and ventral stream are designed to work together seamlessly. When one or both go wrong, visual processing problems occur.
CVI affects each child uniquely and to differing degrees. An affected child may also have an eye condition causing visual impairment, but many people affected by CVI may have “healthy eyes”, yet function as if severely visually impaired. It is vital that difficulties are identified and appropriate strategies are put in place to support the child. Here are some of the ways that CVI might affect a child:
Sometimes there is no obvious cause of CVI. However it is vital that children who may have the following conditions, which sometimes cause or are associated with CVI are carefully monitored for evidence of CVI:
an episode of lack of blood supply or oxygen to the brain, head injury, meningitis and encephalitis, hydrocephalus, low blood sugar at birth, children born prematurely before 34 weeks, lesions as a result of a stroke or “bleed”, cerebral palsy, autistic spectrum disorder, as well as some chromosome disorders and syndromes.
Many of the standard special needs assessments generally fail to identify children with CVI. If anything there is a danger that they may be misdiagnosed as possibly having autistic spectrum condition, due to some of the characteristics being shared. Not being able to maintain eye contact or respond to facial expressions, may be due to poor contrast sensitivity or to the part of the brain that recognises facial expressions being impaired.
Of course CVI and autism can co-exist but it is important that if CVI is present, appropriate educational provision is made and any visuals used for communication meet the child’s needs.
Where a child has a co-existing medical condition all too often this becomes the focus of input and educational support. If the CVI is unrecognised or its impact ignored, potential for habilitation may be lost. It is then all too easy to presume that any developmental delay or other problems are due to the dominant medical diagnosis. This is a missed opportunity to provide an individualised, structured programme of support which may make a huge difference to outcomes for the child.
Whenever CVI is suspected, a thorough eye examination by an ophthalmology team is essential. This can either rule out any ocular reason (relating to the eye) which may explain behaviours, or identify any ocular impairment that is present too. Children should be screened for visual field impairment and contrast sensitivity be checked.
In Brighton we are setting up clinics with our ophthalmology department to ensure children are assessed and refracted properly. Dynamic retinoscopy should be asked for where CVI is suspected, otherwise the child may not receive the correct refraction due to slower accommodation.
If CVI remains undiagnosed and its impact goes unrecognised, this may lead at the very least to a child’s intellectual capacity being underestimated and at worst being misdiagnosed and receiving inappropriate advice and support.
A child who has CVI and their family is entitled to the full support of the local authority visual impairment team of specialist teachers. CVI can affect all aspects of educational, physical, social and emotional development. In the next in this series we will look at the life changing effect that providing appropriate support can have.
Janet Harwood is a qualified teacher of visually impaired children with a passion for CVI, Brighton and Hove Sensory Needs Service