Devising strategies to optimise home and school life for children with visual impairment due to damage to the brain
Presented on 22 January 2008 by Gillian McDaid, Debbie Cockburn and Gordon N Dutton
Strategies for dealing with visual problems due to cerebral visual impairment
The limitations of the human visual system are well recognised. Visual acuity limits the size of the detail which can be seen, contrast sensitivity limits how faint something can be before it becomes invisible, the visual fields limit the extent of the area over which one can see, and the speed of visual processing limits the speed at which something which is moving can be seen.
At a higher level of visual processing both dorsal and ventral stream functions have their limitations. Dorsal stream function limits how easily something can be seen in a visually crowded environment, and limits the accuracy of visually guided movement, while ventral stream function limits whether what is being viewed, whether it be people, objects or the environment for route finding can be recognised.
Pathology of the visual system can interfere with any of these functions, in any combination and degree, and knowledge of which aspects of vision are affected in both nature and degree can assist greatly in determining how they interfere with daily living and education, and devising matched strategies to ensure that an affected child is not inappropriately disadvantaged by being expected to function in a visual world in which crucial elements cannot be seen by the child.
Children who have profound visual impairment due to cerebral damage may show little or no evidence of visual function. However, many show evidence of reflex visual function in response to a moving stimulus, which is known as 'blindsight'. A small population of such children may be mobile and be able to move freely despite no apparent visual function. This has been termed travel vision.
Cerebral visual impairment of lesser degree can occur in many guises. Impaired visual acuities and visual fields comprise the best recognised and understood disorders. Dorsal stream dysfunction, in particular, is common, and if it is not recognised it can easily be misinterpreted or go unrecognised. Inability to find things combined with lack of attention, may be perceived as bad behaviour and impaired visual guidance of movement may be attributed to 'clumsiness' and inappropriately punished. The adverse effects of this, upon a child with CVI, who knows that he is doing his best, can be profound. It is therefore essential to be able to identify and characterise cerebral visual impairment in all its guises and to implement appropriate strategies, matched to the age of the child and the nature and degree of the deficits identified.
Careful structured clinical history taking as described in the last chapter combined with clinical examination of functional vision, assists in making a diagnosis and characterising the visual deficits and the difficulties that they cause from day to day. Matched strategies designed to ameliorate these problems, which are appropriate for age can then be implemented. The approaches described below have been assembled from a combination of audit of our clinical service, information collected at parent conferences and our clinical consultations.
The principles of functional vision
Vision is used for many aspects of daily living. There are three principal elements:
Gaining access to information. Both the near and distant surroundings are monitored and analysed. Distant information, whether, for example, this is the presence of trees or the names of shops, is constantly being assimilated. While prolonged near vision is required for example, to access information from the printed page and for craftwork.
Social interaction is reliant upon vision to a large extent. The ability to identify people in a group, and the ability to recognise them, are necessary to make contact, while social interaction involves the ability to see, recognise and understand the linguistic elements of facial expression, gesture and body language.
Visual guidance of movement not only facilitates reach and manipulation but also guides movement through the visual world, whether by walking, cycling or driving.
Each of these elements has limitations, which are intuitively recognised by the general population in the design of our visual environment, in our social interactions and in our everyday lives. There is no point in printing advertisement hoardings or printed material with too much information, or with images or print, which cannot be seen because it is too small. The size of a crowd in which it is difficult to find someone, or the distance at which it is not possible to see someone are also recognised at an intuitive level and when it comes to moving through the visual world, it is necessary for example, to slow down to drive through a narrow gap, on account of both the spatial and temporal limitations of visual processing. The age at which print size and crowding can be accessed is also recognised by the printers at an intuitive level, with print size diminishing and print crowding increasing progressively as the targeted age increases.
In children with cerebral visual impairment any of these aspects of daily living can be impaired when any of their thresholds for perception have been exceeded.
Strategies matched to specific visual dysfunctions
Impaired visual acuities and contrast sensitivities
Impaired visual acuities and contrast sensitivities limit access to information, both for near and distance. Assessment of these functions while the child is viewing with both eyes open in order to determine what can be seen at maximum speed, is required to ensure the provision of optimal materials, both at home and at school, and to inform families of what can and cannot be seen on a day to day basis.
Access to information
It is the detail within the picture which is lost as acuity decreases, and the overall accessibility of the information which diminishes as contrast diminishes.
The strategies required to assist children with low visual acuities and impaired contrast sensitivities are well recognised and include:
- The provision of high contrast enlarged educational material, books with large clear illustrations in bold contrasting colours, in which the elements of detail within the pictures can all be seen.
- Low vision aids for both near and for distance can prove effective. Provision of magnifiers for young children allows them to be used as part of play and everyday life. Children who are short sighted may choose to take their glasses off and to hold books and objects close to their eyes, as this affords magnification.
- On the other hand, children who are long sighted are afforded magnification by their spectacle correction.
- Computing equipment with software designed to enlarge text and images can also prove helpful.
The 'facial expression recognition distance' can easily be elicited. The examiner adopts different facial expressions, at progressively increasing distances from the child, until they cannot be identified.
The same approach applies to the face recognition distance.
Reduced visual acuity and contrast sensitivity impairs the ability to identify obstacles and variation in the height of the ground. Good lighting enhances contrast and the ability to navigate independently by means of vision.
Visual field deficits, inattention and neglect
The commonest visual field deficits in children with damage to the brain comprise hemianopia and lower visual field impairment. The lower visual field impairment found in children with periventricular white mater pathology may be either absolute or diffuse, characterised by reduced sensitivity of vision in the lower fields. The approaches to circumvent these problems are presented below.
Blindness and blindsightChildren with occipital damage who have no detectable vision by means of formal testing, may react to silent moving objects and may manifest oculokinetic nystagmus (Boyle et al 2006). Adults with damage to the occipital lobes often have a degree of perception of movement which may be either conscious or subconscious.This form of vision has been called 'blindsight' (Weiskrantz 1998). Soldiers who sustained occipital injury during World War I were found to be aware of movement in the 'blind' visual field. (Riddoch 1917) This is known as statokinetic dissociation or the Riddoch phenomenon. Adults with blindness due to cerebral damage may have a relatively subconscious awareness of moving targets, lights, and colours. (Weiskrantz 1998), and rocking to and fro may generate a visual image which is not present when the movement stops. (Dutton) The brain structures that may be responsible for blindsight include residual striate cortex, light scatter from the seeing hemifield, extrastriate cortex, and the superior colliculus and pulvinar (Cowey and Stoerig 1991, Braddick et al 1992, Payne et al 1996, Stoerig et al 1998, Stasheff and Barton 2001). Blindsight can be difficult to elicit in young children and in those with cognitive and physical impairment. Accurate determination of whether or not a child has true blindsight may therefore not be possible. However, it is not uncommon for carers of children with cerebral visual impairment to observe the child reacting to movement in the peripheral visual field, but not centrally. It appears that the phenomenon is fatiguable and may only be present intermittently.
- Movement in the peripheral visual field may elicit a smile in the blind child with quadraplegia and profound intellectual disability.
- Children who are fed with a spoon may intermittently open their mouths to receive food when the spoon is moved in an arc from the peripheral visual fields, but not when it approaches the mouth from straight ahead.
- For those children who understand language stating what is being seen as the child reacts to it may enhance both visual and language development.
- Such children may rock to and fro. Whether this generates an image is difficult to know.
- Rarely children with cerebral blindness who are mobile move slowly around obstacles. This phenomenon has been called travel vision.
Auditory guidance of movement
Children who are totally blind due to damage to the optic chiasm or focal occipital pathology, but who are otherwise fully mobile and have normal intellectual function use a range of auditory cues to analyse their surroundings. Both analysis of the passive soundscape and use of echolocation as an active means of characterising identifying the location of obstacles in the surroundings, has in our experience, proved very helpful in a small number of cases. Echolocation requires the subject to make clicking sounds. It has been claimed that the resultant echoes afford the facility to analyse the surroundings. (Observation of individuals with two glass eyes, who are skilled in this technique, cycling in traffic, leaves little room for doubt.)