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 Approximately
one in a thousand children aged 0-16 years is visually impaired.
This represented 50 children in an average health district with
a population of 250,000 (1), a significant proportion of whom has
cerebral vision impairment.
Children
with multiple disabilities are frequently referred for refraction
and assessment of visual function, so that parents and carers can
be advised about and understand what the child can and cannot see.
The aim of this article is to outline the principle visual problems
and to suggest approaches to their management.
 Assessment
of vision
The
goals of assessment are to determine the functional vision available
for communication, education, navigation and other activities, and
to advise on methods of enhancement and compensation to circumvent
the visual problems and enhance development for each individual
child.
History
A detailed
history can help compensate for what may necessarily be a limited
examination in these children.
The
profoundly visually impaired child may show evidence of "blind sight"
subserved by the collicular visual system (2, 3). Such children
commonly have impaired movement of all four limbs and appear to
react to slowly moving targets at the side. If they are mobile,
they can navigate around obstacles, but paradoxically may show little
evidence of other visual functions.
Severe
visual impairment warrants particular enquiry concerning eye contact,
and the maximum distance from which a silent smile is returned.
Vision
with an acuity of 6/60 or better necessitates questions directed
towards identifying the following problems (fig 1 above). (4, 5).
Estimating
Visual Function
Function
assessment is carried out with both eyes open.
Visual
behaviour is watched carefully. If the child makes eye contact,
move back gradually until it is lost, in order to establish the
distance within which communication must be made. The fixation pattern
if the child looks around is informative. An alert child repeatedly
changes the direction of gaze to fixate on different targets, whereas
the child with impaired vision appears to look past the examiner
with inaccurate and less frequent eye movements.
Visual
acuity may be estimated in a number of ways, the functional significance
of which needs to be distinguished.
- VEP
acuity is the minimum target separation which permits
VEP signal detection.
- Detection
acuity (Catford drum or Stycar balls) estimates the minimum
size visible.
- Resolution
acuity (preferential looking cards) is the minimum separation
which allows discrimination.
- Recognition
acuity (letters or pictures) is the minimum size which
facilitates identification.
Tests
must be appropriate for age and ability. Cardiff cards
afford a rapid and reproducible preferential looking test. The vertical
presentation is helpful in obviating problems due to hemianopia
or horizontal nystagmus and the picture format allows end point
detection. Keeler cards are more suitable for infants
who are severely impaired, and may need to be presented vertically
if hemianopia is present or suspected. Recognition tests can give
a lower visual acuity due to crowding.
As
the visual acuity is a measure at maximum contrast and does not
estimate functional vision, the size of educational material must
be gauged to allow maximum speeds of access to information.
Contrast
sensitivity may be estimated using fading optotypes or in younger
children, low contrast faces revealed from behind a cover*. Reduced
contrast sensitivity necessitates the use of high contrast educational
material.
Colour
vision is commonly intact although some children can match but not
name colours (colour anomia).
Functional
visual field assessment is carried out binocularly, particularly
to elicit homonymous defects. For the young child, the child's attention
is attracted while a target is introduced from behind in each of
the four quadrants, anticipating a head turn.
For
the older, co-operative child small discreet movements of an extended
forefinger in each of the four quadrants, both singly and on both
sides simultaneously, can be made into a game. Homonymous defects
are commonly identified. Inattention (extinction) is common and
functionally can be equally handicapping, e.g. when crossing roads
or attempting to read as the page progressively disappears for a
left hemianopia and jumps into view if it is on the right.
Eye
movement disturbances are common. These include nystagmus (particularly
with additional optic nerve hypoplasia), gaze palsies, oculomotor
apraxia and impaired tracking. The latter may cause children to
have no interest in fast moving cartoons but to prefer TV programmes
with limited movement.
Accommodation
and convergence. Some children with brain damage have reduced or
absent accommodation and therefore poor third dimensional tracking
which leads to difficulty overcoming hypermetropia. Retinoscopy
prior to cycloplegia can identify impaired accommodation and reveal
manifest hypermetropia not corrected by the accommodative reflex.
The provision of spectacles can give gratifying results.
Identification
of Higher Visual Processing Disorders
Observation
and a thorough history may reveal and help explain some of the following
types of visual problem (see fig 1).
- Simultaneous
visual processing problems (9) are common in children
with impaired movement of all four limbs and may manifest as difficulty
in finding a toy on a patterned carpet. Crowding is a clinical
manifestation of the disorder (6). Sorting out a complex visual
scene can take a long time. Such children may read short words
easily but can lose track with long words (4). Enlargement and
simplification of educational material, with sequential presentation
against a plain background can be recommended.
- Recognition
of people, shapes or objects can be selectively impaired particularly
in periventrical leucomalacia (4). A child may be able to see
a small toy in the distance but be unable to recognise a close
relative in a group. Affected children compensate by voice recognition
or by recognising shoes or brooches.
- Problems
with reading are best assessed by an expert in visual
impairment and education who takes into account the optimal size
and lighting required for a maximum comfortable reading speed
and who recognises the nature of specific reading disorders.
- Problems
with orientation occur (5) but are not just visual in
origin. Children blind from eye disease can be adept at navigation,
while children with posterior cerebral pathology (particularly
on the right) may have good acuity but get lost and cannot find
their toys. Limited mobility and constant supervision do not give
opportunities to develop navigational skills. Education strategies
and mobility training which compensate by using, for example,
language memory for standard routes, are essential.
- Perception
of movement is most commonly impaired on account of impaired
tracking, but can rarely be a problem in children with damage
to the part of the brain responsible for movement perception,
area V5 (5.10).
- Visual
memory is used for copying tasks which can prove difficult
for children with cerebral visual impairment. In new environments,
affected young children may have impaired face recognition and
navigation.
- Visual
imagination is also observed by the occipital brain,
and descriptions given by parents may indicate good language processing,
but difficulties in handling imagined visual concepts.
- Lack
of visual attention which is intermittent is common when
tiredness, pre-occupation or distraction lead to behavioural diminution
of visual function. In contract, familiarity with the environment
appears to enhance visual performance.
- Prolongation
of visual tasks is a common feature. Sufficient time
should be given for a child to demonstrate his or her ability,
and the reduced performance speed recognised.
Profoundly
disabled children may suffer from any of the above problems, but
lack of communication can render it impossible to delineate specific
deficiencies.
Conclusion
Vision
Assessment Teams probably provide an optimal service (1). The majority
of such children undergo gradual visual improvement with time, but
early intervention programmes can favourably affect the visual development
of young children with cortical visual impairment. (11-13).
A report
summarising the visual abilities and disabilities with recommendations
for developmental and educational material approaches and, written
in plain English for the parents to give to carers and teachers
can be very helpful in providing a structured addition to the care
plan.
Prof
G N Dutton
References
(1)
The Royal College of Ophthalmologists & the British Paediatric
Association. (1994). Ophthalmic services for children. A report
of a joint working party. Services for children who are partially
sighted or blind. R. C. Ophth. BPA. London. pp. 13-14.
(2)
Haigh D. (1993). Chronic disorders of childhood. In: Visual
problems in childhood. Ed: Buckingham T. Butterworth-Heineman
Ltd., Oxford. pp. 47-62.
(3)
Jan JE., Wong PKH., Groenwell M., Flodmark O., Hoyt CS. Travel
vision "Collicular visual system?" Pediatr. Neurol. 1986.
2: 359-62
(4)
Jacobson L., Ek U., Fernell E., Flodmark O., Broberger U. Visual
impairment in preterm children with periventricular leukomalacia
- visual, cognitive & neuropaediatric characteristics related
to cerebral imaging. Devel. Med. Child Neurol. 1996. 38:
724-735.
(5)
Dutton G., Ballantyne J., Boyd G., Bradnam M., Day R., McCulloch
D., Mackie R., Phillips S., Saunders K. Cortical visual dysfunction
in children: a clinical study. Eye 1996. 10: 302-309.
(6)
Pike MG., Holmstrom G., de Vries LS., Pennock JM., Drew KJ., Sonksen
PM, Dubowitz LMS. Patterns of visual impairment associated
with lesions of the pre-term infant brain. Devel. Med. Child
Neurol. 1994. 36: 849-862.
(7)
Mercuri E., Atkinson J., Braddick O., Anker S., Nokes L., Cowan
F., Rutherford M., Pennock J., Dubowitz L. Visual function
and perinatal focal cerebral infarction. Arch. Dis. Child
1996. 75: F76-F81.
(8)
Harvey EM., Dobson V., Luna B., Scher MS. Grating and visual
field development in children with intra-ventrical haemorrhage.
Devel. Med. Child Neurol. 1997. 39: 305-312.
(9)
Foley J. Central visual disturbances. Devel. Med. Child
Neurol. 1987. 29: 116-120.
(10)
Ahmed M., Dutton GN. Cognitive visual dysfunction in a child
with cerebral damage. Devel. Med. Child Neurol. 1996. 38:
736-743.
(11)
Jan J., Sykanda A., Groenveld M. Habilitation and rehabilitation
of visually impaired and blind children. Paediatrician 1990.
17: 202-207.
(12)
Sonksen P.M. Promotion and visual development of severely
visually impaired babies: evaluation of a developmentally based
program. Devel. Med. Child Neurol. 1991. 33: 320-335.
(13)
Sonksen P., Stiff B. Show Me What My Friends Can See.
1991: John Brown Ltd., Nottingham.
* Available
from Precision Vision, 745 North Harvard Avenue, Villapark, Il.
60191, USA
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