Chapter 57 Perceptual aspects of cerebral visual impairment and their management
Introduction
Cerebral visual impairment (CVI) is the commonest cause of visual impairment in children in developed countries.1,2 Improved perinatal care and survival of young children with profound neurological disease have increased the prevalence of cerebral causes.3 A large proportion of the brain involves visual processing, and, when affected, visual perception and cognition can be disordered. Affected young children are anosagnostic (unaware) for their perceptual deficits, which cause a range of often disabling visual behaviors.
Retrogeniculate damage to the visual brain can impair visual acuity and contrast sensitivity, and restrict visual fields,4,5 while damage to the posterior parietal and temporal lobes and their pathways gives rise to perceptual and cognitive visual impairment.
The pathology may primarily affect gray matter, white matter, or as in some cases of cerebral palsy, no anatomical abnormality is found on MRI scanning. Perceptual visual dysfunction in children is common but easily missed. It is not always accompanied by visual field deficits and poor visual acuity. Strabismus is a frequent association, and affected children may not be identified.6
Patterns of perceptual and cognitive visual dysfunction vary; many cases manifest unique features. The principal elements of perceptual visual dysfunction include impaired visual search (due to limited visual attention), often associated with inaccuracy of visual guidance of movement of the limbs. Peripheral bilateral lower visual field impairment is a common accompaniment due to posterior parietal pathology. Less commonly, impaired recognition due to disordered image processing of people, shape and objects, frequently associated with disordered orientation and route finding, may occur. The visual system alone may be affected, or associated with cerebral palsy and/or other developmental disorders. Perceptual visual dysfunction also contributes to the features of autistic spectrum disorder,7 and Williams’ syndrome.8
Synesthesia (unformed visual hallucinations while listening to music) is a benign condition, which simply warrants explanation. Normal physiological perceptions can cause concern.9 These include blurring of print, words “swimming” when reading (relaxing accommodation), seeing double (physiological diplopia), or colors (after-image effect) or spots (vitreous floaters), and things looking smaller or bigger than they should (image size flux with accommodation or cortical adaptation). The history with normal clinical examination allows the child and family to be reassured, without recourse to needless investigation.
A clinical model of the perceptual visual system
Of the many connections to centers for visual processing, the striate cortex has two principal pathways (Fig. 57.1):
Fig. 57.1 Diagram showing a basic conceptual framework for the areas serving higher visual function, and their connections.
Both are responsible for discrete, separate but closely interconnected perceptual functions (Fig. 57.2).
Fig. 57.2 Diagram in the form of a tree, denoting the range of visual functions which can be impaired by focal damage. Damage at a low level of the tree interferes with all the functions above that level. The branching pattern denotes the common groupings of patterns of dysfunction seen in children sustaining damage to the brain impairing vision and perception.
The dorsal stream and the result of bilateral damage
The dorsal stream and posterior parietal cortex assimilate incoming visual information to bring about moment-to-moment, immediate (“on line”) visual guidance of skilled actions. A constantly refreshed virtual map of the environment is created without conscious awareness. This creates internal, precise, “egocentric coordinates” to continuously stream, coincide with, and emulate the surrounding three-dimensional environment in time and space. This determines the temporal and spatial resolution of body movement and visual search. This internal representation of the surrounding image data provides the substrate for frontal areas to appraise and search the visual scene, to make choices, and facilitates accurate visual guidance of movement of the body.
Bilateral damage affecting the dorsal stream and posterior parietal territory causes inaccurate visual guidance of movement (optic ataxia), despite conscious visuospatial awareness afforded by the ventral stream. It commonly impairs perception of movement and limits the number of entities which can be attended to at once, thus impairing visual search. In its severe form (Balint’s syndrome), it results in great disability using vision to guide movement (despite intact stereopsis in some cases) accompanied by the disability of being able to identify a limited number of items at once. Although eye movements are intact, there may be an inability to elect to move the eyes to visual targets (“egocentric coordinates” – extrageniculostriate vision).
The ventral stream and its damage
The ventral stream and inferotemporal cortices provide a store of previous visual experiences, and serve conscious recognition and understanding of what is seen, by reference to the prior memory base.
Severe focal damage to the inferotemporal cortices and ventral stream pathways impairs visual recognition and route finding, but visual guidance of movement may remain intact. This has been called “travel vision.” Such residual perception of the moving image is known as the Riddoch phenomenon,10 and the intact visual guidance of movement is served by “blindsight.”11
Visual memory is commonly impaired in children with ventral stream dysfunction. This is required for copying, drawing, and remembering where things are. Educational strategies are required for these problems.
Unilateral ventral stream damage
Acquired damage to the right temporal lobe impairs recognition of people (prosopagnosia) and their facial expressions. Less commonly, animals may also be difficult to recognize and differentiate. Impairment in the ability to find the way around and be oriented in place (topographic agnosia) is common.
Acquired damage to the left temporal lobe impairs recognition of shape (shape agnosia), objects (object agnosia), letters (literal alexia), and words causing alexia or dyslexia. The deficits can be relative or absolute.
Very early onset damage to ventral stream structures tends to be bilateral, and impairs both form and shape recognition to similar degrees. When there is additional damage to the occipital lobes causing reduced visual acuity, it can be difficult to determine whether lack of recognition relates solely to poor acuity. Memory impairment is a clue to possible additional visual agnosia.
Diagnosis of perceptual visual dysfunction
History taking
Features of cerebral visual impairment in general
CVI covers a wide range of visual impairment and dysfunction, which can be difficult to quantify when co-existing motor or intellectual dysfunction impair communication. Variability in vision related to impaired attention, visual overload, and fatigue is common. Variation in visual field assessment probably relates to attentional and simultanagnostic visual dysfunction. Light gazing and photophobia have both been described with severe visual impairment.
Features of perceptual visual dysfunction
We have found that structured history taking for behaviors typical of perceptual visual dysfunction helps identify the condition and pathology, as well as guiding habilitation. We have developed targeted question inventories.
A 51 item inventory (divided into seven groups of questions) has been devised to seek dysfunction in the following areas: visual field or localized attention; perception of movement; visually guided movement of the body; visual attention; coping with crowded scenes or environments; and recognition and orientation) (Table 57.1
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