Perceptual aspects of cerebral visual impairment and their management

Chapter 57 Perceptual aspects of cerebral visual impairment and their management


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

The differential diagnosis for perceptual visual dysfunction in children includes cerebellar and labyrinthine disorders (which can cause the horizon to appear tilted), the Pulfrich phenomenon (causing the perception of oncoming targets on the affected side to appear to veer towards them), and the Charles Bonnet syndrome (causing unformed or formed visual hallucinations).

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):

Both are responsible for discrete, separate but closely interconnected perceptual functions (Fig. 57.2).

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).

Diagnosis of perceptual visual dysfunction

History taking

Jun 4, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Perceptual aspects of cerebral visual impairment and their management

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