JAMES E. JAN, MD, FRCP(C)
WILLIAM V. GOOD, MD
CREIG S. HOYT, MD
According to the World Health Organization Criteria, there are 1.5 million blind children younger than 15 years of age: 1 million in Asia, 0.3 million in Africa, 0.1 million in Latin America and 0.1 million in the rest of the world 1-3. The marked differences in the prevalence of blindness in the various regions of the world appear to be based on socioeconomic factors. 90% of visually impaired children live in Third World countries, where loss of sight, in the majority of cases, is preventable and treatable. In Western countries the prevalence of paediatric ocular visual loss declined during the last century, while visual impairment, due to various brain disorders, became the most common cause of visual impairment 4,5. This change has been attributed to improved medical care, which has allowed sick infants and children with severe brain damage to survive.
During the last two decades, as an increasing number of children with neurological visual disorders have been seen by intervention programs, various research groups have begun to study them. The term 'cortical visual impairment' (CVI) was introduced, and was defined as loss of acuity due to bilateral disturbance of the optic radiation and/or occipital lobe. This term replaced 'cortical blindness', because total absence of sight due to neurological insults was extremely rare.
Because children with CVI often tended to have severe neurodevelopmental problems, accurate acuity testing was difficult, and often not done. Thus many children, even without loss of acuity, were diagnosed as having CVI. The term is now widely used by various professionals working with visually impaired children. In most instances, in spite of the strict definition given above, CVI is used to describe the presence of various visual deficits due to neurological disorders of the brain.
At the same time, another diagnostic term, "cerebral visual impairment" has surfaced, and was also abbreviated as CVI. Cerebral visual impairment was diagnosed when individuals exhibited one or more types of neurological visual disorders. Proponents of this term emphasized that some neurological visual deficits were due to damage to the white matter, rather than to visual cortical areas. On the other hand, brain structures outside the cerebrum, such as the cerebellum and the brain stem, may also result in visual disorders. Cerebral visual impairment is now commonly used in some countries, while cortical visual impairment is more popular in others, causing considerable confusion. The two different definitions evolved because the field of neurological visual disorders in children was new and because of inadequate communication between the various research groups.
In June 1998, during a Mac Keith meeting on the assessment and impact of visual impairment, a committee, representing multiple disciplines, was formed to examine the above definitions. It was soon realised that, in view of the increased prevalence of neurological visual disorders in childhood, a classification system needed to be created, by which these various visual conditions could be defined and categorised. Such classification must benefit the child's diagnosis and care. It should be simple, based on sound scientific principles and be useable by all countries. The use of standardised language, without highly technical terminology, should be promoted so it could be used world-wide for epidemiological studies and accurate data collection. This classification should promote inter-disciplinary communication between professionals of various disciplines who are working with visually impaired children. Furthermore, it needs to be flexible and should also mature with time. There is no such existing classification system for neurological visual disorders. The International Classification of Disease (ICD-10)6 and the Diagnostic and Statistical Manual7 do not deal adequately with paediatric neurological visual disorders. In 1980, the World Health Organisation published the International Classification of Impairments, Disabilities and Handicaps (ICIDH)8 but recently revised it (ICIDH-2)9. The ICIDH-2 provides a unified and standard language and framework for the description of human functioning and disability as an important component of health. It organises information into three dimensions: The body dimension, which includes function and structure, the activities performed by an individual and the participation in areas of life in which an individual is involved, has access to and/or for which there are social opportunities or barriers. ICIDH-2 unfortunately does not adequately describe the neurological visual disorders of children either but comments and amendments are invited.
The initial responses of the committee members who were selected during the Mac Keith meeting, showed that vision researchers, physicians in clinical practice, educators, psychologists and other professionals were interested in different aspects of visual impairment. This is not surprising because the training in their fields varied. The main interest of vision researchers is likely to be: normal or abnormal visual function, and what anatomy is affected? Physicians in clinical practice are required to diagnose dysfunction, anatomy and aetiology as well. It makes a major difference to prognosis and rehabilitation when the cause of a visual disorder is a degenerative brain disease, a tumour or a static condition. Educators identify dysfunction in order to offer appropriate educational services. Other professionals need to understand the range of activities of these individuals and how these individuals can be helped in their participation within society.
The development of a comprehensive classification for paediatric neurological visual disorders will be difficult and time-consuming. Some of the reasons are that we still do not understand and accurately diagnose many disturbed visual functions. Due to the widespread impact of brain damage, more than one visual disorder can be present and they can change over the years. It may be difficult to separate ocular from neurological disorders, as for example optic nerve atrophy is clearly a neurological disorder, but generally accepted as an eye abnormality. Visual impairment based only on diminished acuity and field loss does not accurately represent vision in individuals with ocular disorders and much less so with neurological visual problems. The impact of visual disorders, the range of activities and participation in the environment, are not apparent in early childhood. The gap between various disciplines is so wide that a simple classification may not be applicable to all professionals.
The main groups for impaired visual function could be modelled according to Milner and Goodale10. Disorders of eye movements, accommodation, depth perception and others may be added or excluded. Further subheadings will also be created under each group. The organisation of such a structure must be carefully thought through.
The anatomical classification may include a list of structures, such as optic radiation, occipital lobe, dorsal and ventral streams, parietal lobe, basal ganglia, cerebellum and so on. The classification of causes could follow the commonly-used aetiologic groups, such as neoplasms, infections, static encephalopathies, injuries, maldevelopment, prenatal causes, unknown psychogenic causes, etc.
In order to create a comprehensive paediatric international classification of neurological visual disorders, a permanent, international multidisciplinary committee must be set up and supported by appropriate funding. Likely this committee should work closely with the World Health Organisation. The proposed classification will need to be widely circulated for comments and then periodically revised and published. This will be an arduous and expensive undertaking but the future benefits for visually impaired children will be great. A common classification could promote interdisciplinary communication, even if professionals working in different fields would only use part of the classification.