Epidemiology of visual impairment in Britain.

Since vision is the major sensory modality in humans, normal vision is important for the general development of a child. 2 Visual impairment has significant implications for the aVected child and family in terms of education, future employment, and personal and social welfare throughout life. Using the World Health Organisation (WHO) classification of levels of visual impairment (shown in table 1 and used throughout this paper), it is estimated that globally almost one in 1000 children are blind, which is less than a tenth of the prevalence in adults. 5 However, the 1.5 million blind children in the world account for about 75 million person years of blindness, equivalent to the burden due to cataract related blindness, which accounts for 40% of adult blindness worldwide. As the combined categories of low vision in childhood (table 1) are three to 10 times more common than blindness, 6 the total burden of childhood visual impairment is considerable. It has been estimated that globally, the financial costs of childhood blindness incurred by countries, in terms of care and productivity lost, are between 6 and 27 thousand million US dollars. Most of this is accounted for by children in high income countries where childhood blindness is less common but life expectancy and earning capacity are greater than in low income countries. Reduction of avoidable visual impairment remains an important international public health goal. Knowledge of the magnitude and causes of childhood visual impairment is important to the planning, provision, and evaluation of educational and health services for aVected children. This is also essential information for all strategies to reduce visual disability in the population. The purpose of this paper is to review the available data, highlighting relevant sources, on the incidence, prevalence, and causes of childhood visual impairment in Britain which are relevant to both hospital and community based child health. Normal visual development The visual system is relatively immature at birth. Maturation depends on both structural and functional changes. Development is particularly rapid in the first year of life, although some changes continue into late childhood. 9 10 As a child with normal vision only achieves adult levels of visual function at about 5 years, it is diYcult to predict final visual outcome in younger children with visual defects. The plasticity of the visual system is a key principle in the management of ophthalmic disease in children. In early life there are “sensitive periods”, of varying length, during which visual function can be profoundly impaired by a variety of insults such as stimulus deprivation arising from congenital cataract. Additionally “critical periods” exist during which treatment must be instituted to have maximal, or sometimes any eVect. Some adverse changes in the visual system become largely irreversible outside these critical periods.