Vision screening of preschool children by eccentric photorefraction using a digital camera
Aims: To determine the validity of a photorefraction screening protocol using computer-aided techniques; to make recommendations regarding the cost and practicability of establishing a photorefraction screening service for Hong Kong preschool children; and to determine the prevalence of visual problems in Hong Kong kindergarten children.
Materials and methods: We calibrated a digital photorefractor, and a semiautomatic computer program was designed to determine the refractive error based on the image captured. An eye examination was carried out, followed by photorefraction screening, in 854 kindergarten children. We compared the cost of photorefraction with the digital photorefractor with photorefraction with a commercially available photorefractor, the MTl.
Results: Clinical examination showed that 16.9% of children had a visual problem. Myopia was not yet prevalent (only 1.1% of the children had myopia of 1 D or more), and the most common refractive error was astigmatism (10.7%). Strabismus was found in 2.3% and anisometropia in 1.6% ofthe children, so that about 4% of the children were at risk for amblyopia. The photorefractorhad a sensitivity of 71.0% and a specificity of 97.2%. The main cause of false-negative screening results was low astigmatism falling into the null zone of the photorefractor.
Conclusions: A kindergarten-based vision screening scheme for three-year old children is viable, and the calculated cost per child is HK$23.90. The method is much more sensitive than vision measurement. Astigmatism was the most prevalent refractive error in preschool children.
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