Visual screening of children in Sweden : epidemiological and methodological aspects
Sammanfattning: The aim of this thesis was to assess the vision screening system and ocular status in Sweden of today, yesterday and tomorrow and to compare the prevalence of ocular disease before and after screening and treatment with special focus on amblyopia.Screening has been defmed by the United States Commission of Chronic Illness (1957) as "the presumptive identification of unrecognized disease or defect by the application oftests, examinations or other procedures, which can be rapidly applied. Screening tests sort out apparently well persons who probably have a disease from those who probably do not". The screening system for eye disorders was introduced in the whole country in the beginning of 1970 and has not been evaluated in a greater area and for a longer period. Neither has an evaluation been done according to WHO's instructions. Amblyopia is the most common cause to visual impairment in one eye. The visual system is developing mostly in the first years of life and it is important to treat amblyopia in early childhood. The three first papers are retrospective studies and the fourth a prospective study. The study group in the first and second paper consisted of all children born 1982 in three Swedish cities from newborn until the age of 10 years. The children have been tested eight to nine times at the Child Health Care Centres and in school during this time. The sensitivity and specificity of visual screening were 92% and 97% respectively. The prevalence of ametropia was 7.7%, strabismus 3.1%, amblyopia ≤ 7 2.9% and organic lesions 0.2%. We compared the prevalence of amblyopia today with the time before screening was introduced in Sweden. This comparison shows that serious amblyopia has been reduced about 10 times with screening and treatment.Loss of vision in the non-amblyopic eye was investigated by studying patients with amblyopia at four visual rehabilitation centres. Approximately 1.2% of the people with amblyopia ≤ 0.3 will eventually become visually handicapped due to lesions in the better eye.Despite visual screening and treatment there are some children left with residual amblyopia. We investigated ways to improve the system by lowering the age for visual acuity examination from 4 to 3 years and at the same time two vision charts were compared. We found that the testability rate for 3-year-olds was almost the same for the Lea Symbol chart and the HVOT chart (82.8% and 84.8% respectively). Testability was about 10% higher at 4 years. The positive predictive value was lower at 3 years (58%) than has previously been found at 4 years (72%).Conclusion: In these studies we have found that screening is justified for the following reasons: visual screening is efficient in terms of sensitivity and specificity and many important ocular conditions are detected in this process; the prevalence of serious amblyopia is greatly reduced by screening and treatment; loss of vision in the non-amblyopic eye is a significant problem, which can be greatly reduced by screening and treatment, thereby saving expenses for the society.The following has been found regarding the design of visual screening: visual acuity testing is efficient in detecting visual disorders from 4 years and up; visual acuity can be tested at 3 years, but with lower positive predictive value; the most widely used charts in Sweden and internationally, the HVOT chart and the Lea Symbols chart perform equally well in visual acuity testing of 3-year-old and 4-year-old children.
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