***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Purpose : To determine the prevalence of amblyopia and refractive errors among 7 to 12-year-old primary school children in Tehran , Iran . Methods : This population -based cross-sectional study included 2,410 r and omly selected students .  Visual acuity was tested using an E-chart on Yang vision tester .  Refractive errors were measured by photorefractometry and cycloautorefraction . Strabismus was checked using cover test . Direct ophthalmoscopy  was used to assess the anterior segment , lens opacities , red reflex and fundus . Functional amblyopia was defined as best corrected visual acuity ≤20/40 in one or both eyes with no anatomical problems . Results : Amblyopia was present in 2.3 % ( 95 % CI : 1.8 % to 2.9 % ) of participants with no difference between the genders . Amblyopic subjects were significantly younger than non-amblyopic children ( P=0.004 ) . Overall , 15.9 % of hyperopic and 5.9 % of myopic cases had amblyopia . The prevalence of hyperopia ≥+2.00D , myopia ≤-0.50D , astigmatism ≥0.75D , and anisometropia ( ≥1.00D ) was 3.5 % , 4.9 % , 22.6 % , and 3.9 % , respectively . With increasing age , the prevalence of myopia increased ( P<0.001 ) , that of hyperopia decreased ( P=0.007 ) , but astigmatism showed no change . Strabismus was found in 2.3 % of cases . Strabismus ( OR=17.9 ) and refractive errors , especially anisometropia ( OR=12.87 ) and hyperopia ( OR=11.87 ) , were important amblyogenic risk factors . Conclusion : The high prevalence of amblyopia in our subjects in comparison to developed countries reveals the necessity of timely and sensitive screening methods . Due to the high prevalence of amblyopia among children with refractive errors , particularly high hyperopia and anisometropia , provision of glasses should be specifically attended by parents and supported by the Ministry of Health and insurance organizations Purpose : Strabismus and anisometropia are the most common causes of amblyopia . It can be easily prevented or treated if detected early . With the changing socio-cultural-economic milieu of the society , the perspectives of strabismus in society are gradually changing but still adequate knowledge , awareness , and attitude of parents toward strabismus will help in preventing amblyopia and aid in the proper psychosocial adaptation of such children . This study aim ed to assess knowledge and attitude of parents toward children suffering from strabismus . Methods : A prospect i ve study was carried out from January 1 to February 29 , 2016 , through a structured question naire to assess the level of knowledge and attitude of parents of children suffering from strabismus . Results : One hundred and twenty parents of children with strabismus were interviewed through a question naire . Education level of 78 parents was less than graduation ( 60 % ) and of 42 parents ( 40 % ) was graduation or higher . The majority of the parents , i.e. , 116 ( 96.67 % ) were bothered due to strabismus . One hundred and one ( 84.17 % ) parents felt that their child 's strabismus was noticed by others during interaction . Seventy-four ( 61.67 % ) parents felt that their children will have difficulty in making friends . Ninety ( 75 % ) parents felt uncomfortable if someone asked something about their child 's strabismus . One hundred and ten ( 91.67 % ) parents considered strabismus as cosmetic stigma . Conclusion : Some parents , especially from the lower educated segment , had poor underst and ing of strabismus , thus result ing in late presentation and ineffective counter measures . The key to prevent strabismic amblyopia and its psychosocial impacts is to provide health education regarding strabismus PURPOSE To study the prevalence and causes of bilateral and unilateral blindness in an elderly urban Danish population . METHODS Data originated from a Danish epidemiologic cross-sectional r and om sample population eye survey conducted during the years 1986 - 1988 . The population consisted of 1,000 inhabitants aged 60 to 80 years in Copenhagen . The participants underwent an extensive ophthalmologic examination . A participation rate of 96.9 % was achieved . Any blindness was defined as best-corrected visual acuity ( VA ) worse than 0.05 ( the WHO criteria ) and VA of 0.1 or worse ( the National criteria ( NC ) of blindness ) . RESULTS The prevalence rates of bilateral and unilateral blindness were , respectively , 0.53 % and 3.38 % according to WHO , but 1.06 % and 4.44 % using NC . Bilateral blindness rose significantly with age ( p=0.02 ) . According to NC , age-related macular degeneration ( AMD ) was the leading cause of bilateral blindness , accounting for 60 % of all blind persons . Glaucoma , myopic macular degeneration , cataract and retinitis pigmentosa were jointly the second most common cause , each accounting for 10 % of all bilaterally blind persons . Diabetic retinopathy was not a cause of bilateral blindness . Amblyopia was the most frequent , AMD the second most frequent , and diabetic retinopathy was among the third most common cause of unilateral blindness accounting for , respectively , 28.60 % , 16.66 % and 9.52 % of all unilateral blindness . CONCLUSIONS Blindness was associated with increasing age . A calculation indicates that among Danes aged 60 to 80 years 7,736 are bilaterally blind and 35,503 suffer from unilateral blindness . This study highlights AMD as the most important cause OBJECTIVE To determine the prevalence of and factors associated with amblyopia in a rural Chinese population . DESIGN Population -based , cross-sectional study . PARTICIPANTS Six thous and eight hundred thirty Han Chinese aged 30 years or more , recruited from Yongnian County , H and an , Hebei Province , China . METHODS Thirteen villages in the Yongnian County of H and an were selected r and omly , and residents of these selected villages 30 years of age or older were invited to participate in the H and an Eye Study . Participants underwent a comprehensive eye examination , including st and ardized visual acuity ( VA ) tests using logarithm of the minimum angle of resolution charts . Prevalence rates were age- and gender-st and ardized to the 2000 China census . MAIN OUTCOME MEASURES The proportion of rural Chinese population aged 30 years or older with amblyopia . Unilateral amblyopia was diagnosed if best-corrected VA ( BCVA ) was 20/32 or worse in the amblyopic eye and was not attributable directly to any underlying structural abnormality of the eye or visual pathway . Bilateral amblyopia was diagnosed if BCVA was 20/32 or less in both eyes and if there was a history of form deprivation during the sensitive period of visual development , such as media opacities or high , uncorrected ametropia . RESULTS Amblyopia was diagnosed in 205 participants , with an age- and gender-adjusted prevalence of 2.8 % . Of these , 1.7 % were unilateral cases and 1.1 % were bilateral cases . Underlying causes included anisometropia ( 67.3 % ) , strabismus ( 5.4 % ) , mixed strabismus and anisometropia ( 4.4 % ) , visual deprivation ( 9.8 % ) , astigmatism association ( 9.8 % ) , and other ( 3.4 % ) . Of the amblyopia cases , 47.6 % were hypermetropic . CONCLUSIONS In this rural Chinese population , 2.8 % of adults 30 to 80 years of age had amblyopia , a prevalence rate broadly consistent with that of most other studies . One third of the cases were bilateral , and anisometropia was the most common cause of this condition Aim : To determine the prevalence of refractive errors among schoolchildren in urban and rural areas of Dezful County , Iran . Methods : In a cross-sectional study , using r and om cluster sampling , 5721 Dezful schoolchildren were selected from 39 clusters . The participants in the study totalled 5544 ; 3673 elementary and middle school students and 1871 high school students . For the former group , cycloplegic refraction and for the latter , non-cycloplegic refraction was tested . In all participants ,  uncorrected visual acuity and best corrected visual acuity  were determined , and those with a visual acuity of 20/40 or worse , underwent a complete ophthalmic examination to determine the cause of visual impairment . A spherical equivalent of −0.5 diopter ( D ) or worse was defined as myopia , + 2.0 D or more was defined as hyperopia , and a cylinder refraction greater than 0.75 D was considered astigmatism . Results : The uncorrected visual acuity was 20/40 or worse in the better eye of 224 schoolchildren ( 3.8 % of participants ) . This figure ( percentage ) was 14 ( 0.03 % ) based on their best corrected visual acuity and 96 ( 1.7 % ) with their presenting vision . According to results of cycloplegic refraction , 3.4 % ( 95 % confidence interval ( CI ) , 2.5 to 4.4 ) of the primary and middle school students were myopic and 16.6 % ( 95 % CI , 13.6 to 19.7 ) were hyperopic . For high school students , these rates were 2.1 % ( 95 % CI , 0.7 to 3.5 ) and 33.0 % ( 95 % CI , 24.9 to 41.1 ) , respectively , with non-cycloplegic refraction . In the multivariate logistic regression for primary and middle school students , myopia was correlated with age ( p = 0.030 ) , and hyperopia was correlated with age ( p<0.001 ) and area of residence ( p = 0.007 ) . In high school students , hyperopia again showed a correlation with their area of residence ( p = 0.029 ) . Conclusion : The present study reveals the considerable prevalence rates of refractive errors among schoolchildren in Dezful County and the high rate of an unmet need for their correction . Although myopia is not very prevalent , the high rate of hyperopia in the studied population emphasises its need for attention Objective : To estimate the distribution and predictors of some common  visual problems ( strabismus , amblyopia , hypermetropia )  within a population -based cohort of children at the age of 7 years . Methods : Children participating in a birth cohort study were examined by orthoptists who carried out cover/uncover , alternate cover , visual acuity and non-cycloplegic refraction tests . Prospect ively collected data on potential risk factors were available from the study . Results : Data were available for 7825 seven-year-old children . 2.3 % ( 95 % CI 2.0 % to 2.7 % ) had manifest strabismus , 3.6 % ( 95 % CI 3.3 % to 4.1 % ) had past/present amblyopia , and 4.8 % ( 95 % CI 4.4 % to 5.3 % ) were hypermetropic . Children from the lowest occupational social class background were 1.82 ( 95 % CI 1.03 % to 3.23 % ) times more likely to be hypermetropic than children from the highest social class . Amblyopia ( p = 0.089 ) and convergent strabismus ( p = 0.066 ) also tended to increase as social class decreased . Conclusions : Although strabismus has decreased in the UK , it and amblyopia remain common problems . Children from less advantaged background s were more at risk of hypermetropia and to a lesser extent of amblyopia and convergent strabismus . Children ’s eye-care services may need to take account of this socio-economic gradient in prevalence to avoid inequity in access to care Purpose : To determine the prevalence of amblyopia , anisometropia , and strabismus in schoolchildren of Shiraz , Iran . Material s and Methods : A r and om cluster sampling was used in a cross-sectional study on schoolchildren in Shiraz .  Cycloplegic refraction was performed in elementary and middle school children and high school students had non-cylcoplegic refraction .  Uncorrected visual acuity ( UCVA ) and best corrected visual acuity ( BCVA ) were recorded for each participant . Anisometropia was defined as spherical equivalent ( SE ) refraction difference 1.00D or more between two eyes . Amblyopia was distinguished as a reduction of BCVA to 20/30 or less in one eye or 2-line interocular optotype acuity differences in the absence of pathological causes . Cover test was performed for investigating of strabismus . Results : Mean age of 2638 schoolchildren was 12.5 years ( response rate = 86.06 % ) .  Prevalence of anisometropia was 2.31 % ( 95 % confidence interval [ CI ] , 1.45 to 3.16 ) . 2.29 % of schoolchildren ( 95 % CI , 1.46 to 3.14 ) were amblyopic . The prevalence of amblyopia in boys and girls was 2.32 % 
    Output:
    
Conclusions The prevalence of amblyopia in Iran is very different .