***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 OBJECTIVE To compare the prevalence of amblyopia , strabismus , and significant refractive error among African-American , American Indian , Asian , Hispanic , and non-Hispanic white preschoolers in the Vision In Preschoolers study . DESIGN Multicenter , cross-sectional study . PARTICIPANTS Three- to 5-year old preschoolers ( n=4040 ) in Head Start from 5 geographically disparate areas of the United States . METHODS All children who failed the m and atory Head Start screening and a sample of those who passed were enrolled . Study -certified pediatric optometrists and ophthalmologists performed comprehensive eye examinations including monocular distance visual acuity ( VA ) , cover testing , and cycloplegic retinoscopy . Examination results were used to classify vision disorders , including amblyopia , strabismus , significant refractive errors , and unexplained reduced VA . Sampling weights were used to calculate prevalence rates , confidence intervals , and statistical tests for differences . MAIN OUTCOME MEASURES Prevalence rates in each racial/ethnic group . RESULTS Overall , 86.5 % of children invited to participate were examined , including 2072 African-American , 343 American Indian ( 323 from Oklahoma ) , 145 Asian , 796 Hispanic , and 481 non-Hispanic white children . The prevalence of any vision disorder was 21.4 % and was similar across groups ( P=0.40 ) , ranging from 17.9 % ( American Indian ) to 23.3 % ( Hispanic ) . Prevalence of amblyopia was similar among all groups ( P=0.07 ) , ranging from 3.0 % ( Asian ) to 5.4 % ( non-Hispanic white ) . Prevalence of strabismus also was similar ( P=0.12 ) , ranging from 1.0 % ( Asian ) to 4.6 % ( non-Hispanic white ) . Prevalence of hyperopia > 3.25 diopter ( D ) varied ( P=0.007 ) , with the lowest rate in Asians ( 5.5 % ) and highest in non-Hispanic whites ( 11.9 % ) . Prevalence of anisometropia varied ( P=0.009 ) , with the lowest rate in Asians ( 2.7 % ) and highest in Hispanics ( 7.1 % ) . Myopia > 2.00 D was relatively uncommon ( < 2.0 % ) in all groups with the lowest rate in American Indians ( 0.2 % ) and highest rate in Asians ( 1.9 % ) . Prevalence of astigmatism > 1.50 D varied ( P=0.01 ) , with the lowest rate among American Indians ( 4.3 % ) and highest among Hispanics ( 11.1 % ) . CONCLUSIONS Among Head Start preschool children , the prevalence of amblyopia and strabismus was similar among 5 racial/ethnic groups . Prevalence of significant refractive errors , specifically hyperopia , astigmatism , and anisometropia , varied by group , with the highest rate of hyperopia in non-Hispanic whites , and the highest rates of astigmatism and anisometropia in Hispanics Background The present study investigated the prevalence of refractive error , visual impairment , and eye diseases in school-aged children in western China . Methods The survey was done in a representative county ( Yongchuan District , Chongqing Municipality ) of western China . Cluster r and om sampling was used to select children aged 6 to 15 years . We conducted door-to-door surveys and eye examinations including  optometry , stereoscopic vision test , eye position and eye movement , slit lamp examination of the anterior segment , retinoscopy , and fundus examination after cycloplegia with 1 % cyclopentolate . Results Among 3469 children , data were available for 3079 ( 88.76 % ) . The prevalences of eye diseases were , in descending order , refractive error ( 20.69 % ; 637/3079 ) , conjunctivitis ( 11.76 % ; 362/3079 ) , amblyopia ( 1.88 % ; 58/3079 ) , color vision defect ( 0.52 % ; 16/3079 ) , keratitis ( 0.36 % ; 11/3079 ) , strabismus ( 0.29 % ; 9/3079 ) , cataract ( 0.23 % ; 7/3079 ) , pathologic myopia ( 0.19 % ; 6/3079 ) , and ocular trauma ( 0.13 % ; 4/3079 ) . The prevalence of corneal leucoma , corneal staphyloma , optic neuropathy , macular degeneration , and myelinated nerve fibers was 0.03 % ( 1/3079 ) for each . The prevalence of visual impairment was 7.70 % ( 237/3079 ) , and the major causes of visual impairment were uncorrected refractive error ( 86.08 % ; 204/237 ) , amblyopia ( 9.70 % ; 23/237 ) , pathologic myopia ( 1.27 % ; 3/237 ) , congenital cataract ( 0.42 % ; 1/237 ) , and others ( 2.11 % ; 5/237 ) . Conclusions Among school-aged children in a less developed area of western China , refractive error was the most prevalent eye disorder , and uncorrected refractive error was the main cause of visual impairment Purpose To assess the prevalence of refractive error and visual impairment in private school children in Ghana . Methods A r and om selection of geographically defined classes in clusters was used to identify a sample of school children aged 12 to 15 years in the Ashanti Region . Children in 60 clusters were enumerated and examined in classrooms . The examination included  visual acuity , retinoscopy , autorefraction under cycloplegia , and examination of anterior segment , media , and fundus .  For quality assurance , a r and om sample of children with reduced and normal vision were selected and re-examined independently . Results A total of 2454 children attending 53 private schools were enumerated , and of these , 2435 ( 99.2 % ) were examined . Prevalence of uncorrected , presenting , and best visual acuity of 20/40 or worse in the better eye was 3.7 , 3.5 , and 0.4 % , respectively . Refractive error was the cause of reduced vision in 71.7 % of 152 eyes , amblyopia in 9.9 % , retinal disorders in 5.9 % , and corneal opacity in 4.6 % . Exterior and anterior segment abnormalities occurred in 43 ( 1.8 % ) children . Myopia ( at least −0.50 D ) in one or both eyes was present in 3.2 % of children when measured with retinoscopy and in 3.4 % measured with autorefraction . Myopia was not significantly associated with gender ( P = 0.82 ) . Hyperopia ( + 2.00 D or more ) in at least one eye was present in 0.3 % of children with retinoscopy and autorefraction . Conclusions The prevalence of reduced vision in Ghanaian private school children due to uncorrected refractive error was low . However , the prevalence of amblyopia , retinal disorders , and corneal opacities indicate the need for early interventions The aim of this study is to estimate the prevalence of  amblyopia among schoolboys in Abha City and to examine its relations with anisometropia and strabismus . A total of 971 male school children were selected r and omly from the elementary schools for male in the city . Using the criterion of a visual acuity of 6/9 or worse , with no obvious structural disease of the eye , to define amblyopia , 18 cases have been found to have the condition , with a prevalence of 1.85 % . Those cases consisted of 7(47 % ) with anisometropic  amblyopia  and strabismus , and 2(13 % ) with deprivation amblyopia . Amblyopia showed a prevalence equal to 27 % and 31 % among children with anisometropia and those with strabismus respectively . An effective screening program for early detection of amblyopia through the well baby clinics is recommended PURPOSE To assess the prevalence of refractive error and visual impairment in school-age children in Gombak District , a suburban area near Kuala Lumpur city . DESIGN Population -based , cross-sectional survey . PARTICIPANTS Four thous and six hundred thirty-four children 7 to 15 years of age living in 3004 households . METHODS R and om selection of geographically defined clusters was used to identify the study sample . Children in 34 clusters were enumerated through a door-to-door survey and examined in 140 schools between March and July 2003 . The examination included  visual acuity measurements ; ocular motility evaluation ; retinoscopy and autorefraction under cycloplegia ; and examination of the external eye , anterior segment , media , and fundus . MAIN OUTCOME MEASURES Distance visual acuity and cycloplegic refraction . RESULTS The examined population was 70.3 % Malay , 16.5 % Chinese , 8.9 % Indian , and 4.3 % of other ethnicity . The prevalence of uncorrected ( unaided ) , presenting , and best-corrected visual impairment ( visual acuity < or = 20/40 in the better eye ) was 17.1 % , 10.1 % , and 1.4 % , respectively . More than half of those in need of corrective spectacles were without them . In eyes with reduced vision , refractive error was the cause in 87.0 % , amblyopia in 2.0 % , other causes in 0.6 % , and unexplained causes in 10.4 % , mainly suspected amblyopia . Myopia ( spherical equivalent of at least -0.50 diopter [ D ] in either eye ) measured with retinoscopy was present in 9.8 % of children 7 years of age , increasing to 34.4 % in 15-year-olds ; and in 10.0 % and 32.5 % , respectively , with autorefraction . Myopia was associated with older age , female gender , higher parental education , and Chinese ethnicity . Hyperopia ( > or = 2.00 D ) with retinoscopy varied from 3.8 % in 7-year-olds , 5.0 % with autorefraction , to less than 1 % by age 15 , with either measurement method . Hyperopia was associated with younger age and " other " ethnicity . Astigmatism ( > or = 0.75 D ) was present in 15.7 % of children with retinoscopy and in 21.3 % with autorefraction . CONCLUSIONS Visual impairment in school-age children in urban Gombak District is overwhelmingly caused by myopia , with a particularly high prevalence among children of Chinese ethnicity .  Eye health education and screening may help address the unmet need for refractive correction Purpose To examine the prevalence of refractive errors and prevalence and causes of vision loss among preschool and school children in East China . Methods Using a 
    Output:
    
The prevalence of amblyopia varies in different parts of the world , with the highest prevalence in European countries .