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Purpose: To establish the distribution of visual acuity and the prevalence of residual amblyopia and other ocular disorders in a vision-screened population group of 12–13-year-old children.
Methods: In total 1046 children were examined in a field study in Sweden. The examination included visual acuity, stereopsis, cover testing, red reflex, refractive retinoscopy and examination of the posterior pole. In selected cases VEP was also performed.
Results: Visual acuity ≥1.0 in at least one eye was present in 98% of cases. Residual amblyopia (≤0.5) was found in 1.1% of the population. Manifest strabismus was found in 2.7%. There were only a small number of ocular opacities and posterior pole abnormalities. Ocular albinism was found in 7 cases. In 15 children the cause of subnormal VA was unexplained.
Conclusion: Results for visual acuity, residual amblyopia and other ocular disorders are very similar to previous Nordic, vision-screened populations.
Large refractive errors, strabismus, amblyopia and other visual or ocular disorders are common conditions that affect about 7% of young children (Köhler & Stigmar 1978; Macfarlane et al. 1987). Even though some of the conditions are serious and require immediate treatment (e.g. retinoblastoma or glaucoma), the majority are less serious, including for instance anisometropia, strabismus and/or large refractive errors. The early correction of significant refractive errors and treatment of strabismus hold an immediate gain for children: the elimination of visual impairment in their daily life, and in the case of strabismus, abnormalities in binocular vision and cosmetic problems can be treated.
Apart from being numerous, the less serious conditions also involve one other problem: the risk of amblyopia developing. In both cases mentioned above, early detection therefore has a positive “side effect” besides the immediate gain of treatment, namely the prevention of amblyopia, or treatment of the condition if it is already present.
Amblyopia is usually defined as the loss of visual acuity (VA) in one or both eyes, without any obvious structural or pathological anomalies. The origin of the condition is believed to be found in early visual development, where form deprivation or abnormal binocular interaction has led to cortically deprived vision (Flynn 1991). Sometimes treatability is included in the definition, that is, the amblyopic eye should be able to regain some VA if treatment is initiated during childhood (Sjöstrand & Abrahamsson 1997).
Amblyopia is one of the leading causes for unilateral visual loss in childhood (Hansen et al. 1992; Mulvihill et al. 1997). Reduced VA also presents the potential risk of becoming visually handicapped if the good eye is injured (Tommila & Tarkkanen 1981). Amblyopia is treatable and can therefore be considered an avoidable visual defect.
To be able to identify, diagnose and treat visual disorders some countries, such as Sweden and Denmark, have developed vision screening programmes. The main purpose of the programmes has been to identify and treat amblyopia and related conditions, holding a gain for the society in reduction of visual loss in the population. In countries with visual screening programmes, residual amblyopia has decreased and severe amblyopia has become uncommon (Jensen & Goldschmidt 1986; Kvarnström et al. 1998; Sjöstrand & Abrahamsson 1990). The early discovery and treatment of visual disorders, as well as high participation rates in the programmes, have been the cornerstone of success of the visual screening (Köhler & Stigmar 1973, 1978; Sjöstrand & Abrahamsson 1997; Jensen & Goldschmidt 1986).
The first vision screening programmes in Sweden were initiated as early as 40 years ago by ophthalmologist W. Nordlöw and pediatrician S. Joachimsson. Today, more than 99% of 4 year olds undergo visual screening at the Swedish child health care centres (Kvarnström et al. 1998). The objective is to minimize the prevalence of amblyopia and thus of visual handicap due to the condition. Children with an eye pathology other than amblyopia are of course also diagnosed and treated. In actuality the amblyopia screening programme is therefore a screening for a large number of different ocular and visual pathologies.
There is limited knowledge, however, about the prevalence of amblyopia and ocular disorders in children who have been exposed to the screening programme. The aim of this study is to establish the distribution of visual acuity and the prevalence of residual amblyopia and other ocular disorders in a well-defined, vision-screened population of 12–13-year-old children.
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In 1998, there were 125 intermediate schools in Göteborg and four surrounding suburbs (Mölndal, Härryda, Öckerö and Partille). The demographics of the greater Göteborg area made north-eastern Göteborg uninteresting in terms of this study owing to the fact that only a limited number of the children would have been eligible for the survey. Of the remaining schools, 88 were contacted by mail and fax. A total of 42 schools agreed to participate; 22 schools did not wish to participate (e.g. because of participation in other studies, or internal organizational problems at the school) and 24 schools did not respond. Owing to practical problems with the local administration we excluded another six schools from the study. We finally ended up with 36 schools well distributed both geographically and socio-economically over the Göteborg area. The total number of children born in 1985 in the included communities was 6882. All visually handicapped children, except those with additional severe mental retardation, attend the public schools in Sweden.
Two inclusion criteria were used for this study. The children had to have been born in 1985, and they had to be born in Sweden. The latter criterion was used as an easy method of obtaining a screened population since more than 99% of the children born in Sweden take part in child health care screening (Kvarnström et al. 1998). Vision screening based on VA is performed at 4.5 years of age.
Participation in the study was voluntary and informed consent was obtained from the parents and all the children participating in the study. A small questionnaire was added to the permission form in order to gather information about previous contact with ophthalmologists. The eligible population consisted of 1562 children, and 1046 children (513 girls, 533 boys) were examined. Participation rate was therefore 67%.
Of the 516 children not participating, 50 children (3%) had informed consent from their parents but were absent on the examination day, 145 (9%) did not want to participate and 321 (21%) failed to return their forms. We believe that the final sample well represents the population of children born in this area in 1985.
The examination was performed at the schools by an ophthalmologist (GV) and a medical graduate student (JO) in any sufficiently lit (DIN 5036: 200–500 candela/m2) room larger than 5 metres.
Visual acuity: VA was measured monocularly with a Landholt C LogMAR visual chart (2.6′). For approval of a line, 80% of the symbols had to be correctly identified. Children who normally wore prescription glasses used these during the test. If a child failed to see 1.0, we continued with a pinhole (2.0 mm diameter). The pinhole was used as a quick, yet accurate, method of determining whether a child’s weak vision was due to a refractive error or another pathology. Those who failed to see 1.0 with a pinhole were retested with trial glasses after refraction.
The data were therefore truncated above 1.0, since in Sweden this is considered the most common criterion for visual “normality”. The objective of the study was not to determine maximum VA but to find the percentage classified as “normal”.
Stereopsis and strabismus: Stereo vision was examined with the Lang II stereo card. Strabismus was screened for with the cover test for near and distance vision.
Cycloplegia: One drop of 0.5% tropicamide was instilled in each eye. We examined pupil response after 25 minutes, and if there was a pupil response, another drop of tropicamide was instilled and pupil response was re-examined after another 10 minutes. In no case was pupil response found after instillation of a second drop. Tropicamide was chosen because of its rapid onset and short-term effect, and lack of adverse side effects (Garston 1975; Yolton et al. 1980). Egashira et al. (1993) have shown that there is no statistically significant difference between cyclopentholate and tropicamide with regard to either cycloplegic retinoscopy or distance subjective refraction. According to Mutti et al. (1994), the bias from incomplete cycloplegia with tropicamide represents the effect of only 0.20 D. This is less than the 0.25 D usually considered to be the threshold for a clinically significant change.
Refraction, optical media and posterior pole: Refractive retinoscopy (skiascopy) was performed using a Heine retinoscope and lenses. During retinoscopy, the subject was asked to look at an optotype at 5–7 metres’ distance to avoid accommodative pseudomyopia. The red reflex (ocular optical media) was examined, as well as the posterior pole (Heine ophthalmoscope and a 20 D lens).
The examination also included measurement of body weight and height. During the entire study, the same person performed the same test in order to minimize examiner-dependent variations in the results.
Children who did not achieve a 1.0 VA and in whom a refractive error was found were retested with trial lenses added. If they reached a VA of 1.0, they were advised to see an optician. Children with glasses who needed pin-hole or trial glasses to reach a VA of 1.0 were advised to recheck the correction of their own glasses. Children who did not reach 1.0 with trial glasses or who were diagnosed with any other eye pathology were referred to an experienced pediatric ophthalmologist (AS) at the pediatric eye clinic at The Queen Silvia’s Hospital For Childen (formerly Östra Hospital) in Göteborg.
Examination at the eye clinic
At the pediatric eye clinic, a routine clinical examination was performed, including screening for iris transillumination with the slit lamp. Children with transillumination and/or a diagnosis of Subnormal Visual Acuity Syndrome (SVAS) were referred for Visually Evoked Potentials/Visually Evoked Response (VEP/VER) to test for ocular albinism.
For the VEP recordings, a short-latency light flash stimulus of high intensity (i.e. a supramaximal flash stimulus; Grass PS 22, intensity setting 16, duration <100 us) was used bi- and monocularly in each session. Particular attention was paid to completely covering the fellow eye in monocular stimulation. Recordings from five positions in a horizontal row at the occipital level were performed (O1a, O1, Oz, O2, O2a, according to international electroencephalogram standards). The amplified VER activity was averaged (n=20–30) and the results were manually analysed by an experienced VEP neuroscientist (AS), particularly with respect to monocular stimulation asymmetries (Sjödell et al. 1996).