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Keywords:

  • amblyopia;
  • visual acuity;
  • vision screening;
  • child;
  • human;
  • treatment outcome;
  • Sweden/epidemiology

ABSTRACT.

  1. Top of page
  2. ABSTRACT.
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

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.

Methods

  1. Top of page
  2. ABSTRACT.
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

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.

Inclusion criteria

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.

Subjects

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.

Examination

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.

Referral criteria

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).

Results

  1. Top of page
  2. ABSTRACT.
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

Visual acuity

The results of VA testing all reported best correctable VA if nothing else was stated. A VA value equal to or better than 1.0 was considered normal.

Out of the total population, 1000 children (96%) had bilateral VA ≥1.0, and 1026 children (98%) had at least one eye with VA ≥1.0. Sunormal VA was found in 66 eyes, in 46 children (4.4%) (Table 1). Bilateral subnormal VA was found in 20 children (1.9%) (Table 2). Of the 46 children with subnormal VA in one or both eyes, 33 reported previous contact with an ophthalmologist; ten had not been to an ophthalmologist, and three did not respond.

Table 1.  Subnormal visual acuity. 66 eyes in 46 children. Thumbnail image of
Table 2.  Findings in children with bilateral VA ≪1.0. Thumbnail image of

We diagnosed 15 children (1.4%) with obscure subnormal VA. They did not have any known ocular pathology that could have caused the condition. Some had an associated amblyopia, but the VA of the non-amblyopic eye was also subnormal. Eight of the children did not show any interocular difference in VA. We diagnosed these children as Subnormal Visual Acuity Syndrome (SVAS). The cases can be studied in detail in Table 2.

Residual amblyopia

Amblyopia is defined as a VA of <1.0 in one eye, at least two lines of interocular difference and no apparent organic cause.

Amblyopia was found in altogether 29 children, representing 2.8% of the studied population. The VA of the amblyopic eyes is shown in Table 3. Lowering the VA criterion to ≤0.5 allowed easier comparison with previous studies. Using the latter definition, 11 children (1.1%) were diagnosed as amblyopic.

Table 3.  Visual acuity in amblyopic eyes and previous contact with eye clinics among amblyopes. Thumbnail image of

Other ocular disorders

Stereopsis: All images in the Lang II stereo card were recognized by 975 children (93%). Lack of stereopsis (defined as only identifying the monocular star) was found in 21 children (2.0%) and subnormal results, in 50 children (4.8%).

Strabismus: Manifest strabismus was found in 28 children (2.7%). For classification see Table 4.

Table 4.  Types of manifest strabismus. Thumbnail image of

Ocular opacities: A partial cataract was found in two children, one pulverulent and one punctate. In no child did the cataract have a significant visual impact. One child had a unilateral leucocoria (a sequel to a corneal herpes infection), with severe visual impact (VA=0.16).

Refractive errors: Distribution of refractive errors is shown in Table 5. All numbers report right eyes. There was no significant difference between the eyes. Anisometropia (≥1.5 D) was found in 34 children (3.3%). Of these 10 had straight eye amblyopia and three strabismic amblyopia. One child had an abnormal optical nerve in the anisometropic eye, and three children had obscure subnormal VA with no interocular difference.

Table 5.  Distribution of refractive errors, right eye. Thumbnail image of

Posterior pole: Abnormalities in the optic disc were found in five children, all of whom had subnormal vision in the affected eye(s).

Ocular albinism and retro-illumination: Albinism as diagnosed by VEP/VER asymmetries was diagnosed in seven children. Retro-illumination was found in five of these. Two children with retro-illumination were not albinoic, according to the VEP/VER results.

Miscellaneous: Other diagnoses were made in five children. These were congenital paresis of n. oculomotorius (one case), congenital ptosis (one case), nystagmus (one case), ocular melanosis (one case) and accommodative problems (one case).

Evaluation of methods

Of the 66 children with a pathology at clinical examination, 47 would have been found with VA testing alone, 16 using cover testing, two (diagnosed with a partial cataract) with examination of red reflex and one (with a ptosis) on general inspection. Of the 19 children not found with VA testing, no one had a condition needing treatment. The 16 children found using cover testing all had normal VA and 15 had had previous contact with an ophthalmologist. Neither in the two children with a partial cataract nor in the child with the ptosis did the pathology have any visual impact.

Discussion

  1. Top of page
  2. ABSTRACT.
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

Visual acuity

In our study, 98.1% of the children had bilateral VA (best correctable) of ≥1.0. Previous Nordic studies show similar results, including Jensen & Goldschmidt (1986), who found that 98.2% of children in 2nd to 5th grades in Odense, Denmark, had binocular VA of ≥1.0, and Holst & Tjåland (1962), who found that 97% of school children in Oslo, Norway, had binocular VA ≥1.0.

In the present study, 20 children (20%) had binocular VA of <1.0. None of these had binocular VA ≤0.3. In Jensen & Goldschmidt (1986), 0.09% of the children had binocular VA ≤0.3. A binocular VA of <1.0 was found in 0.74% of children. Our figure for a binocular VA of <1.0 is therefore somewhat higher than that reported in Jensen & Goldschmidt. The reason for not finding any children with binocular VA ≤0.3 in our sample could be the sample size.

There appears to have been no major change in best correctable VA in Swedish vision-screened children since previous studies of Nordic populations published over the past few years.

We diagnosed 15 children (1.4%) with Subnormal Visual Acuity Syndrome. Sjödell et al. (1996) introduced SVAS as a general diagnosis used for children with slowly developing VA, bad or no progress on treatment, and no apparent organic cause. Three of the children diagnosed with SVAS had hyperopia of at least +3 D, but all three have worn prescription glasses for several years.

There are no previous reports of SVAS prevalence, but recent reports from a multi-country survey on refractive errors in 5- to 15-year-old children report prevalences of unexplained causes of reduced best vision (≤0.5). These cases had no organic cause of impairment and did not have an amblyopia. Zhao et al. (2000) reports that 4% have unexplained subnormal visual acuity in China, Pokharel et al. (2000) 0.4% in Nepal, and Maul et al. (2000) 4.2% in Chile.

It may be argued that the children with isolated bilateral VA of <1.0 do not have a pathological condition at all – that they merely represent the lower range of normality. Unfortunately, very little has been published on what is to be considered normal VA. Reporting on letter acuity in normal subjects aged 10–19 years, Frisén & Frisén (1981) found a mean VA of 1.23 with a 95% confidence interval of 0.94–1.52 (90% threshold level). Smolek, presenting results of 218 normals aged 10–65 years, reports an average VA of –0.12±0.06 LogMAR (1.15–1.51 Snellen) (Smolek, personal communication).

Residual amblyopia

Amblyopia with VA ≤0.5 was found in 11 children (1.1%). This number is very similar to the prevalence in other screened populations (Table 6).

Table 6.  Prevalence of amblyopia in previous works on screened populations. Thumbnail image of

Amblyopia was associated with anisometropia in 13 cases, with strabismus in seven cases and a combination of the two in three cases. Six children with amblyopia had neither strabismus nor anisometropia, but had either SVAS or ocular albinism as a main diagnosis. The details of these cases can be seen in Table 2 (cases 3, 4, 10, 13, 16 and 20).

Two-thirds (19 out of 29) of the amblyopic children reported having had previous contact with an ophthalmologist (Table 3). This indicates that the prevalence of residual amblyopia may be due to both missed cases in the screening at 4 years of age (or to the condition having developed later) and to unsuccessful treatment (or compliance).

Other ocular disorders

Stereopsis: The results from this study were similar to previously reported findings. For further discussion c.f. Ohlsson et al. (submitted to Acta Ophthalmol Scand).

Strabismus: Manifest strabismus was found in 28 children (2.7%). Nordlöw in 1964 found 3.5% strabismus in a Swedish population of 6-year-old children. Köhler & Stigmar (1973, 1978) found 1.6% manifest strabismus in 4-year-old children and 1.8% in 7-year-old children in Sweden. Other studies show a somewhat higher prevalence than reported in Köhler & Stigmar; Macfarlane et al. (1987), for instance, found 2.5% strabismus in 1000 children in Queensland, Australia. Sorsby et al. (1960) found a prevalence of 4.0% in a population of young British men and Rantanen & Tommila (1971) report a 4.6% incidence in 7-year-old Finnish children.

Refractive errors: There are few reports of measurements of refractive errors in young teenagers.

Astigmatism: Laatikainen & Erkkilä (1980) report 1.7% astigmatism ≥2 D in 11–12-year-olds and Sorsby et al. (1960) found 3.2% astigmatism >2 D in 17–27-year-old men, compared with our 1.2% astigmatism >2 D and 2.4% ≥2 D.

Hyperopia: Defining hyperopia as >4 D, Sorsby found a prevalence of 4.1% in 17–27-year-old men (Sorsby et al. 1960). In our sample, 0.4% had hyperopia of >4 D. Laatikainen & Erkkilä (1980) found 11.7% hyperopia ≥2 D in a population of 11–12-year-olds, compared with our 3.9%. The lower prevalence of hyperopia found in our sample correlates well with the myopic shift that appears to have occurred in young Swedish teenagers.

Myopia: In our sample, 45% of right eyes were myopic, defined as ≤−0.5 D. Using the same definition, Laatikainen & Erkkilä in 1980 found 7.2% of myopia in the same age. In 1936, Strömberg found 8.9% of Swedish recruits to be myopic (viz. −0.25 D) and in 1954, Lundgren found the prevalence of myopia (definition unknown) to be 19–25% in Swedish university students. There appears to have been a large increase in the prevalence of myopia in Sweden, almost reaching numbers more commonly seen in Asian populations. These findings are further discussed in Villarreal et al. (2000).

Ocular optical media and fundus: Two children in our study were diagnosed with a partial cataract. One had a leucocoria as a sequel to a viral herpes infection. Five children (0.5%) were diagnosed with abnormal optic disc(s), all with subnormal vision in the affected eye(s).

An examination of the ocular media and fundus was included in both studies by Köhler & Stigmar (1973, 1978), but no findings are reported. Macfarlane et al. (1987) report 18% abnormal ophthalmoscopic findings and 0.1% abnormalities in refractive media in primary school children in Queensland, Australia.

Ocular albinism: All children with obscure VA reduction (SVAS) and/or retro-illumination of the iris were referred for VEP examination. In total, 12 children showed up for the examination. Ocular albinism was diagnosed in seven children tested with VEP. This gives a prevalence 50–100 times higher than that previously reported. These results are further discussed in Sjöström et al. (submitted to Doc Ophthalmol).

Relationships between pathology and subnormal visual acuity

The most prevalent ocular pathologies in this sample were anisometropia, strabismus, SVAS, and ocular albinism, in order of prevalence (Fig. 1). Residual amblyopia was found in 2.8% of the population studied. This result is similar to results reported previously in screened populations. Figure 1 shows that 29% of the children diagnosed with strabismus were amblyopic and 35% of those diagnosed with anisometropia were amblyopic. Three children had combined anisometropia and strabismus; all three were amblyopic.

image

Figure 1. Total pathology in studied population.

Download figure to PowerPoint

It is interesting to note that bilateral visual impairment, defined as bilateral VA of <1.0, is caused by three conditions, namely: 1. SVAS, 2. ocular albinism, and 3. optic nerve abnormalities. The largest group of children with subnormal VA in our sample were given a general diagnosis (viz. SVAS) without any real known reason stated for their subnormal VA. This is a very interesting finding, and an area with few previous studies. Note that before VEP examination, children with ocular albinism belong to the SVAS-group.

Conclusion

The distribution of visual acuity, residual amblyopia and other ocular disorders in 12–13-year-old children today is very similar to previous Nordic, vision screened populations. Ocular albinism is more common than previously believed and in a substantial number (75%) of children with bilateral subnormal VA we do not have a causal diagnosis.

Acknowledgements

  1. Top of page
  2. ABSTRACT.
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

This study was supported by the Swedish Medical Research Council (grant No. 02226), the Wilhelm and Martina Lundgren Science Foundation, the Sunnerdahl Foundation for the Handicapped, the Margit Thysielius Foundation, the Mayflower Charity foundation for Children, The KMA Foundation, The “De Blindas Vänner” Foundation and the Ahrnberg Foundation.

References

  1. Top of page
  2. ABSTRACT.
  3. Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References
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Correspondence: Josefin Ohlsson Dept. of Ophthalmology SU/Mölndal SE 431 80 Mölndal Sweden e-mail: josefin@oft.gu.se fax: +46 31 41 29 04

Received on January 14th, 2001. Accepted on July 10th, 2001.