Refractive Errors in Children With Vision Impairment.

Emmetropization is the reduction in neonatal refractive errors that occurs after birth. This process uses a visual feedback system with successful emmetropization thought to be dependent on good retinal image quality, accurate focusing, and intact visual pathways. Ocular disease may affect one or more of these requirements, disrupting the emmetropization process and resulting in refractive error persistence or development.

Although the majority of research in the field of refractive error control and myopia development typically involves normally sighted children, the study of the refractive errors of children with ocular and visual pathway defects can provide further insights into the underlying mechanisms of emmetropization. For example, it has been suggested that the location of retinal pathology may play a role in the type of refractive error that occurs. Conditions that cause retinal image degradation such as cataracts and ptosis appear to be associated with myopia and failure of emmetropization. Similarly, children with visual pathway defects are also likely to have large refractive errors, although there is no specific trend in terms of the type of error observed. Because children with vision impairment often have associated mental and/or physical disabilities, characterizing the refractive anomalies present in the pediatric vision-impaired population would assist the clinician in the prescription of suitable vision correction.

The causes of vision impairment in children based on data collected from children attending schools for the visually impaired or records from low-vision clinics vary greatly between regions. For example, the high socioeconomic status of children in countries such as the United States, Nordic region (Denmark, Finland, Iceland, and Norway), and Australia has virtually eliminated the infectious and nutritional diseases that are still the major causes of childhood blindness in developing countries. In third world countries, infections and poor nutrition predominate as factors associated with vision loss. Corneal ulcerations, toxoplasmosis, and other ocular scars account for 43.5% of vision impairment in Brazil and 22.0% in Africa, whereas these causes only account for 2.0% in Australia.

In 1982, Lovie-Kitchin and Bevan reported data from a retrospective study of 398 vision-impaired children from the Pediatric Low Vision Clinic (PLVC), a statewide service for children with vision impairment based in Brisbane, Australia. These authors found that primary congenital nystagmus was the most common cause of vision impairment in children, accounting for 17.6% of the sample size. Other causes were congenital cataracts (15.8%), rubella syndrome (9.3%), albinism (7.5%), primary optic atrophy (6.3%), secondary optic atrophy (5.0%), and the remaining 38.5% consisted of many less frequent conditions such as ectopia lentis, rubella retinopathy, retinoblastoma, and congenital glaucoma. In a more recent study in the United States, the most common causes of vision impairment in 2 to 20 year olds were congenital cataracts (13%), optic atrophy (13%), albinism (13%), glaucoma (8.1%), retinitis pigmentosa (8.1%), and chorioretinal degeneration (8.1%).

DISCUSSION

This study analyzed the ocular conditions and the relative frequency and magnitude of refractive errors in children with vision impairment, and the effect of age and type of ocular disease. This study included data on more children with vision impairment than has been reported in the past in Australia. Few studies have examined the refractive errors of children across a range of different conditions that cause vision impairment.

Cause of Vision Impairment

The relative proportion of children with aphakia or cataract leading to vision impairment has decreased significantly (from 15.8% in 1982 to 5.73% in 2002). Improvements in surgical techniques and compulsory vaccinations (e.g., rubella) are likely factors in reducing the prevalence of rubella cataracts in children. CVI has now become the most frequent cause of vision impairment. In 1982, CVI was virtually unheard of (accounting for 0% of proven diagnoses); within 20 years, it has surged to 27.6%. Advancements in technology in recent times have resulted in increased survival rates of premature low-birth-weight babies, and this presumably accounts for some of the observed increase in CVI and ROP numbers. Some of the increase in the number of children with CVI may also be due to diagnosis as more referrals of these children are now made for support service assessment.

Although the data was not significant, more boys than girls appeared to have conditions such as albinism, retinitis pigmentosa, aphakia, cataracts, and CVI. Fifty-eight percent of the records were from male children and the remaining 42% were from female children. This is consistent with previous reports of vision impairment being more common in boys (percentage of boys in study samples ranges from 54 to 60). This tendency for boys to have more vision problems does not appear to have altered over time. Although some of this difference is clearly the result of the sex-linked genetic nature of some conditions causing vision impairment, other reasons may exist for the gender differences; perhaps male babies are less resilient than female babies or more like to be damaged by adverse environmental conditions during gestation.

Refractive Errors Associated With Vision Impairment

The principal finding of this study is that vision impairment in children is likely to be associated with ametropia. Fewer than 25% of vision-impaired children had refractive errors within ± 1 D. In contrast, almost 80% of normally sighted children satisfy this refractive error criteria (except in Asia, where many develop myopia). The small proportion of children in our study with refractive errors between ± 1 D (23%) is similar to that reported by Nathan and colleagues (30%). The leptokurtotic characteristic of the distribution of refractive states of the normal population is not apparent in the total low-vision group nor, more significantly, within the individual age bands. The refractive error distribution of children with vision impairment resembles a bell-shaped curve rather than the significantly leptokurtotic distribution displayed by normally sighted children.

The large refractive errors of children with vision impairment suggests that the process of emmetropization fails in most cases in which visual quality is reduced, whereas eyes of normally sighted children undergo a process of emmetropization. In normally sighted children, the (Gaussian) distribution of refractive errors at birth changes to a significantly non-Gaussian adult distribution in which there is a preponderance of refractive errors around emmetropia or slight hyperopia. The relative frequency distribution of children with vision impairment begins as a normal distribution, but instead of becoming a narrower distribution, the distribution becomes more Gaussian-like with an associated decrease in kurtosis. Furthermore, the standard deviation of the refractive errors of a group of children with vision impairment is much greater than that of a group of normally sighted children. For the entire group, the standard deviation was 6.00, which was markedly larger than the 0.82 to 0.95 reported for normally sighted children. This suggests that the majority of children with vision impairment do not undergo normal emmetropization.

The mean spherical equivalent refractive errors of children with vision impairment differ from that of normally sighted children. Compared with a recent study of normally sighted children, the mean refractive error of children with vision impairment is less hyperopic or more myopic compared with normally sighted children. The large variation of refractive errors in children with vision impairment and the deviation of the refractive error from the norm demonstrate a relationship among ocular disease, image processing deficits, and inaccurate emmetropization.

Whereas statistical comparison of the variation of refractive errors of the group of children with vision impairment and that of children with normal vision (based on published data) was possible, statistical comparison of the means was not possible as a result of large differences in the standard deviations of the two groups of data. Unpaired two-tailed t-tests could not be used because the two-sample standard deviations were statistically different and hence violated the assumption of t-tests. Wilcoxon signed rank test, the nonparametric version of t-test, could be used if a full dataset for normally sighted children were available. Our study did not include a control group of normally sighted children because this information is already well described in the literature and only children with visual impairment attended this clinic.

Refractive Errors Associated With Specific Conditions

Few reported studies have examined the refractive errors of such a large range of ocular conditions. Nathan and colleagues’ study presents a good base for comparisons owing to its similar subject population (Melbourne, Australia versus Brisbane, Australia) with similar age ranges (to 16 years versus to 17 years) and similar ocular disease classifications. Of the eight conditions in common, only two conditions showed good correlation in spherical equivalent refractive errors: ROP and rod monochromatism. Differences between the studies were observed for the other conditions, for example, coloboma and retinitis pigmentosa. Such differences could be attributed to the difference in time frame of the studies (1985 versus 2002) and thus differences in health care. Our refractive data are similar to that reported in studies that have examined only one ocular condition. For example, our results for albinism (+2.19 ± 2.96 D), CVI (+0.92 ± 3.03 D), and retinitis pigmentosa (-0.97 ± 5.63 D) are similar to that reported by Wildsoet et al. (+1.07 ± 4.67 D), Tuppurainen (+1.54 ± 1.58 D), and Sieving et al. (-2.04 ± 4.07 D) for albinism, CVI, and retinitis pigmentosa, respectively.

The relative frequency distribution of astigmatic refractive errors in children with vision impairment shows a preponderance of low (0 to 1 D) astigmatic errors (69.9%). Nathan and colleagues demonstrated a similar result (64.8%). However, this is quite different from normally sighted children (18.4%). It has been shown that the prevalence of astigmatism in normal infants is high (42%), but the prevalence declines sharply after 6 months of age to reach a minimum of 5% from 6 to 10 years, and then slowly rises after that. Data of Abrahamsson and colleagues also support the view that astigmatism decreases in power as the infant grows older, and this change is especially notable between the first and second year of life. This does not seem to be the case for vision-impaired children. The relative frequency of low amounts of astigmatism decreased with age with an associated increase in higher amounts of astigmatism.

Seven of the 10 most common ocular conditions in the sample population showed higher astigmatic refractive errors in older age groups. Hence, there appears to be a relationship between image processing deficits, i.e., visual impairments, and higher amounts of astigmatism.
Children with vision impairment showed increasing frequency of anisometropia with age (relative frequency of anisometropia more than 1 D: 13.0% at 0 to 2 years, 25.1% at 6 to 8 years, and 30.5% at 12 to 14 years). In normally sighted children, anisometropia is rarely found with prevalence in the range of 1.4 to 7.7%. In cross-sectional studies covering several age groups, the prevalence of anisometropia in normally sighted children is independent of age. This apparent stability is the result of anisometropia decreasing in some children and developing or increasing in others. This difference in anisometropia further supports the conclusions that the coordination of eye growth of visually impaired children is not regulated as well as normally sighted children.

CONCLUSIONS

The causes of vision impairment in children have changed since the 1970s, with CVI now being the most common cause of vision impairment. Children with vision impairment often have an associated ametropia suggesting that the emmetropization system is also impaired.