from Nature, vol. 404, 9 March 2000, pages 143-144
Myopia is a common affliction (one in four adult Americans is near-sighted1), and juvenile-onset myopia is believed to be due to a combination of genetic and environmental factors2. Results from animal experiments indicate that light cycles may affect the development of myopia3,4, and Quinn et al. claim to have extended these to humans5. They reported a strong association between childhood myopia and night-time lighting before the age of two: there were five times more children with myopia among those who slept with room lights on than in those who slept in the dark, and an intermediate number among those sleeping with a dim nightlight5. However, we have been unable to find a link between night-time nursery lighting and the development of myopia in a sample of schoolchildren.
We examined the issue of nursery lighting in a subsample of children from the multicentre Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) Study. Parents reported their use of night-time lighting and their own refractive status, and the child's refractive error was measured by cycloplegic autorefraction. Our sample consisted of 1,220 children with a median age of 10.2 years: 11.5% of them were African-American, 19.1% Asian, 47.9% Caucasian and 21.6% Hispanic; overall, 18.1% of them were myopic (at least -0.50 dioptres spherical equivalent). The proportion of children with myopia did not differ across nursery-lighting groups (X2=2.62, P=0.271). Eighty-four of 417 children (20.0%) who slept in darkness were myopic; 128 of 758 children(16.8%) who slept with a night light before age two were myopic, and 10 of 45 children (22.2%) who slept with full room lights on before age two were myopic.
We found an association between the number of myopic parents and nursery lighting before age two (X2=35.02, P<0.001), as well as an association between ethnicity and room lighting (X2=89.22, P<0.001). This sample carries a statistical power of 0.99 to be able to detect an odds ratio of 2.00 between nursery lighting and childhood myopia.
Our results do not replicate those of Quinn et al.5 In fact, the proportion of myopic children in those subjected to a range of nursery-lighting conditions is remarkably uniform. The association we find between parental myopia and nursery night-time lighting suggests that Quinn et al.'s study should have controlled for parental myopia.
Another possible difference is that Quinn et al.'s sample is not representative of juvenile myopes. It was drawn from a tertiary referral, paediatric ophthalmology outpatient clinic, and the sample had a median age of eight (young for a sample of myopes) with a very high proportion of myopia (30%). Our sample had fewer myopes and fewer hyperopes, and the children were older. Also, the proportion of parents reporting that their infants slept under full lighting is different in our study: more than l5% of their clinic-based sample had full nursery lighting, whereas only 3.7% of our representative, school-based sample had full room lighting at night.
Our results indicate that myopia is unlikely to develop in children
as a result of exposure to night-time lighting as infants.
Karla Zadnik*, Lisa A. Jones*, Brett C. Irvin*,
Robert N. Kleinstein†, Ruth E. Manny‡, Julie A. Shin§,
Donald O. Munix*, for the CLEERE
Study Group
*College of Optometry, Ohio State University,
Columbus, Ohio 43210-1240, USA
†School of Optometry, University of Alabama,
Birmingham, Alabama 35294-0010. USA
‡College of Optometry, University of Houston,
Houston, Texas 77204-6052, USA
§Southern California College of Optometry,
Fullerton, California 92831, USA
email: zadnik.4@osu.edu
Quinn et al, report a strong association
between myopia in children and their exposure to night-time
lighting during their first two years1. We have
been unable to confirm this surprising result, but we find that
myopic parents are more likely to employ
night-time lighting aids for their children.
Moreover, there is an association between
myopia in parents and their children2,3.
We acquired child and parent refraction information as part of a 24-year longitudinal study of visual development in children. These children were research subjects and are not a clinical population. Refractions from 213 children and their parents are included; all children were refracted in the laboratory by non-cycloplegic retinoscopy.
One limitation of Quinn et al.'s study is a lack of information about the refractive status of the parents. Parents in our study were either tested in the laboratory or their spectacle prescriptions were used; if they had never worn glasses and could see clearly at a distance, they were classed as non-myopic.
Subjects (100 females and 113 males) ranged in age from 2 to 24 years, with a mean of 11 years. The data were divided into two groups: myopes, with a spherical equivalent refractive error ranging from -9.0 to -0.5 dioptres (mean, -2.50 dioptres), and non-myopes, with a spherical equivalent refractive error more positive than -0.5 dioptres (range, -0.38 to +4.38 dioptres; mean, +0.87 dioptres). Answers to questionnaires on nursery lighting conditions at night were collected from parents over the telephone, using the questions of Quinn et al. and a few extra ones. One asked parents to rate their confidence in the reliability of their recall of night-time lighting conditions from years earlier: 98% were confident in their responses.
The prevalence of myopia in our sample of children was not associated with ambient light exposure at night during their first two years, or later in life: 20% of those who slept with night lights before age 2 were myopic -- the same incidence as in children who slept in the dark. There were no myopes among the small group who slept with full room illumination. This result was not related to either age of onset (mean, 10.5 years) or the severity of myopia.
Families with two myopic parents, however, reported
the use of ambient lighting at night significantly more
than those with zero or one myopic parent (X2=7.42,
P<0.025). This could be related either to
their own poor visual acuity, necessitating
lighting to see the child more easily at night,
or to the higher socio-economic level of
myopic parents, who use more child-monitoring devices. Myopia in
children was associated with parental myopia, as reported
previously2,3. The proportion of myopic
children with two myopic parents was significantly greater than
the proportion of myopic children with zero or one myopic
parent (X2=4.42, P<0.05).
Based on these results, we question whether parents need to be
concerned about causing myopia in their children by
lighting their nurseries at night.
J. Gwiazda, E. Ong, R. Held, F. Thorn
New England College of Optomertry, 424 Beacon Street,
Boston, Massachusetts 02115, USA
Quinn et al. reply -- In not being able to
find the strong association reported by us1 of
childhood myopia with night-time ambient
lighting before age 2 years, Zadnik et al. and
Gwiazda et al. ascribe our results to a tendency of
myopic parents to illuminate their
children's rooms at night. Family studies of
myopia typically have difficulty separating
environmental from genetic factors, however, as sib-sib
correlations for myopia
decrease with increasing age difference2 and
within-family refractive similarities decrease
with adjustment for the 'classic' environmental factors of
education and close work3. Thus, shared
inter-generational behaviour(such as use of night lighting)
cannot be excluded a priori as contributing to any
familial association for myopia.
There are major differences among the studies. Our subjects were younger (mean age, 8 years) and had a considerably higher myopia prevalence of 28% -- itself quite high for a United States population of this young age. Accordingly, early-onset myopes, who ultimately tend to become more severely affected, are overrepresented in our tertiary-care population. Thus, it remains to be determined whether the lack of a daily period of darkness during infancy either accelerates myopia onset or provokes the condition in a subset of children who may be predisposed to a more severe form of the condition.
Neither of the subsequent studies considers possible reporting bias. Our findings received widespread publicity, and parents of myopic children might not accurately report or may even under-report a behaviour they fear could have harmed their children. Misclassification errors may also have been introduced into the later results, from non-cycloplegic childhood refractions in one and from self-reported parental refractions4 in the other.
Our results1 and others demonstrating
the influence of lighting on ocular development in animals5
support the notion that disrupting the daily light-dark illumination
cycle may affect eye development in children. Rather than offering
reassurance to parents at this time, the disparities in the
available clinical reports are better directed
to guiding the design of future research into
the interactions of light, dark and refractive
development.
Richard K. Stone*, Maureen G. Maguire*,
Graham E. Quinn*†
*Department of Ophthalmology, Scheie Eye
Institute, and †Children's Hospital of Philadelphia,
University of Pennsylvania School of Medicine,
Philadelphia, Pennsylvania 19104-6075, USA
e-mail: stone2@mail. med. upenn.edu