By Alma I. Murphy, MD
The results of this prospective study indicate that myopia progression, axial growth rate and corneal power vary according to annual daylight exposure in Danish children with myopia.
The authors investigated whether myopia progression, axial eye growth and corneal steepening vary with the length of the day in 235 Danish children with myopia (< 0 diopters) aged 8 to 14. They evaluated the children at baseline and six months later using cycloplegic autorefraction and biometry with the IOLMaster.
The accumulated daylight hours during each child's six-month study period were calculated using an astronomical table. As a result of Denmark's northern latitude, day length varies from seven to 17.5 hours throughout the year. The children were divided into seven groups according to the accumulated daylight hours during the study period. Accumulated hours of daylight ranged from 1,660 to 2,804 hours.
Myopia progression, axial eye growth and corneal power change were all found to increase significantly (P ≤ 0.01) during periods of less daylight. These findings are consistent with other studies that suggest a beneficial role for outdoor light in the development of myopia in children. The authors suggest indoor light sources simulating daylight should be evaluated for the treatment of myopia.
They note that accumulated hours of daylight only represent the potential exposure to daylight, not the actual exposure. Thus, these data do not allow evaluation of the effect of staying more or less indoors, of staying outdoors in open land or in areas with dense vegetation or many high-rise buildings, of the sky being more or less overcast, of the ground being covered by snow, and so forth. Other possible explanations could be variation in dietary habits or ambient temperature, or different levels of work activity during winter and summer periods.
They also note that the average day length for the highest daylight group was 15.3 hours. Thus, daylight lasting almost all waking hours apparently is not sufficient to completely normalize the eye elongation rate, perhaps because most of the day is spent indoors. They write that it would be interesting to know whether the axial growth rate also fluctuates with light exposure in emmetropic children, or whether the phenomenon is restricted to myopic children and thus reflects some underlying disturbance that causes myopia to develop.
They conclude that these findings suggest that both emmetropic and myopic children should be encouraged to spend more time outdoors. If this is not possible, it may be worth considering the use of indoor light sources that imitate daylight as a means of reducing myopia.