by Brian McDaniel
Stephen F. Austin State University, Spring 2000
Return to Perception, Spring 2000 frontpage.
An impairment in vision can lead to a very challenging life. Many Americans suffer from abnormalities in their vision. They suffer with diseases such as glaucoma, hyperopia, and myopia. Myopia, or nearsightedness, is fairly prevalent in the United States of America. In fact, it is estimated that twenty-five percent of the population of the United States is nearsighted (Kolata, 1985). Myopia occurs when the resting eye becomes focused on distant objects. The lens of the eye must become thicker and the radius of curvature must be increased in order for the eye to look at nearby objects. In individuals with myopia, the eyes are excessively long. This causes the image to form in front of the retina as opposed to on it (Kolata, 1985). Myopia frequently results from excessive postnatal eye growth. Typically, it develops in the early school years; however, some cases do not develop until early adulthood (Quinn, Shin, Maguire, & Stone, 1999).
Myopia is a very serious disease that can have a tremendous effect on the life of a child. Approximately 5.6% of blindness among American school children can be attributed to myopia. Furthermore, it predisposes individuals to retinal detachment, retinal degeneration, and glaucoma (Quinn et. al., 1999).
The cause of myopia has not yet been determined. However, children of parents with myopia tend to develop myopia more frequently than children with nonmyopic parents (Young et.al., 1998a). The two highest factors that contribute to myopia are myopic parents and doing "near work". Other indications for the development of myopia include childhood illnesses, low birth weight, and nearsighted siblings (Review of Optometry, 1999).
Since a definite cause for myopia has not been determined, researches are divided as to whether the disease is caused by genetic factors or environmental factors. The debate for the genetic theory begins with the thought that myopia may be due to a dominant gene that is inherited by the child from the parent (Young et.al., 1998a). There is evidence to suggest that genetic heterogeneity attributes to high myopia ( Young et.al., 1998b). This implies that the child who inherits diversified genes from his or her parents is more likely to develop myopia. However, it is also thought that myopia may be caused by influences from the nervous system that are not normal and have a negative impact on the developing eye. Thus, it may, as well, be genetically determined (Kolata, 1985). The basis for the genetic theory is supported by the fact that myopia tends to run in families. The genetic theory has ample evidence for support; however, it has not been directly linked to myopia as a definite cause for its development.
While the genetic argument tends to be extremely practical, the argument that myopia is caused by environmental factors is also valid. Some researchers believe that myopia is caused by too much "close work", consisting of reading, for example. A study was completed in 1960 by Francis Young at the Primate Research Center of Oregon State that indicated when monkeys are forced to view objects at a distance of only 20 inches or less, they tend to become myopic (Kolata, 1985). Furthermore, children tend to become myopic at the time they begin schoolwork. There have also been studies conducted that indicate that children with illiterate parents tended to be hyperopes. Other studies conducted conclude that men working in the close quarters of a missile launch facility have a tendency to develop myopia (Wallman & Turkel, 1978). The purpose of this paper is to introduce the theory that night lights may cause myopia in infants, provide the impact of this information, and explain the validity of the research as viewed by professionals in the field.
Past research that has been conducted on myopia has been done so on animals, particularly chicks, and has paved the way to myopia research with human subjects. The first published study that addressed the development of myopia in chicks was conducted by Wallman and Turkel (1978). Wallman and Turkel concluded that limiting the vision of chicks to only their frontal visual field leads them to be myopic. Limiting their visual field allows for definite and extreme changes in their ocular refraction. These researchers began limiting the vision of the chicks when the animals were 4 to 7 weeks old. They then would measure the refraction of the chicks' eyes. It was concluded that the animals that had extreme restriction in their visual field became myopic.
The reasoning behind these findings seems to be that the chicks that were constricted to vision only in their frontal field were forced to view life close up. Their vision was limited to the area around their beak, so this area consisted of their only means for eating and exploring their environment. Thus, the majority of the objects they were able to see was close to them, whereby the animals that did not have their vision limited were able to see using a greater visual field while focusing on objects that were further away from them. The results of this study by Wallman and Turkel (1978) suggest that the neuronal connectivity in the brain as it influences vision can be greatly affected by the environment. Furthermore, the development of vision is not only influenced by the established or absent visual experiences of the animal, but also the nature of these visual experiences. This study of the development of myopia using chicks paved the way for further research on the same topic with different circumstances, such as the effect of artificial light on chicks.
Within the past year, the nation has been horrified at a study conducted that indicated that children who sleep with a night light have more of a tendency to develop myopia. According to Quinn, et. al. (1999), their research on the effect of artificial light on infants was based on a study connecting the light exposure of chicks to the development of myopia. They based their study on the fact that postnatal eye growth, as well as refractive development, is determined by visual experience of the retinal mechanisms. They believe this is the case with the brain and neural pathways only having limited affect on the development. With the understanding that the daily light period that chicks are exposed to can effect their eye growth, it was to be determined if light would have the same effect on infants.
The study by Quinn, et. al. (1999) was conducted between January and June 1998 using children who were seen in a pediatric ophthalmology clinic. The study consisted of 479 children between the ages of 2 and 16. Fifty-five percent of those studied were males, 70% were Caucasian, 30% were African-American, and less than 1% were Asian-American. Children with a history of prematurity and children with amblyopia, cataract, or glaucoma were not included in the study. Parents of the children were given a questionnaire on the light exposure of the child at the present time and before the age of 2 years.
The results of this experiment were astounding. Ten percent of the children who slept in darkness before the age of 2 years were found to be myopic. Thirty-four percent of the children had myopia when the study was conducted. Fifty-five percent of the children who slept with some light source before the age of 2 years were found to be myopic. Thus, myopia was strongly associated with light exposure while the child was sleeping before the age of 2. Children who were exposed to room lighting during sleep rather than a night light had a higher prevalence of myopia. It seems that the more light the child was exposed to, the greater his or her chance for developing myopia. There was no correlation between light exposure and myopia with the children at their current age when the study was conducted.
The results of the study seem to be plausible because the eyelids of the children are able to transmit some visible light. By the age of 18 weeks, children have good light sensitivity as compared to the light sensitivity of adults. (Quinn et. al., 1999). The fact that their eyes are as sensitive to light as those of an adult implies that artificial light may have an effect on the developing eye. Children who are above the age of 2 do not seem to be at risk for myopia as a result of the night light. Thus, the artificial light may influence changes primarily in the developing eye (Rose, 1999).
This study suggests that the absence of darkness during the time of early childhood may be a factor in the development of myopia. While this study is not conclusive, parents should be encouraged to allow their children to sleep at night in darkness, with no artificial lighting (Quinn et. al. 1999). The best implication that can be reasoned from this experiment is the fact that there may be a "critical period" in the time of the developing infant that is important for refractive development, as it is needed for visual function (Quinn et. al. 1999).
While this study is interesting and may provide a new theory as to the cause of myopia, it needs further research in order to be considered valid. This study did not rule out parental myopia. Perhaps myopic parents are more likely to light their children's bedrooms. Research already suggests they are more likely to have myopic children (Tonks, 1999). Furthermore, the study needs to be extended to other ethnic groups, especially Asian, since the incidence of myopia tends to be higher in the Asian population (Quinn et. al. 1999). This study did not use variables that would provide for a random sample of participants. The researchers studied people who were patients of an eye clinic that may already be predisposed to myopia. Confusion also surrounds the time at which the development of myopia takes place in these children. The question is how the infant's eye is able to recall the damage from infancy that is not manifested until preadolescence (Review of Optometry, 1999). In other words, since the damage to the eye of the child occurred very early in life, there seems to be no explanation as to the reason for this damage to become apparent so much later in life. Furthermore, the study is dependent on the ability of the parent to recall information that is provided for the study. Thus, the effect of nightlights needs further research (Burke, 1999).
While this research on myopia may be the beginning of a lead to the cause of the disease, it is still a beginning. More research needs to be conducted on this topic before every parent in the United States begins to throw away his or her night lights. While the study of night lights is interesting, there are too many questions and concerns raised about its validity for it to be considered a fact.
References
Burke, M.G. (1999). Lights and nearsightedness. Contemporary Pediatrics, 16, 119.
Kolata, G. (1985). What causes nearsightedness? Science, 229, 1249-1250.
Quinn, G.E., Shin, C. H., Maguire, M.G., & Stone, R.A. (1999). Myopia and ambient lighting at night. Nature, 399, 113.
Rose, V.L. (1999). Near-sightedness and light exposure during sleep. American Family Physician, 60, 328.
Study on myopia and night lights is in the dark. (1999). Review of Optometry, 136, 4.
Tonks, A. (1999). Children who sleep with light on may damage their sight. British Medical Journal, 318, 1369.
Young, T.L., Ronan, S.M., Drahozal, L.A., Wildenberg, S.C., Alvear, A.B., Oetting, W.S., Atwood, L.D., Wilkin, D.J., & King, R.A. (1998). Evidence that a locus for familial high myopia maps to chromosome 18p. American Journal of Human Genetics, 63, 109-119.
Young, T.L., Ronan, S.M., Alvear, A.B., Wildenberg, S.C., Oetting, W.S., Atwood, L.D., Wilkin, D.J., & King, R.A. (1998). Second locus for familial high myopia maps to chromosome 12q. American Journal of Human Genetics, 63, 1419-1424.
Wallman, J., & Turkel, J. (1978). Extreme myopia produced by modest change in early visual experience. Science, 201, 1249-1251.