by Brian McDaniel
Stephen F. Austin State University, Spring 2000
Return to Perception, Spring 2000frontpage.
An impairment in vision can lead to a very challenging life. ManyAmericans suffer from abnormalities in their vision. They suffer withdiseases such as glaucoma, hyperopia, and myopia. Myopia, ornearsightedness, is fairly prevalent in the United States of America.In fact, it is estimated that twenty-five percent of the populationof the United States is nearsighted (Kolata, 1985). Myopia occurswhen the resting eye becomes focused on distant objects. The lens ofthe eye must become thicker and the radius of curvature must beincreased in order for the eye to look at nearby objects. Inindividuals with myopia, the eyes are excessively long. This causesthe 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 casesdo not develop until early adulthood (Quinn, Shin, Maguire, &Stone, 1999).
Myopia is a very serious disease that can have a tremendous effecton the life of a child. Approximately 5.6% of blindness amongAmerican school children can be attributed to myopia. Furthermore, itpredisposes individuals to retinal detachment, retinal degeneration,and glaucoma (Quinn et. al., 1999).
The cause of myopia has not yet been determined. However, childrenof parents with myopia tend to develop myopia more frequently thanchildren with nonmyopic parents (Young et.al., 1998a). The twohighest factors that contribute to myopia are myopic parents anddoing "near work". Other indications for the development of myopiainclude childhood illnesses, low birth weight, and nearsightedsiblings (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 geneticfactors or environmental factors. The debate for the genetic theorybegins with the thought that myopia may be due to a dominant genethat is inherited by the child from the parent (Young et.al., 1998a).There is evidence to suggest that genetic heterogeneity attributes tohigh myopia ( Young et.al., 1998b). This implies that the child whoinherits diversified genes from his or her parents is more likely todevelop myopia. However, it is also thought that myopia may be causedby influences from the nervous system that are not normal and have anegative impact on the developing eye. Thus, it may, as well, begenetically determined (Kolata, 1985). The basis for the genetictheory is supported by the fact that myopia tends to run in families.The genetic theory has ample evidence for support; however, it hasnot been directly linked to myopia as a definite cause for itsdevelopment.
While the genetic argument tends to be extremely practical, theargument that myopia is caused by environmental factors is alsovalid. Some researchers believe that myopia is caused by too much"close work", consisting of reading, for example. A study wascompleted in 1960 by Francis Young at the Primate Research Center ofOregon State that indicated when monkeys are forced to view objectsat a distance of only 20 inches or less, they tend to become myopic(Kolata, 1985). Furthermore, children tend to become myopic at thetime they begin schoolwork. There have also been studies conductedthat indicate that children with illiterate parents tended to behyperopes. Other studies conducted conclude that men working in theclose quarters of a missile launch facility have a tendency todevelop myopia (Wallman & Turkel, 1978). The purpose of thispaper is to introduce the theory that night lights may cause myopiain infants, provide the impact of this information, and explain thevalidity of the research as viewed by professionals in the field.
Past research that has been conducted on myopia has been done soon animals, particularly chicks, and has paved the way to myopiaresearch with human subjects. The first published study thataddressed the development of myopia in chicks was conducted byWallman and Turkel (1978). Wallman and Turkel concluded that limitingthe vision of chicks to only their frontal visual field leads them tobe myopic. Limiting their visual field allows for definite andextreme changes in their ocular refraction. These researchers beganlimiting the vision of the chicks when the animals were 4 to 7 weeksold. They then would measure the refraction of the chicks' eyes. Itwas concluded that the animals that had extreme restriction in theirvisual field became myopic.
The reasoning behind these findings seems to be that the chicksthat were constricted to vision only in their frontal field wereforced to view life close up. Their vision was limited to the areaaround their beak, so this area consisted of their only means foreating and exploring their environment. Thus, the majority of theobjects they were able to see was close to them, whereby the animalsthat did not have their vision limited were able to see using agreater visual field while focusing on objects that were further awayfrom them. The results of this study by Wallman and Turkel (1978)suggest that the neuronal connectivity in the brain as it influencesvision can be greatly affected by the environment. Furthermore, thedevelopment of vision is not only influenced by the established orabsent visual experiences of the animal, but also the nature of thesevisual experiences. This study of the development of myopia usingchicks paved the way for further research on the same topic withdifferent circumstances, such as the effect of artificial light onchicks.
Within the past year, the nation has been horrified at a studyconducted that indicated that children who sleep with a night lighthave more of a tendency to develop myopia. According to Quinn, et.al. (1999), their research on the effect of artificial light oninfants was based on a study connecting the light exposure of chicksto the development of myopia. They based their study on the fact thatpostnatal eye growth, as well as refractive development, isdetermined by visual experience of the retinal mechanisms. Theybelieve this is the case with the brain and neural pathways onlyhaving limited affect on the development. With the understanding thatthe daily light period that chicks are exposed to can effect theireye growth, it was to be determined if light would have the sameeffect on infants.
The study by Quinn, et. al. (1999) was conducted between Januaryand June 1998 using children who were seen in a pediatricophthalmology clinic. The study consisted of 479 children between theages of 2 and 16. Fifty-five percent of those studied were males, 70%were Caucasian, 30% were African-American, and less than 1% wereAsian-American. Children with a history of prematurity and childrenwith amblyopia, cataract, or glaucoma were not included in the study.Parents of the children were given a questionnaire on the lightexposure of the child at the present time and before the age of 2years.
The results of this experiment were astounding. Ten percent of thechildren who slept in darkness before the age of 2 years were foundto be myopic. Thirty-four percent of the children had myopia when thestudy was conducted. Fifty-five percent of the children who sleptwith some light source before the age of 2 years were found to bemyopic. Thus, myopia was strongly associated with light exposurewhile the child was sleeping before the age of 2. Children who wereexposed to room lighting during sleep rather than a night light had ahigher prevalence of myopia. It seems that the more light the childwas exposed to, the greater his or her chance for developing myopia.There was no correlation between light exposure and myopia with thechildren at their current age when the study was conducted.
The results of the study seem to be plausible because the eyelidsof the children are able to transmit some visible light. By the ageof 18 weeks, children have good light sensitivity as compared to thelight sensitivity of adults. (Quinn et. al., 1999). The fact thattheir eyes are as sensitive to light as those of an adult impliesthat 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 formyopia as a result of the night light. Thus, the artificial light mayinfluence changes primarily in the developing eye (Rose, 1999).
This study suggests that the absence of darkness during the timeof early childhood may be a factor in the development of myopia.While this study is not conclusive, parents should be encouraged toallow their children to sleep at night in darkness, with noartificial lighting (Quinn et. al. 1999). The best implication thatcan be reasoned from this experiment is the fact that there may be a"critical period" in the time of the developing infant that isimportant for refractive development, as it is needed for visualfunction (Quinn et. al. 1999).
While this study is interesting and may provide a new theory as tothe cause of myopia, it needs further research in order to beconsidered valid. This study did not rule out parental myopia.Perhaps myopic parents are more likely to light their children'sbedrooms. Research already suggests they are more likely to havemyopic children (Tonks, 1999). Furthermore, the study needs to beextended to other ethnic groups, especially Asian, since theincidence of myopia tends to be higher in the Asian population (Quinnet. al. 1999). This study did not use variables that would providefor a random sample of participants. The researchers studied peoplewho were patients of an eye clinic that may already be predisposed tomyopia. Confusion also surrounds the time at which the development ofmyopia takes place in these children. The question is how theinfant's eye is able to recall the damage from infancy that is notmanifested until preadolescence (Review of Optometry, 1999). In otherwords, since the damage to the eye of the child occurred very earlyin life, there seems to be no explanation as to the reason for thisdamage to become apparent so much later in life. Furthermore, thestudy is dependent on the ability of the parent to recall informationthat is provided for the study. Thus, the effect of nightlights needsfurther research (Burke, 1999).
While this research on myopia may be the beginning of a lead tothe cause of the disease, it is still a beginning. More researchneeds to be conducted on this topic before every parent in the UnitedStates begins to throw away his or her night lights. While the studyof night lights is interesting, there are too many questions andconcerns raised about its validity for it to be considered a fact.
References
Burke, M.G. (1999). Lights and nearsightedness. ContemporaryPediatrics, 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 duringsleep. American Family Physician, 60, 328.
Study on myopia and night lights is in the dark. (1999). Reviewof Optometry, 136, 4.
Tonks, A. (1999). Children who sleep with light on may damagetheir 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 tochromosome 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). Secondlocus for familial high myopia maps to chromosome 12q. AmericanJournal of Human Genetics, 63, 1419-1424.
Wallman, J., & Turkel, J. (1978). Extreme myopia produced bymodest change in early visual experience. Science, 201,1249-1251.