Association Between Breastfeeding and Likelihood of Myopia in 6-7 Years Old Children

For proper vision, eyes need to focus exactly on images. The two main determinants of refraction are focus power of cornea and the eye's axis length. In normal eyes, images are formed exactly on retina (emmetropia), but farsightedness (hyperopia) is the situation in which, the eye forms images in the backside of retina and nearsightedness (myopia) is a situation in which images are formed in front of retina, and it may have two reasons: 1) the lens convexity is more powerful than normal and 2) the eye axis is longer than normal (axial myopia).

Refractive errors are measured in diopter (D), and myopia is specified with a negative sign. Severity of myopia is categorized into 3 levels. 1) Mild (0 to -1.5D), 2) moderate (-1.5D to -6.0D) and 3) and severe myopia (-6.0D or above). Pathogenic myopia develops, when this condition advances to -8.0 D and above. Visual disorders - moderate retina disorders, cataract, and glaucoma - may be seen in patients with moderate and severe myopia. At birth, most infants are hyperopic but this condition lowers when they grow up and by the age 5-8, eyes must be emmetropic (normal and without any refractive error) (2).

Nearsightedness is seen in 30.4 million adults in USA (1). It's prevalence in adolescents is remarkably increased in last decades and today its prevalence in industrialized western and eastern societies are increased into 10-25 and 60-80 percent respectively (3). For American society, 60 percent of myopia can be categorized as early myopia (in childhood/school ages) which is usually in ages 9-11 years, and increases in adolescence years, and decreases in late adolescence and beginning of 20th decade of life, and usually stops at -3 to -4 D(9,10). A study in Tehran (4) showed that prevalence of myopia is 21.8 and 56.6 percent based on manifest and cycloplegic refraction respectively. Another study showed a high prevalence of myopia in schoolchildren from Kashan, Iran (11). Also a study in Dezful, Iran showed that prevalence of myopia in 7-15y and 14-18y children are 3.4 and 33 percent respectively (12).

Factors related to myopia can be divided into two groups: 1) hereditary and 2) environmental. Relationship between heredity and likelihood of myopia has been shown in many studies (13, 14). These studies obviously showed the effect of parental myopia on moderate levels of childhood myopia. Today different areas on several chromosomes are known for severe myopia (such like 18p, 12q, 7q36, TGIF) (15-17). Although it seems that two of these chromosomes (18q, 12q) do not have any relationship with childhood myopia (18).

Environmental factors which studies have shown that may be in relation with myopia are: near work (19, 20), stature (21, 22), night time light exposure (10) [however this relationship was not confirmed neither in American (20) and English children and nor in Indian monkeys (23, 24)], day time light exposure (25), refined sugar consumption [It has been shown that carbohydrate consumption can stimulate a series of events which lead to eye's axial length increase and consequently myopia. So refined sugar consumption which has been increased nowadays may play a role.] (26), IQ (Intelligence quotient) (27-33), birth weight, social and economic condition (7), physical activity (32), and breastfeeding (7).

Increasing prevalence of myopia, particularly in Asian urban children, confirms that life style in childhood and infancy may play a role (3). Today potential effect of early life nutrition on myopia development in later life is becoming an attractive field of study. Breastfeeding has been related to neural development which affects visual acuity. Up to weaning, breastfeeding is the main source of many micronutrients [including docosahexanoic acid (DHA)], which are important for light receptors and brain neural development (6). A study which was done in Singapore showed that breastfeeding is independently related to likelihood of myopia after controlling for cofactors (7). But this study had some limitations which may affect the results. This study had a small population size and its participants were from one school. All factors relating to myopia were not controlled by this study. However, data pulled from three English birth cohorts done in 10-11y and 15-16y children did not show any effect (8). Hence, results about the relationship between breastfeeding and likelihood of childhood myopia are controversial.

To examine the association between breastfeeding pattern and likelihood of myopia in 6-7y children, a retrospective case-control study is designed in National Nutrition and Food Technology research Institute of Iran. This study will try to assess almost all cofactors which may relate to myopia; such as child's age, sex, height, birth weight, maternal age at delivery, IQ (intelligence quotient), near work, parental education and ethnicity, parental myopia, child n-3(omega-3) fatty acid and refined carbohydrate intake, maternal n-3 fatty acid intake, n-3 fatty acid intake while infancy, physical activity and other confounding factors which may have an effect on the relationship among breastfeeding pattern and myopia.