Association Between Breastfeeding and Likelihood of Myopia in 6-7 Years Old Children
Condition: Myopia; Nearsightedness
MeSH terms: Myopia
Study type: Observational

Summary: 
      Myopia is the main reason of vision loss in the world which is seen in 30.4 million adults in
      USA (1). In a myopic eye, images are focused in front of retina because, cornea and
      crystalline lens become more powerful in focusing the image or the eye axis become too long
      (axial myopia) (2).

      Myopia prevalence in adolescents has been raised in recent years and nowadays it has reached
      to 10-25% and 60-80% in western and eastern countries respectively (3). Tehran eye study
      showed that the prevalence of myopia in Tehran is 21.8% and 17.2% based on manifest and
      cycloplegic refraction respectively (4). One study in Dezful, Iran showed that 3.7% of 7-15
      years old children are myopic (5).

      Today potential role of early life nutrition in myopia development in later life is becoming
      an attractive field of study. It has been shown that until weaning; breast milk is the main
      source of many nutrients [such as docosahexaenoic acid (DHA)] which are important for
      photoreceptors and cortical neuronal development (6). But results about the relationship
      between breastfeeding and likelihood of childhood myopia are controversial. A
      hypothesis-generating study (7) showed an independent and inverse relationship between
      breastfeeding and likelihood of myopia in 10-12 year old Singaporean children. However, data
      pulled from three English birth cohorts done in 10-11y and 15-16y children did not show any
      effect (8).

      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 breastfeeding pattern
      and almost all cofactors which may relate to myopia.
    

Detailed description: 
      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.
    

Observational model: Case-Control
Time perspective: Retrospective
Sampling method: Non-Probability Sample
Study population: 
        This study is a retrospective case-control trial, in which subjects for case and control
        groups will be chosen based on inclusion and exclusion criteria, from 6-7 years old school
        children living in north Tehran.
      
Keywords: breastfeeding; myopia; nearsightedness; children