?2.LITERATURE REVIEW
2.1TYPES OF REFRACTIVE ERRORS
Refractive errors occur when the shape of the eye prevents light from focusing directly on the retina. The length of the eyeball (longer or shorter), changes in the shape of the cornea, or aging of the lens can cause refractive errors.

The most common types of refractive errors are near-sighted (myope), far-sighted (hyperope), astigmatism and presbyopia.

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Near-sightedness (also called myopia) is the condition where objects up close appear clearly, while objects far away appear blurry. With near-sightedness, a light comes to focus in front of the retina instead of on the retina.

Far-sightedness (also called hyperopia) is a common type of refractive error where distant objects may be seen more clearly than objects that are near. However, people experience far-sightedness differently. Some people may not notice problems with their vision, especially when they are young. For people with significant far-sightedness, vision can be blurry for objects at any distance, near or far.

Astigmatism is a condition in which the eye does not focus light evenly onto the retina, the light-sensitive tissue at the back of the eye. This can cause images to appear blurry and stretched out.

Presbyopia is an age-related condition in which the ability to focus up close becomes more difficult. As the eye ages, the lens can no longer change shape enough to allow the eye to focus close objects clearly.

2.2PREVALENCE OF REFRACTIVE ERROR IN CHILDREN GLOBALLY
2.2.1PREVALENCE OF REFRACTIVE ERROR IN CHILDREN IN AFRICA
2.3PROGRESSION OF MYOPIA IN CHILDREN
Myopia can begin at any age but begins in most cases at school age. The prevalence of myopia and its rate of progression vary widely according to several factors (e.g. ethnicity, population, age, gender, occupation, and education). Hereditary factors have been reported in relation to refraction and myopia (Teikari et al. 1991; Parssinen et al. 2010).

Recent genome-wide studies have shown several gene loci which are associated with ocular refraction and myopia (Verhoeven et al. 2012, 2013). However, the worldwide trend towards an increase in the prevalence of myopia is hard to explain outside of environmental factors (Vitale et al. 2009; Parssinen 2012). Several studies have connected the prevalence of myopia with higher education and occupational status (Parssinen 1987; Teasdale et al. 1988; Kinge et al.1998).

There is some evidence that myopic progression could be connected with more time spent on reading and close work and less time spent on sports and outdoor activities among children. Saw et al. (2002) found that children aged 7–9 years with greater current reading exposure were more likely to be myopic. A follow-up study by Yi & Li (2011) in 7- to 11-year-old school children showed a connection between slower myopic progression and more outdoor activities, more time spent wearing glasses, more time spent in natural light and less time using a computer. However, the causative role of these variables on myopic progression has not gone unquestioned in all studies (Jones-Jordan et al. 2011). Studies on the connections between time spent watching TV and myopia have not shown signi?cant correlations (Czepita et al. 2010; Wu et al. 2010).

To date, it remains unclear precisely what factors in?uence the increase in myopia prevalence, and in what way, and which of the two factors, reading and close work or time spent on sports and outdoor activities, is more connected with the progression of myopia. Do the time spent on sports and outdoor activities per se prevent myopia or are the connection between these factors explained simply by the absence of reading and near-work activities?

The results of the ?rst 3-year follow-up study of the subjects of the present study showed that the factors with the most signi?cant relationships to myopic progression were female gender, young age of onset and the high degree of myopia at Baseline (Parssinen & Lyyra 1993).

Faster myopic progression and higher myopia at the end of 3-year follow-up were related to more time spent on reading and close work and to short reading distance but not, however, to accommodation stimulus (Parssinen & Lyyra 1993).
When the same subjects were studied 10 years later, the rate of myopic progression was related to the level of education among those whose myopia begun at the ?fth grade of school, but not among those whose myopia begun 2 years earlier at the third grade (Parssinen 2000).

The aim of this study was to examine myopic progression in the same subjects from its onset at school age onwards into adulthood and to study to what extent the rate of myopic progression is explained by individuals’ patterns of spending time and parental myopia.

This study consisted of 240 myopic schoolchildren with a mean age of 10.9, range 8.7–12.8 years, who were recruited during the years 1983–1984 to a randomized clinical trial of myopia treatment (Hemminki ; Parssinen 1987). The children from grades III or V of basic school who had a poor distant vision at screening were sent for an ophthalmological examination. All participants were born Finn’s resident in the Central Hospital of Central Finland Health Care District.

The main inclusion criteria were spherical refraction > -3D, astigmatism ? -2 D, spherical equivalent (SE) ? -3 D, no other eye diseases and no previous glasses for myopia. The more detailed inclusion and exclusion criteria have been described earlier (Hemminki & Parssinen 1987).

One hundred and nineteen boys and 121 girls were randomly allocated to three di?erent treatment groups according to the recommended use of spectacles: continuous use, only for distant use and bifocals with a 1.75 D add. An annual examination (Follow-ups 1, 2 and 3) was conducted for 3 years.

Follow-up 3 at the mean age of 13.9 years was conducted for 238 of them (Parssinen & Lyyra 1993). The next clinical follow-up (age 24 follow-up) was subsequently conducted about 13 years after the Baseline for 179 (74.6%) subjects (Parssinen 2000). The mean age of the subjects at that examination was 23.7 years, ranging from 20.9 to 26.9 years.
The last clinical examination (age 35 follow-up) was carried out for 134 (55.3%) subjects. Their mean age was 34.7 years (ranging from 31.9 to 37.4 years.). Mean years of education was 15.6 ± 3.3, and in 93% of cases, it was >12 years.

There were no statistically signi?cant di?erences in SE between the right, -1.43 D ± 0.59 (SD), and the left eye, -1.47 D ± 0.60, at the beginning of the study (p = 0.153, paired-samples test) or at the end of the follow-up (25–39 years), SE -5.02 D ±2.23 and -5.06 D ± 2.14 (p = 0.599, paired samples t-test). Next, only the SE values of the right eye were used. Refractive surgery had been performed for 17 subjects. Their refraction in childhood (Follow-up 3) was -3.20 D ±1.19 as compared to 3.06 D ± 1.19 (p = 0.638, paired-samples t-test) in non-operated persons.

In our long-term follow-up of myopic children, in nearly half of the cases, myopic progression starting at a young school-age continued in adulthood. Higher adulthood myopia was mainly associated with female sex, parental myopia and less time spent on sports and outdoor activities in childhood. Reading without glasses or use of bifocals to reduce accommodation stimulus during childhood did not correlate with adulthood refraction. Short reading distance in childhood predicted higher adulthood myopia among females but not males. Time spent on reading and close work in childhood was associated with myopic progression during the ?rst 3 years but did not predict myopia in adulthood.

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