Emmetropia (Figure 1) is when a distant object is in sharp focus with the eye in its relaxed or neutral state and is achieved when the refractive power of the cornea (and lens) and the axial length of the eye balance, which focuses the rays on to the retina. A mismatch between the power and axial length of the eye results in myopia or hypermetropia. In hypermetropia parallel rays of light are brought into focus behind the retina, usually due to an eye that is too short (Morgan et al., 2012)(Figure 1). Myopia is the condition where parallel rays of light are brought into focus in front of the retina (Curtin, 1985) (Figure 1) and as a result distant objects appear blurred.
Emmetropic eye Myopic eye Hypermetropic eye
Figure 1. Parallel rays of light in focus on the retina in the emmetropic eye, focused in front of the retina in the elongated myopic eye and focused behind the retina in the hypermetropic eye (from www.lensoftheeye.ga).
1.2 Emmetropisation
Neonates are usually born hypermetropic (with a normal distribution of refractive errors) (Cook and Glasscock, 1951) and through the emmetropisation process become less so. Refractive error is when the shape of the eye causes light from an image to be incorrectly focussed. The mean range of refractive power reduces to between +1.00 and -2.00 dioptres during the first two years of life (Mayer et al., 2001). The axial length of the human eye increases from 17 to 24mm between birth and 14 years of age (Curtin, 1985) (Figure 2). The mechanism involved in eye growth involve both passive and active (lens thinning and the role of defocus) mechanisms (Gwiazda et al., 2003). To achieve normal vision, control of the axial elongation of the eye during development is vital. Emmetropisation is guided by visual input,as well as genetic factors.
1.3 Optical changes in myopia
Myopia is primarily due to the axial elongation of the eye, or more specifically the vitreous chamber depth. 1mm of increase equates to 3 dioptres of myopia. However, a wide range of axial lengths are associated with emmetropia (Mallen, 2002) because corneal curvature also has an impact. There is a 90 percent certainty of knowing the refraction if the axial length is divided by corneal curvature (Mallen 2002). There is thus an inverse relationship between axial length and corneal curvature to myopia. Longer eyes are more likely to be myopic than shorter ones (Benjamin et al., 1957). Myopes also have thinner lenses and flatter corneas (Ip et al., 2008; Gwiazda et al., 2003; Zadnik et al., 1990; McBrien et al., 1997).
If one or more of the optical components of the eye fall outside the normal range for emmetropia component ametropia results and can lead to high levels of myopia. Correlation ametropia involves optical components, which are within a normal range, but the optical components are mismatched and is linked to lower levels of myopia (Mutti and Zadnik, 1997).
Myopic children exhibit axial elongation of typically 24-28mm (Trier et al., 2008). Most myopia develops in children between the ages of 8 and 14 (Zadnik et al., 1994) but can occur from the age of 6 progressing until 20 years of age, and even into early adulthood (Cumberland et al., 2007). Myopia progression (Figure 2) increases at a linear rate up to the age of 14 (Jones et al., 2005), after which there is a slow deceleration in progression (Thorn et al., 2005).
Figure 2. Myopia progression curve from an early onset, typical onset and later onset myope (Data from Thorn et al., 2005).
The axial length of an emmetropic eye is equal along its axial length and at 30 degrees in the periphery, as it is spherical (Figure 3). A myopic eye, however, has a prolate elliptical shape and so has a much larger axial measurement in comparison to that taken at 30 degrees. This characteristic ‘egg-shape’ occurs because the myopic eye grows greatest in length (0.35mm per diopter) compared to in height (0.19mm per diopter) and width (0.1mm per diopter) (Atchinson et al., 2004). There are ethnic variations to this rule as the East Asian eye is globally expanded equatorially as the peripheral and axial lengths are similar. Western eyes have more curved nasal retinas, whereas the temporal retina shows asymmetry and a more oblate shape (Singh et al., 2006)(Figure 3).
Figure 3. Different eye shapes including; the spherical emmetropic eye, an oblate hyperopic eye, the prolate myopic eye, the diffuse equatorial expansion of the East Asian myopic eye, and the asymmetrical (bulge nasally) Western myopic eye (from Stone et al., 2004).
Excessive axial elongation involves the outer coat of the eye, the sclera, whichhas a pivotal role in the control of eye size and the development of myopia. Aberrant mechanisms of scleral remodelling underlie the development of myopia,leading to scleral thinning, loss of scleral tissue, the weakening of the sclera’s mechanical properties, and posterior staphyloma (localised retinal elongation) (Figure 4) (McBrien et al., 2003).
Figure 4. A 3D MRI scan of a myopic eye with posterior staphyloma. The posterior globe is asymmetrical and shows protrusion (from www.retinalphysician.com).
Axial elongation and ocular globe enlargement is achieved by active remodelling of the sclera rather than a passive stretch (Christensen and Wallman, 1991; McBrien and Gentle 2003). In response to altered visual experience there is local scleral growth regulation. Remodelling of the scleral structure results in a decrease in the average diameter of collagen fibers and consequent biochemical weakening of the sclera. An enlarged globe is associated with ectasia (bulging and thinning) of the posterior sclera (Elsheikj and Phillips, 2013).
1.4 Ocular morbidity of myopia
Myopia control is important because of the comorbidity and risk for severe visual impairment associated with it (Saw et al., 2005). Ocular diseases associated with myopia include; primary open-angle glaucoma (with odds ratios of 1.77 with low myopia and 1.88 with high myopia) (Marcus et al., 2011), retinal detachment and degenerations (Chou et al., 2007), posterior staphyloma (Figure 4) (Saw et al., 2005), cataract (Leske et al., 1991) (Jeganathan et al., 2011) and maculopathy (Cheng et al., 2012). Myopia is negatively associated with age-related macular degeneration (Lavanya et al., 2010). Increased myopia severity poses an increased risk of complications due to retinal and choroidal pathologies (Vongphanit et al., 2002) and the prevention of the associated vision loss requires costly ophthalmic treatment (Morgan et al., 2012).
The incidence of retinal detachment is estimated at 0.015% for less than 4.74 dioptres of myopia and increases to 0.07% with up to 5 dioptres of myopia, and further increases to 3.2% in over 6 dioptres (Arevalo et al., 2000; Arevalo and Azar-Areval, 2000). Myopes have a double risk of developing macular choroidal neovascularisation with 1 to 2 dioptres of myopia, a four times increased risk with 3 to 4 dioptres of myopia and a nine times increased risk with 5 to 6 dioptres (Avila et al., 1984; Steidl et al., 1997; Vongphanit et al., 2002).
Myopia is associated with major social, economic and educational consequences and impacts significantly on the quality of life (Saw et al., 1996). The estimated cost for correction of myopia by spectacles and contact lenses in the USA alone is 2.5 to 4.6 billion dollars per year (Javitt and Chiang, 1994) and so interventions to retard myopia progression would reduce these costs worldwide.
About 2 % of Western populations have myopia greater than 6 diopters, termed high myopia (McBrien et al., 2003), which is associated with a marked increased risk of sight threatening disease. Highly myopic eyes exhibit a thin, stretched, sclera which is expanded under the force of intraocular pressure (Harper and Summers, 2015) and have an increased risk of retinal detachment, glaucoma and macular degeneration (Pan et al., 2013; Qiu et al., 2013). In the USA 2 to 4% of the population have high myopia (Jones and Luensmann, 2012), whereas around 20% of Taiwanese school age children have sight threateningly high myopia (Lin et al., 2004). Children are presenting with myopia at a younger age, which puts them at a greater risk of developing high myopia and its associated complications (Vitale et al., 2004). Consequently, high myopia and its associated co-morbidities are a worldwide leading cause of registered blindness and partial sight (Arevalo et al., 2000; Arevalo and Azar-Areval, 2000; Avila et al., 1984; Steidl et al., 1997; Vongphanit et al., 2002; Mitchell et al., 1999).
Myopia can be largely corrected with glasses, contact lenses or refractive surgery. However, these interventions may not be possible in developing countries and the World Health Organisation (WHO) analyses demonstrate that uncorrected refractive error is the major cause of visual impairment in the world (Resnikoff et al., 2008).
With an increased myopic population comes a smaller pool of candidates for specific careers, such as the armed forces and airline pilots. Myopia is a global public health concern (Vitale et al., 2008) and one of the most important causes of visual impairment (Buch et al., 2001; Yamada et al., 2010), and so strategies to attempt to reduce its prevalence and minimize its effects should be investigated.
1.5 Prevalence of myopia
Myopia affects approximately 1 in 6 of the World’s population (Norton et al., 2005). The prevalence of myopia is 25 to 40% percent of the population in the USA (Kempen et al., 2004; Vitale et al., 2008, 2009), 30 to 50 percent in Europe (Logan et al., 2005; Jobke et al., 2008), and up to 75-80 percent in the East Asia. (Morgan and Rose, 2005). There is an epidemic of myopia across East and South East Asia (Morgan et al., 2012) including China (He et al., 2004; Qian et al., 2009), Hong Kong (Goh and Lam, 1994), Japan (Matsumura and Hirai, 1999), Korea (Junge et al., 2012), Singapore (Wu et al., 2001; Pan et al 2011; Saw et al., 2008; Wong et al., 2000; Wu et al., 2001) and Taiwan (Lin et al., 2004; Cheng et al., 2003).
1.5.1 The prevalence of myopia is on the increase
In recent years the prevalence of myopia, especially in East Asia, has been increasing (Fan et al., 2004: Sawada et al., 2008: Won et al., 2000; Morgan et al., 2005; Pan et al., 2012; Fredrick, 2002; Cheng et al., 2003; Lin et al., 2004; Saw et al., 1996; Sperduto, 1983; Bar et al., 2005; Virale et al., 2009; Pan et al., 2012). The prevalence of myopia was also two-thirds higher in 1999-2004 than in 1971-1972 in the USA (Vitale et al., 2008; Vitale et al., 2009). In Singapore the prevalence of myopia has increased from 1987-92 in all three ethnic groups (Chinese, Indians and Malays) (Figure 5). In Taiwan the prevalence of myopia has stopped increasing and plateaued at a high level, although there are increases in severity of myopia (Cheng et al., 2003).
Figure 5. Increases in the prevalence of myopia in Singapore in the three major ethnic groups from 1987-92 to 2009-10 (Vitale et al., 2009; Tay et al., 1992; Saw et al., 2011) (from Morgan et al., 2012).
1.5.2 Prevalence of myopia increases with age
Myopia prevalence increases with age, rising from 2.8 % in Northern Irish children aged 6 to 7 to 17.7% in those aged 12 to 13 years of age (O’Donaghue et al., 2010), and, in English children, myopia prevalence increases from 9.4% at age 6 to 7 to 29.4% at 12 to 13 years of age (Logan et al., 2011) (Figure 6).
Figure 6. The prevalence of myopia in Northern Irish and English children (data from O’Donaghue et al., 2010; Logan et al., 2011).
In Singapore, myopia increases from 9-15% in preschool children (Saw et al., 2001, Tan et al. 2000) to 29% in primary school children (Saw et al., 2001). In Taiwan the prevalence of myopia increases from 6% in 6 year olds to 70% at age 15 (Lin et al., 2004) and up to 90% in other parts of East Asia (Morgan et al., 2012), showing that late onset myopia (aged 15 and above) is also on the increase.
1.5.3 Prevalence of myopia and ethnicity
Myopia varies with ethnicity (Figure 7). 28% of Singaporean 7 year olds have myopia (Saw et al., 2002), in comparison to less than 5% in Western populations (Mayer et al., 2001; Young et al., 1954; Junghans et al., 2005; Amigo et al., 1976; Matsumura et al., 1999; Ojaimi et al., 2005). 2% of Western populations have high myopia, but in Singapore up to 10% of the population are so affected (Wu et al., 2001; Wong et al., 2000).
1.5.4 Prevalence of myopia and location
Differences in prevalence have been noted with lower incidences of myopia in rural areas to urban areas (Figure 7) (Pi et al., 2010; Zhao et al., 2000; He et al., 2004; Watanabe et al., 2009; Matsumara et al., 1999; Gao et al., 2012; Yingyong et al., 2010; Dandona et al., 2002l Murthy et al., 2002; Czepita et al., 2008).