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Epidemiology

There have been several studies on the prevalence of myopia in specific countries, and there appears to be a very wide distribution of prevalence. Unfortunately, there is no universally agreed statistical grouping point, in part owing to the lack of a formal classification system. This makes comparisons between epidemiological data difficult, with some groups classifying myopia as any negative mean sphere, others as an arbitrary amount, such as -0.50D or -1.00D. One group[1] notes that changing the minimum definition of myopia from -0.50DS to -0.75DS in their cohort would result in a 22% lower myopia prevalence. This dissimilitude extends to defining ‘high’ myopia, most typically associated with sight threatening debility. This would cause disparate inter-study groups, and dissimulate any true differences in between populations, or even longitudinal studies on the same populations. This could lead to inappropriate comparisons between the risks of associated conditions in high myopia, for example, the true rate of retinal detachment in ‘high’ myopia.

An additional difficulty of determining longitudinal prevalence changes is the natural course of myopia over a lifespan. There is a natural tendency for infants to be hyperopic, and then shift myopically towards emmetropia[2]. During the schooling years, there is a superfluous shift towards myopia, which stabilises for the majority of the working ages, then retrogrades in the fifth to sixth decade[3]. Any cross-sectional study is going to show age-related variations due to these variations, which will tend to indicate increasing myopia prevalence in a younger age group[4]. There are also discrepancies in the protocols used to measure myopia, both in technique (auto-refractor, retinoscopy, subjective refraction), and in control of accommodation (dosing, drugs, confirmation of cycloplegia). There is a current movement to standardise the methodology using ≥-0.50D mean sphere as the definition of myopia, measured with auto-refractometry and retinoscopy on a cyclopleged eye[5, 6].

Worldwide, there appears to be very large variations in myopia prevalence between countries, and even within geographical cohorts within a country. In South Africa, 2.9% of 5-15 year old low to middle socioeconomic area Africans had myopia[7], correlating well with 2.9% of 6-19 year old Vanuatuan Melanesians [8]. In Morocco, 6.1% of school aged children had myopia[9]. Contrast these less developed countries with those experiencing rapid social development and population growth. Taiwanese 16-18 year olds have a myopia prevalence of 84% [10], as do 83% of Singaporean medical students[11]. Another Taiwanese study of first year university students (using the rather low myopia threshold of -0.25DS) shows that myopia prevalence is still increasing, increasing from 91.3% in 1988 to 95.9% in 2005 [12]. Interestingly, there was a gap between the sexes in 1988, which had neutralised by 2005, which could suggest phenotype saturation in this cohort. This wide international range is likely to due to the numerous implicated risk factors associated with increased urbanisation, and the spread of myopia-genic genes, discussed in the following section.

The prevalence of myopia is increasing in zz (all studied?) advanced countries. In the United States, a group repeated an epidemiological study of one conducted in 1971-1972. They originally used somewhat non-standard methodology by today’s standards, of dividing groups based on unaided vision, and measuring (or estimating) refractive error differently in each group. The methodology was kept reasonably constant (retinoscopy switched for autorefraction) in the 1999-2004 data set, allowing longitudinal comparison between the data sets, but not easy comparison with other epidemiological studies. It should be noted that a threshold of myopia of any negative mean sphere, which would tend to overestimate myopia compared to other studies. The total prevalence of myopia increase from 25% in 1971-1972, to 41.6% in the 1999-2004 period. A significant increase was present when the population was divided into race (black and white), sex, age, and severity of myopia.

Unfortunately, despite on-going anti-myopiagenic research, there is no published epidemiological data for New Zealand. Sharing a similar societal composition with our Australian neighbours, it is unlikely to be vastly different. In Australia, myopia prevalence increases from 1.43% at 6.7 years old to 17% in the general population [13]. There does not appear to be much evidence of an epidemic in Australia, and the prevalence in New Zealand is anecdotally not increasing at the same rate as Asia [14]. There is the possibility that we are merely where Asia was several decades ago, and Grosvenor[3] provides several reasons why we are likely to trend towards myopia, including the earlier commencement of, and increasingly competitive education.There have been several studies on the prevalence of myopia in specific countries, and there appears to be a very wide distribution of prevalence. Unfortunately, there is no universally agreed statistical grouping point, in part owing to the lack of a formal classification system. This makes comparisons between epidemiological data difficult, with some groups classifying myopia as any negative mean sphere, others as an arbitrary amount, such as -0.50D or -1.00D. One group[1] notes that changing the minimum definition of myopia from -0.50DS to -0.75DS in their cohort would result in a 22% lower myopia prevalence. This dissimilitude extends to defining ‘high’ myopia, most typically associated with sight threatening debility. This would cause disparate inter-study groups, and dissimulate any true differences in between populations, or even longitudinal studies on the same populations. This could lead to inappropriate comparisons between the risks of associated conditions in high myopia, for example, the true rate of retinal detachment in ‘high’ myopia.

An additional difficulty of determining longitudinal prevalence changes is the natural course of myopia over a lifespan. There is a natural tendency for infants to be hyperopic, and then shift myopically towards emmetropia[2]. During the schooling years, there is a superfluous shift towards myopia, which stabilises for the majority of the working ages, then retrogrades in the fifth to sixth decade[3]. Any cross-sectional study is going to show age-related variations due to these variations, which will tend to indicate increasing myopia prevalence in a younger age group[4]. There are also discrepancies in the protocols used to measure myopia, both in technique (auto-refractor, retinoscopy, subjective refraction), and in control of accommodation (dosing, drugs, confirmation of cycloplegia). There is a current movement to standardise the methodology using ≥-0.50D mean sphere as the definition of myopia, measured with auto-refractometry and retinoscopy on a cyclopleged eye[5, 6].

Worldwide, there appears to be very large variations in myopia prevalence between countries, and even within geographical cohorts within a country. In South Africa, 2.9% of 5-15 year old low to middle socioeconomic area Africans had myopia[7], correlating well with 2.9% of 6-19 year old Vanuatuan Melanesians [8]. In Morocco, 6.1% of school aged children had myopia[9]. Contrast these less developed countries with those experiencing rapid social development and population growth. Taiwanese 16-18 year olds have a myopia prevalence of 84% [10], as do 83% of Singaporean medical students[11]. Another Taiwanese study of first year university students (using the rather low myopia threshold of -0.25DS) shows that myopia prevalence is still increasing, increasing from 91.3% in 1988 to 95.9% in 2005 [12]. Interestingly, there was a gap between the sexes in 1988, which had neutralised by 2005, which could suggest phenotype saturation in this cohort. This wide international range is likely to due to the numerous implicated risk factors associated with increased urbanisation, and the spread of myopia-genic genes, discussed in the following section.

The prevalence of myopia is increasing in studied advanced countries. In the United States, a group repeated an epidemiological study of one conducted in 1971-1972. They originally used somewhat non-standard methodology by today’s standards, of dividing groups based on unaided vision, and measuring (or estimating) refractive error differently in each group. The methodology was kept reasonably constant (retinoscopy switched for autorefraction) in the 1999-2004 data set, allowing longitudinal comparison between the data sets, but not easy comparison with other epidemiological studies. It should be noted that a threshold of myopia of any negative mean sphere, which would tend to overestimate myopia compared to other studies. The total prevalence of myopia increase from 25% in 1971-1972, to 41.6% in the 1999-2004 period. A significant increase was present when the population was divided into race (black and white), sex, age, and severity of myopia.

Unfortunately, despite on-going anti-myopiagenic research, there is no published epidemiological data for New Zealand. Sharing a similar societal composition with our Australian neighbours, it is unlikely to be vastly different. In Australia, myopia prevalence increases from 1.43% at 6.7 years old to 17% in the general population [13]. There does not appear to be much evidence of an epidemic in Australia, and the prevalence in New Zealand is anecdotally not increasing at the same rate as Asia [14]. There is the possibility that we are merely where Asia was several decades ago, and Grosvenor[3] provides several reasons why we are likely to trend towards myopia, including the earlier commencement of, and increasingly competitive education.

 

References

1. Wensor, M., C.A. McCarty, and H.R. Taylor, Prevalence and risk factors of myopia in Victoria, Australia. Archives of Ophthalmology, 1999. 117(5): p. 658-663.
2. Kleinstein, R.N., et al., Refractive error and ethnicity in children. Archives of Ophthalmology, 2003. 121(8): p. 1141-1147.
3. Grosvenor, T., Why is there an epidemic of myopia? Clinical & experimental optometry : journal of the Australian Optometrical Association, 2003. 86(5): p. 273-275.
4. Park, D.J. and N.G. Congdon, Evidence for an "epidemic" of myopia.[see comment]. Annals of the Academy of Medicine, Singapore, 2004. 33(1): p. 21-6.
5. Negrel, A.D., et al., Refractive error study in children: Sampling and measurement methods for a multi-country survey. American Journal of Ophthalmology, 2000. 129(4): p. 421-426.
6. Ojaimi, E., et al., Methods for a population-based study of myopia and other eye conditions in school children: The Sydney myopia study. Ophthalmic epidemiology, 2005. 12(1): p. 59-69.
7. Naidoo, K.S., et al., Refractive error and visual impairment in African children in South Africa. Investigative Ophthalmology and Visual Science, 2003. 44(9): p. 3764-3770.
8. Garner, L.F., et al., Refraction and its components in Melanesian schoolchildren in Vanuatu. American Journal of Optometry and Physiological Optics, 1988. 65(3): p. 182-189.
9. Anera, R.G., et al., Prevalence of refractive errors in school-age children in Morocco. Clinical and Experimental Ophthalmology, 2009. 37(2): p. 191-196.
10. Lin, L.L.K., et al., Prevalence of Myopia in Taiwanese Schoolchildren: 1983 to 2000. Annals of the Academy of Medicine Singapore, 2004. 33(1): p. 27-33.
11. Woo, W.W., et al., Refractive errors in medical students in Singapore. Singapore Medical Journal, 2004. 45(10): p. 470-474.
12. Wang, T.J., et al., Changes of the ocular refraction among freshmen in National Taiwan University between 1988 and 2005. Eye, 2009. 23(5): p. 1168-1169.
13. Ojaimi, E., et al., Distribution of ocular biometric parameters and refraction in a population-based study of Australian children. Investigative Ophthalmology and Visual Science, 2005. 46(8): p. 2748-2754.
14. Rose, K., et al., The increasing prevalence of myopia: Implications for Australia. Clinical and Experimental Ophthalmology, 2001. 29(3): p. 116-120.
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