Both the optical and tabloid press are eager to inform us that we are in the midst of a ‘myopia epidemic’. In their systematic review published in 2016 Holden, Fricke, Wilson, et al1 estimated that the number of myopic individuals in 2000 constituted 23% of the global population. Their analysis predicts that by 2020 the myopic population will have increased to 34% of the total and that, by 2050, 50% of the world may be myopic (≥-0.50DS).

These projections obviously vary by geographical location and within population demographics. This potential twofold increase in the prevalence of myopia by 2050 has huge implications for the optometric profession. The optometrist’s traditional role in the supply of refractive correction is likely to expand to include the provision of a variety of interventions to arrest myopia development. The various interventions currently proposed for myopia prevention and their impact will be discussed in the second article in this series.

What is myopia?

If we define emmetropia as the condition in which the focal plane of the refractive system of the eye is conjugate with the foveal plane in an unaccommodated eye, then myopia is the condition where the focal plane of the refractive system forms in front of the foveal plane (figure 1). This condition occurs when there is a mismatch between the refractive structures of the eye, predominantly the cornea and lens, and the axial length of the eye. The radii of curvature of the cornea and lens as well as their refractive indices are the principal elements which contribute to their overall power. Typically, the myopia seen in the early stages of keratoconus can be traced to the steepening of the radii of curvature of either the anterior or posterior surfaces of the cornea. The anterior chamber and posterior chamber depths also have a small part to play in the overall refractive status of the eye.

Figure 1: Schematic myopic eye

The infant eye

Wood, Mutti and Zadnik,2 in their study of 19 infants, found the cornea to have an average corneal radius of 7.76mm (7.35 to 8.46mm) and an average power of 43.5DS (45.9 to 39.9DS). Sorsby, Benjamin & Sheridan,3 in their 1961 study of the growth of the eye, found that the infant cornea was 3 to 5DS steeper than that of the adult. The average adult cornea has an anterior radius of 7.8mm, and 6.7mm for the posterior radius, giving rise to an average corneal power of 42.00DS. The reduction in corneal power which occurs as a child grows should be matched by the change in the axial length if the eye is to achieve emmetropia.

Analysis of the longitudinal data from the Orinda Longitudinal Study of Myopia by Zadnik, Mutti, Mitchell, et al,4 produced data on normal eye growth in emmetropic children (refractive error of between -0.25 and +1.00DS). All the children in the study were aged between six and 14 years of age. The group found the anterior chamber depth, the vitreous chamber depth (VCD) and the axial length (AL) increased as the children aged. The corneal power reduced by 0.06DS on average, and lens power reduced by between 1.08 and 2.11DS, dependent upon the method of calculation, over the same period. They also found the lens thickness and refractive index decreased. Changes of this magnitude in the two refractive elements of the eye accompanied by the average increase of 0.61mm in VCD and 0.73mm in AL allowed the eyes to maintain their emmetropic status. Axial growth of the eye without the corresponding change in the cornea and lens would create a myopic refractive error of approximately 2.00DS (2.67D per 1mm of axial length)5.

A recent study of myopia in full term and premature children found that, while an increase in axial length was the main contributor to myopia in children delivered at full term (39 weeks’ gestation), in children born at less than 30 weeks’ gestation corneal curvature and lens thickness were the predominant factors in their myopia development.6 The lens thickness in these preterm children meant they also had shallower anterior chamber depths. In their paper on structural models for emmetropic and myopic eyes, Scott and Grosvenor pointed out the paradox that structural modelling would indicate that eyes with a greater axial length should have a flatter corneal radius.7 This is clearly not reflected in practice, since myopes are generally found to have steeper corneas than hypermetropes.

It has been suggested the axial length/corneal radius (AL/CR) ratio may be a better predictor of the progression towards myopia than if just the axial length were to be considered. Grosvenor and Scott in their 1994 study suggested in emmetropia the AL/CR ratio should be in the region of 3:1.8 A number of cross-sectional studies found no relationship between this ratio and the presence of either juvenile or young adult myopia.9 It seems there is no single parameter which can be interpreted as being indicative of myopia. An individual’s axial length, however, must be seen as the strongest correlate with their final refractive status.

Categories of myopia

The definition of myopia, simply in terms of the position of the image plane, fails to provide a full description of the condition. Studies into the impact of myopia vary in their definition of the level of refractive error which they consider to constitute myopia. Typically errors of less than or equal to -0.50DS are taken to be an indication of the presence of myopia for research purposes.1 The myopia spectrum has been further sub-divided into;

  • moderate myopia; -3.00 to -6.00DS
  • high myopia; -6.00DS and over

Holden et al used a value of ≥ -5.00DS for their evaluation of the prevalence of high myopia.1 They projected that the number of myopes falling into this category by 2020 could be 5% of the world’s population, rising to 10% by 2050. The American Academy of Ophthalmology suggests high myopia could be defined as occurring in eyes with an axial length greater than or equal to 26.5mm.10

In other research studies investigating the development of myopia, the condition is often defined in terms of its age of onset. Juvenile or early onset myopia are terms used to describe myopia with an initial presentation between the ages of nine to 11 years of age and that continues to progress throughout the teenage years. In contrast, adult or late onset myopia is that which is first seen from 17 years of age and continues to develop into the twenties. Pointer and Gilmartin reported a small number of individuals developed a myopic shift during the incipient phase of presbyopia.11 In this study, individuals aged 35 to 44 years showed a change of -0.50 to -0.75DS; in all cases the individual was already myopic prior to the change in refraction. The authors suggest these data could indicate another group of late onset myopia which is not related to the pathology of ageing, such as nuclear sclerosis of the lens.

A number of myopia studies have included the descriptive term of either physiological or pathological myopia. In this instance, physiological myopia is myopia which remains at a level which requires refractive correction but leaves the sufferer with no increased risk of other pathology. Pathological myopia is that in which the magnitude of the correction and the consequent effect on the tissues of the eye leave the individual open to the full spectrum of myopia complications. The American Academy of Ophthalmology defines pathological myopia as being -8.00DS or more and with an axial length of 32.5mm or more.10

The term pathological myopia is often used when the economic burden of health interventions on the individual sufferer or the population as a whole is to be considered. It is easy to envisage that the economic burden of pathological myopia is an ever increasing challenge due to the associated complications. The cost of the provision of refractive correction for physiological myopes continues to increase. Zheng, Pan, Chay, et al, estimated the economic burden of myopia for the adult population of Singapore was $755 million per year.12 Since the prevalence of myopia in Singapore is 80%, this burden is clearly in excess of that seen in the UK adult population.

Myopia correction methods

We are all familiar with the use of spectacles for the correction of myopia. The development of lens materials with increasing refractive index, both glass and plastic, have provided improved cosmesis. The use of aspheric and bi-aspheric lens designs and the plethora of lens coatings enable the optometrist to provide their myopic patients with spectacles which meet both their visual and aesthetic requirements. Many myopic individuals elect to use contact lenses for some or all of their optical correction. The development of contact lens polymers, both soft and rigid, and the surface treatments of these materials have increased the range of myopic correction available. In the next article we will examine the efficacy of the use of either spectacles or contact lenses in the control of the progression of myopia.

A number of surgical procedures for the correction of myopia have been developed since the 1940s.13 Radial keratotomy, in which a number of radial incisions were made into the anterior or posterior cornea, produced flattening of the corneal curvature. The availability of laser technology since the 1960s has allowed a further range of surgical procedures to be developed to address myopia correction. Photorefractive keratectomy (PRK) was the first of these procedures (figure 2) followed by LASIK and LASEK techniques.

Figure 2: Radial keratotomy

These latter two surgical options are now seen as safe procedures for the correction of myopia (up to -14.25DS and -4.50DCyl).14,15 The visual outcomes for both LASIK and LASEK have been enhanced with the use of wave front guided technology to reduce the impact of higher order aberrations.

In response to concerns regarding the creation of the flap and consequent corneal ectasia in the LASIK procedure a new surgical technique of small incision lenticule extraction (SMILE) has been developed. In this case, a small lenslet of tissue is removed from the corneal stroma via a peripheral corneal tunnel and no flap is produced.16 Zheng, Zhou, Zhang, et al, compared the visual outcomes for LASIK, wave front guided LASIK and SMILE and found that after three months’ refractive results were comparable for all three modalities.17 They did note that mesopic contrast sensitivity and higher order aberrations were improved by wave front guided LASIK.

Intrastromal corneal rings received FDA approval in 1999 for treatment of myopia between -1.00 and -3.00DS. This reversible procedure involves the insertion of two bio-compatible sectoral sections which cover 150° of the circumference of the cornea. The procedure is limited by central corneal thickness with a lower limit of 480µm. The rings allow the central cornea to flatten; the degree of flattening is governed by the thickness of the inserted rings.13,18 The American Academy of Ophthalmology confirm that intrastromal corneal rings are a safe surgical procedure for the treatment of low myopia (-1.00 to -3.00DS).19

In individuals with high myopia (<-6.00) the use of phakic intraocular lenses (figure 3) allows the patient to retain accommodative function. These lenses may be angle or iris supported in the anterior chamber or sulcus supported in the posterior chamber.20 Gimbel, Norton and Amritanand evaluated the use of angle supported lenses in 51 myopes of between -6.50 and -19.50DS. They found the lens provided a refractive outcome of within +/-1.00D of the target refraction in 92% of subjects.21 Kohnen, Maxwell and Holland found similar results in their follow up of more than 500 eyes over five years.22 Miraftab, Hashemi and Asgari found that phakic IOLs provided a better refractive outcome than mitomycin C enhanced PRK in their group of high myopes.23 The implantation of phakic IOLs is clearly limited by a patient’s anterior chamber depth and in these circumstances PRK-MMC is a suitable alternative in high myopes. Refractive lens or clear lens exchange has also been used to correct high levels of myopia. While the use of intraocular lenses addresses the myopic defect in young patients they are rendered prematurely presbyopic unless some form of multifocal implant is used. Researchers have concluded that refractive lens exchange provides a predictable surgical outcome in moderate to high myopes.24,25 Individuals undergoing clear lens extraction are at risk of any of the sight-threatening complications which may occur as a result of intraocular surgery.

Figure 3: Optics of a diffractive intraocular lens

Inheritance

Anecdotal evidence from optometric practice would indicate that myopia ‘runs in the family’. The CREAM consortium, an international group of scientists, has recently published its report on childhood gene and environment interactions.26 The report states 39 genetic loci have been identified in the human genome which may be associated with refractive error and myopia. Work is ongoing to look at how known environmental risk factors interact with these genetic markers. They concluded that certain genes were active at specific times in a child’s life giving rise to the pattern of myopia development mentioned earlier, ie juvenile or young adult onset. Numerous twin-based studies have looked at the inheritance of refractive error over the years. Individuals recruited to these studies are monozygotic or identical twins. Since each pair has the same genetic pattern then differences in refractive error between the pairs can be put down to environmental influences.27 Guggenheim, St Pourcain, McMahon, et al, concluded genetic factors produced only 35% of the variation in refractive error in unrelated individuals.28

The influence of parental myopia has been evaluated by many groups of researchers. Most have concluded that the number of myopic parents influences the development of myopia. The Sydney Myopia study found that the influence of one myopic parent was double the increase in the risk of myopia while having two myopic parents increased that risk by eight times.29 -32 Xiang, He and Morgan, in their studies of children in China, found children there became myopic irrespective of their parents’ myopia status.33,34 They also concluded that the level of myopic development did not necessarily reflect that seen in their parents, ie highly myopic children did not always have a parent with high myopia. Ip, Huynh, Robaei, et al, found the ethnic background of a child, as well as the parental history influenced myopia development.29 They concluded that in children of East Asian ethnicity the number of myopic parents had greater influence on myopia progression then in Caucasian children.

The Northern Ireland Childhood Errors of Refraction (NICER), whose cohort is composed of predominantly Caucasian children, also found that the number of myopic parents influenced their refractive error. They found that children with two myopic parents were 7.79 times more likely to be myopic.35 It is clear from the results of these genetic studies that the question of myopia development is not simply a matter of inheritance but is also influenced by environmental risk factors. The timescale involved whereby the prevalence of myopia has increased exponentially over a single generation confirms that genes are not the whole story. In the next section we will look at the environmental effects which are said to influence the development of myopia. It may be that the reason myopic parents have myopic children is that they recreate the myopigenic environment they experienced as children.

Environmental risk factors

For many years the amount of near work a child was involved in was felt to be a big driver of myopia development. In the Sydney Myopia Study, the group found that a greater incidence of myopia was seen in children whose parents reported they read for periods of more than 30 minutes continuously.36 A similar response was seen in children who read at a distance less than 30cm. Those children who read for extended periods of time and at a short working distance had the greatest levels of myopia. A similar study of Singaporean children noted that children who read more than two books a week were three times more likely to be myopic.37 These children were found to have an increase of 0.17mm in their axial length. Despite these findings, the Orinda Longitudinal Study of Myopia (OLSM) found the amount of near work did not significantly compound the risk of myopia from the number of myopic parents a child had.38 Many of the studies of the interaction between near work and myopia development rely upon parental recall of the amount of time a child has spent reading, this recall is always open to unintentional bias.

In conjunction with near work, accommodation and AC/A ratio have also been implicated in the development of myopia.39,40 A high AC/A ratio (5.84Δ/D) compared to the normal AC/A ratio of (3Δ/D) was associated with a greater risk of myopia development. It appears that this high AC/A ratio precedes the onset of myopia. It has been hypothesised that the error in accommodation may give rise to peripheral hyperopic defocus with its consequent influence on myopia development. Studies have looked at the provision of progressive power lenses to children with a large lag of accommodation and a near esophoria in order to slow their rate of myopia progression.41,42 The efficacy of this treatment and other modalities will be considered in a later article.

It could be argued that the amount of near work an individual participates in would be reflected in their level of educational achievement. The E3 consortium evaluated the impact of education on myopia prevalence in Europe.43 They found individuals who continued in education until age 20 years or more were twice as likely to be myopic than those who completed their education before the age of 16. The group suggest that the increasing prevalence of myopia in Europe may be a consequence of the increasing level of education achieved. It could also be argued that those individuals who remain in education until age 20 or more have been involved in greater levels of concentrated near work. These findings are reflected in myopia studies in East Asia.44,45 Morgan and Rose cite the length of the school day and the increasing use of after school tutoring as possible myopigenic factors which may be driving the worldwide increase in myopia.45 Dirani, Shekar and Baird looked at educational attainment in the Genes in Myopia (GEM) study.46 They concluded that the genes involved in refractive error development may also be involved in educational attainment and so education should not be considered solely as an environmental risk factor. In a study in 2004 Saw, Tan, Fung, et al, examined non-verbal IQ scores and myopia.47 They concluded this specific type of IQ measure may be a more significant indicator for myopia than the number of books read per week.

Outdoor activity and light levels

Suggestions have been made that parents, particularly if they are myopic, should be advised to encourage their children to take up outdoor activities in order to reduce the risk of myopia onset (figure 4).39

Figure 4: Outdoor classroom in China

This has often been translated as requiring children to take up outdoor physical activity. Recent studies have looked at the influence of the level of physical activity and myopia onset.48,49 It was found that emmetropic and myopic children had similar levels of physical activity but myopic children spent less time outdoors.50 Read, Collins and Vincent looked at light exposure and axial length changes and found that children with greater light exposure had smaller axial growth.51 Using conjunctival autofluoresence as an objective predictor of sunlight exposure the Raine Cohort Eye Study, from Western Australia, also found that higher levels of myopia were associated with lower levels of sunlight exposure.52 A recent study involving classroom lighting in north east China concluded that increased levels of lighting had a significant effect on the onset of myopia in children aged between six and 14 years of age 53. The Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error (CLEERE) study have however found that in their group of children the level of outdoor activity did not provide a clinically significant benefit against myopia progression.54

Urban vs rural

There has been a suggestion that children in urban environments have a greater prevalence of myopia than those in rural environments. These findings have been seen in large countries such as China and Cambodia where rural populations may be remote from conventional eye care systems.55-57 Children in these rural environments may also only receive education to primary level and spend more time in the outdoors. Countries where both rural and urban populations have equal access to healthcare and education do not demonstrate an equivalent effect. Where ethnic groups have migrated from rural to urban communities their myopia prevalence reflects that of the population they enter rather than of their country of origin. This can have either a detrimental or beneficial effect for example; children of Chinese ethnicity who live in Australia show a lower prevalence of myopia than Chinese children resident in Singapore.58 The group suggests this difference may be environmental in origin with the Singaporean children entering school from age three in many cases and spending less time outdoors.

The effect of peripheral refraction has also been implicated as a driver of myopia onset and progression. This has led to the development of a number of refractive strategies to modify this effect. This mechanism will be discussed in the next article.

Dr Annette Parkinson is a senior lecturer at the University of Bradford responsible for the teaching of investigative techniques and visual impairment and rehabilitation

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