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suggest that the prevalence of

development appears to be influ-

the condition starts to increase at

enced by a complex interplay

the age of six, when children start

between multiple genetic and envi-

school, and continues to increase

ronmental risk factors. 2

throughout the school-age years,

The introduction of large

but then begins to slow as children

genome-wide association studies

reach adulthood. 8

has improved the understanding

The prevalence of myopia is

of genetic links, with almost 200

greater in urban settings, which may

genetic loci identified as associated

be the result of different lifestyle factors that increase the risk of myo-

with refractive error and myopia. 10 Most carry a low risk and are found

pia development such as less time

in the general population. 10

spent outdoors. 2, 4

People with a high polygenic

AETIOLOGY AND RISK FACTORS

MOST studies into the aetiology and

risk factors for myopia have been

risk score( where risk is determined by all genes contributing to a trait) can have up to 40 times greater risk than those of low genetic risk. 10 While most genes causing syndro-

conducted in children of school age.

mic forms of myopia have not been

There is less research in children

implicated in common myopia,

aged under five and in those aged

there is some overlap.

over 18. Even though it is less com-

A 2019 consortium combined

mon, myopia is known to develop

their findings on 161 common

and progress in these less com-

genes identified for refractive error

monly studied age groups. From

and found these only explained

the limited evidence available, it

about 8 % of the variance, suggest-

appears the aetiology and risk fac-

ing environmental factors play

tors may differ between the less commonly studied groups and those of school age.

a key role in the development of myopia. 10 The strong role environmental factors play in the devel-

Infants and children under five years

When a baby is born, the eyes are

opment of myopia is evident by the dramatic rise in the prevalence of myopia over the past 50 years, which genetics alone cannot

not fully developed, and the axial

explain.

length tends to be relatively short.

The most influential and con-

Emmetropisation is the process

sistent environmental risk factor

by which a child’ s eye develops to

is education. People with higher

achieve normal or‘ emmetropic’

levels of education, children with

vision, which means having clear

higher academic performance

or near-perfect vision without

and countries with higher educa-

the need for corrective lenses like spectacles or contact lenses. This process relies on input from the

tional pressures have a higher risk of developing myopia. 2 The exact mechanisms involved is unclear

environment and involves the eye’ s

but near work is thought to be

growth and adjustment over time. The process of emmetropisation

Figure 1. Refractive errors.

contributory. 2 There is considerable evidence

starts from birth and continues

that increased time outdoors is pro-

until 7-9 years of age. During this

tective against the development of

process, ocular components such

myopia, with children who spend

as the cornea, lens and axial length

less time outdoors developing

undergo growth and changes to

myopia. 2

ensure that light entering the eye

Existing evidence is unclear

focuses precisely on the retina.

as to whether increased time out-

Visual input from the environ-

doors continues to have signifi-

ment plays a crucial role in guid-

cant beneficial effects in reducing

ing the emmetropisation process.

the progression of myopia once the

As the baby starts to explore the

development has started. 2

world, their eyes receive signals,

The mechanisms for the ther-

which help the eye adapt to the

apeutic effect of outdoor time

appropriate shape for clear vision.

are unclear; however, one theory

The measured refractive state can

asserts that the brighter light out-

vary during this phase, for exam-

doors stimulates the release of

ple, there may be some hyperopia

dopamine, which inhibits axial

or a small amount of myopia as the

elongation through a cellular medi-

eye continues to correct itself, with the aim of emmetropia. Therefore, small amounts of refractive error are not normally optically cor-

Figure 2. Retinal tears with haemorrhages( the arrows indicate horseshoe tears with associated haemorrhages).

ated response. 11

More research in the area of outdoor time is required, although outdoor time for children may have

rected before the age of five. How-

other benefits in terms of a child’ s

ever, clinical judgement is required

of those with severe ROP likely to

work) and outdoor exposure. 7

Ehlers-Danlos syndrome( joint

health and development, for exam-

because a large refractive error,

develop the condition, compared

The second one involves sin-

hypermobility, skin hyperexten-

ple, increased physical activity and

if left uncorrected, will limit the visual stimulus required for normal visual development, resulting in amblyopia, that is, a lazy eye.

Both environmental and genetic factors may cause high levels of

with those with mild or regressing ROP. 7 The prevalence and severity of myopia in children with ROP varies widely and not all children with ROP will develop myopia. 9 In very low birthweight infants( less

gle gene / monogenic mutations that significantly affect refractive development. 7 Monogenic high myopia may occur in isolation or be associated with various ophthalmic and / or non-ophthalmic related

sibility and fragility, chronic joint and muscle pain) and other disorders causing corneal or lens malformations such as keratoconus( see figure 5), microspherophakia and congenital glaucoma. High levels of

vitamin D synthesis, and is therefore encouraged.

Children of parents with myopia are more likely to develop the condition, although it is unclear if the cause is genetic, related to expo-

myopia in infants and young chil-

than 1250g), the prevalence of myo-

features; the latter is referred to as

myopia can be detected at the age

sure to similar environment influ-

dren. Retinopathy of prematurity

pia has been reported as 19 % in

syndromic myopia. These include

of five in syndromic forms of myo-

ences, or a combination of both. 2

( ROP) is the most notable environ-

eyes with any degree of ROP at age

retinal dystrophies, connective tis-

pia, but there is little progression

Children who read at a very

mental risk factor for infants and

two, and in only 6 % of eyes with

sue disorders such as Stickler syn-

after that. This may be related to

close range( less than 30cm) and for

young children. This is distinct

no ROP. 9

drome( ocular findings, hearing

the development of ocular struc-

continuous periods of time without

from the environmental factors

Two genetic aetiologies have

loss, midfacial underdevelopment

tures, for example, the myopia

a break( longer than 30 minutes)

that appear to be stronger drivers

been identified for the develop-

and cleft palate and early-onset

may be mostly corneal or lenticular

are at greater risk of developing

for the development of myopia in

ment of myopia. The first involves

degenerative joint disease), Mar-

rather than axial. This may also be

myopia. 12

older children, such as inadequate time outdoors. 7 The risk of development of myopia in children with ROP varies depending on the sever-

the interaction between numerous known genetic risk factors( polygenic) and environmental factors, like prolonged time on the visual

fan syndrome( ocular lens dislocation [ see figure 4 ], tall and thin build, skeletal and cardiovascular issues, chest deformities, joint

the case in ROP.

School-aged children

Myopia most commonly presents

There is no consistent evidence for the role of gender on the risk of myopia, with older studies showing males are at increased risk( when

ity and extent of ROP, with 70 %

tasks of reading and writing( near

and back pain, dental problems),

in school-aged children and its

educational pressures were greater