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Controlling the growing problem of myopia in children

Published on: 
Ophthalmology Times Europe, Ophthalmology Times Europe November 2022, Volume 18, Issue 09

Axial length growth is a sensitive means of assessing myopia progression in children. Drops and special lenses are options for inhibiting excessive axial length growth.

The prevalence of myopia is increasing worldwide: in 2050, half of the world’s population is expected to be myopic, with the estimated prevalence highest for Asian regions.1 The proportion of the population with the condition is also increasing in Europe,2 especially in young people.3

Uncorrected myopia does not only cause far objects to appear blurry; with increasing severity of the disorder, the risk for serious eye diseases increases. For example, retinal detachment, choroidal neovascularisation and glaucoma are linked to severe myopia and can cause irreversible blindness.4–6

Myopia is caused by a mismatch of refractive power and axial length of the eye, so that the focal point of the eye lies in front of the retina, creating a blurry image. This mismatch usually develops (and increases) due to excessive axial growth of the eyeball during childhood. Over recent years, several therapies to inhibit myopia progression at this critical age have been developed.

The long-established therapy: Low-dose atropine

The most popular therapy, especially in Asia, is the daily application of atropine eye drops ranging from 0.01% to 0.05%.7 Owing to the higher numbers of myopic children in Asian countries, most of the randomised and placebo-controlled studies of low-dose atropine were performed with Asian children, with only a handful of studies including European children.8–10

The key message concerning atropine therapy is that the higher the dose, the more myopia progression is inhibited. After 3 years of daily application of 0.05%, 0.025% or 0.01% atropine eye drops, children showed mean myopia progressions of −0.73 D, −1.31 D and −1.60 D, respectively. Accordingly, mean axial length increases of 0.50 mm, 0.74 mm and 0.89 mm were observed after 3 years.11

Atropine eye drops at 0.01% may not show beneficial effects in every case, so increasing the dose should be considered.8 However, the higher the dose, the stronger the possible side effects. Therefore, one needs to carefully evaluate the highest tolerable dose for each individual case.

In the Netherlands, drops of 0.5% and 1% are applied to children with progressive myopia. This shows great success in terms of inhibiting myopia progression but also causes strong side effects, such as disturbances of accommodation or photosensitivity, which need to be tackled with photochromic multifocal spectacles.12

Apart from pharmacological intervention with atropine, there are also optical therapies with specially designed contact or spectacle lenses. These are usually based on the theory of myopic defocus, in which focus in front of the retina is created in order to stop/inhibit further axial length growth. This is realised by adding a second image shell focused in front of the retina to the primary image shell focused on the fovea, or by ‘bending’ the image shell so that it lies centrally on the fovea but in front of the retina in the periphery.

The invisible: Contact lenses

In addition to the central zone with normal correction, multifocal contact lenses have a second focus with positive refractive power, which creates a second image shell in front of the retina. Here we can see an addition-dependent effect: higher addition leads to less myopia progression and axial length growth.

After 3 years of contact lens wear with high addition (+2.50 D) or medium addition (+1.50 D), children showed mean myopia progression of −0.60 D and −0.89 D, respectively, compared with −1.05 D in the group with monofocal contact lenses. Mean axial length growth was 0.42 mm, 0.58 mm and 0.66 mm, respectively, for the groups with high addition, medium addition and monofocal contact lenses.13

Orthokeratology lenses follow the same optical principle: by flattening central corneal radii to reduce corneal refractive power and thus correct myopia, the mid-peripheral corneal radii are steepened. Therefore, the refractive power mid-peripheral cornea increases, and a second image shell is created in front of the retina in the midperiphery.

Orthokeratology lenses are worn at night; during sleep, the cornea is modulated so that daytime vision is possible without any optical correction. Unlike refractive surgery, this process is reversible. A detailed review concluded that orthokeratology lenses are effective in inhibiting myopia progression,14 but high compliance is needed to not only inhibit myopia progression but also achieve the desired effect of myopia correction during the day.

Other soft contact lenses specifically designed to inhibit myopia progression showed a duration-dependent effect. The treatment effect correlated positively with daily wearing time: the longer the daily wearing time, the less myopia progression in terms of increasing refractive error.15

All types of contact lenses place special demands in terms of handling the lenses, hygiene and a care routine, which must be considered when contact lenses are recommended for children.

The newcomer: Spectacle lenses with multisegments

Like contact lenses, specially designed spectacle lenses offer optical correction and therapy of myopia at the same time. However, spectacle lenses have additional advantages over contact lenses: they do not require special handling and they can be worn all day, every day, which has shown a positive effect in terms of myopia progression.16

Although spectacle lenses and contact lenses both use the principle of myopic defocus, its implementation is fundamentally different. Because spectacle lenses have a higher vertex distance than contact lenses, they offer the possibility to implement the defocus, which should primarily act on the periphery of the retina, outside the central zone of the lens.

Therefore, with spectacle lenses, when looking straight ahead through the defocus-free central zone, the defocus acts only in the periphery, not on the fovea, and the patient has clear vision. The situation is different with contact lenses: because they sit directly on the cornea, the defocus is always present.17

Most promising in terms of inhibition of myopia progression and tolerability appear to be spectacle lenses with numerous tiny lenses, with positive refractive power that are incorporated in the mid-periphery of the front surface and thus create a second image shell in front of the peripheral retina. The tiny additional lenses are hardly visible; looking at someone wearing spectacles with these, one cannot tell that they are special lenses with a therapeutic function.

After 2 years wearing spectacle lenses with highly aspherical (HAL) and slightly aspherical lenslets (SAL), children experienced average myopia progression of −0.66 D and −1.04 D, respectively, and average axial length growth of 0.34 mm and 0.51 mm, which, again, shows the addition-dependent effect of optical therapies on inhibiting myopia progression.16

Children wearing spectacle lenses with defocus incorporated multiple segments (DIMS) technology (see Figure 1) experienced an average myopia progression after 3 years of −0.52 D and an average axial length growth of about 0.31 mm.18 Owing to the different implementation of myopic defocus, these spectacle lenses provide better visual acuity and contrast sensitivity in primary gaze than multifocal contact lenses.17

Axial length is important

All myopia therapies should be carried out over many years because the critical phase of axial eye growth extends from childhood until adolescence.19 Control examination should take place on a regular basis, at least every year. In addition to the standard determination of refraction and, if necessary, adjustment of the optical correction, it is essential to measure axial length at each examination with high accuracy.

Axial length growth is a more sensitive means of assessing myopia progression than looking at the actual refraction change. By measuring axial length, myopia progression can be detected at an earlier stage, even before it would be reflected in a change in refractive state (see Figure 2).

As the eye grows from birth to adulthood, not all axial length growth detected can be attributed to progressive myopia: only axial length growth that is excessive over the age-dependent physiological growth of the eye leads to myopia. As a specific goal for any myopia therapy, it is proposed to focus on the underlying physiological axial length growth versus the axial length growth in excess thereof that brings about the progressive myopia.20

Hakan Kaymak
E: dr.h.kaymak@gmail.com
Dr Kaymak is head surgeon with a focus on retina, macula and vitreous body at Breyer, Kaymak & Klabe, Duesseldorf, which is one of the largest private ophthalmology practices in Germany. His personal interest is myopia therapy. Currently, he receives consulting fees from Hoya Lens Deutschland GmbH.
References
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