myopia in children and to see them frequently to keep their
prescription up to date. However, it is thought that single
vision spectacles can themselves promote myopic elongation of
the eye. Standard lens designs cause hyperopic defocus in the
peripheral retina placing the image shell behind the peripheral
retina. It is thought this promotes elongation of the eye as
the eye attempts to place the peripheral image on the retina.
Standard aspheric single vision lenses also produce a similar
image shell behind the peripheral retina and are not therefore
an option to control myopia.
Existing spectacle lens designs, such as large segment
bifocals and progressive addition lenses designed to correct
presbyopia have been shown to have a very small effect on
myopia progression 12 . The limitations are believed to be due
to the small area of reading corridor which only corrects the
hyperopic defocus in the corresponding sector of retina, rather
than a full 360˚ correction of the mid-peripheral field defocus.
Manufacturers of spectacle lenses have attempted to
develop products that offer a myopic peripheral defocus to
resolve this issue and create a new product genre designed
to reduce myopic progression. However, these attempts to
create peripheral myopic defocus in spectacle lenses have
had significant limitations due to induced peripheral visual
distortion. This increases when viewing obliquely through
the mid-periphery of the lens causing problems with visual
comfort and proprioception.
Correcting myopia with contact lenses
Single vision contact lenses can be used to correct myopia
in the same way as spectacles. However, the extensive work of
Jeff Walline over the last decade has shown that existing soft
multifocal contact lenses with a centre-distance portion, such
as Biofinity and Proclear centre-distance lenses can produce a
statistically significant reduction in progression of myopia 13 .
Soft multifocal, centre-distance contact lens wear resulted in
a 50% reduction in the progression of myopia. A study using a
‘dual focus’ daily disposable contact lens, not yet commercially
available in the UK, has released very favourable interim
results showing a better-than 50% reduction whist having
good acceptance in terms of visual comfort and clarity.
Another useful option that has shown good results, with one
study showing a reduction in myopia progression of 57% 14 is
the technique known as orthokeratology, often abbreviated
to ortho-k. Here, rigid contact lenses are worn over night and
are removed in the morning a technique which has earned the
lenses the nickname ‘optical retainers’. The overnight corneal
re-shaping effectively reduces central corneal curvature. This
temporary flattening of the central cornea and mid-peripheral
steepening reduces myopia and the vision correction can be
retained for 24 – 48 hours after lens removal. Ortho-K has
few clinically significant side effects although, as with other
contact lenses worn overnight it has been linked with an
increase in microbial keratitis 15 , and its long-term impact on
corneal nerves and corneal stability is questioned by some
researchers 16 . Importantly though, several studies have
shown that fitting ortho-k lenses can slow the progression of
myopia in school age children. Manufacturers are currently
researching new lens designs that might enhance the potential
for myopia control.
Soft, centre-distance multifocal or orthokeratology?
The mechanism of myopia control is believed to be the same
with both soft and rigid options, namely that of the peripheral
defocus mechanism. Similar levels of myopia control have
been found with both modalities in many studies. A recent
study to compare the relative efficacy of myopia progression
control of orthokeratology and multifocal contact lenses found
no significant difference in the efficacy of the two methods 17 .
Of the 110 patients reviewed in this study, 56 were prescribed
corneal reshaping contact lenses and 32 were prescribed dual
14
etCETera | January 2017
Figure 3 Standard myopic correction with focus at the macula
will have a hyperopic defocus where the image shell is behind
the peripheral retina. Myopia control strategies alter the optics
to provide a myopic defocus and the image shell is in front of
the retina.
Figure 4 Orthokeratology can be used to correct myopia and to
slow its progression (image courtesy of Tressa Larson OD FAAO)
focus contact lenses. The remaining 22 received advice only.
One adverse event was reported over a 4-year period. The
authors concluded that both orthokeratology and dual focus
soft contact lenses are effective strategies for targeting myopia
progression, stating that there are very few barriers for any
contact lens practitioner to start actively promoting myopia
control treatment to at-risk patients.
Likewise the work of Huang 18 , comparing intervention
options in myopia control using a meta-analysis of 32 papers,
showed a similar effectivity of both forms of vision correction.
Some studies show slightly better control of myopia with one
technique over another, but these variations can be explained
through project/research design variations rather than one
modality being better than another.
There is a volume of evidence that, using currently available
lenses, centre-distance soft multifocal or orthokeratology, we
can achieve a reduction in myopia progression in the order
of 50%. It is calculated that a 50% reduction in progression
across the whole population would reduce high myopia in the
UK population by an order of 90% 19 .
Practicalities of Contact Lens
selection and consultations
As with any contact lens fitting, the choice of contact lens
is based on a number of factors. Clinicians need to use their