Taken from an article with the permission of the author Nick Dash. References on request.
Much of the research published around Myopia Control is based on the use of optical devices. This seems logical as these patients require some form of optical correction and so it seems intuitive to prescribe a device which not only corrects the refractive error but also limits one of the mechanisms believed to cause myopic progression. The emphasis of this article will be based around these optical methods and the issues surrounding how best to prescribe and follow up optical corrections for myopic children.
Some of the optometric professional bodies and organisations, such as the Association of Optometric Professionals (AOP), have released statements regarding the implementation Myopia Control within UK practices. The AOP’s statement, entitled ‘Juvenile Myopia Control (7), states that several evidence-based approaches have been shown to potentially slow myopia progression and goes on to say
‘The use of dual focus contact lenses and varifocal spectacles lenses are both approaches that can be safely practiced to manage myopia. Ortho-k works through a different mechanism but can also be safely administered in an optometric practice’, acknowledging that these areas are within the competences of an optometrist.
This online statement and accompanying evidence supports implementing a strategy for myopia control within the consulting room but falls short of suggesting a ‘best clinical practice’ for the myopic child.
Contact lens options
Correction of a child's myopia has historically been the domain of single vision spectacles; however, research has shown that by correcting the myopic eye with standard single vision lenses, a hypermetropic defocus is produced behind the peripheral retina and this drives the myopic elongation of the eye as the eye attempts place the image shell on the retina. Likewise the use of some aspheric single vision contact lenses produce an image shell that is more hyperopic in the periphery, in other words is placed behind the retina. Whilst these offer small gains in terms of visual quality it might be at the expense of myopia progression.
As with any contact lens fitting, the choice is based on a number of factors. Clinicians need to use their professional judgment based on evidence, patient anatomy and physiology, refractive status, lifestyle and parental influence.
Standard myopic correction with focus at the macula will have a hyperopic defocus where the image shell is behind the peripheral retina.
Myopia Control strategy is to alter optics to provide a myopic defocus and the image shell is in front of the retina.
The extensive work of Jeff Walline over the last decade has shown that existing soft multifocal contact lenses with a centre distance portion (SMF(cd)), such as Biofinity and Proclear Centre-Distance lenses can produce a statistically significant reduction in progression (15). Soft multifocal contact lens wear resulted in a 50% reduction in the progression of myopia. A study using a ‘dual focus’ daily disposable contact lens, yet not commercially available in the UK, has released very favorable interim results showing effectivity better than 50% whist having good acceptance in terms of visual comfort and clarity.
Another useful option which has shown good results, with one study showing a reduction in myopia progression of?is the technique known as Orthokeratology, (18) often abbreviated to OrthoK. Here, rigid contact lenses are worn over night (and are removed in the morning producing corneal re-shaping that effectively reduces central corneal curvature, a technique which has earned the lenses the nickname ‘optical retainers’. This temporary flattening of the central cornea and mid-peripheral steepening reduces the myopia and the vision correction can be retained for 24 – 48 hours after lens removal. Ortho-K has few clinical significant side effects although its impact on corneal nerves and its stability (15 Swarbrick) of the cornea is questioned by some researchers. It has also been shown that fitting OrthoK lenses can slow the progression of myopia in school age children (17). Manufacturers are currently researching new lens designs that manipulate effective zone sizes and offer the potential optimisation of myopia control.
The question exists whether to choose Soft Multifocal CD or OrthoK?
The mechanism of myopia control is believed to be the same with both soft and rigid options, namely that of the peripheral defocus mechanism and similar levels of myopia control with both modalities have been found in many of the studies. A recent study to compare the relative efficacy of myopia progression control of Orthokeratology and multifocal contact lenses (22) found no significant difference in the efficacy of the two methods. Of the 110 patients reviewed in this study, 56 were prescribed corneal reshaping contact lenses and 32 were prescribed dual focus contact lenses, and the remaining 22 received advice only. One adverse event was reported over the 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 be actively promoting myopia control treatment to at risk patients.
Likewise, the work of Huang (23) 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, however these variations can might be explained through project/research design variations rather than one modality being better than another.
There is a volume of evidence using currently available lenses suggest an achievable reduction in myopia progression in the order of 50%. This is a meaningful reduction as a 50% reduction in progression would reduce high myopia in the population by the order of 90%. (24).
Practicalities of Contact Lens selection and consultations
The benefits of OrthoK are mainly the parental involvement in handing and maintenance of lens and having the daytime free from contact lens wear, which is most especially beneficial when involved in swimming activities at school.
Some limitations are imposed by the range of available refractive error correction by OrthoK, which is believed to be in the order of -1.00D to a maximum of -4.50D, although this is patient dependent.
The effect on peripheral refraction is based on the inherent myopic error, so to achieve a forward shift in the peripheral refractive shell it is thought that very low myopes might not have an effective topographic change to deliver the peripheral myopic defocus required. This means child with a refraction of -1.00DS might be more effectively treated with a SMF(cd)CL of distance power -1.00DS with a +2.00D addition. However, because a -3.00D OrthoK lens will deliver an Add in the order of +3.00D *. Above -4.50D or in cases of astigmatism over spectacles can be worn by the child, mindful that the reason for fitting is to reduce myopic eye extension rather than solely the vision correction.
Public awareness of OrthoK for myopia control in China and the Far East has preconditioned some parents towards this lens modality and so parental influence might affect lens choice, whereas soft contact lenses dominate the contact lens markets in the western world. It is highly likely that many patients, parents and professionals will choose soft multifocal SMF(cd) as the lens of choice. The child’s acceptance to these lenses in terms of handling and comfort is extremely good and concerns over the effect on loss of contrast sensitivity is rarely sited by the younger child but may be more of a problem in the older teenager. Most studies show dropout rates to be in the order of 30%, which is less than normal contact lens wearing population.
The concerns about microbial infection need to be addressed and repeated reinforcement at aftercare to minimise this risk. However, this risk is small when compared to sight threatening complications associated with high myopia.
A near addition +2.00D, or greater, centre distance lens design offers will often provide a myopic image shell that is in front of the peripheral retina and so provide a mechanism of myopia control. This is often a good first choice lens option for many. However higher add or different lens design can be customised based on a number of optometric measures as patients peripheral defocus is variable.