MYOPIA CONTROL
Learning Outcomes
After completing this module you should:
- Understand the optic diameter size and altering it can impact on visual correction
TREATMENT ZONE IN MYOPIA CONTROL
Correcting the refraction of myopes allowing clear distance vision is the primary purpose of optical correction. This applies to myopes of all ages. In childhood when myopia is increasing, orthokeratology slows axial elongation (the increase in myopia) on average by 63%.1
This is due to the optical properties of the treatment zone on the cornea, which induces myopic defocus on the peripheral retina. Myopic defocus is now incorporated on spectacles and day lenses.(Fig. 1)
After orthokeratology, the most effective soft lens for myopia control is MiSight, manufactured by CooperVision and the two best spectacle lenses are MiyoSmart, manufactured by Hoya, and Stellest, manufactured by Essilor. 2
Subtractive topography shows us the refraction change after night lens wear. Because corneas are stable and do not change from early childhood, we can monitor the night lens correction at every review with the subtraction topography.3 This shows any change to the effect of the treatment, for example if lenses had not been worn we would measure reduced power change. It also shows change of the myopia over time, if we measure the same treatment zone power change, but the patient needs more correction the myopia is increasing. Stable myopia is shown over time when the treatment zone power is unchanged with the patient remaining corrected for distance. (Figure 2). An increase in myopia is shown over time where the lens has been altered to increase the treatment zone power in order to keep the patient corrected for distance. (Figure 3).
These two examples with one myope who has stabilised and the other who has progressed illustrates why setting appropriate expectations with patients for myopia control is important.
A quoted value from peer reviewed research like “-63% reduction in axial length” is the group average. Each individual in the group will have progressed a different amount to that. It is currently impossible to know how any individual will respond.
Correction with myopic defocus shows approximately 90% of patients get a benefit with a reduction in axial elongation, 20% will stabilise, but 10% will not respond (figure 4).4,5 If a patient shows progression of their myopia whilst having good compliance to their correction, they may one of the 10% group.
Topography enables us to check that the patient is getting treatment zone properties that are beneficial in slowing axial elongation.
TOPOGRAPHY METRICS USEFUL IN MYOPIA
In addition to the TZ power change discussed above, the following metrics can be of interest to individually assess patients when myopia control is a motivation with night lenses. Clinical studies are made up from patients with a broad range of the following metrics.
Treatment Zone Decentration
Whilst we strive to get a perfect bullseye treatment zone, it is shown that decentration slows axial elongation.6 Decentration can reduce visual acuity, so a poor lens fit should be corrected. Decentration expected from the baseline corneal displacement where the treatment zone is >0.52mm decentred can be tolerated if the VA is adequate.
Treatment Zone Size
Treatment zone size (diameter) does not correlate well to slowing axial elongation. Small (<3.8mm) and large (>3.8mm) sizes have been shown to be statically similar.6
Myopic Defocus Dose (MDD)
The MDD can be understood if considering the treatment zone similar to the ADD on a soft multifocal day lens. It is the difference between the minimum and maximum power in the pupil. This value shows significant association with slowing axial elongation. Higher the myopic correction the higher the MDD, typically the MDD value will be similar to the baseline SE correction. This is why high myopic correction can result in increased glare with a large ‘multifocal’ power difference going through the pupil. There is a correlation between higher baseline corrections, which have higher MDD values and slower axial elongation.7 Whilst we typically want to minimise the effects of a high MDD (glare and induced Rx) when the baseline correction is low we may want to ensure there is a critical amount. A MDD value greater than +2.50 would correlate to the effective threshold shown with distance centre multifocal soft lenses.8
Spherical Aberration (SA)
Higher order aberrations alter with the treatment zone properties outlined above. There is a correlation between increased HOAs and reduced axial elongation. Although spherical aberration is not fully understood, like MDD it can be a useful metric to track. In increase of >0.8um is associated with slower axial elongation. It may indicate any lens changes will have a beneficial effect on axial elongation.
Pupil Size
Pupil size impacts on the amount of MDD and SA that will get to the retina. Average pupil size for children in 5.4mm. A very small pupil may limit the defocus that can get to the retina.
TOPOGRAPHY METRICS
LENS DESIGN FOR MYOPIA CONTROL
Standard night lens designs are effective at reducing axial elongation.1,2,4,6 These designs with optic diameters between 6.0-6.5mm originated in the early 2000s, providing the best VA for patients, and effective myopia control.
As we start to understand and measure MDD and SA we are staring to understand that smaller optic diameters can increase those values and slow axial elongation. Smaller optic diameters can heve negative effects. Reducing optic diameter from 6.0 to 5.0mm can reduce vision significantly in 30% of patients.10 The slowing of axial length with smaller optics remains to be established, currently the benefit being limited to the initial six months rather than a sustained effect demonstrated.11
NocturnalTM MC+ can be specified when ordering with 6.0, 5.5 or 5.0mm optic diameters.
CASE HISTORY 1 MYOPIA MONITORING
The visual axis power shows the power correction of the night lens. The baseline cornea should remain unchanged during the patients life. The subtractive power change combined with any residual refraction indicates if there is an increase in power needed.
In this example the TZ power is constant at -2.30D. The UVA is 6/6 with no refraction. This indicates the myopia is fully corrected and stable.
CASE HISTORY 2 MYOPIA MONITORING
This patient presented in 2014 for review having worn a newly issued lens.
Patient is asymptomatic, reports the vision is the same as with the previous lenses and reports wearing the new lenses nightly for the last week without issues. Previous reviews have been 6/6.
UVA: 6/12
rRx: -0.75 6/6.
Action: Do we alter the lens to correct the -0.75?
On further questioning the patient reports not wearing the lenses the night previous.
A TZ power of -2.35D would be expected. The cornea should not change, the lens was the same as previous (NocturnalTM night lenses power is checked 4 x during production). The TZ power only being -1.82D indicates the patient is not getting their full correction. The most likely causes for this are; failure to wear, reduced sleep, wrong lens worn or solution/allergic reaction.
In this case failure to wear was indicated. The UVA returned to 6/6 following further nights wear. The following years for the patient are shown below with the patient continuing to get full correction and stable myopia.
MYOPIA MONITORING
TREATMENT ZONE IN MYOPIA CONTROL
CASE HISTORY 3 MYOPIA MONITORING
New 8 year old patient present with Mo interested in night lenses for holidays, swimming and general. Myopia control is a motivation, already wearing spectacles with myopia control .
Rx:
R: -2.50/-0.25 x 180 6/7.5+2 L: -3.50 6/6
(increased -0.50 R&L in 6/12, reports full time spec wear =13/hrs/day)
FH: Mo myope, laser eye, previous day lens soft CL wear.
GH: Good, no allergies.
CL Suitable: Suitable patient. Patient keen for night lenses.
Due to myopia progression with MC spectacles, first lenses ordered Nocturnal MC+5.50 OZ.
REVIEW (4 weeks)
BASELINE
L: -3.50 6/6 KFLAT: 8.17. SA: 0.26
Px reports adequate vision. Aware not quite as sharp as specs. Worn: Nocturnal MC+ 5.5
Bin UVA: 6/12
L: 6/12 rRx: -0.75 6/7.5-3
SA: 1.30 (+1.02)
TZ Assessment
Position: Bullseye
MDD: 3.33D
Action
Under correction necessitates exchange.
BCVA has reduced from 6/6 to 6/7.5-3. TZ diameter small, due to reduced optic.
Given KFLAT, age, previous progression with MC specs and reduced VA. Increase OZ to 6.5 to improve VA.
REVIEW (following lens change)
BASELINE
L: -3.50 6/6 KFLAT: 8.17 SA: 0.26
Px reports improved VA. “Sees everything”
Worn: Nocturnal 6.5 (ATL 10um, Power -4.50)
L: 6/6 rRx: Plano 6/6
SA: +1.78 (was +1.52) – increased with power increase
TZ Assessment
Position: Bullseye
MDD: 3.50D (was 3.33D)
Outcome
Optimum correction continue with wear.
Standard optic diameter NocturnalTM, providing full Rx correction, good VA, and appropriate metrics for slow AE.
Accurate topography provides reassurance that appropriate TZ properties are in place for reducing AE. Enabling individualized patient treatment.
References Myopia Control
1. 63% reduction in AE over 2 year meta-analysis. Li, X., Xu, M., San, S. et al. Orthokeratology in controlling myopia of children: a meta-analysis of randomized controlled trials. BMC Ophthalmol 23, 441 (2023).
2. Erdinest N, London N, Lavy I, Berkow D, Landau D, Morad Y, Levinger N. Peripheral Defocus and Myopia Management: A Mini-Review. Korean J Ophthalmol. 2023 Feb;37(1):70-81.
3. Lisa A. Jones, G. Lynn Mitchell, Donald O. Mutti, John R. Hayes, Melvin L. Moeschberger, Karla Zadnik; Comparison of Ocular Component Growth Curves among Refractive Error Groups in Children. Invest. Ophthalmol. Vis. Sci. 2005;46(7):2317-2327
4. Tetsuhiko Kakita, Takahiro Hiraoka, Tetsuro Oshika; Influence of Overnight Orthokeratology on Axial Elongation in Childhood Myopia. Invest. Ophthalmol. Vis. Sci. 2011;52(5):2170-2174
5. Chamberlain P, Bradley A, Arumugam B, Hammond D, McNally J, Logan NS, Jones D, Ngo C, Peixoto-de-Matos SC, Hunt C, Young G. Long-term Effect of Dual-focus Contact Lenses on Myopia Progression in Children: A 6-year Multicenter Clinical Trial. Optom Vis Sci. 2022 Mar 1;99(3):204-212
6. Lin W, Li N, Gu T, Tang C, Liu G, Du B, Wei R. The treatment zone size and its decentration influence axial elongation in children with orthokeratology treatment. BMC Ophthalmol. 2021 Oct 12;21(1):362
7. Wang, Bingjie & Naidu, Rajeev & Qu, Xiaomei. (2017). Factors related To axial length elongation and myopia progression in orthokeratology practice. PLOS ONE
8. Walline JJ, Walker MK, Mutti DO, et al. Effect of High Add Power, Medium Add Power, or Single-Vision Contact Lenses on Myopia Progression in Children: The BLINK Randomized Clinical Trial. JAMA. 2020;324(6):571–580.
9. Lau JK, Vincent SJ, Cheung SW, Cho P. Higher-Order Aberrations and Axial Elongation in Myopic Children Treated With Orthokeratology. Invest Ophthalmol Vis Sci. 2020 Feb 7;61(2):22.
10. Vuong, Connie Chi Linh; Mullinax, Constance; and Bui, Cang, “The effects of optic zone diameter in orthokeratology” (2001). College of Optometry. 1384.
11. Guo B, Cheung SW, Kojima R, Cho P. Variation of Orthokeratology Lens Treatment Zone (VOLTZ) Study: A 2-year randomised clinical trial. Ophthalmic Physiol Opt. 2023; 43: 1449–1461.
MYOPIA CONTROL
SUMMARY
- Visual acuity and subtractive topography can indicate when reduced optic size is not optimum correction
- Metrics from topography can indicate appropriate visual correction and metrics monitored for myopia control
- Smaller optic diameters do not always improve myopia control metrics