What are the unique challenges in monocular (unilateral) or anisometropic myopia management? We know that whilst a very complex process, myopic eye growth is caused by a combination of genetic factors, lifestyle risk and visual conditions. However in some people, one eye can have significantly more myopia than the other, despite the reasonable assumption the two eyes share genetic and environmental influences.
Studying this unique group of people with anisometropia, or a refractive difference between the eyes of 1.00D or more,1 gives us special insight into the factors behind myopic development. A recent large population-based study indicated that the prevalence of anisometropia among school-aged children was 5.3%.2 Anisometropia is a significant risk factor for amblyopia in children, with at least 25-30% of children with anisometropia having a degree of amblyopia.3 Hence, in cases of unilateral or anisometropic myopia, it is very important that your first step is addressing amblyogenic concerns and risks before thinking about management of myopic progression.
What could drive the anisometropia?
There seems to be less correlation between monocular myopia and parental anisometropia than with typical iso-myopia and family history risk.1 The modality of inheritance also seems more variable. Twin studies have shown that genetics may play a role, with several case reports showing similar levels of anisometropia. Reports focused on heritability suggest that when the anisometropia is present or emerging from very early childhood, there is more likely to be a genetic explanation, with family members having similar levels of refractive error. After onset, anisometropic myopia tends to follow a similar pattern of progression and eventual stabilization as isometropic myopia.1
As we know from animal studies, any type of form deprivation can cause severe and rapid myopic growth.4 Conditions such as congenital cataract, ptosis, corneal scarring and retinal conditions can induce poor vision and accelerate axial length growth in the affected eye(s).1
A review conducted on 48 children with unilateral high myopia found that 90% had an abnormality with the more myopic eye, a family history of high monocular or binocular myopia, or a central nervous system abnormality.5 A significant proportion (31%) had an optic nerve abnormality, such as optic nerve hypoplasia, driving the refractive difference. The authors caution the need for careful assessment of the health of the unilaterally high myopic eye - full or segmented optic nerve hypoplasia can present diagnostic difficulties with an appearance like a tilted disc, and due to magnification effects of the high myopia.5
Retinopathy of prematurity is another frequent cause of anisometropic eye development. The CYRO-ROP study, a study on cryotherapy for babies born prematurely. found up to 49% of the children had spherical difference of -2.00D or more.6 In this circumstance however, it is the eyes more severely affected with ROP that is abnormally small, and the more myopic eye that presents with better visual acuity and a more normal retinal appearance.
It has been suggested that an internal failure of homeostasis enables one eye to grow faster, and once that eye is larger the local vision-stimulant mechanisms may then play a role. This may be the case for lower level anisometropia (1-3D) which is less likely to be associated with a syndromic cause.1
Unilateral high myopia (more than 5D) is highly likely to have a developmental and/or pathological cause and this eye may not achieve normal acuity. Careful investigation and management of eye health is imperative. Anisometropic or unilateral myopia of 1-3D may be genetic, especially with early childhood onset.
Can we alter growth patterns in one eye?
The ability to manipulate the growth in one eye has been demonstrated in contact lens studies. For example, 40 children with reasonably equal refractive error were fitted with one multifocal contact lens (hypothesized to control myopic growth) and a single vision lens in the other eye. There was a significant reduction in myopic growth in the treated eye, demonstrating that growth can be manipulated in a monocular way.7
Orthokeratology studies also provide an understanding on unilateral treatment approaches. A trial of 26 children wearing an orthokeratology lens in one eye, and an alignment fit RGP lens in the other, demonstrated significant difference in the growth between the treated and untreated eyes.8
These studies were undertaken on children without anisometropia, though, demonstrating the potential to alter growth in one eye. How does this apply to management of children with anisometropic or unilateral myopia?
What evidence is there for unilateral or anisometropic myopia control?
Our traditional first line option when correcting children for myopia, spectacles for anisometropia can lead to aniseikonia: a perceived difference in the size of images causing difficulties in vision and stereopsis.9 There are no studies on reduction of myopia progression using any type of myopia controlling spectacles in children with anisometropia at this stage.
Contact lenses to correct aniso-myopia
Alignment fit, standard RGP lenses reduce the likelihood of aniseikonia when children have more than 3D of refractive difference between the eyes. RGP lenses can also provide superior vision in cases of high myopia with anisometropia. A case report on 15 young children with high myopia and amblyopia who were fit with RGP lenses showed visual acuity improvement in the amblyopic / more myopic eye, as well as an improvement in stereopsis. Importantly, there was also no adverse events that occurred during lens wear. Myopia progressed significantly in these young children10 - this was a study of myopia correction and not myopia control.
Contact lenses to control aniso-myopia
Several studies have investigated the influence of orthokeratology (OK) on unilateral and aniso-myopia development - enough, in fact, for a recent meta-analysis to be published. There is currently no other data on contact lens interventions for myopia control in anisometropic or unilateral myopes.
Ten cohort studies (nine retrospective and one prospective) have together indicated that OK slows myopic eye growth in unilateral myopia, as well as reduces progression to a greater degree in the more myopic eye in aniso-myopia. For unilateral myopia, OK reduced the inter-eye difference from 0.27mm after one year to 0.17mm after two years. In anisometropic myopes, OK controlled axial growth by 0.06mm more after one year and 0.13mm more after two years in the more myopic eye.11
Orthokeratology appears to consistently slow myopic eye growth in the more myopic eye in both unilateral and anisometropic myopes.
What should you do in practice?
- Address amblyogenic risk factors: considering the link between anisometropia and amblyopia, address these risk factors with full refractive correction and other measures first, before considering myopia control.
- Manage any ocular pathology: High levels of aniso-myopia can be associated with ocular pathology and reduced acuity outcomes - ensure any reduction in acuity or ocular heatlh concerns are addressed in co-management with ophthalmology as needed.
- Consider orthokeratology: treating the myopic eye / both eyes if indicated with orthokeratology holds evidence for aniso-myopic control of axial growth. Contact lenses also have the advantage of improved visual comfort through reducing the likelihood of aniseikonia experienced in spectacles.
- Vincent SJ, Collins MJ, Read SA, Carney LG. Myopic anisometropia: ocular characteristics and aetiological considerations. Clin Exp Optom. 2014 Jul;97(4):291-307. (link)
- Lee CW, Fang SY, Tsai DC, Huang N, Hsu CC, Chen SY, Chiu AW, Liu CJ. Prevalence and association of refractive anisometropia with near work habits among young schoolchildren: The evidence from a population-based study. PLoS One. 2017 Mar 8;12(3):e0173519. (link)
- Abrahamsson M, Fabian G, Andersson AK, Sjöstrand J. A longitudinal study of a population based sample of astigmatic children. I. Refraction and amblyopia. Acta Ophthalmol (Copenh). 1990 Aug;68(4):428-34. (link)
- Flitcroft DI. The complex interactions of retinal, optical and environmental factors in myopia aetiology. Prog Retin Eye Res. 2012 Nov;31(6):622-60. (link)
- Weiss AH. Unilateral high myopia: optical components, associated factors, and visual outcomes. Br J Ophthalmol. 2003 Aug;87(8):1025-31. (link)
- Mills MD. Evaluating the Cryotherapy for Retinopathy of Prematurity Study (CRYO-ROP). Arch Ophthalmol. 2007 Sep;125(9):1276-81. (link)
- Anstice NS, Phillips JR. Effect of dual-focus soft contact lens wear on axial myopia progression in children. Ophthalmology. 2011 Jun;118(6):1152-61. (link)
- Swarbrick HA, Alharbi A, Watt K, Lum E, Kang P. Myopia control during orthokeratology lens wear in children using a novel study design. Ophthalmology. 2015 Mar;122(3):620-30. (link)
- South J, Gao T, Collins A, Turuwhenua J, Robertson K, Black J. Aniseikonia and anisometropia: implications for suppression and amblyopia. Clin Exp Optom. 2019 Nov;102(6):556-565. (link)
- Wang B, Naidu RK, Qu X. The use of rigid gas permeable contact lenses in children with myopic amblyopia: A case series. Cont Lens Anterior Eye. 2018 Apr;41(2):224-228. (link)
- Tsai HR, Wang JH, Chiu CJ. Effect of orthokeratology on anisometropia control: A meta-analysis. J Formos Med Assoc. 2021 Jun 9:S0929-6646(21)00239-4. (link)