To the best of our knowledge, there is no exclusive prescribing guideline for anisomyopic pediatric cohort as on date. The addressed pediatric anisomyopes spectacle prescription had two challenges. First, the aniseikonic5 effect reaches a maximum when it’s fully corrected by spectacles, which leads to diplopia or the absence of binocular vision.5 Second, pediatric myopia prescribing guidelines recommend optimal or full correction6 and it is contraindicated if it’s under corrected. The under correction may lead to myopia progression from moderate to high-myopia. High-myopia may cause several other ocular complications including low-vision.7,8
Anisomyopia is usually accompanied by strabismus and increases as the degree of anisomyopia increases.9 Interesting, the twins had BCVA equal in both the eyes and orthophoric for distance and exophoria for near. The amount of aniseikonia (in percentage) was calculated based on the refractive errors.10 In twins 1 and 2, the amount of aniseikonia was 7.9% and 11.4% respectively. South J et al5 reported that the amount of aniseikonia greater than 5% would cause almost no stereopsis, no measurable contrast sensitivity summation and suppression. However, the twin 1 and 2 had normal fusion with stereopsis 50 arc and 200 arcs.
Due to the low socio-economic status and rural background, the twins couldn’t afford to go for any other mode of correcting the aniseikonia (Contact lens).
How anisomyopia develops is still unclear and is important to investigate further considering its complications. Premature birth and optical component defects like unilateral cataracts, pseudophakia and vitreous haemorrhage could give rise to a difference in interocular growth.11 However, other than refractive errors and premature birth, the twin sisters in this case report had normal eyes, thus the above-mentioned causes are unlikely factors. As the father of the twin sisters had high myopia, the anisomyopia among twin sisters in this case report could be more due to genetic inheritance.11
Effective methods are to be implemented to reduce or impede the interocular refractive error difference early in life. In a recent study, Orthokeratology and low dose atropine were compared on anisomyopic children. When followed up after two years, the interocular difference in axial length significantly decreased in the Ortho-k group, whereas it remained stable in atropine groups. Hence it was suggested that Ortho-k might be a better choice in reducing the degree of anisomyopia and the progression of myopia.2 Depending on how the twin sisters respond to low dose atropine, the Ortho-k or combined treatment strategies could be tried during the subsequent visits, as the twin sisters are at risk of developing further retinal complications due to pathological myopia.
In conclusion, the co-occurrence of anisomyopia in siblings is extremely rare. This case suggests that anisometropic amblyopia if treated early in life could be avoided and phoria status and binocularity could be improved with optimal refractive correction among anisomyopic individuals. When anisomyopia is in progress, the correction shall be aimed to control myopia management than balancing the aniseikonic effect. This report also warrants a need for prescribing guidelines for anisomyopic subjects in different age groups.