From the Centre for Eye Research Australia, University of Melbourne, Royal Victorian Eye and Ear Hospital; and the Vision Cooperative Research Centre, Sydney, Australia.
The authors have no financial or proprietary interest in the materials presented herein.
This study would like to acknowledge the Australian Twin Registry for acting as the main referral source for twin recruitment. This research was supported by the Australian Government Cooperative Research Centre Program.
Address correspondence to Maria Schache, BSc(Hons), PhD, Centre for Eye Research Australia, 32 Gisborne Street, East Melbourne 3002, Australia.
Monozygotic twins are commonly referred to as identical twins and arise from a single fertilized egg (zygote) that splits into two early in the gestational period. Monozygotic twins are genetically identical and often share many physical characteristics. Any differences between the twins may result from non-genetic (environmental) factors. We previously described a pair of female monozygotic twins with discordant myopia who were recruited as part of the Genes In Myopia (GEM) Twin Study in Melbourne, Australia. We now describe a second monozygotic twin pair from the GEM Twin Study who presented with discordant refraction, in this instance hypermetropia. Both twins from this pair also reported amblyopia, but the cause of this differed between the individual twins. The phenomenon of refractive discordance is relatively uncommon in monozygotic twins, with this being the second reported case. The explanation for this discordance is yet to be determined.
A total of 612 twin pairs were recruited through the Australian Twin Registry as part of the GEM Twin Study.1 Each twin pair underwent a general questionnaire and a series of vision tests. The zygosity of the twins was determined by standard questions and evaluation of their physical characteristics.2 Zygosity was later confirmed by standard genotyping of 11 polymorphic markers performed by the Australian Genome Research Facility, Melbourne.3 Uniocular and binocular vision was assessed in all twins with and without spectacle correction using the logarithm of the minimum angle of resolution chart at 3 m followed by a cycloplegic (tropicamide 1%) refraction using the Topcon auto-refractor (KR 8100 model; Topcon Inc., Paramus, NJ). We describe one twin pair with discordant refraction.
Both twins from this pair reported a similar birth weight (1.82 kg), and neither twin reported any health problem during childhood. Both twins were 67 years old at recruitment. Both twins are currently being medically treated for hypothyroidism and hypertension with no other significant medical history. The eldest daughter of twin 1 is wears spectacles and has a history of childhood strabismus, but there is no other significant family history of ocular pathology or motility disorders in either parent or other family members.
Both individuals from this twin pair were diagnosed as having amblyopia at the age of 6 years. Total direct full-time occlusion was employed as the first line of treatment, but compliance was poor and only slight improvement in vision was achieved for either twin. Twin 1 had a history of micro-strabismus, which correlated with reduced vision (6/15) in his right eye. Past ocular notes were used to confirm the presence of microtropia. Twin 2 had a history of hyper-anisometropia since childhood. Although the basis of the amblyopia appears to differ between the twins, the time of onset of amblyopia is identical. In addition, more detailed information on ocular history would be needed to ascertain whether twin 2 had an underlining micro-strabismus.
For twin 1, uncorrected visual acuity was 6/15 in the right eye and 6/4.8 in the left eye with a cycloplegic autorefraction of +0.25/+0.75 × 105 and +0.25/plano, respectively. A modest improvement in visual acuity (6/12) was achieved in the right eye with a correction of +0.50/–0.50 × 90°. Twin 1 had used reading spectacles since the age of 54 years. Ocular motility assessment for twin 1 was unremarkable with no signs of strabismus or muscle weakness.
For twin 2, uncorrected visual acuity was 6/7.5 in the right eye and 6/60 in the left eye, with improvement to 6/30 with a pinhole in the left eye. A hypermetropic correction of +1.5/0.50 × 50 in the right eye and +4.50 diopter sphere in the left eye improved visual acuity to 6/4.8 and 6/30+, respectively. Cycloplegic auto refraction was +2.25 diopter sphere in the right eye and +5.75 diopter sphere in the left eye. Left aniso-hypermetropic amblyopia with a difference of 3.50 diopter sphere in refraction between the eyes was diagnosed. Twin 2 had worn reading spectacles since the age of 12 years. Ocular motility assessment for twin 2 revealed a small exophoria (3°) with moderate recovery; no manifest strabismus or ocular muscle imbalance was detected.
We report discordant refraction in a pair of monozygotic twins. The twin pair described here was recruited as part of the GEM Twin Study. All twins in the GEM Twin Study were examined by trained eye professionals using extensive ophthalmic measures that enabled a clear and precise diagnosis. Complete details of the full refractive status of all of the GEM Study twins have been reported elsewhere.1 Of all twins examined, the pair reported here represents the second case of discordant refraction.4 The discordant refraction between the twins may be a result of inter-uterine insult or birth trauma, or it may have an epigenetic origin. There is no clear difference in environmental exposures between the twins that can explain the discordant refraction.
The twins in this report both exhibited amblyopia, but the underlying cause of this differs. The amblyopia in twin 1 seemed to be primarily caused by the presence of a right microtropia and may have worsened during childhood due to poor compliance with occlusion therapy. The amblyopia in twin 2 essentially reflected the difference in hypermetropia between the eyes, but a micro-strabismus during childhood years cannot be excluded.
Because monozygotic twins share 100% of their genes, it would be expected that refractive errors should be highly concordant if genetically determined. Previous twin studies have found that refraction measurements are more concordant in monozygotic (> 0.8) than in dizygotic (< 0.5) twins, suggesting that refractive errors can in part be attributed to genetic factors.5 However, this reported case of discordant monozygotic twins in conjunction with that previously published for the GEM Twin Study demonstrates the complexity of refractive error due to the involvement of both genetic and environmental risk factors.4 The report provides some insight into the complexity of the etiology of refraction.
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- Martin NG, Martin PG. The inheritance of scholastic abilities in a sample of twins: I. Ascertainments of the sample and diagnosis of zygosity. Ann Hum Genet. 1975;39:213–218. doi:10.1111/j.1469-1809.1975.tb00124.x [CrossRef]
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