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Omar Abdul Rahman Mahroo, Pirro G Hysi, Obeda Kailani, Juliet Thompson, Christopher J Hammond; Right eyes are longer than left eyes: axial length findings from a large cataract cohort with consistent refractive findings from a large twin cohort. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3610.
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Small inter-ocular differences exist in the incidence of certain conditions; for example, retinal detachments affect right eyes slightly more frequently. This cross-sectional study explored, in two large cohorts, whether right and left eyes differ in terms of axial length or myopia (both linked risk factors for retinal detachment).
For the cataract cohort, axial lengths previously measured for 12,766 eyes of 8,195 consecutive patients over a 6.5 year period were analysed. Right and left eyes were compared (t test: paired for patients with measurements for both eyes; unpaired for those with measurements available for one eye). For the twin cohort, refractive error was compared between right and left eyes (paired t test) for 5,755 twin subjects from 3,199 families from the TwinsUK database. To adjust for relatedness within families, re-sampling was performed with one random member of each family (running 10,000 permutations). For 1,186 twins, eye dominance data were also available.
For 4,571 patients for whom axial lengths were available for both eyes, mean (SD) axial lengths were 23.55 (1.40) mm and 23.50 (1.40) mm for right and left eyes respectively (p=4.7x10-20). For 3,624 patients undergoing unilateral surgery, mean axial lengths were 23.56 (1.28 mm) and 23.47 (1.23) mm respectively (p=0.04). For twin subjects, right eyes were significantly more myopic than left eyes (p = 0.04). 65% of twin subjects for whom eye dominance data were also available were right-eye dominant. Although right eyes were on average more myopic for right eye dominant subjects, the laterality was reversed for left eye dominant subjects, but differences did not reach significance.
Right eyes appear to be, on average, slightly, but significantly, longer than left eyes. Longer eyes tend to be more myopic and our refractive data were consistent with this. This represents a novel finding and, in terms of clinical significance, may explain the slight laterality imbalance in retinal detachments. An understanding of mechanisms underlying small differences in development of the two eyes would shed important light on the development of myopia; our finding that differences might be reversed for left eye dominant subjects suggests an interesting interaction with mechanisms driving eye dominance.
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