Anterior scleral thickness changes with accommodation in myopes and emmetropes

Abstract

Although a range of changes in anterior segment structures have been documented to occur during accommodation, the quantification of changes in the structure of the anterior sclera during the accommodation process in human subjects has yet to be examined. This study therefore aimed to investigate the presence of short-term changes in anterior scleral thickness associated with accommodation in young adult myopic and emmetropic subjects. Anterior scleral thickness was measured in 20 myopes and 20 emmetropes (mean age 21 ± 2 years) during various accommodation demands (0, 3 and 6 D) with anterior segment optical coherence tomography (AS-OCT). A Badal optometer was mounted in front of the objective lens of the AS-OCT to allow measurements of the anterior temporal sclera (1, 2 and 3 mm posterior to the scleral spur) to be obtained while fixating on an external accommodation stimulus. Anterior scleral thickness was not statistically different between refractive groups at baseline, but thinned significantly with the 6 D accommodation demand (-8 ± 21 μm, p < 0.05), and approached a statistically significant change with the 3 D demand (-6 ± 20 μm, p = 0.066). While both refractive groups thinned by a statistically significant amount at the 1 mm location with the 3 D demand; significant (p < 0.001) refractive group differences occurred at 3 mm, where the thinning found in the myopic group reached statistical significance with both the 3 D (-12 ± 21 μm) and 6 D (-19 ± 17 μm) demands, and the emmetropes showed no significant changes. This demonstrates the first evidence of a small but statistically significant thinning of the anterior sclera during accommodation. These changes were more prominent in myopes, particularly 3 mm posterior to the scleral spur. These regional differences may be associated with previously reported regional variations in ciliary body thickness between refractive groups, regional differences in the contraction of the ciliary muscle with accommodation, or differences in the response of the sclera to these biomechanical forces.

Scleral changes with accommodation

“…Our work further contributes to these results by considering the effect of accommodation on the scleral shape. A significant thinning of the anterior sclera during accommodation has been previously suggested, but in that work no consideration was given to the shape of anterior sclera. We found that scleral shape undergoes changes during accommodation process but we did not find any substantial change in corneal shape during accommodation.…”

Understanding Myopia: Pathogenesis and Mechanisms

“…Consequently, posterior reinforcement surgery using donor scleral tissue has been refined over the years to arrest further axial elongation and scleral thinning in highly myopic eyes [96,97]. Although anterior scleral thickness is similar between myopic and non-myopic eyes [98][99][100], there is growing evidence that the anterior sclera thins slightly during accommodation, particularly in myopic eyes [101,102], most likely due to biomechanical forces of the ciliary muscle. A greater thinning observed in myopic eyes may be a result of a thicker posterior ciliary muscle [103][104][105] or changes in biomechanical properties of the sclera reported in animal models of form-deprivation myopia [106,107].…”

Association of range of accommodation with fully corrected refractive errors

“…By the year 2020, it is estimated that 2.5 billion people, or one third of the world's population, will be affected by myopia alone. 1,2 The refractive error profile of the United States adult population is less than 5% hyperopic, about 30-40% myopic, and slightly more than half are emmetropic. This study describes the difference range of accommodation in all refractive errors (myopes, hypermetropes) and normal population.…”

“…1 The refractive error profile of the United States adult population is less than 5% hyperopic, about 30-40% myopic, and slightly more than half are emmetropic. 2,3 Accommodation is the mechanism used by the pre-presbyopic eye to produce clear images on the retina of objects at different distances. This reflective mechanisms initiated by signals that activate the ciliary muscle, changing the shape of the crystalline lens and thus the power of the eye.…”