Novel method using 3-dimensional segmentation in spectral domain-optical coherence tomography imaging in the chick reveals defocus-induced regional and time-sensitive asymmetries in the choroidal thickness

Abstract

Studies into the mechanisms underlying the active emmetropization process by which neonatal refractive errors are corrected, have described rapid, compensatory changes in the thickness of the choroidal layer in response to imposed optical defocus. While high frequency A-scan ultrasonography, as traditionally used to characterize such changes, offers good resolution of central (on-axis) changes, evidence of local retinal control mechanisms make it imperative that more peripheral, off-axis changes also be tracked. In this study, we used in vivo high resolution spectral domain-optical coherence tomography (SD-OCT) imaging in combination with the Iowa Reference Algorithms for 3-dimensional segmentation, to more fully characterize these changes, both spatially and temporally, in young, 7-day old chicks (n = 15), which were fitted with monocular +15 D defocusing lenses to induce choroidal thickening. With these tools, we were also able to localize the retinal area centralis, which was used as a landmark along with the ocular pectin in standardizing the location of scans and aligning them for subsequent analyses of choroidal thickness (CT) changes across time and between eyes. Values were derived for each of four quadrants, centered on the area centralis, and global CT values were also derived for all eyes. Data were compared with on-axis changes measured using ultrasonography. There were significant on-axis choroidal thickening that was detected after just one day of lens wear (∼190 µm), and regional (quadrant-related) differences in choroidal responses were also found, as well as global thickness changes 1 day after treatment. The ratio of global to on-axis choroidal thicknesses, used as an index of regional variability in responses, was also found to change significantly, reflecting the significant central changes. In summary, we demonstrated in vivo high resolution SD-OCT imaging, used in combination with segmentation algorithms, to be a viable and informative approach for characterizing regional (spatial), time-sensitive changes in CT in small animals such as the chick.

Assessment of Choroidal Vascularity and Choriocapillaris Blood Perfusion in Anisomyopic Adults by SS-OCT/OCTA

“…In addition, studies in chicks showed greater choroidal thickening in the central region than in the peripheral region in response to myopic defocus. 49 These findings indicate that the central retinal (including macular) regions are particularly responsive to visual stimulus conditions, leading to a decrease in choroidal vascularity and choriocapillaris blood perfusion during myopia development.…”

IMI – Report on Experimental Models of Emmetropization and Myopia

“…Chick retinas do not have a fovea, but have a largely rod-free area centralis that provides a visual acuity of approximately 7 cyc/deg 79–83. Optical coherence tomography (OCT) imaging shows that the retina is thickest in the region of the area centralis 84. The chick inner retina is avascular and is supplied with oxygen and nutrients by the pecten oculi, which is a vascular structure continuous with the choroid and projecting into the vitreous chamber.…”

Regional alterations in human choroidal thickness in response to short‐term monocular hemifield myopic defocus

“…To date, human studies have only examined foveal or nasal and temporal parafoveal choroidal thickness changes in response to full‐field defocus and suggest that the defocus mediated changes in choroidal thickness occur uniformly across the central macular area. However, studies in chicks have revealed that greater changes in choroidal thickness occur centrally, rather than peripherally, in response to full‐field optical defocus . Further, the altered ocular growth and refractive errors that are induced when form deprivation or optical defocus is imposed on only half of the retina (in both avian and mammalian animal models) are largely localised to the hemiretinal region exposed to image blur.…”

“…Research with animal models suggests that eye growth is influenced by retinal image defocus, with the choroid playing a significant role in mediating the effects of defocus on ocular growth and the development of refractive errors . Studies in a range of species show that a thinning or thickening of the choroid in response to hyperopic or myopic defocus results in a rapid movement of the retina towards the defocused image plane, followed by alterations in scleral growth that eventuates in the development of experimental myopia or hyperopia, respectively . In humans, short‐term exposure to full‐field myopic and hyperopic defocus in both children and adults has also elicited a transient increase or decrease in choroidal thickness respectively, implying that the choroid may contribute to the mechanisms involved in the longer‐term development of refractive errors in humans …”

“…2,3 Studies in a range of species show that a thinning or thickening of the choroid in response to hyperopic or myopic defocus results in a rapid movement of the retina towards the defocused image plane, followed by alterations in scleral growth that eventuates in the development of experimental myopia or hyperopia, respectively. [4][5][6][7][8][9][10] In humans, short-term exposure to full-field myopic and hyperopic defocus in both children and adults has also elicited a transient increase or decrease in choroidal thickness respectively, implying that the choroid may contribute to the mechanisms involved in the longer-term development of refractive errors in humans. [11][12][13][14][15][16][17][18] To date, human studies have only examined foveal [11][12][13][14][15] or nasal and temporal parafoveal 14,16,17 choroidal thickness changes in response to full-field defocus and suggest that the defocus mediated changes in choroidal thickness occur uniformly across the central macular area.…”