IMI-The Dynamic Choroid: New Insights, Challenges, and Potential Significance for Human Myopia

Abstract

The choroid is the richly vascular layer of the eye located between the sclera and Bruch's membrane. Early studies in animals, as well as more recent studies in humans, have demonstrated that the choroid is a dynamic, multifunctional structure, with its thickness directly and indirectly subject to modulation by a variety of physiologic and visual stimuli. In this review, the anatomy and function of the choroid are summarized and links between the choroid, eye growth regulation, and myopia, as demonstrated in animal models, discussed. Methods for quantifying choroidal thickness in the human eye and associated challenges are described, the literature examining choroidal changes in response to various visual stimuli and refractive error-related differences are summarized, and the potential implications of the latter for myopia are considered. This review also allowed for the reexamination of the hypothesis that short-term changes in choroidal thickness induced by pharmacologic, optical, or environmental stimuli are predictive of future long-term changes in axial elongation, and the speculation that short-term choroidal thickening can be used as a biomarker of treatment efficacy for myopia control therapies, with the general conclusion that current evidence is not sufficient.

Figure 1.

Schematic overview of innervation to the choroid in the human eye. BV, blood vessel; CG, ciliary ganglion; Chor, choroid; ICN, intrinsic choroidal neuron; III/VII, brainstem nuclei of cranial nerves III and VII; IML, intermediolateral nucleus; NVSMC, nonvascular smooth muscle cell; PPG, pterygopalatine ganglion; Ret, retina; SCG, superior cervical ganglion; Scl, sclera; TRI, trigeminal ganglion. Adapted with permission from Rucker F, Taylor C, Kaser-Eichberger A, Schroedl F. Parasympathetic innervation of emmetropization. Exp Eye Res. 2022;217:108964. © 2022 Elsevier Ltd.

Figure 2.

Example of an OCT B-scan (Spectralis; Heidelberg Engineering, Heidelberg, Germany; top) and corresponding optical biometry A-scan (Lenstar; Haag-Streit, Köniz, Switzerland for the whole eye, from which axial length is defined (bottom) and a closeup of the A-scan from the posterior eye (middle). The OCT B-scan from the same participant demonstrates concordance between the posterior peaks (RPE: P3, Ch/Sc: P4) identified in the optical biometry A-scan and the retinal and choroidal features in the OCT B-scan.

Figure 3.

Example OCT images and analyses from a healthy young subject, including (A) a single OCT B-scan captured with standard focus, (B) a single OCT B-scan captured with enhanced depth imaging, (C) an averaged OCT B-scan captured with enhanced depth imaging (the average of 30 individual B-scans captured from the same retinal location), and (D) the same averaged OCT B-scan following segmentation analysis (to delineate the posterior boundary of the retinal pigment epithelium [blue line] and the inner boundary of the choroidoscleral interface [red line] to allow choroidal thickness [yellow arrow] to be calculated) and binarization analysis to allow the calculation of choroidal vascularity metrics (where black pixels indicate choroidal vascular luminal tissue and white pixels indicate choroidal vascular stromal tissue).

Figure 4.

Mean (± standard error) 24-hour change in axial length (µm, black), choroidal thickness (µm, red), IOP (mm Hg, green), and mean ocular perfusion pressure (MOPP, mm Hg, blue) for children ages 10.06 ± 2.53 years, with spherical equivalent refractive errors of +0.35 ± 0.38 D (n = 18); solid lines are cosinor fits to the data. Figure reproduced with permission from Ostrin LA, Jnawali A, Carkeet A, Patel NB. Twenty-four hour ocular and systemic diurnal rhythms in children. Ophthalmic Physiol Opt. 2019;39(5):358-369. © 2019 The College of Optometrists.