Structural and Elemental Evidence for Edema in the Retina, Retinal Pigment Epithelium, and Choroid during Recovery from Experimentally Induced Myopia

Abstract

The results suggest that occluder removal induces edema across the retina and choroid and that this fluid may be the vector eliciting choroidal expansion during recovery from form deprivation possibly driven by the hyperosmolarity in the choroid, RPE, and photoreceptor outer segments that accompanies deprivation.

Ionic control of ocular growth and refractive change

“…This up-regulation of fluid flow into the choroid is consistent with the rapid morphological changes in transverse thickness of retina and choroid during recovery (4). Because the choroidal reservoir vessels are lymphatic in nature but without obvious visible drainage vessels to the venous system (26), it is not surprising that the lymphatic vessels of the choroid expand rapidly before the return of full function in the choriocapillaris and its vascular fenestrations (from 48 h after occlusion onwards) (27,28).…”

“…The pattern of changes in the ratios of K, Na, and Cl abundance during the time after occluder removal approached unity at the same time as refractive recovery reached emmetropia. The recovery of refractive state also coincides with the return of choroidal thickness to fellow eye values (4). This finding gives credence to the proposal that ionically driven fluid movements across the RPE (together with the Müller cells bridging outer and inner limiting membranes) could provide the vector effecting ocular volume changes through transfer of fluid between vitreous chamber and the choroidal lymphatics (6).…”

“…Certainly, if the balance of intracellular to extracellular K, Na, and Cl had been disturbed in such a way as to significantly alter the relative membrane potentials, then massive cell death would have been expected. Although we have not seen substantial morphological evidence for such cell necrosis, Liang et al (4) described retinal and choroidal thinning and reversible compromise of the integrity of the RPE in an ultrastructural study over the same time period. It is also likely that the RPE and neural retina are specialized to withstand substantial changes in ionic abundance because circadian rhythms and nightly sleep practices must regularly lead to conditions of 7-8 h of maintained low illuminance and reduced temporal modulation.…”

“…The correlations observed between P and K in myopic eyes could thus relate to changes in cellular metabolism. The significantly lowered potassium in the RPE-SRS-OS region during refractive recovery is accompanied by rapid morphological responses previously described after occluder removal (4). When coupled with the strict regulation of K in this region by the ionic channels and cotransporters of the apical membrane of the RPE and the spatial buffering of the Müller cells (19), this finding suggests that the K concentration in the SRS may determine the direction or amount of transfer of Na and Cl and associated fluid across the RPE at any point in time.…”

“…If, however, vision is prevented by occlusion of an eye, excessive eye growth and myopia result such that 2 weeks of occlusion in hatchling chickens leads to Ͼ20 diopters of myopia, abnormal vitreal chamber growth, and severe thinning of retinal and choroidal tissue (3,4). These gross morphological changes highlight the inverse relationship between change in vitreal depth and choroidal thickness in refractive error development.…”