Ciliary muscle changes provide direct evidence of accommodation stimulation. Past research has reported that the ciliary muscle changes with accommodation using UBM, and OCT25–26. The ciliary muscle showed a contractile shortening and a thickening of the anterior portion with accommodation27. In this study, the CM1 increased significantly with accommodation state and CM3 decreased with accommodation state, consistent with past results.
In addition, we observed that the SSL increased with increasing accommodation force. We speculate that the changes in the SSL under different accommodation states may be due to several causes. On the one hand, the SS is a wedge structure formed by the projection of the inner sclera, the posterior border of the corneoscleral portion of the TM and the anterior border of the longitudinal fibers of the ciliary muscle. The ciliary muscle was thicker and showed a greater contractile response on the anterior portion27. Thus, the force of accommodation derived from the ciliary muscle mainly acts on the base of the SS rather than simply pulling on the tip of the SS. Moreover, elastic fiber tendons from the longitudinal fibers of the ciliary muscle are continuous anteriorly into the SS and attach to the posterior elastic fibers of the SS8. The wedge-shaped structure results in more longitudinal fibers of the ciliary muscle attached to the base of the SS, which results in more accommodation force derived from the ciliary muscles. On the other hand, the SS may possess contractile ability and compliance. The SS contains circumferentially oriented collagenous and elastic fibers that provide rigidity. Within the aggregated fibers of the SS, a population of circularly oriented and spindle-shaped cells (scleral spur cells) stain intensely for α-smooth muscle actin and smooth muscle myosin28. In addition, scleral spur cells are innervated by nerve endings, which contain granular and agranular vesicles that are regarded as typical for adrenergic terminals29. This evidence indicates that, in the state of accommodation stimulation, the contraction force of the ciliary muscle can posteriorly and internally pull the SS, which increases the SSL.
The TM and SC are other important structures in mediating aqueous humor outflow3–4. In this study, the SCA and SCL increased significantly with accommodation stimulation, and these changes showed a significantly positive association with the SSL. The previous study has reported the SC size correlated with outflow facility and had an effect on outflow resistance2. The SC inner wall and the juxtacanalicular tissue (JCT), which are the majority of outflow resistance, also respond to changes in mechanical tension30. Although IOP or aqueous outflow could not be simultaneously monitored during the OCT scans in this study. However, earlier research reported that accommodation stimulation could promote aqueous humour outflow and decreases IOP13–14. In addition, the TML increased significantly with accommodation stimulation. We speculate that the contractile force of the ciliary muscle with the accommodation stimulation is transmitted to the TM via pulling SS, causing the TM to be stretched, which increases the TML, thus leading to an expansion of the spaces of SC. Clinically, pilocarpine is one treatment for lowering IOP in POAG, and it is a drug that increases aqueous outflow by inducing contraction of the ciliary muscle, leading to an expansion of the spaces between the beams of the TM31. Furthermore, the posterior part of the SC where the SS exerts the most force is wider than the anterior part, consistent with the function of the SS in maintaining the opening of the SC23. These evidences suggest that the SS can regulate the morphology of the TM and SC.
The previous histological sections study by Swain DL et al. found that the significantly shorter SS in POAG eyes than in age-matched normal eye, which could not provide sufficient support for the SC and TM10. In addition, Li M et al. also found the scleral spur length was significantly shorter in POAG eyes compared with healthy eyes by SS-OCT in real-time and in vivo11. These results suggest that the shorter SS than in normal eyes contains fewer ciliary muscle fibers and TM attachments, circumferentially oriented collagenous and elastic fibers, and circularly oriented and spindle-shaped cells. The shorter SS may compromise its contractile ability and compliance. When the ciliary muscle contracts and pulls the SS of POAG eyes, it moves only a short distance posteriorly, opening few layers of meshwork beams and failing to support the SC lumen; however, this hypothesis needs further confirmation. Additionally, high myopia is an independent risk factor for open-angle glaucoma, but the reason why myopic eyes appear to be more susceptible to glaucomatous damage is unclear32. High myopia is characterized by a marked thinning of the sclera, choroid and retina as well as elongation of the axial length33. High myopia patients have a series of collagen fiber changes, including a predominantly laminar collagen fiber bundle arrangement, loss of fiber cross-links, and reductions in collagen and glycosaminoglycan synthesis34. Whether these changes will affect the collagen fiber structure, biochemistry or biomechanical properties of the SS in high myopia, resulting in changes in the contractile ability and compliance of SS, needs further investigation. Thus, further revealing the effect of SS contractile ability on the SC and TM may help to elucidate the underlying pathophysiological mechanisms involved in open-angle glaucoma.
There were several limitations in this study. First, we only observed SSL, SC, and TM changes in normal subjects. It is unclear whether similar changes would be observed in high myopic subjects or POAG patients; thus, further verification is needed. Second, in a previous study, it was found that age had an impact on structural compliance. However, in this study, the subjects’ ages ranged from 7 to 14 years (10.91 ± 1.99 years), so whether smaller changes would occur in adults and older people requires further confirmation. Third, in the current study, when provided with accommodation stimulation, IOP or aqueous outflow could not be simultaneously monitored during the OCT scans. Future studies and additional advanced and convenient methods to evaluate outflow function may provide more insight into the mechanisms underlying the pathology of glaucoma.
In conclusion, by SS-OCT, we found that the SSL increased with different accommodation stimulation, and SSL was significantly correlated with SC size. These findings suggest that the contractile ability and compliance of the scleral spur play an important role in maintaining the morphology of the SC. Moreover, the force of accommodation regulates the SC size by changing the length of SS.