June 2020
Volume 61, Issue 6
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Letters to the Editor  |   June 2020
Intralenticular Hydrostatic Pressure Increases During Ciliary Muscle Contraction: A Finding Consistent With the Schachar Mechanism of Accommodation
Author Affiliations & Notes
  • Ronald A. Schachar
    Department of Physics, University of Texas at Arlington, La Jolla, California, United States
  • Ira H. Schachar
    Department of Ophthalmology, Horngren Family Vitreoretinal Center, Byers Eye Institute, Stanford University School of Medicine, Palo Alto, California, United States.
Investigative Ophthalmology & Visual Science June 2020, Vol.61, 34. doi:https://doi.org/10.1167/iovs.61.6.34
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      Ronald A. Schachar, Ira H. Schachar; Intralenticular Hydrostatic Pressure Increases During Ciliary Muscle Contraction: A Finding Consistent With the Schachar Mechanism of Accommodation. Invest. Ophthalmol. Vis. Sci. 2020;61(6):34. doi: https://doi.org/10.1167/iovs.61.6.34.

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      © ARVO (1962-2015); The Authors (2016-present)

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Chen et al.1 measured ex vivo mouse intralenticular hydrostatic pressure following the topical administration of either pilocarpine 0.2% or tropicamide 0.1%. They observed that intralenticular hydrostatic pressure increased with pilocarpine and decreased with tropicamide. In addition, the lens diameter after pilocarpine (n = 8) increased 80 µm, 3.8%, to 2.17 ± 0.12 mm compared to the lens diameter of 2.09 ± 0.07 mm that occurred following tropicamide (n = 6). The minimal 3.8% increase in lens diameter was not, and would not be expected to be, statistically significant in view of the authors’ small sample size. 
Chen et al.1 also noted that the circumlental space between the ciliary processes and the lens equator decreased with pilocarpine to 124 ± 14 µm and increased with tropicamide to 174 ± 7 µm. Moreover, as one can observe from their images reproduced below, the valleys between the ciliary processes deepened and the distance between the lens equator and the sclera decreased with pilocarpine compared to that obtained with tropicamide (Fig.). 
Figure.
 
Reproduced from Chen et al.1 Figs. 1F and 1G. Because the sclera does not significantly change during ciliary muscle contraction, Fig. 1G is shifted down to align the sclera of the two images. Annotations and arrows have been added. The valleys between the ciliary processes are deeper and the distance between lens equator and sclera is decreased following pilocarpine induced ciliary muscle contraction (F), compared to tropicamide induced ciliary muscle relaxation, where the valleys of the ciliary processes are shallow and the distance between the lens equator and sclera is increased (G).
Figure.
 
Reproduced from Chen et al.1 Figs. 1F and 1G. Because the sclera does not significantly change during ciliary muscle contraction, Fig. 1G is shifted down to align the sclera of the two images. Annotations and arrows have been added. The valleys between the ciliary processes are deeper and the distance between lens equator and sclera is decreased following pilocarpine induced ciliary muscle contraction (F), compared to tropicamide induced ciliary muscle relaxation, where the valleys of the ciliary processes are shallow and the distance between the lens equator and sclera is increased (G).
The authors found that during pilocarpine-induced ciliary muscle contraction intralenticular hydrostatic pressure increased. The authors did not have an explanation for this increased intralenticular hydrostatic pressure. Increased tension on the equatorial zonules during pilocarpine induced ciliary body activation would be expected to increase intralenticular hydrostatic pressure. This is consistent with the Schachar mechanism of accommodation, which predicts an increase in stress on the lens capsule during ciliary muscle contraction as occurs during human accommodation.3 
According to the Schachar mechanism, during accommodation the anterior and posterior radial muscle fibers assist the anterior and posterior longitudinal muscle fibers in pulling on the scleral spur and pars plana, respectively, whereas the circular muscle fibers isometrically contract. This results in notching, outward movement of the anterior ciliary muscle2,58 with deepening of the valleys between the ciliary processes causing an increase in equatorial zonular tension, which minimally increases equatorial lens diameter, decreases the distance between the lens equator and sclera, and, counterintuitively, minimally increases central lens thickness with an associated large increase in central lens optical power.9,10 The forward movement of the ciliary processes decreases the circumlental space. 
In fact, all of the authors’ findings are consistent with the Schachar mechanism of accommodation.2 Following the administration of pilocarpine, there was < 5% increase in equatorial lens diameter,4 the valleys between the ciliary processes deepened, the circumlenticular space decreased, the distance between the lens equator and the sclera decreased, and intralenticular pressure increased from equatorial zonular tension. 
In addition to explaining the pilocarpine and tropicamide induced changes in intralenticular hydrostatic pressure, ciliary body, and lens, the Schachar mechanism of accommodation elucidates the etiology for the reported lack of mouse accommodation. According to Schachar, vertebrates with lenses that have an aspect ratio (minor axis/major axis) > 0.6 have minimal accommodative amplitude.2,11 Therefore, the lack of accommodation in the mouse is not due to a ciliary muscle that is “smaller and less developed than the primate ciliary muscle,”1 but rather related to the spherical shape of its lens. 
Acknowledgments
Disclosure: R.A. Schachar, None; I.H. Schachar, None 
References
Chen Y, Gao J, Li L, et al. The ciliary muscle and zonules of Zinn modulate lens intracellular hydrostatic pressure through transient receptor potential vanilloid channels. Invest Ophthalmol Vis Sci. 2019; 60: 4416–4424. [CrossRef] [PubMed]
Schachar RA . The Mechanism of Accommodation and Presbyopia. Amsterdam, The Netherlands: Kugler Publications; 2012: 21–50.
Schachar RA, Koivula A. The stress on the anterior lens surface during human in vivo accommodation. Br J Ophthalmol. 2008; 92: 348–350. [CrossRef] [PubMed]
Schachar RA, Tello C, Cudmore DP, Liebmann JM, Black TD, Ritch R. In vivo increase of the human lens equatorial diameter during accommodation. Am J Physiol. 1996; 271(3 Pt 2): R670–R676. [PubMed]
Schachar RA, Anderson DA. The mechanism of ciliary muscle function. Ann Ophthalmol. 1995; 27: 126–132.
Schachar RA . Human accommodative ciliary muscle configuration changes are consistent with Schachar's mechanism of accommodation. Invest Ophthalmol Vis Sci. 2015; 56: 607 [CrossRef]
Ke B, Mao X, Jiang H, et al. The relationship between high-order aberration and anterior ocular biometry during accommodation in young healthy adults. Invest Ophthalmol Vis Sci. 2017; 58: 5628–5635. [CrossRef] [PubMed]
Schachar RA, Schachar IH. Comment on ‘Prolonged nearwork affects the ciliary muscle morphology’ by Wagner S. et al. [Exp Eye Res 186 (2019) 42-51]. Exp Eye Res. 2019; 187: 107785. [CrossRef] [PubMed]
Schachar RA . Qualitative effect of zonular tension on freshly extracted intact human crystalline lenses: implications for the mechanism of accommodation. Invest Ophthalmol Vis Sci. 2004; 45: 2691–2695. [CrossRef] [PubMed]
Grzybowski A, Schachar RA, Gaca-Wysocka M, Schachar IH, Kamangar F, Pierscionek BK. Mechanism of accommodation assessed by change in precisely registered ocular images associated with concurrent change in auto-refraction. Graefes Arch Clin Exp Ophthalmol. 2018; 256: 395–402. [CrossRef] [PubMed]
Schachar RA, Pierscionek BK, Abolmaali A, Le T. The relationship between accommodative amplitude and the ratio of central lens thickness to its equatorial diameter in vertebrate eyes. Br J Ophthalmol. 2007; 91: 812–817. [CrossRef] [PubMed]
Figure.
 
Reproduced from Chen et al.1 Figs. 1F and 1G. Because the sclera does not significantly change during ciliary muscle contraction, Fig. 1G is shifted down to align the sclera of the two images. Annotations and arrows have been added. The valleys between the ciliary processes are deeper and the distance between lens equator and sclera is decreased following pilocarpine induced ciliary muscle contraction (F), compared to tropicamide induced ciliary muscle relaxation, where the valleys of the ciliary processes are shallow and the distance between the lens equator and sclera is increased (G).
Figure.
 
Reproduced from Chen et al.1 Figs. 1F and 1G. Because the sclera does not significantly change during ciliary muscle contraction, Fig. 1G is shifted down to align the sclera of the two images. Annotations and arrows have been added. The valleys between the ciliary processes are deeper and the distance between lens equator and sclera is decreased following pilocarpine induced ciliary muscle contraction (F), compared to tropicamide induced ciliary muscle relaxation, where the valleys of the ciliary processes are shallow and the distance between the lens equator and sclera is increased (G).
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