INTRAOCULAR ACCOMMODATIVE MOVEMENTS IN MONKEYS; RELATIONSHIP TO PRESBYOPIA

Abstract

Our goal was to quantify the age-related changes in the dynamic accommodative movements of the vitreous and aqueous humor in iridic, aniridic, phakic and aphakic primate eyes.

Six bilaterally iridic and four bilaterally iridectomized rhesus monkeys, ranging in age from 6–25 years, received a stimulating electrode in the midbrain Edinger-Westphal nucleus to induce accommodation, measured by a Hartinger coincidence refractometer. One of the four iridectomized monkeys underwent unilateral extracapsular and another monkey underwent intracapsular lens extraction. Eyes were imaged utilizing specialized techniques and contrast agents to resolve intraocular structures.

During accommodation the anterior hyaloid membrane and the posterior lens capsule bowed backward. Central vitreous fluid and structures/strands moved posteriorly toward the optic nerve region as peripheral vitreous, attached to the vitreous zonule, was pulled forward by ciliary muscle contraction. Triamcinolone particles injected intravitreally were also observed in the anterior chamber and moved from the anterior chamber toward the cleft of the anterior hyaloid membrane and then further posteriorly into the vitreous-filled cleft between the vitreous zonule and the ciliary body pars plana. These accommodative movements occurred in all eyes, and declined with age.

There are statistically significant accommodative movements of various intravitreal structures. The posterior/anterior fluid flow between the anterior chamber and the vitreous compartments during accommodation/disaccommodation represents fluid displacement to allow/facilitate lens thickening. The posterior accommodative movement of central vitreous fluid may result from centripetal compression of the anterior tips of the cistern-like structure attached to the vitreous zonule, and posterior displacement of the central trunk of the cistern during ciliary muscle contraction and centripetal muscle movement. The findings may have implications for presbyopia.

1. Introduction

The rhesus monkey provides an excellent model to study human accommodation and presbyopia.(Croft 2006,Croft 2008,Croft 2009,Croft et al., 2006a,b,Croft et al., 2015,Croft et al., 2016a,b,Glasser et al., 2001,Glasser and Kaufman 1999,Lütjen-Drecoll et al., 2010,Wasielewski 2008) (Croft et al., 2016a,b) Although there are minor differences between the species, the accommodative mechanism is virtually identical,(Lütjen-Drecoll et al., 2010) and both species develop presbyopia on the same timescale relative to lifespan.(Bito et al., 1982,Duane 1922) We have studied dynamic accommodation and the magnitude of the movements made by components of the accommodative apparatus in both human and monkey eyes with comparable results. In this study we quantitatively evaluated the anterior vitreous, vitreous cistern and vitreous strands in iridectomized and iridic monkey eyes to further elucidate the influence of the above structures on the development of presbyopia and the influence of presbyopia on other functional systems of the eye.

The Helmholtz theory of accommodation posits that accommodation occurs with ciliary muscle contraction, releasing tension on the anterior zonula that are attached to the lens and allowing the lens to thicken and increase in curvature. (von Helmholtz 1909) Fincham postulated that the capsule molds the lens into the accommodated state. This is the generally accepted basic mechanism of accommodation. Presbyopia, the loss of the eye’s ability to accommodate as it ages, has been attributed to hardening of the lens (Fisher 1971,Fisher 1977,Glasser and Campbell 1998,Glasser and Campbell 1999,Pau and Krantz 1991) and/or decreased ability of the ciliary muscle to undergo configurational changes (forward and centripetal movement) (Bito and Miranda 1989,Croft 2009,Tamm et al., 1992). Existing evidence supports the theory that the lens plays a major role in presbyopia.(Bito and Miranda 1989,Croft 2006,Fisher 1969,Fisher 1971,Fisher 1977,Glasser and Campbell 1998,Glasser and Campbell 1999,Heys et al., 2004,Koretz et al., 1989,Pau and Krantz 1991) Indeed, age-related loss of deformability in the excised human lens can entirely account for presbyopia.(Glasser and Campbell 1998,Glasser and Campbell 1999) However, there is a significant correlation between the age-related decrease in accommodative forward muscle movement and lens movement (e.g., lens thickening, centripetal lens equator movement). Thus lens hardening might occur secondarily, consequent to maintained zonular traction on the lens during attempted accommodation.(Croft 2006,Croft 2009) The age-related loss in accommodative forward muscle movement is more pronounced than the loss in centripetal muscle movement, perhaps due to the direction of restriction of an increasingly inelastic posterior attachment of the muscle, i.e., the choroid.(Croft 2006,Tamm et al., 1992,Tamm et al., 1992) Another source of ciliary muscle posterior restriction could be the buildup or accumulation of vitreous strands that attach to the vitreous zonule, which is in turn attached to the anterior and posterior ends of the muscle. (Croft et al., 2016) The pronounced loss in forward ciliary muscle movement during attempted accommodation occurs by middle age in both live monkey and human eyes.(Croft 2006) (Croft 2009,Croft et al., 2013,Croft et al., 2013) While the muscle loses mobility with age, it does not lose the ability to contract with age, thus undergoing an increasingly isometric contraction that perhaps places increased tension on the optic nerve region via the “age-stiffened” choroid. (Croft et al., 2016) (Kaufman et al., 2019) (See EER 00002). Another factor in presbyopia may be the age-related inward bowing of the scleral/muscle contour in the limbal region, that may change the geometric relationship between the muscle/zonule complex and the lens, and thereby dampen accommodation.(Croft et al., 2013)b

Thus, the aging lens is the primary optical reason for decreased accommodation and presbyopia. However, the lens capsule,(Krag and Andreassen 2003,Krag et al., 1997) ciliary muscle,(Tamm et al., 1992) (Croft 2006,Croft 2009,Croft et al., 2006,Croft et al., 2013,Lütjen-Drecoll et al., 2010,Wasielewski 2008) vitreous zonule,(Wasielewski 2008) vitreous compartment,(Green and Sebag 2001,Sebag 1992,Sebag 1998,Sebag and Balazs 1985,Smith et al., 1990) and the scleral limbal contour (Croft et al., 2013)b all undergo age-related changes that can potentially cause accommodative loss. Plausibly, manipulating them may enhance accommodation.

After pars plana vitrectomy in human patients aged 33 to 41 years, there is a loss of accommodative ability of 2.25 diopters.(Smith et al., 1990) Earlier research (Coleman 1970,Coleman 1986,Coleman and Fish 2001) pointed to a role for the vitreous in accommodation and presbyopia. More recent findings also indicate a role for the vitreous,(Croft et al., 2016,Croft et al., 2013) but not as previously proposed.(Coleman 1970,Coleman 1986,Coleman and Fish 2001) We have demonstrated that there is an accommodative backward bowing of the anterior hyaloid (decorated with triamcinolone) [Croft, Kaufman et al., ARVO 2012] (Croft et al., 2013)b (Kaufman et al., 2019) and the posterior lens capsule^{ARVO 2013]} (Croft et al., 2013)b (Croft et al., 2016), and fluid flow from the anterior chamber toward the anterior hyaloid(Croft et al., 2013)b and then further into the cleft between the intermediate vitreous zonule and the ciliary muscle pars plana. (Kaufman et al., 2019). Although we have reported intravitreal accommodative movements qualitatively, (i.e., anterior hyaloid, central vitreous, lacunae (cistern) tips, Cloquet’s canal),(Croft et al., 2016,Croft et al., 2012,Croft et al., 2013,Kaufman et al., 2019) we have not reported quantitative measures of these movements with the exception of the vitreous zonule.(Croft et al., 2013,Lütjen-Drecoll et al., 2010) Further, the bulk of our previous work has been in the iridectomized monkey eye, rather than in the iridic eye.

In previous publications we reported qualitative movements of intravitreal movements (i.e. anterior hyaloid backward bowing, central vitreous posterior movement).(Kaufman et al., 2019) In the current paper we provide quantitative measures of the accommodative movements of various intravitreal structures and fluid flow and the changes with age. Thus, we further elucidate the role of the vitreous, using a technique to visualize and measure fluid movement and the structures/strands within the vitreous compartment (anterior hyaloid, central vitreous, lacunae (cistern tips), Cloquet’s canal) at rest and during accommodation, to determine their accommodative function and relationship to the lens and ciliary muscle in both young and aging rhesus monkey eyes. We also determined whether the findings are the same in the presence and the absence of the iris.

2. Materials and methods

The methods have all been described in previous publications as referenced; thus, only brief descriptions are included here.

2.1. Monkeys

Six bilaterally iridic and four bilaterally iridectomized rhesus monkeys (Macaca mulatta) of either sex weighing 5.0 kg - 15.0 kg and ranging in age from 5 years to 25 years were randomly selected and housed as previously described.(Lütjen-Drecoll et al., 2010) In all 10 monkeys, a bipolar stimulating electrode was inserted into the Edinger-Westphal nucleus (midbrain) to induce accommodation.(Crawford et al., 1989,Croft 2006) Two of the four iridectomized monkeys underwent unilateral extracapsular (n=1) or intracapsular (n=1) lens extraction, and the eyes were imaged by B-scan ultrasound biomicroscopy (UBM; 50, 35, 20 MHz) before and after lens extraction. All procedures conformed to the ARVO Statement for the Use of Animals in Research, were in accordance with institutionally approved animal protocols and the guidelines of the National Institute of Health (NIH) guide for the care and use of laboratory animals were followed.

2.2. Refractometry

A Hartinger coincidence refractometer measured the total refractive error of the eye in the resting and accommodated states.(Croft 2006) Accommodative amplitude was taken as the difference in refractive error between the two states.

2.3. Ultrasound Biomicroscopy (UBM)

UBM was used to image the entire globe, the vitreous membrane, the entire extent of the ciliary body, the anterior and posterior lens surfaces, and the zonula during accommodation. UBM in the rhesus monkey has been described in detail elsewhere.(Croft 2006,Croft et al., 2006,Glasser et al., 2001) Briefly, each anesthetized (pentobarbital Na 10–15 mg/kg IV, supplemented with 0.5–10 mg/kg IV, as needed) monkey was placed supine with the head stabilized facing upward in a head holder, and a saline fluid well was placed around the eye.(Glasser et al., 2001,Pavlin and Foster 1995) The eye was rotated using a suture passed beneath the lateral rectus muscle.(Glasser et al., 2001) During UBM imaging, the eye was stabilized with additional extraocular muscle sutures, so that during accommodation there was minimal, if any, convergent eye movement. The stabilizing arm of the ultrasound instrument held the transducer in place. Thus there was very little, if any, change in angle of the transducer to the eye during accommodation. Dynamic UBM images were obtained during central stimulation of accommodation and then recorded to videotape or computer hard drive.(Croft 2006)

2.4. Instrumentation

High-resolution contact imaging dynamic goniovideography and UBM were performed during the dynamic accommodation response in anesthetized monkeys implanted with a midbrain-stimulating electrode.(Croft 2006,Croft et al., 2006,Croft et al., 1998,Glasser et al., 2001,Glasser and Kaufman 1999)

The UBM-H (Humphrey Instruments, Model # 840, 80 MHZ (50 μm lateral resolution, 37 μm axial resolution, 5 mm field of view), San Leandro, CA) imaged the anterior portion of the ciliary body, vitreous zonule, vitreous strand, iris and lens equator (video clip #1). The UBM-ER (Model # MHF-1 Ultraview System Model P60, 50 MHz, 35 MHz, 13 mm field of view) and the UBM-VM (VuMax, SonoMed Escalon, New Berlin, WI, 50 MHz, 35 MHz, 13 mm field of view) imaged the entire sagittal extent of the ciliary body, from the region of the ora serrata to the cornea, the vitreous zonule, vitreous strand, lens equator, iris, and the anterior and posterior lens surfaces (Fig. 1, video clip #1 – #6). In addition, the UBM ER 20 MHz probe, which enables a 25 mm field of view, imaged the entire extent of the globe. Images that provided the best definition of the accommodative apparatus, showing clearly defined edges of the ciliary body, lens equator, anterior and posterior lens surfaces, vitreous zonule and cornea were accepted for analysis.

Figure 1.

Ultrasound biomicroscopy (UBM) Images of 6 and 19-year-old rhesus monkeys in the unaccommodated and accommodated states. A/P distances (green arrows) were measured perpendicular to the anterior plane of the iris (yellow lines) near the region of the circumlental space (red arrows). During accommodation, the iris bows backward in the region of the circumlental space—more so in the older eye than in the young eye.

2.5. Monkey surgical procedures

One of the 4 iridectomized monkeys (age 13 years) underwent unilateral intracapsular lens extraction (ICLE: removal of the lens nucleus and cortex within the intact capsule). α-chymotrypsin was allowed to remain in the eye for <30 seconds to lyse the connection to the zonula before fluid rinsing and lens removal with a cryoprobe. The reduced exposure time to the α-chymotrypsin minimized the disruption of the zonular attachments to the pars plana zonula and vitreous zonule. Wieger’s ligament and the vitreous strand remained largely intact in these eyes post-ICLE, and mechanical vitrectomy was not required. Another one of the 4 iridectomized monkeys (age 7 years) underwent extracapsular lens extraction (ECLE: ~4 mm anterior capsulorrhexis, removal of the lens nucleus and cortex). Both surgical procedures were performed using standard clinical techniques,(Croft 2008) with the minor technical variations noted above. The remaining two iridectomized monkeys with the lens intact were aged 8 and 25 years.

2.6. Intravitreal injection

Triamcinolone acetonide (50 μl of 10% Triesence®, Bristol-Meyers Squibb Company, Princeton, NJ), which adheres to the vitreous membranes/fibers, was administered via trans-pars plana intravitreal injection to enhance visualization (by either UBM or endoscopy) of the intravitreal structures (e.g., anterior hyaloid). A 50-μl intravitreal injection of 0.02% aminofluorescein dye was used to enhance visualization of intravitreal structures by endoscopy.(Schumann and Rentsch 1998) (Croft et al., 2013)b

2.7. Statistical Analysis

A two-tailed paired t-test was used to detect significant differences between various intraocular measurements taken in the resting and accommodated states (e.g., lens thickness). A p-value ≤ 0.05 was considered significant; 0.05 ≤ p ≤ 0.10 was considered to indicate a trend, given the small number of monkeys. Simple linear regression (i.e., central vitreous vs age) was undertaken to determine if there was a significant relationship between variables and the results reported as the slope of the regression equation (coefficient) ± standard error of the coefficient.

3. Results

3.1. Triamcinolone intraocular injection enabled visualization of vitreous fibers/strands, membranes and fluid flow by UBM

Triamcinolone particles were suspended in the vitreous fluid and also clung to the vitreous fibers/strands and membranes as visualized by UBM. Although the triamcinolone particles were injected into the vitreous compartment only, some particles were also observed within the anterior and posterior chambers one day post-injection.

3.2. Aniridic and iridic eyes

In the iridectomized eye during accommodation, the triamcinolone particles (suspended in the ocular fluid) flowed around the lens equator from the unified anterior chamber into the vitreous compartment toward the anterior hyaloid (as previously reported (Croft et al., 2013)b (Kaufman et al., 2019)), and then further into the cleft between the vitreous zonule and pars plana. The same was true during accommodation in the presence of the iris; the posterior chamber collapsed as the iris (both anterior and posterior surfaces) bowed backward in the region of the circumlental space (CLS), and the ciliary muscle moved forward and inward (see video clips # 1, 2). During accommodation the iris root (insertion point of the iris into the anterior aspect of the ciliary body) moved with accommodative forward and inward muscle movement and contributed to pupil constriction. The iris was thicker and more full-bodied in the young eye versus the older eye (Fig. 1). In the young eye, the anterior chamber and the iris plane at the CLS region were deeper (A/P distance with respect to the corneal apex) compared to the older eye (Fig.1; see Section 4.5 of the Discussion). The backward bowing of the iris in the region of the CLS was more pronounced in the older eye than in the young eye (Fig. 1, Table 1). In the young eye the accommodative backward bowing of the iris was in parallel with the accommodative backward bowing of the anterior hyaloid; the higher the accommodative amplitude the more pronounced was the backward bowing of the iris and the anterior hyaloid (see section 3.3 below and video clip #2).

Table 1.

Data are mean ± s.e.m. A/P perpendicular distance from the central cornea (as defined by the green arrow in Fig. 1) to the anterior iris position (yellow line in Fig. 1) near the region of the circumlental space (CLS; red arrow in Fig. 1). Reading horizontally, during accommodation the anterior iris in the region of the CLS moves posteriorly in the young iridic monkey eyes, and the amount of posterior movement increases slightly with age (p ≤ 0.05 between young versus older age group by two sample t-test). Reading vertically, the paired observation was accommodative posterior iris movement (anterior iris position in the region of the CLS in the resting minus accommodated state) within each eye, grouped by age. One eye was randomly selected from each monkey.

Anterior Iris Position with Respect to the Cornea
A/P Distance of the Anterior Iris Position in the Region of the CLS from the Central Cornea (mm)
		n	Young (6–7 yr)	n	Older (17–19yr)	Young versus Older
Resting	Mean	3	3.58	3	3.28	p = 0.22
	s.e.m.		0.044		0.17
Accommodated	Mean	3	3.63	3	3.37	p = 0.17
	s.e.m.		0.040		0.17
Resting Minus Accommodated	Mean	3	−0.05	3	–0.10	p < 0.05
	s.e.m.		0.005		0.011
			p < 0.01		p < 0.01

3.3. Accommodative movements of the anterior hyaloid

Backward bowing of the anterior hyaloid occurred in iridic, aniridic, phakic, and aphakic eyes. Accommodative backward bowing of the anterior hyaloid and accommodative amplitude were significantly correlated (Fig. 2); the higher the electrically-induced accommodation, the higher the backward bowing of the anterior hyaloid. The slopes of the correlations for four monkey eyes are shown in Figure 2.

Figure 2.

Accommodative backward bowing of the anterior hyaloid with respect to the scleral spur vs accommodative amplitude in one eye of each of 4 rhesus monkeys, aged 8, 8, 13 and 19-years old. Accommodative backward bowing of the anterior hyaloid was significantly related to accommodative amplitude; as the accommodative amplitude increased the backward bowing of the anterior hyaloid increased.

At maximum accommodative stimulation the mid region of the anterior hyaloid bowed backward by 0.26 ± 0.02 mm (p=0.001; n=8; Table 2) in 8 monkeys ranging in age from 7 to 25 years old; the reverse was true during disaccommodation. The amount of backward bowing did not significantly change with age (p = 0.30; Table 2) and was not significantly correlated with the decline in accommodative amplitude with age (p=0.16) (Data not shown; see Discussion Section 4.2).

Table 2.

Data are mean s.e.m accommodative backward movments of the anterior hyaloid and central vitreous, and accommodative forward and centripetal movements of the peripheral lacunae (cistern branch) tips. The young vs. older age groups were compared by an unpaired 2-tailed 2-sample t-test. N.S.= not significant.

		Accommodative Backward Bowing of the Anterior Hyaloid		Accommodative Posterior Movements of the Central Vitreous		Accommodative Movements of the Lacuane Peripheral (Cistern Branch) Tips
						Forward Movement		Centripetal Movement
		(mm)	paired t	(mm)	paired t	(mm)	paired t	(mm)	paired t
Young	Mean	0.29	p=0.001	0.34	p=0.003	0.54	p=0.008	0.15	p=0.014
Ages 7–13 yr	s.e.m.	0.02		0.05		0.11		0.04
	n	5		5		5		5
Older	Mean	0.21	p=0.029	0.15	p=0.030	0.22	p=0.052	0.13	p=0.030
Ages 19–25 yr	s.e.m.	0.04		0.01		0.05		0.02
	n	3		2		3		3
Young vs. Older two sample-t		N.S.		p=0.025		p=0.049		N.S.
Young + Older	Mean	0.26	p=0.001	0.29	p=0.001	0.42	p=0.003	0.14	p=0.001
	s.e.m.	0.02		0.05		0.09		0.02
	n	8		7		8		8

3.4. Accommodative movements of the central vitreous, peripheral tips of the intravitreal lacunae (cistern tips) and vitreous fibers/strands

Intravitreal lacunae could be visualized, beginning in the mid-region of the vitreous, with the peripheral (cistern (Jongbloed and Worst 1987)) tips of these lacunae extending to and interconnected with the vitreous zonule (see video clip #3). During accommodation, the mid-vitreous portion of the lacunae (central vitreous) as far back as the optic nerve moved posteriorly with the vitreous fluid by 0.29 ± 0.05 mm (p=0.001; n=7; Table 2). The posterior movement of the central vitreous during maximum accommodation was significantly related to maximum accommodative amplitude (p=0.019; Fig. 3) and declined significantly with age (p=0.031; Fig. 3). The posterior movement of the central vitreous fluid/fibers extended from the posterior lens pole to the posterior pole of the eye. The movements were in the opposite direction during disaccommodation. This occurred in the presence or absence of the lens substance, the lens capsule and/or the iris (see video clip # 4). Following ECLE, the central lens capsule moved posteriorly (by 0.66 +/− 0.07mm) as previously reported. (Croft et al., 2016)

Figure 3.

Accommodative posterior movement of the central vitreous with respect to the retina plotted vs accommodative amplitude (left panel) or age (right panel) in 7 rhesus monkeys. Triamcinolone particles were not visualized in the central vitreous in one monkey post injection, thus n=7.

During maximum accommodation, while the central vitreous moved posteriorly, the peripheral lacunae (cistern) tips, adjacent to and interconnected with the vitreous zonule, were pulled forward by 0.42 ± 0.09 mm (p=0.003; n=8; Table 2) and centripetally by 0.14 ± 0.02 mm (p=0.001; n=8; Table 2.). The amount of forward movement of the peripheral lacunae tip was significantly positively correlated with accommodative amplitude (p=0.047; the higher the amount of forward movement of the lacunae tip the higher the accommodative amplitude) and the amount of forward movement of the lacunae tip tended to decline with age (p=0.08; Fig. 4, Table 2). However, the amount of centripetal movement of the lacunae tip did not decline significantly with age (p=0.52) and was not significantly related to accommodative amplitude (p=0.33; Table 2). The anterior edge of the anterior lacunae contributed to the anterior hyaloid membrane. During accommodation the posterior edges of the anterior lacunae did not bow backward and the width of the peripheral lacunae narrowed by 0.28 mm ± 0.001 mm (p=0.02; n=3) (see video clip #3). The fluid inside the anterior lacunae moved centripetally during the accommodative response as the anterior hyaloid bowed backward.

Figure 4.

Accommodative forward movement of the peripheral lacunae (cistern) tip with respect to the scleral spur plotted vs accommodative amplitude (left panel) or age (right panel) in 8 rhesus monkeys.

3.5. Accommodative posterior movement of Cloquet’s canal

Triamcinolone particles were suspended inside Cloquet’s canal, allowing its visualization. The entire Cloquet’s canal moved posteriorly or anteriorly during accommodation or disaccommodation respectively, in proportion to accommodative amplitude (see video clip #4) and similar in magnitude to the accommodative posterior movement of the central vitreous.

In one 6 year old monkey eye particles of tramcinolone accumulated on the layers of vitreous fibers that extended from the retina to the lens. These vitreous fiber layers moved centripetally during the accommodative response and then returned to the resting state (more centrifugal position) during disaccommodation (video clip #5). Video clip #5 shows dynamic movements of three parallel vitreous fibers/strands highlighted by triamcinolone. In this one eye measurements were repeated 4 times (4 separate stimulus trains) and during accommodation the fiber/strands moved centripetally by 0.315 ± 0.039 mm (p=0.001) and the space between the two fibers/strands increased by 0.047 ± 0.010 mm (p=0.02)(Video Clip #5).

In the older monkeys, there was an age-related accumulation of vitreous membranes/fibers in the region of the ora serrata and the vitreous zonule, as indicated by the accumulation of triamcinolone (Fig. 5). These accumulated vitreous membranes/fibers also moved forward during the accommodative response, consistent with their connections to the vitreous zonule, the strand that extends between the posterior vitreous zonule insertion zone and the lens equator (PVZ INS-LE) (Croft et al., 2016,Croft et al., 2015), and to the ora serrata region (video clip #6).

Figure 5.

Ultrasound biomicroscopy (UBM) images in iridectomized rhesus monkey eyes aged 7-years-old (left panel) and 17-years-old (right panel) following intravitreal injection of triamcinolone. With age, vitreous fibers aggregate and build up peripherally in the ora serrata region in close juxtaposition and attachment to the vitreous zonule, as evidenced by the accumulation of triamcinolone particles.

3.6. The lens

The lens thickened during accommodation in both the aniridic and the iridic monkey eyes (Table 3). During accommodation the anterior lens surface moved anteriorly, the posterior lens surface moved posteriorly, and both lens surfaces became more sharply curved. In the unaccommodated state, the radius of curvature of the anterior lens pole surface was 7.0 mm ± 0.6 mm in the aniridic eye and 6.3 mm ± 0.4 mm in the region of the pupil in the iridic eye. In the accommodated state the anterior lens radius of curvature was 3.6 mm ± 0.3 mm and 3.8 mm ± 0.8 mm in the aniridic and iridic eye, respectively. In the young eye, the posterior lens pole moved posteriorly by 0.28 mm ± 0.03 mm (p<0.02; Table 3, video clip #2), allowed/facilitated by the posterior movement of the central vitreous (see Section 3.4 of the Results section above), while the lens equator and the PVZ INS-LE moved forward. The accommodative posterior movement of the posterior lens pole declined with age, but the older posterior lens pole resided in a progressively more posterior position both at rest and during accommodation, encroaching on the anterior vitreous as previously reported. (Croft et al., 2013)a

Table 3.

Data are mean ± s.e.m. accommodative amplitude, accommodative lens thickening and accommodative lens posterior pole movement in the iridic monkey eye^*. The young versus older age groups were compared by an unpaired 2-tailed 2-sample t-test. The accommodation, lens thickening (lens thickness in the accommodated state minus lens thickness in the resting state) or accommodative lens posterior pole movement (accommodated lens posterior pole position minus resting lens posterior pole position with respect to the cornea) within the younger and older groups were 2-tailed paired t-test comparisons.

Accommodative Amplitude, Accommodative Lens Thickening and Lens Posterior Pole Movement (UBM)
Monkey Iridic**
Age (years)		n	Accommodative Amplitude (D)		Accommodative Lens Thickening (mm)		Accommodative Lens Posterior Pole Movement (mm)*
Young	Mean	3	15.3	p < 0.01	0.61	p < 0.01	0.28	p < 0.02
(6–7 yr)	s.e.m.		1.15		0.01		0.03
Older	Mean	3	5.8	NS	0.19	p < 0.05	0.09	NS
(17–19 yr)	s.e.m.		1.8		0.05		0.03
Young vs Older			p<0.01		p < 0.01		p < 0.01

Lens Thickening: During accommodation in the young iridic monkey eyes, the lens thickens but the amount of accommodative lens thickening decreases with age, as previously reported for the human eye and similar to the iridectomized monkey eye (data not shown).

Accommodative Lens Posterior Pole Movement: During accommodation in the young iridic monkey eyes, the lens posterior pole moves posteriorly but the amount of posterior movement decreases with age, as previously reported for the human eye and similar to the iridectomized monkey eye (data not shown).

^*positive number = posterior movement of the posterior lens pole.

^**Monkey Iridic = Current study. Accommodation induced by EW stimulation.

3.7. The lens capsule

Figure 6 shows the typical shape of the accommodated capsule without the lens substance. During accommodation following ECLE, in the central capsular region around the capsulorrhexis, the anterior and posterior capsule edges separated from each other: the anterior central capsule bowed forward and the posterior central capsule bowed backward. Peripherally, the anterior and posterior capsules were adjacent to each other and moved forward during accommodation, supported by the PVZ INS-LE, consistent with previous reports.(Croft et al., 2016)

Figure 6.

Typical ultrasound biomicroscopy (UBM) image of the accommodated capsule shape following extracapsular lens extraction (ECLE).

4. Discussion

Our previous publications reported qualitative descriptions (i.e., direction of intravitreal flow, backward bowing of the anterior hyaloid etc.) of fluid or structural movements without quantitative measurements or statistical analysis.(Kaufman et al., 2019) (Croft et al., 2013) In this current paper we quantify the accommodative movements of various intravitreal structures and iris (Figs. 2,3 and 4; Tables 1 and 2) and intravitreal fluid flow and provide a full description of how we obtained the data in the Materials and Methods. In addition, we provide statistical analysis of the data obtained and provide correlations between the variables and the changes with age. We include new videos never before published. In addition, we show that these movements occur in both the iridic and aniridic monkey eye.

4.1. UBM and intravitreal contrast agents

UBM has proven to be the best technique for imaging intraocular structures and their dynamic movements during accommodation, facilitated by the injection of contrast agents that cling to the various membranes and that highlight fluid flow. Although magnetic resonance imaging (MRI) (Strenk 2006,Strenk et al., 1992,Strenk et al., 1999,Strenk et al., 2010) has proved useful for imaging in the human eye, MRI resolution is limited and cannot consistently image the vitreous zonule or the dynamic movements inside the eye during accommodation.(Strenk 2006) (Strenk et al., 2010) Similar issues are true for OCT and, in the case of OCT, the images need correction for distortion.(Martinez-Enriquez et al., 2017) (Sheppard and Davies 2011,Sheppard et al., 2010)

4.2. Accommodative movements

The data demonstrate that there are statistically significant accommodative movements of the anterior hyaloid, central vitreous and the lacunae (cistern) peripheral tips (Table 2, Figs. 2, 3, 4). During accommodation, the anterior hyaloid bows backward, the central vitreous moves posteriorly while the lacunae (cistern) tips move forward and inward (centripetally) (Table 2). The accommodative posterior movements of the central vitreous increased with increasing accommodative amplitude (p=0.019; Fig. 3); the more the central vitreous moved posteriorly during accommodation the higher the accommodative amplitude. The accommodative posterior movements of the central vitreous declined significantly with age (p=0.031), suggesting either a possible role in presbyopia pathophysiology or as ‘innocent bystanders’. In addition, the accommodative forward movement of the lacunae (cistern) tips increased with increasing accommodative amplitude (p = 0.047; Fig. 4) and tended to decline with age (p = 0.08). The loss in accommodative movements of the lacunae (cistern) tips might also be part of presbyopia pathophysiology, given their attachment to the vitreous zonule and the age-related accumulation of peripheral vitreous aggregates that might dampen forward muscle movement. As above, one cannot exclude the ‘innocent bystander’ explanation. The accommodative backward bowing of the anterior hyaloid did not significantly decline with age, perhaps because most of the centripetal muscle movement (73%)(Croft 2006) (Croft 2009) is retained with age and is still sufficient to allow the accommodative relaxation (backward bowing) of the anterior hyaloid. In addition, there is inward bowing of the sclera in the region of the limbus, and thus tension to the anterior hyaloid may be already somewhat relaxed in the disaccommodated state, and more so in the accommodated state. Thus, an age-related change in accommodative backward bowing of the anterior hyaloid and any correlation with the age-related accommodative amplitude decline either is not present or is obscured by the inward bowing of the sclera that occurs with age.

The accommodative movement of the anterior/posterior intravitreal parallel strands observed in one monkey (Video Clip #5) supports the idea of microchannels for fluid flow; these may influence trans-vitreal flow of aqueous humour and systemic or topical drugs. (Sebag and Balazs 1985) It was extremely rare for the triamcinolone injection needle to precisely traverse in between the leaves of the cistern and highlight the parallel fibers, so we only have this one observation. Given the experiment’s cost, the setup’s tediousness, the slim chance of repeating the exact placement of the triamcinolone and the slim chance of capturing the accommodative dynamic video image, we have included the single observation. Further study of the accommodative movements of the anterior hyaloid and the intravitreal parallel strands is needed to make definitive conclusions, but is beyond the scope of this paper.

The cistern tip is attached to the vitreous zonule which is pulled forward and inward (centripetally) during accommodation by the ciliary muscle in the young eye. However, with age only 39% of the forward muscle movement remains (due to posterior restriction).(Croft 2009) This may explain why the forward movement of the cistern tip tends to decline with age while overall the centripetal (inward) movement of the cistern tip does not. Yet another possibility is an ‘intermediate’ theory: something elsewhere in the system is askew, the vitreous movement decline is secondary to that, and then the lens movement decline is a tertiary consequence of the decreasing vitreous movement.

4.3. Accommodative movements are the same in iridic vs. aniridic eyes

The UBM dynamic imaging showed that absence of the iris does not change the normal accommodative fluid movements or the movements of the accommodative apparatus itself compared to the iridic eye, and that the movements are similar to those in the human eye. The lens thickened during accommodation in both the aniridic and the iridic monkey eyes (Table 3; video clip #2), similar to the thickening previously reported in the human eye.(Croft et al., 2013)a Further, in both the aniridic and iridic monkey eyes, accommodative posterior movement of the posterior lens pole declined with age, but the older posterior lens pole resided in a more posterior resting position that encroached on the anterior vitreous with age, similar to the human eye. (Croft et al., 2013)a (Croft et al., 2016) In the young human eye the posterior pole of the lens moved posteriorly by 0.08 ± 0.01 mm (p<0.05). (Croft et al., 2016) (Croft et al., 2013) In the older human eye, the mean accommodative movement of the posterior lens pole was −0.08 mm ± 0.04 mm, (Croft et al., 2016) demonstrating a slight but not significant average forward movement (p<0.14). This has been a controversial point historically, but larger human data sets have shown that there is a small posterior movement of the posterior lens pole.(Coleman 1970) (Wendt and Glasser 2010) Age may affect the magnitude and direction of the posterior lens pole movement, perhaps accounting for the discrepancies.(Koretz et al., 1989) (Croft et al., 2016) (Koretz et al., 1987) In any event, the iridic or aniridic monkey eye is a useful model for human accommodation and presbyopia.

4.4. Iris pupil constriction and anterior lens curvature

Montes-Mico et al.(Montes-Mico et al., 2010) reported a 1/3 diopter hyperopic shift in equivalent sphere after iris pupil constriction (without accommodation) in the human eye (exposed to varying light conditions) and posited that this was due to change in lens position or curvature.(Montes-Mico et al., 2010) One might expect that the iris would produce a slightly convex central lens curvature that would increase spherical aberration,(Crawford et al., 1990,Crawford et al., 1990) but that was not the case in our current study. Here, the anterior lens pole surface in the unaccommodated state appeared more sharply curved in the iridic eye than in the aniridic eye, but the difference was not significant. Further, in the accommodated state, there was virtually no difference in the anterior lens pole curvature in the iridic eye versus the aniridic eye; thus the presence of the iris had little if any effect on lens curvature. Surgical aniridia also seems to have no effect on the conventional or unconventional aqueous outflow pathways (Kaufman and Lütjen-Drecoll 1975) (Kaufman et al., 1977) (Kaufman 1979) (Kaufman and Rentzhog 1981) (Kaufman 1986) (Crawford et al., 1990)

4.5. Accommodative backward bowing of the iris declines with age

That the backward bowing of the iris was more pronounced in the older eye than in the young eye might seem counterintuitive. However, the older iris is less robust and perhaps more flaccid (more prone to backward movement) than the young iris. The reduced forward ciliary muscle movement in the older eye may allow more posterior movement of the older iris (Fig. 1, Table 1), especially since the older iris in the CLS region is in a more forward position at rest than in the young eye. In the young eye, the anterior chamber and the iris plane at the CLS region is deeper compared to the older eye, likely due to age-related central lens thickening.

Although we did not observe iris thinning in the region of the CLS (video clips 1–2), it is possible that this occurs and may affect the magnitude of the backward bowing of the iris that was measured. Iris A/P thickness (over the entire radial iris length), radial iris length, iris contour, area of the iris and their changes during accommodation are all important parameters to measure in a subsequent study centered on the iris, but are beyond the scope of this paper. What is important here is that both the anterior and posterior iris surfaces bow backward in the region of the CLS and the backward bowing of the iris contributes to the collapse of the posterior chamber.

4.6. Accommodative lens shape change is aided by several facets of the accommodative apparatus

During accommodation the capsule helps to mold the lens into the accommodated shape, based on excised human lens data. (Glasser and Campbell 1999) The typical shape of the accommodated capsule without the lens substance (Fig. 5) can give insight into the accommodative compressive/directional forces that the capsule exerts on the lens substance (Fig. 6). Following ECLE, the anterior and posterior leaves of the peripheral capsule are adjacent to each other, while at the center they separate from each other during accommodation. This suggests that, during accommodation, the peripheral lens would be compressed by the peripheral capsule while the central capsular leaves separate from each other, expanding the distance between them to allow a thicker central lens (Fig. 6). The young excised lens is in an accommodated form due to the forces exerted by the capsule and the loss of zonular centrifugal tension.(Glasser and Campbell 1998,Glasser and Campbell 1999) However, this does not negate the evidence we report here and elsewhere that other dynamics facilitate lens shape change in the living eye. (Croft et al., 2016) A recently found structure that extends from the vitreous zonule posterior insertion zone to the posterior lens equator (termed the PVZ INS-LE) may act as a strut to the posterior lens/capsule equator. (Croft et al., 2016) During accommodation the muscle pulls the PVZ INS-LE forward, likely facilitating forward lens equator movement and lens shape change. Other researchers have posited a “vitreous support” component in accommodation. Our current data suggest a role for the vitreous in accommodation/disaccommodation, but it may not be as posited previously. (Coleman and Fish 2001)

In addition, the capsule/lens complex does not/cannot drive the accommodative movements that are seen inside the vitreous (video clips #2–6, Section 3.4 of the Results), since these intravitreal accommodative movements occur in the presence or absence of the lens and capsule complex. Vitreous fluid/fiber movement in the posterior direction in the central and posterior vitreous (including Cloquet’s canal), extending posteriorly to the optic nerve region, occurs during the accommodative response, while the peripheral vitreous adjacent to and interconnected with the vitreous zonule (including lacunae tips) moves forward. Factors that drive the accommodative fluid redistribution and membrane reconfiguration of the vitreous are the ciliary muscle contraction/repositioning and the connections between the accommodative apparatus (muscle, lens, zonula) and the vitreous. These connections (i.e., and video clips 2–6) need further morphological study.

Glasser et al.1999 reported that the young excised lens is in an accommodated form, (Glasser and Campbell 1999) demonstrating that the capsule has sufficient force to mold the lens substance. According to Krag, the load deformation curves for a 3.2-mm diameter anterior capsule in the human were in the range of 40–45 mN (~4 mg) for a deformation of ~4 mm.(Krag and Andreassen 2003) In the monkey eye, the amount of pharmacologically induced force generated by longitudinal ciliary muscle contraction is 80 mg for a 5 × 4 mm tissue strip.(Poyer et al., 1993) The muscle drives the accommodative response (in response to blur stimulus), and it seems intuitive that the muscle is required to do far more than simply release tension to the lens, given the available forces just described. The in vivo situation—e.g., accommodation to a speeding fastball—may require far more than just the capsular forces to rapidly change lens shape. It may require the forward push of the PVZ INS-LE to the posterior lens equator (to facilitate forward lens equator movement), (Croft et al., 2016) rapid fluid redistribution at the lens equator, and posterior movement at the posterior lens pole facilitated by the posterior movement of the central vitreous. (Croft et al., 2016)

4.7. The cistern structure and accommodation

How can the muscle contraction and movement so quickly affect fluid/fiber movement throughout the vitreous? This suggests that there is a structure attached to the accommodative apparatus that is affected by ciliary muscle contraction and inward movement. Jongebloed and Worst reported the existence of a cistern-like structure within the vitreous (left panel next to video clip #3).(Jongbloed and Worst 1987) (Sebag 1992) The main “trunk” of this cistern-like structure lies in the central posterior region of the vitreous, while the branches of the cistern-like structure extend from the cistern “trunk “ in an anterior/outward direction toward the peripheral vitreous. The lacunae we report could be the branches of the cistern-like structure (left panel next to Video Clip #3).(Jongbloed and Worst 1987) Further study of the structural composition of the vitreous is required to make definitive conclusions, but it is clear that the vitreous cannot be simply considered as only a gel-like substance given our findings and those in the literature.

The cistern-like structure may play a role in the movements within the vitreous during the accommodative response. The peripheral tips of the lacunae (branches) of the cistern-like structure that are attached to the vitreous zonule are pulled forward and centripetally as the ciliary muscle contracts–while the main trunk of the cistern-like structure, which extends to the optic nerve region, is pushed slightly backward during accommodation. This may place a suction-type posterior pull on the posterior lens pole, facilitating accommodative lens thickening. There is a 50% and 85% loss in forward muscle movement in the middle-aged and older eye, respectively, while the amount of centripetal muscle movement associated with a given amount of centripetal lens movement and a given amount of accommodation was increased.(Croft 2009) Geometrically, this means that in the older eye the peripheral tips of the lacunae (cistern branch tips, video clip #7) attached to the vitreous zonule are not pulled forward but rather in a predominately centripetal direction; this in turn increases the backward pressure on the central cistern “trunk.”

Our data show accommodative posterior movement of fluid and fibers in the central vitreous (including Cloquet’s Canal) all the way to the optic nerve region. This perhaps places more pressure on the optic nerve region. Further, in a previous study we showed that the contour of the nasal scleral/corneal complex at the limbus bowed inward with age (in both monkeys and humans) and the inward bowing was more pronounced during the maximum accommodative response (induced pharmacologically by 2 drops of 4% pilocarpine in human subjects and by central electrical stimulation in monkeys).(Croft et al., 2013)b This phenomenon may also contribute to an increase in backward pressure on the optic nerve region. Ruggeri et al.,^{[Ruggeri et al., ARVO 2018]} using OCT imaging, reported no change in the shape of the conjunctiva/scleral complex during accommodation in five subjects. However, the subjects were young (ages 22 through 39 years), the OCT field of view only included the region posterior to the scleral spur, the OCT image may or may not have been in a completely sagittal plane, and the amplitude of the induced voluntary accommodative response was only 4 diopters.

Whether the accommodative movements inside the vitreous are via the cistern-like structure or not, the posterior movement of the central vitreous toward the optic nerve region suggests that there is a fluid current and pressure gradient (pressure increase) toward the optic nerve region during accommodation and a decrease during disaccommodation. If one thinks of the ‘zeroing in’ that must occur during normal maintenance of focus, there must be a cyclical increase/decrease pressure sequence, perhaps with millisecond frequency, at the optic nerve head. In addition, the posterior lens pole in the resting older eye is in a more posterior position versus the young eye (due to the aged thickened lens) and in turn the depth of the vitreous is decreased. The reduced distance between the posterior lens pole and the posterior pole of the eye may result in reduced central vitreous fluid movement but not necessarily a reduced pressure gradient toward the optic nerve. These phenomena, in combination with the predominantly centripetal direction of the muscle’s accommodative movement in the aged eye, may also increase pressure toward the vitreous compartment/optic nerve. The accommodative movements may decrease with age, and the age at which they essentially completely “time out”—and when accommodation is completely gone—is the approximate age when the incidence of POAG begins to appear and increase. (Kaufman et al., 2019) (Hitzl et al., 2007,Tham et al., 2014,Tuck and Crick 1998,Weinreb and Khaw 2004)

The accommodative posterior/anterior segment fluid movements between the anterior chamber and the vitreous compartments during accommodation/disaccommodation represents fluid displacement in response to muscle contraction and allows for lens thickening. The fluid movement between the anterior chamber and the vitreous compartments may allow for fluid exchange and also allow for the elimination of waste particles/material/molecules from the vitreous by transfer to the anterior chamber and then out of the eye through the trabecular meshwork and uveoscleral aqueous outflow pathways.

How can the cistern-like structure occupy the vitreous space when histological sections show Cloquet’s Canal and numerous vitreous strands that extend in a straight course from the posterior pole of the eye to the vitreous base and/or anterior face of the vitreous/lens/ciliary muscle complex? The answer may lie in the exact location of the section obtained within the globe. The cistern-like structure is layered, and the layer of vitreous strands and Cloquet’s Canal may lie between the layers/branches of the cistern. The vitreous becomes increasingly liquid with age, possibly due to biochemical changes within the vitreous separating collagen from hyaluronic acid (HA).(Sebag 1989) Another possibility is that many years of rapid eye movements, including accommodation, could have “mechanical” effects and could also separate collagen from HA, increasing the liquid portion of the vitreous.(Sebag 1989)

With age, the vitreous strands begin to adhere to one another and accumulate at the peripheral vitreous, while the central vitreous becomes progressively more liquid.(Sebag 1992,Sebag 1998,Sebag and Balazs 1985) Age-related aggregation of vitreous fibers peripherally might contribute to posterior restriction of the ciliary muscle, vitreous zonule and the PVZ INS-LE—and dampen both the accommodative lens shape change and the accommodative fluid dynamics. Eliminating such restrictions might restore muscle mobility, fluid exchange, and facilitate accommodating IOL function. Identification and study of all the intraocular accommodative structures will increase understanding of how the eye accommodates earlier in life, and might change how we understand and treat presbyopia.

4.8. The mechanism of accommodation

Beyond the “basic” mechanism of accommodation, we can now write a new and more complete description of accommodation (Animation video clip #7) The ciliary muscle contracts, releasing tension on the anterior zonula and allowing the capsule/lens to become more spherical, and the iris and the anterior hyaloid to bow backward (video clips 2, 7). During accommodation, the ciliary muscle pulls all of these structures forward and centripetally: 1) the vitreous zonule; 2) the PVZ INS-LE; 3) the cisternal branch tips; and 4) the choroid at the ora serrata. The PVZ INS-LE acts like a strut to the posterior lens equator and facilitates forward lens equator movement and lens shape change. The cistern branch tips are pulled forward and centripetally during accommodation. and the central vitreous moves posteriorly and facilitates posterior lens pole movement and lens shape change. During accommodation in the young eye, the muscle moves forward and inward, placing tension on the elastic choroid all the way back to the optic nerve region and pulling the choroid centrifugally, with the force centered around the optic nerve. In the older eye, the choroid stiffens, dampening muscle forward mobility, but muscle contractility remains, and the stiffening of the choroid may increase accommodative tension spikes on the optic nerve as the muscle contracts. Forward muscle movement is diminished more than centripetal movement, since the restriction is from the posterior direction in direct opposition to forward muscle movement. Thus, centripetal movement is still available and largely maintained in the older eye, and the need for centripetal movement is increased to achieve a given level of zonular relaxation and a given level of accommodative amplitude. Geometrically, the increased demand for centripetal muscle movement may also generate more pressure toward the optic nerve during accommodation, due to the attachments of the cisternal structure to the vitreous zonule/muscle complex, i.e., as if fitting the ”hypotenuse” formed by the cisternal branches into a shorter A/P distance to the optic nerve region, displacing vitreous fluid posteriorly toward the optic nerve head. The base of the cistern may also be subject to the accommodative tensional forces exerted by the choroid at the optic nerve region.

The accommodative movements of the lens/capsule complex cannot explain the accommodative movements within the vitreous, given the small amount of force that can be generated by the capsule and given that the vitreous movements occur in absence of the lens/capsule complex. Rather, we hypothesize that, in addition to the capsule molding the lens, the accommodative movements of the vitreous (driven by ciliary muscle contraction) also facilitate accommodative lens thickening. The anterior lens pole moves anteriorly, the anterior chamber shallows, and the posterior lens pole moves posteriorly during accommodation.(Croft et al., 2016,Croft et al., 2013,Croft et al., 2013) For this to happen, the accommodative central vitreous movement has to allow for or perhaps even facilitate the posterior movement of the lens posterior pole, and at the same time the PVZ INS-LE provides accommodative forward strut-like support to the peripheral posterior lens (Croft et al., 2016) to support the accommodative forward movement of the lens equator and lens shape change (thickening).(Croft et al., 2016)

4.9. Conclusion

Accommodation, presbyopia and accommodative pressure and tension spikes

During accommodation the entire central vitreous instantaneously moves posteriorly, implying a pressure gradient toward the optic nerve head region—perhaps inducing an accommodative pressure spike toward the optic nerve head and/or adding another biomechanical stressor to the nerve or the feeding retinal ganglion cell axons. In addition, the intravitreal fluid and structural movements during accommodation need to be considered in relation to shear stress, especially given that there are also accommodative tension spikes on the optic nerve head via the choroid, induced by ciliary muscle contraction (Croft et al., 2022). Studies are needed to measure pressure within different regions of the eye during accommodation. (Kwon et al., 2019) Further, it has long been posited that choroidal tension may reduce ciliary muscle movement in the aged eye; (Tamm et al., 1992) thus, measurement of age-related choroidal tension changes in monkeys may provide insight into presbyopia. Measurement of accommodative pressure changes and their age-related changes at accommodation-relevant sites may provide further insight into the accommodation mechanism and presbyopia.

5.0. Comment

There has been an arbitrary anatomic division of ocular physiology, pathophysiology and clinical ophthalmology into anterior and posterior segment dichotomies over the past few centuries. This dichotomy may be necessary for hyperspecialized clinical training, but it may be counterproductive for understanding basic disease pathophysiology for presbyopia, the world’s most common ocular disease, and glaucoma, the world’s most common cause of irreversible visual loss.