Introduction
“There is no other portion of physiological optics where one finds so many differing and contradictory ideas as concerns the accommodation of the eye, where in the most recent time have we actually made observations, where previously everything was left to the play of hypotheses.” Hermann von Helmholtz (1909)
It is primarily due to Helmholtz that we owe our current “basic” understanding of the accommodative mechanism of the human eye ( Fig. 3.1 ). His insight came from his own work and from pioneers before him. Thomas Young was instrumental in demonstrating that accommodation occurs, not through changes in corneal curvature or axial length as those before him believed, but through changes in the curvature of the lens. Young’s painstaking anatomical investigations were insufficient for him to rule out the possibility that the crystalline lens received direct innervation from a branch of the ciliary nerves to allow it to contract as a muscle. It was only after the work of Crampton, who first described the ciliary muscle from his investigation of bird eyes, that a mechanistic description of how the ciliary muscle might alter lens curvatures was proposed by Müller. Understanding of human accommodation was mired by confusion from numerous investigations of the eyes of birds and other vertebrates, studied for their comparatively large size to gain insight into the human accommodative mechanism ( Box 3.1 ). However, these species are now known to accommodate through mechanisms quite different from humans. Current understanding of human accommodation stems from the work of many early investigators including Brücke, Cramer, Hess, Müller, Helmholtz, and Gullstrand. This path was made tortuous by the diversity of accommodative mechanisms of the various vertebrates studied. The wide diversity of avian visual habitats (aerial, aquatic, terrestrial), eye shapes (tubular, globose, and flattened), and feeding behaviors in all likelihood dictates their accommodative needs. Corneal accommodation, of considerable value to terrestrial birds, is of no value to aquatic birds where the corneal optical power is neutralized under water. The evolutionarily divergent accommodative mechanisms, or the absence of accommodation in other vertebrates, is, by reasonable conjecture, determined by feeding behaviors. Herbivorous animals (sheep, horses, cows, etc.), those that forage and dig for food primarily using olfactory cues (pigs), or those with nocturnal eyes and relatively poor visual abilities (mice, rats, rabbits) have little need for accommodation. Carnivores have better-developed ciliary muscles than these other species, but still have relatively little accommodative ability; the raccoon is the only nonprimate terrestrial mammal with substantial accommodative amplitude. Cats are suggested and raccoons and fish shown to translate the lens forward without lenticular thickening. Other adaptations in the lens, iris, or retina allow other lower vertebrates functional near and distance vision, although these cannot be classified as true accommodation as they rely on static optical adaptations. Among the vertebrates that do accommodate, amplitudes vary considerably. Diving birds have among the largest amplitudes, with cormorants having ~50 diopters (D) and diving ducks suggested to have 70 to 80 D. Among the mammals, vervet and cynomolgus monkeys have approximately 20 D, young rhesus as much as 40 D and raccoons about 20 D. Humans, for only a few short childhood years, may have a maximum of about 10 to 15 D measured subjectively or about 7 to 8 D measured objectively, but find much less accommodation adequate for most visual tasks. Although accommodative amplitude gradually declines until completely lost by about age 50 years, to most individuals the deficit appears to be of sudden onset when the accommodative amplitude is diminished to a few diopters as presbyopia develops. Although full presbyopes may read at intermediate distances, this is almost certainly due to depth of field (see section Depth of Field) resulting from pupil constriction rather than active accommodation. The word presbyopia (Greek, presbys meaning an aged person and opsis meaning vision) possibly derives from Aristotle’s use of the term presbytas to describe “those who see well at distance, but poorly at near.” Historically the term was used to describe the condition in which the near point has receded too far from the eye due to a diminution in the range of accommodation. Despite the wealth of studies of accommodation on vertebrates, only primates are shown to systematically lose the ability to accommodate with increasing age. It may be no coincidence that although absolute life spans differ considerably, the relative age course of the progression of presbyopia is similar in humans and monkeys ( Fig. 3.2 ).
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Accommodation is a dioptric change in optical power of the eye due to ciliary muscle contraction.
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The basic mechanism of accommodation occurs largely in accordance with the mechanism originally proposed by Helmholtz.
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Ciliary muscle contraction moves the apex of the ciliary body towards the axis of the eye and releases resting zonular tension around the lens equator.
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When zonular tension is released, the elastic lens capsule molds the young lens into a more spherical and accommodated form.
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During accommodation, lens diameter decreases, lens thickness increases, the anterior lens surface moves anteriorly, the posterior lens surface moves posteriorly, and the lens anterior and posterior surface curvatures increase; the thickness of the nucleus increases, but without a change in thickness of the cortex.
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The increase in curvature of the lens anterior and posterior surfaces results in an increase in optical power of the lens.
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The physical changes in the lens and eye result in an increase in optical power of the eye to focus on near objects.
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During accommodation the ciliary muscle pulls forward the vitreous zonule, the PVZ-INS-LE and the cistern branch tips while the anterior hyaloid bows backward and the central vitreous moves backward, including the central cistern.
Accommodation
Accommodation is a dynamic optical change in the dioptric power of the eye allowing the point of focus of the eye to be changed from distant to near objects. Accommodation changes the focus of the eye from the far point to the near point. The far point is the position of the object that is imaged sharply on the retina when the eye is unaccommodated. The near point is the position of the object that is imaged sharply on the retina when the eye is under maximal accommodation. In primates this is mediated through a contraction of the ciliary muscle, release of resting zonular tension around the lens equator, a decrease in lens equatorial diameter, and a “rounding up” of the crystalline lens through the force exerted on the lens by the lens capsule. The increased optical power of the lens is achieved through increased anterior and posterior surface curvatures and overall increased thickness. In an emmetropic eye (an eye without refractive error), the far point is a distant object located at or beyond what is considered optical infinity for the eye (6 m or 20 ft). When an object is brought closer to the eye, the eye must accommodate to maintain a clearly focused image on the retina. Myopic eyes, typically too long for the optical power of the lens and cornea combined, are unable to attain a sharply focused image for objects at optical infinity unless optical compensation is provided, such as through negative-powered spectacle lenses. The far point of young myopes is an object at a distance closer to the eye than optical infinity, i.e., myopes can see sharply at a near distance when the eye in unaccommodated. Young hyperopic eyes, however, are typically too short for the optical power of the lens and cornea and require correction with a positive spectacle lens to view objects at infinity sharply. Young hyperopes can focus on objects at optical infinity by increasing the optical power of the eye through accommodation.
Optics of the eye
Light from the environment enters the eye at the cornea and, in an emmetropic eye, is brought to a focus on the retina through the combined optical power of the cornea and the lens (see Chapter 1 ). Specific details for schematic eyes are given in Bennett and Rabbetts. Accommodation is caused by a change in power of the lens. In the unaccommodated eye, the lens represents approximately 30% of the total power of the eye. The lens power is determined by the curvature of the anterior and posterior surfaces and by the presence of a refractive index gradient inside the lens. The lens refractive index progressively increases from ~1.379 at the surface of the cortex to ~1.410 at the center of the nucleus of the lens. The gradient refractive index (GRI) of the lens adds additional optical power because the gradient results in refraction of light throughout the lens. This results in light taking a curved path rather than a straight path through the lens. For simplified optical calculations, the more complex GRI of the lens is often substituted with a single equivalent refractive index value. An equivalent refractive index lens would need to have a value that is greater than the highest refractive index value at the center of the GRI lens, in order to have the same shape and optical power as the GRI lens.
The posterior lens surface is more steeply curved, and therefore has higher refractive power than the anterior surface. The lens anterior and posterior radii of curvature and the GRI change with age because of continuous lens growth, with a more pronounced change in the anterior surface. It is these surfaces that become more steeply curved to allow the accommodative increase in optical power of the lens to occur. Historically, it was suggested that the posterior lens surface does not move and that the posterior lens surface curvature does not change appreciably with accommodation. , , However, it is now known that the posterior lens surface does undergo an increase in curvature and moves posteriorly during accommodation as the lens thickness increases. Gullstrand suggested that the lens equivalent refractive index must change during accommodation. As the lens shape, axial thickness, and equatorial diameter change during accommodation, this dictates that the form of the GRI of the lens must also change during accommodation. , However, this does not require a change in the equivalent refractive index of the lens during accommodation, at least to the extent that resolution limits of currently available technology can detect.
The optical requirements for accommodation
The optical power of the crystalline lens increases (i.e., the lens focal length decreases) during accommodation. As a consequence, the eye changes focus from distance to near so the image of a near object is brought to focus on the retina. The dioptric change in power of the eye defines accommodation, and accommodation is measured in units of diopters (D). A diopter is a reciprocal meter and is a measure of the vergence of light. Light rays from a point object diverge and are by convention designated to have negative vergence. Light rays converging toward a point image are designated to have positive vergence (see Chapter 1 ). An object at optical infinity subtends zero vergence at the cornea. The optical interfaces of the eye (the cornea and lens) add positive vergence to draw light rays toward a focus on the retina ( Fig. 3.3 ). When an object is moved from infinity to a point closer to the eye, the near object subtends divergent rays on the cornea. To focus on the near object, the optical power of the eye must increase to add positive vergence to the now divergent rays to bring the refracted rays to a focus on the retina. When an emmetropic eye is focused on a distant object the eye is considered unaccommodated. If the eye accommodates from an object at optical infinity to an object 1.0 m in front of the eye, this represents 1.0 D of accommodation. If the eye accommodates from infinity to 0.5 m in front of the eye, this is 2 D of accommodation; from infinity to 0.1 m is 10 D, and so on. The accommodative amplitude (in diopters) is close to, but not exactly equal, to the increase in optical power of the eye.
Depth of field
Clinically, the nearest point of clear vision is typically measured subjectively in an eye corrected for distance vision. This is done using the push-up test: a near reading chart is moved toward the eyes while the subject is asked to report when they can no longer sustain clear vision on the near target or when the near target first becomes blurred.
Although the reciprocal of this near reading distance expressed in meters is clinically referred to as the accommodative amplitude, this is technically inaccurate. The push-up test is a subjective measure of the near point expressed in units of diopters. However, this is not a measure of the true dioptric change in power of the eye because the eye’s depth of field results in an overestimation of the dioptric change in the eye’s optical power by about 1 to 2 D compared with the objectively measured accommodative response amplitude. Subjective testing of this nature in complete presbyopes might lead one to believe that about 1 D of accommodation is present, but this is not a true change in optical power of the eye and is sometimes called pseudoaccommodation. , ,
Visual acuity
In addition to the depth of field of the eye, acuity and contrast sensitivity of the eye affect the subjective measurement of the near point of clear vision. Increased illumination provides higher contrast on the target, thus, smaller changes in focus or blur of the target are more easily detected. While increasing the level of illumination will help to improve the contrast sensitivity and acuity, this will also decrease the pupil size and will thereby increase the depth of field of the eye, resulting in a nearer point of perceived clear vision. Further, in cases of cataract or other opacities of the ocular optical media, the image of a near object is not seen clearly, thus small changes in the focus of the image are less readily detected. With increasing age, the optical clarity of the lens decreases and the prevalence of cataract increases. Retinal disease can also affect visual acuity. Elderly patients often have reduced visual acuity and/or reduced contrast sensitivity, not solely owing to decreased optical performance.
The anatomy of the accommodative apparatus
Grossly, the accommodative apparatus of the eye consists of the ciliary body (including the ciliary muscle and the ciliary pigmented and nonpigmented epithelium), the choroid, the anterior and posterior zonular fibers, the lens capsule, and the crystalline lens ( Fig. 3.4 ). The ciliary muscle forms the active component of the accommodative apparatus. This specialized smooth muscle, with cellular elements of fast striated muscle (see section The ciliary muscle) is attached anteriorly to Schwalbe’s line by collagenous tendons and posteriorly to the choroid by elastic tendons. These elastic tendons then insert into the elastic lamina of Bruch’s membrane and the choroid, and thus includes their ultimate functional insertion into the elastic ring around the optic nerve head. The muscle is comprised of three muscle fiber groups, oriented respectively longitudinally, radially (obliquely), and circularly in the outer, middle, and inner portions of the ciliary muscle. Contraction of the muscle stretches the posterior elastic tendons, the muscle moves anteriorly and inwardly along the curved inner scleral surface, and the muscle’s circular portion thickens so that the distance between lens and ciliary body shortens. The elastic zonula between the ciliary body and lens equator is thus relaxed, loosening the tension at the lens equator. The crystalline lens, consisting of a central nucleus, a surrounding cortex, and an outer collagenous elastic lens capsule, “rounds up,” thereby increasing optical power. In addition to lens rounding a recent study in human subjects demonstrated that the lens equator moves forward during accommodation and is correlated with accommodative amplitude.
Improved optical methods have shown that the zonula consists of different portions, serving different functions. The anterior zonular fibers span the circumlental space extending from the valleys of the ciliary processes and inserting all around the lens equator and the anterior vitreous membrane. They constitute the suspensory elements of the crystalline lens. The posterior zonular fibers (or pars plana zonules that line the inner aspect of the pars plana ) intermingle with the anterior zonula at the transition between posterior pars plicata and anterior pars plana (zonular plexus), and extend from the plexus and the pars plana to the region of the ora serrata and the vitreous membrane facing the pars plana ( Fig. 3.4 ). Presumably, the posterior zonula tenses as the anterior zonula relaxes, helping to prevent and smooth larger movements of the ora serrata during anterior movement of the ciliary muscle. The posterior zonula is distinct from zonular fibers inserting into the outer vitreous membrane (intermediate vitreous zonule) thus forming a fluid-filled cleft between the vitreous and the pars plana zonule, ( Figs. 3.4 and 3.5 ). Attached posteriorly at the insertion zone (near the ora serrata), both the intermediate vitreous zonules and the PVZ-INS LE ( Fig. 3.6 ) extend forward in a straight course to attach to the zonular plexus or the posterior lens equator, respectively ( Fig. 3.4A ). The cleft between the pars plana zonule of the ciliary body and the vitreous membrane is bridged by bands of intermediate vitreous zonule fibers. The cleft could provide a low-friction interface between the ciliary epithelium/pars plana zonule and vitreous membrane when the ciliary muscle moves anteriorly and posteriorly during accommodation and disaccommodation, increasing the efficiency of the system. Thus, the vitreous zonula allows a sliding of the ciliary body during accommodation/disaccommodation, with only a little friction.
Theoretical suggestions that the vitreous plays a role in accommodation exist, although empirical evidence implies no need for the vitreous in accommodation. A recent study found evidence, including quantifiable intravitreal accommodative movements, indicating a vitreous role in the mechanism of accommodation (also see section The vitreous). In addition, accommodative choroidal movements that extend to the optic nerve region have been quantified.
The ciliary body
The ciliary body occupies a triangular-shaped region that lies between the scleral spur and retina. It is bounded on its outer surface by the anterior sclera and on its inner surface by the nonpigmented ciliary epithelium. The anterior ciliary body begins at the scleral spur at the angle of the anterior chamber. The base of the iris inserts into the anterior ciliary body. Posterior to the iris, the ciliary processes are found at the anterior-innermost point of the ciliary body and form the corrugated pars plicata of the ciliary body. The smooth surface of the ciliary body, the pars plana, is posterior to the pars plicata. The most posterior aspect of the ciliary body is joined to the ora serrata of the retina. The outer surface of the ciliary body beneath the anterior sclera is the suprachoroidal lamina, or supraciliarus, and is formed by a thin layer of collagen fibers, fibroblasts, and melanocytes. The ciliary epithelium has two layers of secretory cells. The inner layer, the nonpigmented epithelium (NPE), is made up of columnar cells and faces the posterior chamber and the vitreous body. The NPE is apposed apex-to-apex to the outer layer of ciliary epithelium made up of cuboidal cells, the highly pigmented epithelium (PE). The PE and the NPE express ultrastructural differences that seem to result from different functional demands. Ultrastructural differences exist between the ciliary NPE cells at the tips of the processes and those in the valleys, the former being adapted for fluid secretion and the latter for mechanical anchoring of the zonule. The length of the ciliary body, from the tips of the ciliary processes to the ora serrata, is longest temporally and shortest nasally.
The ciliary muscle
The ciliary muscle occupies a triangular-shaped region within the ciliary body beneath the anterior sclera ( Fig. 3.7 ). It has an anterior origin at the scleral spur in close proximity to Schlemm’s canal. , Anterior ciliary muscle tendons insert into the scleral spur, the elastic network of the trabecular meshwork, the inner wall endothelia of Schlemm’s canal, and Schwalbe’s line, which collectively serve as a fixed anterior anchor against which the ciliary muscle contracts. Posterior to the scleral spur, the outer surface of the ciliary muscle is attached only loosely to the inner surface of the anterior sclera, allowing the muscle to slide forward and backward as it contracts and relaxes during accommodation and disaccommodation. The posterior ciliary muscle attaches to the stroma of the choroid. Here the muscle forms true elastic tendons, which insert into the choroidal elastic network. The anterior and inner surfaces of the ciliary muscle are bounded anteriorly by the stroma of the pars plicata and posteriorly by the pars plana of the ciliary body. The ciliary muscle fiber bundles beneath the sclera are oriented such that a contraction of the ciliary muscle results in a forward and inward redistribution of the mass of the ciliary body and a narrowing of the ciliary ring diameter owing to sliding ciliary muscle movement along the inner surface of the sphere formed by the anterior sclera. This causes the choroid to be pulled forward not only in the region of the ora serrata, but also in the region of the optic nerve, perhaps putting tension on the optic nerve. , ,
The three muscle fiber groups that comprise the ciliary muscle, identified by their relative positions and orientations as detailed previously, form a morphologically and functionally integrated three-dimensional structure. The major group of muscle fibers is the peripheral meridional or longitudinal fibers, or Brücke’s muscle. They extend longitudinally between the scleral spur and the choroid adjacent to the sclera. Located inward to the longitudinal fibers are the reticular or radial fibers. These fibers are branching V- or Y-shaped fibers and constitute a relatively smaller proportion of the ciliary muscle. They are attached anteriorly to the scleral spur and the peripheral wall of the anterior ciliary body at the insertion of the iris. Beneath the radial fibers and positioned more anteriorly in the ciliary body and closest to the lens are the equatorial or circular fibers, or Müller’s muscle. These constitute the smallest proportion of the ciliary muscle. The division of the ciliary muscle into three muscle fiber groups is somewhat artificial. In reality, there is a gradual transition from the outermost longitudinal muscle fibers to the radial fibers to the innermost circular muscle fibers, with some intermingling of the different fiber types. A contraction of the ciliary muscle results in a contraction of all three muscle fiber groups together. When the ciliary muscle contracts there is a gradual rearrangement of the muscle bundles, with an increase in thickness of the circular portion and a decrease in thickness of the radial and longitudinal portions of the muscle. Contraction of the ciliary muscle as a whole pulls the anterior choroid forward (inducing centrifugal choroidal movements around the optic nerve , ), resulting in a forward and inward redistribution of the mass of the ciliary body and a narrowing of the ciliary ring diameter. The valleys of the ciliary processes with the inserted zonules move toward the lens equator and serve the primary function of releasing resting zonular tension at the lens equator to allow accommodation to occur. The ciliary muscle bundles, each containing around 6 to 12 individual muscle fibers, collectively are surrounded/bound by a sheath composed of thin flattened fibroblasts or connective tissue cells. ,
The ciliary muscle is a smooth muscle, with a dominant parasympathetic innervation causing accommodative contraction mediated by M3 muscarinic receptors. Upon disaccommodation, there is β2-adrenergic receptor–mediated relaxation of the ciliary muscle, so that the posterior elastic tendons and the elastic choroid can pull the muscle backward. The stretched tendons and choroid retract, the muscle is pulled backward, and the inner circular edge of the muscle is diminished.
Only in the inner circular portion are there nitrergic ganglia that innervate the circular muscle cells. Connections to mechanoreceptors in the inner ciliary body facing the attachment of anterior zonular fibers indicate that this innervation is important for fine regulation of accommodation.
In general the ciliary muscle is atypical for smooth muscles in its speed of contraction, the large size of its motor neurons, the distance between the muscle and the motor neurons, and the unusual ultrastructure throughout the ciliary muscle cells, which in some ways resembles skeletal muscles (indeed, in birds of prey it is a striated skeletal muscle). There are also regional differences in ultrastructure and histochemistry of the primate ciliary muscle, suggesting that the longitudinal portion may be acting like a fast skeletal muscle to “set” or “brace” the system rapidly, for the contraction of the inner portion to be most effective. Thus, the ciliary muscle fibers have some intracellular elements analogous to fast striated muscle; indeed, in primates this is the fastest smooth muscle in the body.
In the young eye both forward (anterior; Fig. 3.8A ) and inward (centripetal; Fig. 3.8B ) muscle movement is required for efficient and maximal accommodation to occur ( 
; Fig. 3.8C ). The amount of centripetal lens equator movement parallels the centripetal ciliary muscle movement, but the amount of centripetal muscle movement required is greater than centripetal lens equator movement. Thus, the system requires a greater amount of centripetal muscle movement than centripetal lens equator movement. The amount of muscle movement in both vectors is similar in humans and monkeys. , ,
.The zonular fibers
The zonular apparatus is a complex meshwork of fibrils 70 to 80 nm in diameter and grouped into fiber bundles, which are estimated to be between 4 to 6 and 40 to 50 microns (μm) in diameter. , From these bundles fine fibrils separate to attach to the basement membrane (inner limiting membrane) of the nonpigmented ciliary epithelium. The principal constituent of the zonule is fibrillin-1, but there are also noncollagenous carbohydrate–protein mucopolysaccharide and glycoprotein complexes that are secreted by the ciliary epithelium. The fibrillin-rich elastic zonular microfibrils are thought to be much more elastic than the lens capsule. Their primary function is to stabilize the lens and allow accommodation. Because the zonule is not a continuous tissue but is composed of fibers, it allows fluid flow from the posterior chamber behind the iris to the vitreous chamber.
Anterior zonule
The attachment of the zonular fibers to the lens capsule is superficial, with few fibers penetrating into the capsule to form a mechanical (possibly similar to Velcro) or chemical union. From scanning electron microscopy this anterior zonule crossing the circumlental space and extending to the lens is alternatively described as:
- (1)
consisting of three fiber strands running to the anterior, equatorial, and posterior lens surfaces, or
- (2)
fibers that insert along a circular line on the anterior and posterior surface of the lens with some fibers inserting directly on the equator, , , or
- (3)
a zonular fork with two main fiber groups extending to the lens anterior and posterior surfaces with finer bundles seemingly of relative unimportance running to the lens equator, or
- (4)
successive sagittal lines of insertion from lens anterior to posterior surface and two coronal lines of insertion, one where the fibers insert onto the capsule around the anterior surface and another where the fibers insert onto the capsule around the posterior surface.
Although no systematic crossing of anterior zonular fiber was observed by McCulloch, crossing of anterior zonular fibers has been observed in other preparations , , and was documented in early diagrams from histology ( Fig. 3.9 ) and in live monkeys in which the iris was totally removed and a fluorescent dye injected into the anterior chamber. From histologic preparations, when an appropriate plane of section is obtained, a continuous line of zonular insertion into the entire lens equator is seen. Unfixed, dissected human eye specimens show a continuous meshwork of fibers uniformly covering the entire lens equator, and show crossing of zonular fibers.
Zonular plexus and posterior zonule
In the attachment zone, the lateral walls of the ciliary processes and the transition zone between pars plicata and pars plana, the zonular fibers form broad, flattened strands crossing and joining each other and the attaching posterior zonular fibers to form the zonular plexus. This plexus functions as a fulcrum. During disaccommodation the fulcrum moves posterolaterally, exerting traction directly to the anterior zonular fibers. During accommodation the pars plana zonule (tension fibers) is stretched, relaxing the anterior zonule.
Anteriorly located zonule as a tool for fine regulation in accommodation
An additional system of fine zonular fibers derive from the valleys of the anterior pars plicata, opposite to structures resembling mechanosensors in the ciliary body, and join the main anterior zonule at their insertion to the lens. Changes in tension of these fibers can stimulate the mechanoreceptors that show nervous connections to the nitrergic nerve cells, relaxing innervation of the circular portion of the muscle. This zonular system could help to fine-regulate accommodation. ,
Vitreous zonule
The vitreous zonule connects the vitreous membrane in the region of the ora serrata with the ciliary plexus (intermediate vitreous zonule), as well as the plexus with the anterior vitreous membrane posterior to the lens capsule (anterior vitreous zonule). Observations of the ciliary region during accommodation show that the posterior ciliary body slides forward against the curvature of the anterior sclera, moving the posterior insertion zone of the intermediate vitreous zonular fibers forward ( Figs. 3.4 and 3.5 ; 
). In addition, contraction of the ciliary muscle pulls the muscle’s posterior attachment forward owing to forward and inward movement of the ciliary muscle and ciliary process tips. This suggests that in addition to allowing nearly resistance-free gliding of the muscle during contraction, the intermediate vitreous zonular fibers may similarly assist in pulling the ciliary muscle back to the unaccommodated configuration after cessation of an accommodative effort.
The PVZ-INS LE strand
In addition to the insertion to the vitreous membrane there are strands of vitreous zonules attaching to the lens equator (PVZ-INS LE). The PVZ-INS LE strand is attached to the vitreous membrane at the insertion zone posteriorly (near the region of the ora serrata) and extends forward to attach directly to the posterior lens equator ( Fig. 3.4A ). , The PVZ-INS LE strand is pulled/pushed forward by the ciliary muscle during accommodation and remains straight during the accommodative response (i.e., it does not relax) ( Figs. 3.4 and 3.6 ). The lens equator also moves forward during the accommodative response. The vitreous is partially composed of collagen type materials and the PVZ-INS LE may be “stiff,” thereby supplying forward push to the lens equator as the muscle, the PVZ-INS LE, and insertion zone move forward during accommodation. In an older eye with decreased accommodative forward muscle movement, the PVZ-INS LE not only supplies less “forward push,” but may also provide a direct drag against its forward movement and thereby against lens thickening during accommodation.
The vitreous
Recent advances have allowed visualization of the vitreous by ultrasound biomicroscopy, contrast agents (in nonhuman primates) ( Fig. 3.10 ; 
and ), and endoscopy ( Fig. 3.11 ; 
) during dynamic imaging of the accommodative response, providing empirical evidence for a role of the vitreous in accommodation. , , There are numerous connections between the vitreous and the accommodative apparatus, from the front to back of the eye. , , , Histologic and other research has examined the vitreous structure and its possible role in accommodation and presbyopia. , , , Jongbloed and Worst reported on the cistern structure within the vitreous compartment. The base of the cistern resides in the optic nerve region and the branches of the cistern extend forward to the anterior vitreous. In the rhesus monkey the tips of the cistern branches in the anterior vitreous attach to the intermediate vitreous zonule , ( Figs. 3.12 and 3.13 ; 
and ). As the ciliary muscle contracts and moves forward and inward, the lens thickens (the anterior lens pole moves anteriorly and becomes more sharply curved and the central posterior lens pole/capsule moves backward [ Fig. 3.14 ; 
]), , , and the fibrillar structures within the central vitreous (including the anterior hyaloid, Cloquet’s canal, and possibly the cistern trunk ) move posteriorly toward the optic nerve head ( Figs. 3.12, 3.13, 3.15, and 3.16 ; 
,,,,,,, ). The accommodative posterior movements of the central vitreous correlate with accommodative amplitude; the greater the accommodative posterior movement of the central vitreous the higher the accommodative amplitude ( Fig. 3.17 ). These movements have been quantified ( Table 3.1 ; Figs. 3.17 and 3.18 ) and decline significantly with age ( Fig. 3.17 ). The accommodative posterior movement of the central vitreous includes the region of the vitreous very near the optic nerve head ( 
,,, ). This strongly suggests that there is a fluid wave—and consequently a pressure change—impacting the nerve head. Simultaneously, the fibrillar peripheral vitreous, some of which is attached to the intermediate vitreous zonule (including the tips of the cistern branches near the anterior vitreous ; 
and ), moves anteriorly ( Fig. 3.18 ) and inwardly. , The accommodative forward movement of the cistern branch tips correlate with accommodative amplitude; the greater the forward movement of the cistern branch tips, the higher the accommodative amplitude ( Fig. 3.18 ). These movements are reversed in disaccommodation. The accommodative forward movement of the cistern branch tips tends to decline with age ( Fig. 3.18 ). In addition to pressure gradients, the fluid movements may generate shear stress at the nerve head. Whether these vitreal forces are bad, good, or irrelevant for the nerve is impossible to say, but they likely gradually decrease with age (see further in this section) and become small once presbyopia becomes complete, again at about the age when primary open angle glaucoma begins to appear. , Of course, the above effects may not be on the nerve directly, but rather on the astrocytes and other glial elements associated with the nerve head and the lamina. Several other phenomena occur with age in addition to the likely significant reduction in accommodative vitreous movements, including an age-related increase in lens thickness. In the older eye the anterior lens pole encroaches on the anterior chamber, and the posterior lens pole is in a more rearward position, encroaching on the anterior central vitreous. With age, there is a large decrease in the forward movement of the ciliary muscle (65% and 85% loss in monkeys and humans, respectively), but less reduction in movement in the centripetal direction (i.e., ~20%). , Furthermore, the central vitreous liquifies with age, perhaps allowing more pressure on the optic nerve via the fluid current, lens position, and accommodative pressure spikes. In the older eye, to achieve zonular relaxation and maximum accommodation, the ciliary muscle moves more in a centripetal direction (rather than forward direction), forcing the tips of the cistern branches backward. Thus, the cistern trunk would be pressed backward toward the optic nerve head (i.e., the hypotenuse of the right triangle (cistern branch) fits into a smaller space).
for movement of the vitreous membrane during accommodation visualized using an endoscopic camera in a 19-year-old rhesus monkey eye. Note anterior hyaloid configuration bowing toward the lens and pars plicata in the resting state, whereas in the accommodated state the anterior hyaloid bows in a posterior direction. An aminofluorescein dye was used as a contrast agent to facilitate visualization of the vitreous membrane or zonula.
. Ultrasound biomicroscopy image in the nasal (left panel) and temporal (right panel) quadrant of a 10-year-old iridectomized rhesus monkey eye. During accommodation the triamcinolone particles (white dots) , suspended in the ocular fluid, flow around the lens equator and into the vitreous compartment toward the anterior hyaloid (left and right panels) and then further into the cleft between the intermediate vitreous zonule (vz) and pars plana (pp) (right panel) . When the red dot appears in the video clip (left panel) , or when the channel (CH) indicator (right panel, lower left) changes from 00 to 11, the stimulus to induce accommodation is on.
. Ultrasound biomicroscopy in the nasal and temporal quadrants of a 7-year-old iridic rhesus monkey eye injected with triamcinolone, which clings to the vitreous membranes. In the presence of the iris, the anterior hyaloid still bows backward during accommodation and does so in parallel with the accommodative backward bowing of the iris. For both left and right panels, when the red dot appears, the stimulus to induce accommodation is on.
. Typical ultrasound biomicroscopy images taken from two separate phakic rhesus monkeys: one with an aniridic eye ( left panel ; age 6) and the other monkey with an iridic ( right panel ; age 7) eye. A red dot indicates when the stimulus to induce accommodation is on. The iris bows backward during accommodation and the posterior lens pole moves posteriorly.
,, . Ultrasound biomicroscopy images in normal ( right panel , iris and lens present), and aphakic and iridectomized ( left and middle ) rhesus monkey eyes following intravitreal injection of triamcinolone. Ages of monkeys posted on the figure and videos. During accommodation following extracapsular lens extraction (ECLE) ( 
), the capsule bows backward and the central vitreous moves posteriorly following ECLE ( 
) and intracapsular lens extraction (ICLE) ( 
; thick green arrows ), and also when the iris and the lens are in place ( 
). Cloquet’s canal also moves posteriorly during accommodation ( 
). AZ , Anterior zonula attached to Wieger’s ligament possibly with capsular remnant ( 
). A red dot indicates when the stimulus to induce accommodation is on for each respective video clip.
. Ultrasound biomicroscopy image in a normal (iris and lens present) rhesus monkey eye (age 19) following intravitreal injection of triamcinolone. The central vitreous moves posteriorly during accommodation all the way to the optic nerve. A red dot indicates when the stimulus to induce accommodation is on.
| Accommodative Backward Bowing of the Anterior Hyaloid | Accommodative Posterior Movements of the Central Vitreous | Accommodative Movements of the Lacunae Peripheral () 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 | 0.540.54 | p=0.008 | 0.15 | p=0.014 |
| Ages 7-13yr | s.e.m.n | 0.025 | 0.055 | 0.115 | 0.045 | ||||
| Older | Mean | 0.21 | p=0.029 | 0.15 | p=0.030 | 0.22 | p=0.052 | 0.13 | p=0.30 |
| Ages 19-25yr | s.e.m.n | 0.043 | 0.012 | 0.053 | 0.023 | ||||
| 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.n | 0.028 | 0.057 | 0.098 | 0.028 | |||||
The sclera
During accommodation in both monkeys and humans, there is a slight change (a small notch) in the scleral contour in the region of the limbus when they are young ( Fig. 3.19 ). In the older resting eye, there is inward bowing of the sclera in the region of the limbus (increased concavity), and with it, significant changes in the geometry of the ciliary muscle/sclera/lens complex. The inward bowing of the sclera in the resting older eye is more pronounced during accommodation.
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