eMedicine Specialties > Ophthalmology > Presbyopia
Presbyopia - Cause and Treatment
Updated: Jan 26, 2010
Introduction
The scleral expansion band procedure for the surgical reversal of presbyopia (SRP) is a new technique. The scleral expansion band procedure has been developed for SRP. The effects of the scleral expansion band are based on a recently developed theory by Schachar that states that the crystalline lens is under increased equatorial zonular tension during accommodation. An understanding of demonstrable clinical effects of the scleral expansion band procedure, based upon the Schachar theory, requires a revision of historically held views concerning the mechanism of accommodation.
Helmholtz Theory of Accommodation
According to Helmholtz, during accommodation, when the optical power is greatest, the zonules are relaxed and the crystalline lens can shift. The lens would not be stable while reading or examining close objects. The instability of the crystalline lens during near vision did not seem physically correct. When viewing through an optical system, the higher the magnification, the more stable the system needs to be.
According to Helmholtz's hypothesis, since the equatorial diameter increases with age (ie, since the crystalline lens equator is getting closer to the ciliary muscle), the zonules should relax. As one ages, the power of the crystalline lens should increase while viewing distant objects in the accommodated state. One should become more myopic and the crystalline lens should become unstable, but in fact, one becomes slightly hyperopic and the crystalline lens remains stable. The Helmholtz theory also is not consistent with the decrease in spherical aberration, that is, the negative shift that occurs during accommodation.
Helmholtz attributes the universal linear decrease in the amplitude of accommodation with age to hardening of the crystalline lens. No tissue in the body uniformly hardens in a linear fashion with age. During cataract extraction, it commonly is observed that crystalline lenses have different degrees of hardness and no uniform loss of water content of the crystalline lens with age has been demonstrated.
Even though the elastic modulus of pieces of thawed lenses, following freezing in liquid nitrogen for 2 weeks, appears to be significantly higher than that of fresh lenses, these measurements do not demonstrate any significant change in elastic modulus of lenses from patients younger than 40 years. Furthermore, the speed of ultrasound through the lens is directly related to the elastic modulus of the lens. The speed of ultrasound remains constant with age. Surgeons use a constant ultrasonic speed to accurately predict the required optical power of an intraocular lens for cataract surgery. The in vivo optical density of lenses of patients younger than 39 years is constant. Only about 40% of patients develop nuclear sclerosis. The time constant and the peak velocity of accommodation are a constant in subjects younger than 40 years. These observations all suggest that hardness of the lens is not linearly related to age and, therefore, cannot be the basis for presbyopia.
Schachar Theory of Accommodation
An outward equatorial displacement of the crystalline lens produces central steepening. This counterintuitive phenomenon is demonstrated readily by pulling on the equator of a biconvex air-filled Mylar balloon, prolate vesicles, or a freely floating water droplet in space. Once a biconvex object has a long oval shape, similar to the human lens, the surface of the object will steepen centrally and flatten in its periphery in response to equatorial traction. A small increase in the equatorial diameter of the biconvex long oval object will induce a large change in its radius of curvature.
This can be readily demonstrated by observing the reflections from the center of a balloon as equatorial traction is applied. The central reflection becomes smaller or minifies, as shown in the image below.
Reflection in the center of the balloon.
The reflection from its periphery enlarges, as depicted in the image below.
Reflection in the periphery of the balloon.
This counterintuitive phenomenon occurs independent of the wall thickness of the object or the compressibility of the enclosed material.
The equatorial displacement of the crystalline lens occurs as a result of increased tension on the equatorial zonules, produced by contraction of the anterior radial muscle fibers of the ciliary muscle; this is shown in the following images.
In the unaccommodated state, all the zonules are under tension (a). According to the Schachar theory, in the accommodated state, the equatorial zonules are under increased tension, and the anterior and posterior zonules are relaxed (b).
Schema of the configuration of the eye in the unaccommodated state.
Schema of the configuration of the ciliary body in the accommodated state according to the Schachar theory.
Since an active force is involved in accommodation, the amount of force that the ciliary muscle can apply is dependent on how much the ciliary muscle is stretched.
The crystalline lens is of ectodermal origin and continues to grow throughout life. Except for the progressive myope, the dimensions of the scleral shell do not change significantly after 13 years. The distance between the ciliary muscle and the equator of the lens decreases throughout life. Therefore, the effective force that the ciliary muscle can apply to the lens equator is reduced in a linear fashion with age. The amplitude of accommodation decreases linearly with age resulting in presbyopia and is a consequence of normal lens growth.
Augmentation of Accommodative Function
Any procedure that increases the distance between the lens equator and the ciliary muscle should reverse presbyopia. In the mid 1980s, the author performed scleral expansion based on these concepts by making multiple incisions in the sclera over the ciliary muscle. The scleral incisions produced accommodative amplitude increases of only +1.25 diopters (D) in young presbyopes and the effect regressed. In the mid 1990s, Fukusaku independently confirmed these observations and predictions.
In 1992, the first scleral expansion band procedure was performed using a plastic polymethyl methacrylate (PMMA) circular band sutured to the sclera; this band is shown in the image below.1
Polymethyl methacrylate band.
The results were dramatic. Presbyopic patients had as much as 10 D of accommodation. Since that time, the scleral expansion band procedure has been modified and improved, so that now a separate PMMA segment is placed in each of the 4 oblique quadrants of the eye, as shown in the images below.
Incisions for placement of the polymethyl methacrylate band.
Placement of the polymethyl methacrylate band.
To date, the worldwide experience with the scleral expansion band procedure for the SRP involves more than 500 eyes with a range of accommodative recovery of 1.3-7 D with a mean of 3.25 D. In general, the response has been favorable with no change in distance refraction, best-corrected visual acuity, or axial length. Common adverse effects that resolve in 6-8 weeks include subconjunctival hemorrhage, transient astigmatism, fluctuating near vision, and dry eyes.
Comparison of the Helmholtz Theory and the Schachar Theory
The Schachar theory of accommodation has met considerable reaction and discussion, especially from those subscribing to the Helmholtz theory. Many past experiments have been published that are in disagreement with Schachar's conclusions that the crystalline lens diameter increases during accommodation. A careful examination of these experiments reveals that a systematic error exists. Movement occurs between the imaging device and the eye. Measurement of the thickness of the cornea in the accommodated and unaccommodated states of these experiments reveals a change in corneal thickness and curvature. Since corneal curvature and corneal thickness do not change during accommodation, these experiments are flawed and cannot be used to reveal the mechanism of accommodation.
Since accommodation involves a small displacement of the lens equator, to make accurate measurements, experiments must have controls, high-resolution instrumentation, triangulation, and eye tacking to ensure image correspondence for proper image registration. Unfortunately, almost all of the experiments involving accommodation have not satisfied these requisites.
Experiments by Glasser and Kaufman2 and their groups are similarly flawed. Although they placed sutures in the cornea as a reference point, neither the sutures, nor the corneal Purkinje images, of the accommodated and unaccommodated images register, demonstrating that eye movement occurred between the imaging device and the eye. They stated that the small amount of eye movement observed in their experiments cannot account for the changes in the crystalline lens size and configuration during Edinger Westphal or pharmacologic stimulation; however, they offer no controls to prove that this statement is true.
Interestingly, when they fixated the lateral rectus, so that eye movement was reduced, they observed that the crystalline lens equator moved toward the sclera, with anterior and posterior zonular relaxation. They state that the movement of the crystalline lens equator toward the temporal sclera is caused by lateral translation of the crystalline lens. This is mechanically impossible. Since the crystalline lens is denser than water and vitreous, when the anterior and posterior zonules are relaxed, the crystalline lens equator can only move toward the temporal sclera by an active generated force (eg, by the pull of the equatorial zonules). The author has used computer image analysis to correct for the spurious eye movement and registered the images from the experiments of Glasser and Kaufman. When this was done, the lens equator moved toward the sclera, less than 100 µm, as predicted by the Schachar theory.
The importance of eye movement relative to the imaging device is exemplified by MRI studies performed on accommodating patients by Strenk et al.3 An MRI image of the patient's eye during accommodation revealed that the eye is turned nasally and that a change in the configuration of the orbital bones occurred. Therefore, both the head and eye moved during accommodation.
Measurement of the transverse diameter of the globe, the corneal diameter, and the equatorial diameter of the crystalline lens in the unaccommodated and accommodated states of the MRI images by Strenk et al demonstrates that all these measurements decrease during accommodation. This means that the image plane of the eye in the unaccommodated and accommodated states was not the same. Their observations are due to artifact, and any conclusion that they make concerning the mechanism of accommodation from this MRI study is not valid. The consequence of not using controls and image registration is that it leads to erroneous conclusions. For example, Strenk et al have claimed that the lens compresses during accommodation and that the equatorial diameter of the lens does not change with age.
The lens consists of 35% water and 65% protein. The speed of ultrasound through the lens in vivo is a constant (approximately 1641 m/s). The bulk modulus is a measure of the compressibility of a material. The higher the bulk modulus is, the less the compressibility of the object. Water has a bulk modulus of 2.2 GPa. The bulk modulus is related to the speed of sound by the following formula:
Bulk Modulus = Density of the Lens X (Speed of Sound in the Lens)2
Since the density of the lens is equal to 1064 kg/m3 and the speed of sound in the lens is 1641 m/s, and both remain constant with age, the bulk modulus of the lens is approximately 2.8 GPa throughout life. The speed of ultrasound does not significantly change with accommodation. Therefore, the lens is negligibly compressible with a bulk modulus similar to water.
The lens stroma is totally of ectodermal origin and, therefore, grows throughout life. The stem cells of the lens are located at the lens equator. The embryonic lens is essentially spherical, and, at birth, the lens is a long oval with a minor-to-major axis ratio equal to approximately 0.6, which decreases throughout life as a result of lens equatorial growth. The equatorial diameter of an infant's lens is approximately 7 mm, and the equatorial diameter of a 70-year-old patient's lens is approximately 9.6 mm.
Linear and nonlinear finite element mathematical analyses have been performed on the human crystalline lens. Nonlinear finite element analysis is used routinely to reliably predict reality. The mathematical analysis demonstrates that only equatorial stretching of the equator of the crystalline lens by the equatorial zonules can produce the clinically observed increase in central optical power accompanied by a decrease in spherical aberration. The models demonstrate that the Helmholtz theory of accommodation is physiologically and anatomically impossible. The Helmholtz theory requires more force than is physiologically possible and a larger equatorial circumlental space than is anatomically present.
Scanning electron microscopy has shown the following 3 types of zonules: anterior, posterior, and equatorial. The equatorial zonules act similar to a skeletal muscle tendon and are the components that transduce the force of the ciliary muscle to change the focal power of the crystalline lens during accommodation. The anterior and posterior zonules are tense during distance vision and relax during accommodation. The anterior and posterior zonules act similar to the ligaments of skeletal joints and are stabilizing components, predominately for distance vision.
Since only the anterior and posterior zonules can be visualized with a slit lamp in vivo during accommodation, it is understandable how incorrect deductions have been made. The equatorial zonules have a separate and distinct insertion into the ciliary body. Because of the increased tension applied by the equatorial zonules, the crystalline lens remains stable during accommodation. Investigators have demonstrated that the crystalline lens is stable and that gravity does not affect the amplitude of accommodation. While experiencing 8 times the Earth's gravitational force (or 8 g's), astronauts have normal accommodative amplitudes.
In vivo measurements of the position of the crystalline lens equator of young human research subjects during pharmacologically controlled accommodation using high-frequency, high-resolution anterior segment ultrasound revealed that the crystalline lens moves toward the sclera during accommodation. The mean movement was 6.8±1 µm/D. This amount of equatorial movement during accommodation was consistent with the predictions of the nonlinear finite element models and demonstrated that accommodation is a small displacement phenomenon (ie, <5% change occurs in the equatorial diameter of the crystalline lens during accommodation).
The small amount of equatorial crystalline lens movement explains the problems and the systemic errors that have occurred during previous experiments that try to determine the position of the crystalline lens equator during accommodation. Eye movements are much larger than the movement of the crystalline lens equator; therefore, proper controls are essential to interpret any measurements.
Table 1. Comparison of the Helmholtz Theory and the Schachar Theory
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Table
| Test | Helmholtz | Schachar | Observation |
| Equatorial traction | Decrease in central optical power | Large increase in central optical power | Large increase in central optical power |
| Spherical aberration with accommodation | Positive shift | Negative shift | Negative shift |
| Gravity effects accommodation | Yes | No | No |
| Refractive change with presbyopia | Myopic | Hyperopic | Hyperopic |
| Anterior disinsertion ciliary muscle | Myopic | Hyperopic | Hyperopic |
| Change in circular ciliary muscle with aging | Atrophy | Hypertrophy | Hypertrophy |
| Change in anterior radial ciliary muscle with aging | No effect | Atrophy | Atrophy |
| Required force | >300 mN 10 X ciliary muscle capacity | <50 mN | Ciliary muscle capacity <50 mN |
| Required change in lens diameter | >4000 m m | <300 m m | Must be <2000 m m |
| Etiology of presbyopia | Sclerosis | Normal equatorial lens growth | Lenses <40 years are soft not sclerotic; equatorial diameter grows throughout life |
| Effect of tight 12:00 corneal suture | Cornea flattens in vertical meridian "against the rule astigmatism" | Cornea steepens in vertical meridian "with the rule astigmatism" | Cornea steepens in vertical meridian "with the rule astigmatism" |
| Theory has widespread applications | No | Yes | Profiles of: balloons, oil films, vesicles, magnetic fluids, ocean tides, spiral galaxy |
| Test | Helmholtz | Schachar | Observation |
| Equatorial traction | Decrease in central optical power | Large increase in central optical power | Large increase in central optical power |
| Spherical aberration with accommodation | Positive shift | Negative shift | Negative shift |
| Gravity effects accommodation | Yes | No | No |
| Refractive change with presbyopia | Myopic | Hyperopic | Hyperopic |
| Anterior disinsertion ciliary muscle | Myopic | Hyperopic | Hyperopic |
| Change in circular ciliary muscle with aging | Atrophy | Hypertrophy | Hypertrophy |
| Change in anterior radial ciliary muscle with aging | No effect | Atrophy | Atrophy |
| Required force | >300 mN 10 X ciliary muscle capacity | <50 mN | Ciliary muscle capacity <50 mN |
| Required change in lens diameter | >4000 m m | <300 m m | Must be <2000 m m |
| Etiology of presbyopia | Sclerosis | Normal equatorial lens growth | Lenses <40 years are soft not sclerotic; equatorial diameter grows throughout life |
| Effect of tight 12:00 corneal suture | Cornea flattens in vertical meridian "against the rule astigmatism" | Cornea steepens in vertical meridian "with the rule astigmatism" | Cornea steepens in vertical meridian "with the rule astigmatism" |
| Theory has widespread applications | No | Yes | Profiles of: balloons, oil films, vesicles, magnetic fluids, ocean tides, spiral galaxy |
For the first time, the theory has predicted methods to surgically reverse presbyopia, to produce a single element variable focus lens that can have rapid large optical power changes from small equatorial displacement, and to treat and to prevent ocular hypertension and primary open-angle glaucoma. The continued challenge will be to perform properly controlled experiments and to see how this theory will provide new tools and better methods for improving the visual performance of patients.
Historical Perspective of the Theories of the Mechanism of Accommodation
The healthy, young human (<40 y) or young primate eye can rapidly focus on near and distant objects (ie, it can change focus or accommodate). The mechanism by which the eye can accomplish this amazing task has been speculated upon for centuries. Initially, it was suggested that the eye was divinely created; therefore, it did not follow known physical laws of optics.
In 1619, Scheiner, a Jesuit priest, proved that accommodation occurred as a result of a change in the optical power of the eye and that the eye obeyed the laws of optics. His experiment easily is duplicated and consists of making 2 vertical pinholes in a card, which are separated by less than the diameter of the pupil of the eye. The observer looks through both holes simultaneously and focuses on a needle held perpendicular to the plane of the holes. When focusing on the needle, it will appear single; however, if the observer focuses on a more distant or near object, the needle will appear doubled. This simple elegant experiment demonstrates that the eye functions as an optical system.
The explanation of Scheiner's experiment is demonstrated in the image below.
Scheiner experiment.
Consider a point source of light as the object. A convex lens converges the rays of light to a point. By placing a card containing 2 holes between the point source and the convex lens, only 2 rays are brought to a focus. If the power of the convex lens is changed, then the 2 rays are brought to a focus at a different distance. The point source appears doubled at all other distances. If the card has 3 or 4 holes, the point source will triple or quadruple.
Some of the most famous philosophers and scientists were interested in how the eye accommodates. In 1611, Kepler and others thought the crystalline lens moved forward and backward. In 1677, Descartes suggested that the shape of the crystalline lens changed. In 1742, Lobe postulated that the shape of the cornea changed. Sturm and Listing believed that the eye elongated.
In 1801, Thomas Young, using ingenious experiments, provided evidence that accommodation occurs as a result of changes in shape of the crystalline lens. He had very prominent eyes. Without anesthesia (which had not been discovered yet) he placed a caliper, that had rings attached to each side, around his eye. With his eye rotated nasally, he placed 1 ring on his cornea and the other ring over his macula. He could see a circular entopic ring induced by the ring on his macula. As he changed his point of focus, the entopic ring did not change size. This proved that the eye does not elongate during accommodation.
Next, he calculated the amount the cornea would have to move forward to account for his accommodative amplitude. Using candles and a front surface mirror engraved with a scale, he did not observe any corneal movement as he changed his point of focus. He further proved that the radius of curvature of the cornea does not play a role in accommodation. He attached a convex lens possessing the optical power of the cornea to the bottom of an eyecup. He filled the eyecup with saline and placed it over his cornea (the forerunner of contact lenses). The saline in contact with the cornea eliminated the refractive power of the cornea; yet, he was still able to fully accommodate.
Young demonstrated that accommodation did not occur in aphakes. He realized that accommodation had to result from a change in position or shape of the crystalline lens. He was convinced that accommodation could not occur because of forward or backward movement of the crystalline lens. He calculated that the crystalline lens would have to move 10 mm to account for his amplitude of accommodation. This would be impossible.
Young observed that spherical aberration decreased during accommodation. He concluded that accommodation occurs as a result of a change in shape of the crystalline lens. Since the ciliary body had not been discovered yet, he postulated that the change in shape of the crystalline lens is induced by a muscular mechanism within the crystalline lens.
In 1823, Purkinje noted the reflected images of a candle from the anterior and posterior crystalline lens surfaces. In 1849, Langenbeck was able to observe in a patient that the Purkinje image from the anterior surface of the crystalline lens became smaller during accommodation by using a candle and a magnifying glass. He correctly concluded that the anterior surface of the crystalline lens becomes more convex during accommodation. He proposed that the ciliary muscle, which had been discovered independently by Bruecke and Bowman in 1847, squeezes the crystalline lens.
In 1851, Cramer followed up on Langenbeck's observation and improved on it by making a device that incorporated a telescope to allow accurate observations of the Purkinje images during accommodation. He observed that the anterior surface of the crystalline lens became more convex, but the posterior surface did not change shape.
In 1855, Helmholtz improved on the Cramer device by placing crossed glass plates between the patient's eye and the telescope, so that the Purkinje images were doubled and could be measured more accurately. In addition to observing that the anterior and posterior surfaces of the crystalline lens became more convex, he noted that the lens became thicker during accommodation. He hypothesized that the ciliary muscle relaxes during accommodation allowing the lens to become more spherical under the influence of its own elasticity. According to his hypothesis, the equatorial diameter of the lens should decrease as it becomes more spherical during accommodation. He postulated that presbyopia, the loss of accommodation with age, occurred as a result of lens sclerosis (ie, loss of elasticity of the lens with age).
In 1864, Donders studied the change of the amplitude of accommodation with age. He found that the amplitude of accommodation declined in a linear fashion with age. This decline occurs universally and predictably. If patients are corrected properly for distance vision, their age can be determined within 1.5 years by measuring their amplitude of accommodation. Donders also observed that patients become slightly hyperopic when they become presbyopic.
In 1901, Tscherning examined the curvature changes of the anterior crystalline lens surface by observing the changes in the Purkinje images when 4 lights are used as objects. He placed the lights so that 2 formed reflected images from the central anterior surface and 2 formed reflected images from the peripheral anterior surface of the crystalline lens. He observed that the central images moved closer together during accommodation, while the peripheral images moved further apart. He concluded that the crystalline lens was becoming more convex centrally but was becoming flatter in the periphery during accommodation. This was consistent with Young's observation that the spherical aberration of the eye decreases during accommodation.
The Helmholtz theory did not explain the peripheral flattening of the crystalline lens without additional assumptions. For example, the iris constricts during accommodation and it was imputed to produce the peripheral flattening of the crystalline lens. However, von Graefe had demonstrated accommodation in a patient with a total iridectomy.
Tscherning postulated that during accommodation the ciliary muscle exerted tension on the crystalline lens, pressing the crystalline lens against the anterior vitreous. The resistance of the vitreous transmitted sufficient force to effect central bulging of the anterior surface of the crystalline lens. His theory predicts that the central thickness should decrease during accommodation. He did not accept Helmholtz's measurements of increasing crystalline lens thickness with accommodation. Tscherning thought that presbyopia was the result of enlargement of the crystalline lens nucleus. All subsequent theories Gullstrand (1911), Fincham (1937) used Helmholtz's hypothesis that the zonules are relaxed during accommodation. Helmholtz's hypothesis and subsequent modifications attribute presbyopia to sclerosis of the crystalline lens stroma or capsule, atrophy of the ciliary muscle, or stiffening of the ciliary muscle attachments.
Based on these theories for the mechanism of accommodation, the amplitude of accommodation could be increased only by softening the lens stroma and/or capsule, rejuvenating the ciliary muscle by somehow reversing ciliary muscle atrophy, or reversing ciliary body fibrosis. Since none of these methods are clinically possible there has been no surgical therapy for increasing the amplitude of accommodation and reversing the symptoms of presbyopia.
Methods for Treating Presbyopia
Presbyopia initially was treated with near vision optical aids using magnifying lenses, reading glasses, and monocles. Patients were constantly removing reading glasses and losing them because the reading glasses interfere with vision at all other distances. Benjamin Franklin fused the distance lens with the near reading lens to give us bifocals that were later modified to trifocals. The problem with these reading aids is that they only allow sharp near vision at a given distance and the near visual field is limited by the lens. Patients must learn to rotate their eyes downward when reading with bifocals instead of rotating their head. It usually takes 2-3 weeks for patients to get used to wearing bifocals. Trifocals can even be more of a problem for many patients.
To avoid the problems of bifocals and trifocals, bifocal contact lenses have been developed. The bifocal contact lenses generally have been unsuccessful because the distance and near powers of the contact lens must be crowded into an area that can barely cover the pupil. The patient must learn how to shift the contact lens and to ignore the distant or near image according to the visual task.
Multifocal glasses and multifocal contact lenses also are generally not satisfactory. Multifocal lenses produce multiple images at various focal points. Light reflected or emitted by an object must be dispersed by the multifocal lens over all the focal points. Therefore, the intensity at any given focal point will be reduced and the contrast sensitivity diminished. To avoid prismatic effects, the visual field of a multifocal lens is reduced. In addition, the patient must learn to select the appropriate image.
The problems with bifocal and multifocal contact lenses forecast the problems that have, and will continue to occur, with attempts at making a bifocal or multifocal cornea using LASIK or using intracorneal lenses, phakic intraocular lenses, or aphakic intraocular lenses.
The so-called accommodating intraocular lenses are based on the Helmholtz theory of accommodation or a variation of it in which an increase in vitreous pressure is assumed to occur during accommodation. Patients have normal accommodative amplitude after vitrectomy; therefore, the vitreous is not required for accommodation. Intraocular pressure decreases during accommodation. Partial coherence interferometry (PCI) has definitively demonstrated that the length of the vitreous chamber does not increase during accommodation and that the entire lens does not move forward during accommodation. The decrease in depth of the anterior chamber that occurs during accommodation is due to the increase in central thickness of the lens and not to a forward movement of the whole lens.
The proposed mechanism of action of the Human Optics ICU accommodating lens is based on the Helmholtz theory of accommodation. The concept is that, with accommodation, the diameter of the lens capsule should decrease, which would squeeze the ICU and cause it to move forward. PCI has demonstrated that the ICU lens does not move forward during normal accommodation.
The Eyeonics Crystalens AT45 is based on the erroneous idea that there is increased vitreous pressure during accommodation. The vitreous pressure supposedly moves the intraocular lens forward during accommodation. Contrary to these presumptions, PCI has demonstrated that the Eyeonics Crystalens AT45 does not move forward during normal accommodation.
Monovision as a treatment for presbyopia generally is accepted by fewer than 30% of the population. The loss of stereopsis and learning to ignore a blurry image from one half of the binocular visual field easily accounts for the patient's distress with monovision.
Summary
Treatments for presbyopia have not been very good because the physiological mechanism of the crystalline lens has not been restored. The Schachar theory of accommodation states the following:
- Increased equatorial zonular tension occurs during accommodation.
- Presbyopia is due to a decrease in the effective working distance of the ciliary muscle as a result of normal crystalline lens growth.
Based on this theory, the accommodative amplitude of presbyopes can be increased.
Any technique that increases the effective working distance of the ciliary muscle, the distance between the ciliary muscle and the crystalline lens equator, increases the amplitude of accommodation physiologically.
Table 2. Comparison of Methods for Treating Presbyopia
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Table
| Focus at Multiple Near Distances | Full Clear Visual Field | Reversible | Large Range of Correction | Effect Regresses | Normal Stere-opsis | Negative Cosmetic Impli-cations | Physio-logical | Involves a Surgical Technique | Halos at Night | Potential for Serious Compli-cations | |
Reading glasses | No | No | Yes | Yes | N/A | Yes | Yes | No | No | No | None |
Bifocals glasses | No | No | Yes | Yes | N/A | Yes | Yes | No | No | No | None |
Trifocals glasses | No | No | Yes | Yes | N/A | Yes | Yes | No | No | No | None |
Monovision | No | No | Yes | Yes | N/A | No | No | No | No | No | None |
Multifocals glasses | Yes | No | Yes | Yes | N/A | Yes | No | No | No | No | None |
Bifocal contacts | No | No | Yes | Yes | N/A | Yes | No | No | No | No | Minimal |
Multifocal contacts | Yes | No | Yes | Yes | N/A | Yes | No | No | No | No | Minimal |
Intracorneal lenses | No | No | Yes | Yes | No | Yes | No | No | Yes | No | Significant |
Intracorneal multifocal lenses | Yes | No | Yes | Yes | No | Yes | No | No | Yes | Yes | Significant |
Phakic intraocular lenses | No | No | Yes | Yes | No | Yes | No | No | Yes | Yes | Significant |
Phakic multifocal intraocular Lenses | Yes | No | Yes | Yes | No | Yes | No | No | Yes | Yes | Significant |
LASIK produced bifocal cornea | No | No | No | Yes | No | Yes | No | No | Yes | Yes | Significant |
LASIK produced multifocal cornea | Yes | No | No | Yes | No | Yes | No | No | Yes | Yes | Significant |
Scleral incisions | Yes | Yes | Yes | No | Yes | Yes | No | Yes | Yes | No | Minimal |
Scleral expansion band | Yes | Yes | Yes | Yes | No | Yes | No | Yes | Yes | No | Minimal |
| Focus at Multiple Near Distances | Full Clear Visual Field | Reversible | Large Range of Correction | Effect Regresses | Normal Stere-opsis | Negative Cosmetic Impli-cations | Physio-logical | Involves a Surgical Technique | Halos at Night | Potential for Serious Compli-cations | |
Reading glasses | No | No | Yes | Yes | N/A | Yes | Yes | No | No | No | None |
Bifocals glasses | No | No | Yes | Yes | N/A | Yes | Yes | No | No | No | None |
Trifocals glasses | No | No | Yes | Yes | N/A | Yes | Yes | No | No | No | None |
Monovision | No | No | Yes | Yes | N/A | No | No | No | No | No | None |
Multifocals glasses | Yes | No | Yes | Yes | N/A | Yes | No | No | No | No | None |
Bifocal contacts | No | No | Yes | Yes | N/A | Yes | No | No | No | No | Minimal |
Multifocal contacts | Yes | No | Yes | Yes | N/A | Yes | No | No | No | No | Minimal |
Intracorneal lenses | No | No | Yes | Yes | No | Yes | No | No | Yes | No | Significant |
Intracorneal multifocal lenses | Yes | No | Yes | Yes | No | Yes | No | No | Yes | Yes | Significant |
Phakic intraocular lenses | No | No | Yes | Yes | No | Yes | No | No | Yes | Yes | Significant |
Phakic multifocal intraocular Lenses | Yes | No | Yes | Yes | No | Yes | No | No | Yes | Yes | Significant |
LASIK produced bifocal cornea | No | No | No | Yes | No | Yes | No | No | Yes | Yes | Significant |
LASIK produced multifocal cornea | Yes | No | No | Yes | No | Yes | No | No | Yes | Yes | Significant |
Scleral incisions | Yes | Yes | Yes | No | Yes | Yes | No | Yes | Yes | No | Minimal |
Scleral expansion band | Yes | Yes | Yes | Yes | No | Yes | No | Yes | Yes | No | Minimal |
Multimedia
 | Media file 1: Atopic keratoconjunctivitis. |
 | Media file 2: Reflection in the center of the balloon. |
 | Media file 3: Reflection in the periphery of the balloon. |
 | Media file 4: In the unaccommodated state, all the zonules are under tension (a). According to the Schachar theory, in the accommodated state, the equatorial zonules are under increased tension, and the anterior and posterior zonules are relaxed (b). |
 | Media file 5: Schema of the configuration of the eye in the unaccommodated state. |
 | Media file 6: Schema of the configuration of the ciliary body in the accommodated state according to the Schachar theory. |
 | Media file 7: Polymethyl methacrylate band. |
 | Media file 8: Incisions for placement of the polymethyl methacrylate band. |
 | Media file 9: Placement of the polymethyl methacrylate band. |
 | Media file 10: Scheiner experiment. |
Keywords
vision loss, visual deficit, presbyopia cause, presbyopia treatment, scleral expansion band procedure, surgical reversal of presbyopia
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References
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Further Reading
Keywords
vision loss, visual deficit, presbyopia cause, presbyopia treatment, scleral expansion band procedure, surgical reversal of presbyopia
